Medicine – Risk Factors for Osteoarthritis

Osteoarthritis (OA) is a chronic degenerative joint disorder characterised by progressive loss of articular cartilage, changes in subchondral bone, and varying degrees of synovial inflammation. Although ageing is one of the strongest risk factors, osteoarthritis usually develops through a combination of mechanical stress, genetic susceptibility, previous joint damage, metabolic factors, and abnormalities of joint structure.

1. Increasing Age

Age is one of the most important risk factors for osteoarthritis. With advancing age, articular cartilage becomes less able to repair itself after repeated mechanical stress. Changes in cartilage composition, subchondral bone, muscle strength, and joint mechanics all contribute to the increasing prevalence of OA in older adults.

2. Female Sex

Women have a higher risk of osteoarthritis, particularly after middle age and after menopause. The reason is likely multifactorial and may involve hormonal influences, anatomical differences, and differences in joint biomechanics. Hand and knee osteoarthritis are especially common in women.

3. Genetic Predisposition

A family history and genetic susceptibility can increase the likelihood of developing OA. Genetic factors appear particularly important in osteoarthritis of the hands, hips, and spine. Inherited differences in cartilage structure, bone shape, or connective tissue may make certain joints more vulnerable to degeneration.

4. Obesity

Obesity is a major modifiable risk factor, especially for osteoarthritis of weight-bearing joints such as the knees and hips. Excess body weight increases the mechanical load passing through these joints and accelerates cartilage wear. Adipose tissue may also contribute through metabolic and inflammatory mediators, so the effect is not purely mechanical.

5. Joint Hypermobility

Hypermobility can increase abnormal movement and mechanical stress within a joint. Repeated excessive movement may reduce joint stability and expose articular surfaces to uneven loading, thereby increasing the risk of degenerative change over time.

6. Previous Joint Trauma

A history of significant joint injury is an important cause of secondary osteoarthritis. Fractures that extend through the articular surface are particularly important because they can permanently alter joint congruity and load distribution. Ligament injuries and meniscal damage may also increase later OA risk by producing chronic instability or abnormal biomechanics.

7. Chondrocalcinosis

Chondrocalcinosis, usually related to calcium pyrophosphate deposition, is associated with osteoarthritic change. Crystal deposition can damage cartilage and alter joint mechanics, sometimes producing a pattern of degeneration that differs from typical primary OA.

8. Previous Septic Arthritis

Septic arthritis can cause rapid and severe destruction of articular cartilage. Even after the infection has been successfully treated, residual cartilage damage and joint deformity can predispose the affected joint to secondary osteoarthritis.

Developmental Conditions

9. Congenital or Developmental Hip Abnormalities

Abnormal joint development can produce uneven loading of articular surfaces and markedly increase later OA risk. Developmental dysplasia or congenital dislocation of the hip can alter the relationship between the femoral head and acetabulum, leading to premature degeneration of the hip joint.

Perthes’ disease, in which childhood avascular necrosis affects the femoral head, can also leave residual deformity. The altered shape of the hip may cause abnormal mechanical stress and predispose to osteoarthritis in adult life.

Bone Disorders

10. Paget’s Disease of Bone

Paget’s disease produces abnormal bone remodelling and can change the shape and mechanical alignment of bones near a joint. These structural abnormalities alter the distribution of forces across articular cartilage and may lead to secondary osteoarthritis.

Endocrine and Metabolic Conditions

11. Acromegaly

In acromegaly, excess growth hormone and insulin-like growth factor 1 cause enlargement of bones, cartilage, and soft tissues. These changes alter joint structure and mechanics and may eventually result in a characteristic degenerative arthropathy.

12. Haemochromatosis

Haemochromatosis can produce a characteristic arthropathy through iron deposition and associated metabolic effects on cartilage. It is also strongly associated with calcium pyrophosphate deposition, which can further contribute to joint degeneration.

13. Wilson’s Disease

Wilson’s disease has historically been associated with degenerative joint abnormalities in some patients. Disturbances in copper metabolism may contribute to skeletal and joint changes, although this is a much less common risk factor than age, obesity, prior trauma, or developmental abnormalities.

Key Clinical Concept

The major risk factors for osteoarthritis include increasing age, female sex, genetic predisposition, obesity, hypermobility, and previous joint injury. Secondary OA can also result from conditions that damage cartilage or alter joint anatomy, including septic arthritis, chondrocalcinosis, developmental hip disorders, Paget’s disease, and selected endocrine or metabolic diseases.

Medicine – Synovial Fluid Findings in Common Joint Disorders

Synovial fluid analysis is an important investigation in patients presenting with an acutely swollen joint. Examination of the fluid can help distinguish non-inflammatory arthritis, inflammatory arthritis, crystal arthropathy, and septic arthritis. Important characteristics include the fluid’s appearance, clarity, viscosity, white blood cell count, microbiological culture, and the presence or absence of crystals.

1. Normal Synovial Fluid

Normal synovial fluid is typically clear and yellow, with a high viscosity because of its high concentration of hyaluronic acid. Bacterial culture is negative, and the white blood cell count is very low, traditionally below approximately 200 cells/mm³ in the table shown. These findings reflect the absence of significant joint inflammation.

2. Osteoarthritis

In osteoarthritis, synovial fluid is generally classified as non-inflammatory. It remains clear and yellow, has relatively high viscosity, and bacterial culture is negative. The supplied table gives a white blood cell count of less than 200 cells/mm³, although in clinical practice non-inflammatory osteoarthritic fluid can have counts higher than this while generally remaining below about 2,000 cells/mm³.

The relatively preserved clarity and viscosity help distinguish osteoarthritis from inflammatory disorders such as rheumatoid arthritis or septic arthritis.

3. Rheumatoid Arthritis

Synovial fluid in rheumatoid arthritis is inflammatory. It may appear yellow to yellow-green and range from relatively clear to turbid depending on the degree of inflammation. Its viscosity is reduced because inflammatory processes break down hyaluronic acid.

The table gives a white blood cell count of approximately 3,000–50,000 cells/mm³. Bacterial culture is negative because rheumatoid arthritis is an autoimmune inflammatory disease rather than an infection.

4. Bacterial Septic Arthritis

Bacterial arthritis typically produces markedly inflammatory synovial fluid. The aspirate may be purulent and turbid, with low viscosity and a predominance of neutrophils. Bacterial culture may be positive and is crucial for identifying the causative organism and guiding antimicrobial treatment.

The table gives a typical white blood cell count of approximately 50,000–100,000 cells/mm³. However, the count alone cannot confirm or exclude septic arthritis, because lower counts can occur in infection and similarly high counts can occur in crystal arthritis. Suspected septic arthritis therefore requires urgent microbiological assessment and clinical management.

5. Gout

In gout, the synovial fluid is inflammatory and may appear yellow or whitish, while viscosity is reduced. Culture should be negative unless infection is simultaneously present. The table shows a broad white blood cell count of approximately 10,000–150,000 cells/mm³, illustrating that severe crystal-induced inflammation can produce very high cell counts.

The key diagnostic finding is the presence of needle-shaped monosodium urate crystals that demonstrate strong negative birefringence under polarised light microscopy. Importantly, finding urate crystals does not completely exclude simultaneous septic arthritis.

6. Pseudogout

In calcium pyrophosphate deposition disease (CPPD or pseudogout), the synovial fluid is also inflammatory, with low viscosity. It may appear yellow or whitish and can occasionally be bloodstained. The table gives a white blood cell count of approximately 50,000–75,000 cells/mm³, although the actual count varies considerably between patients.

Microscopic examination demonstrates calcium pyrophosphate crystals, which are typically rhomboid-shaped and weakly positively birefringent. As with gout, crystal identification is important for establishing the diagnosis, but infection must still be considered when clinically suspected.

Key Clinical Distinction

The general pattern is that normal and osteoarthritic synovial fluid is relatively clear and viscous with a low white cell count, whereas rheumatoid arthritis, gout, and pseudogout produce inflammatory fluid with reduced viscosity and higher white cell counts. Septic arthritis is particularly important to recognise urgently, as it often produces markedly turbid or purulent fluid with a high neutrophil count and may have a positive bacterial culture.

A major examination point is that synovial white cell counts overlap substantially between septic arthritis and crystal arthropathies, so a high count should not be interpreted in isolation. Gram staining, culture, crystal examination, and the overall clinical presentation must all be considered.

Medicine – Bacteria Associated with Septic Arthritis

Septic arthritis is an infection of a synovial joint caused by invasion of the joint space by a pathogenic microorganism, most commonly bacteria. It is a medical emergency because rapid inflammation can cause irreversible destruction of articular cartilage and subchondral bone. Patients typically present with an acutely painful, swollen, warm joint with markedly restricted movement, sometimes accompanied by fever and systemic illness. The causative organism varies according to age, immune status, underlying disease, and route of infection.

1. Staphylococcus aureus

Staphylococcus aureus is the most common cause of native-joint septic arthritis in adults. Infection usually reaches the joint through haematogenous spread, although direct inoculation following trauma, surgery, or joint procedures can also occur. Risk is increased in patients with diabetes, immunosuppression, intravenous drug use, prosthetic joints, or pre-existing joint disease such as rheumatoid arthritis.

Methicillin-resistant S. aureus (MRSA) should also be considered when appropriate risk factors are present. Because S. aureus can rapidly destroy cartilage, early joint drainage and appropriate antimicrobial therapy are essential.

2. Streptococcus pyogenes

Streptococcus pyogenes, or group A streptococcus, is another recognised cause of septic arthritis. Joint infection may develop following bacteraemia originating from skin, soft-tissue, or other streptococcal infections. The disease can progress rapidly and may be associated with significant systemic toxicity.

3. Streptococcus pneumoniae

Streptococcus pneumoniae is a less common but important cause of septic arthritis. It may occur following pneumococcal bacteraemia associated with infections such as pneumonia or meningitis. Patients with impaired immunity, advanced age, or asplenia may be particularly vulnerable to invasive pneumococcal disease.

4. Mycobacterium tuberculosis

Mycobacterium tuberculosis produces a more chronic form of infectious arthritis rather than the typical rapidly progressive presentation of acute bacterial septic arthritis. Tuberculous arthritis usually develops through haematogenous dissemination from another site of infection and commonly affects a single large weight-bearing joint, particularly the hip or knee.

Symptoms tend to develop gradually, with persistent pain, swelling, stiffness, and progressive loss of joint function. Constitutional manifestations such as fever, night sweats, or weight loss may be absent. Untreated disease can eventually cause extensive joint destruction.

5. Neisseria gonorrhoeae

Neisseria gonorrhoeae is an important cause of infectious arthritis, particularly in sexually active adolescents and younger adults. Disseminated gonococcal infection may initially produce a characteristic combination of migratory polyarthralgia, tenosynovitis, and pustular skin lesions.

Some patients subsequently develop a more localised purulent monoarthritis, frequently involving the knee, wrist, ankle, or elbow. A sexual history and appropriate testing from genital and other relevant mucosal sites are important because cultures obtained directly from the affected joint may sometimes be negative.

6. Salmonella Species

Salmonella species are particularly associated with septic arthritis and osteomyelitis in patients with sickle cell disease or other haemoglobinopathies, although infection can occur in other immunocompromised individuals. Bacteraemia can allow the organism to seed a joint and produce an acute inflammatory arthritis.

Diagnosis and Clinical Importance

When septic arthritis is suspected, urgent arthrocentesis and synovial fluid examination are central to diagnosis. Synovial fluid should generally undergo cell count, Gram staining, culture, and other appropriate microbiological investigations. Blood cultures are also important, particularly when bacteraemia is suspected.

Treatment usually requires prompt antimicrobial therapy together with drainage of the infected joint. Whenever clinically feasible, synovial fluid and blood cultures should be obtained before antibiotics are started, but treatment should not be unnecessarily delayed in a severely ill patient.

Key Clinical Pattern

The major bacterial causes to remember are Staphylococcus aureus, Streptococcus pyogenes, Streptococcus pneumoniae, Neisseria gonorrhoeae, and Salmonella species, while Mycobacterium tuberculosis is an important cause of chronic infectious arthritis. Among these, S. aureus is the most important and common cause of acute native-joint septic arthritis in adults.

Medicine – Gout

Gout is an inflammatory crystal arthropathy caused by the deposition of monosodium urate crystals within joints and surrounding tissues. These crystals form when serum urate levels are persistently elevated, leading to crystal precipitation and an intense inflammatory response. Gout most commonly affects the first metatarsophalangeal joint, producing the classic presentation known as podagra, although virtually any joint can be involved.

1. Monosodium Urate Crystal Deposition

The underlying abnormality in gout is the accumulation of monosodium urate crystals within articular and periarticular tissues. Crystal deposition stimulates a strong inflammatory reaction, resulting in sudden attacks of severe pain, swelling, warmth, and erythema. Repeated attacks can eventually lead to chronic joint damage and the development of tophi.

2. Diagnosis by Joint Aspiration

The most definitive way to diagnose gout is by identifying urate crystals in synovial fluid obtained from an affected joint. Joint aspiration is particularly important when the diagnosis is uncertain or when septic arthritis must be excluded.

3. Crystal Appearance

Under polarised light microscopy, monosodium urate crystals are typically needle-shaped and strongly negatively birefringent. This finding helps distinguish gout from pseudogout, in which calcium pyrophosphate crystals are usually rhomboid-shaped and positively birefringent.

4. Typical Joint Involvement

The first metatarsophalangeal joint is the most characteristic site of gout. An acute attack often presents with sudden, intense pain, swelling, redness, and extreme tenderness, sometimes developing overnight. However, gout can also involve the ankles, knees, wrists, fingers, elbows, and other joints.

Causes of Gout

Hyperuricaemia can result from either increased urate production, decreased renal excretion, or drug-related effects. In many patients, more than one factor contributes.

1. Increased Production of Urate

Increased Purine Synthesis

Some patients overproduce urate because of increased purine synthesis. This may be idiopathic, meaning that no specific cause is identified.

A rare inherited cause is Lesch–Nyhan syndrome, an X-linked disorder of purine metabolism caused by deficiency of hypoxanthine-guanine phosphoribosyltransferase. This leads to excessive purine breakdown and marked hyperuricaemia.

Increased Turnover of Preformed Purines

Conditions associated with rapid cell turnover can produce excessive breakdown of nucleic acids and therefore increased urate formation. These include lymphoproliferative and myeloproliferative disorders, chronic haemolytic anaemias, and other states with accelerated cell destruction.

Cytotoxic chemotherapy can also cause a sudden rise in urate because of rapid tumour cell breakdown, particularly in tumour lysis syndrome.

2. Decreased Urate Excretion

Idiopathic Reduced Excretion

In many patients with gout, the main abnormality is reduced renal excretion of urate without an identifiable secondary cause.

Chronic Renal Failure

Chronic kidney disease reduces the ability of the kidneys to eliminate urate efficiently, leading to progressive hyperuricaemia and an increased risk of gout.

Increased Organic Acids

Certain metabolic states raise the concentration of organic acids that compete with urate for renal tubular excretion. This can occur with alcohol excess, starvation, vigorous exercise, and ketoacidosis, all of which may contribute to acute attacks in susceptible individuals.

3. Drugs

Diuretics

Thiazide and loop diuretics, such as furosemide, can reduce renal urate excretion and increase the risk of hyperuricaemia and gout.

Low-Dose Salicylates

Low-dose salicylates can also reduce urate excretion and therefore contribute to hyperuricaemia. Their effect differs from that of very high salicylate doses, which may increase urate excretion.

Treatment of Gout

Acute Gout

1. NSAIDs

Non-steroidal anti-inflammatory drugs (NSAIDs) are commonly used to treat acute attacks by rapidly reducing joint inflammation and pain. Drugs such as naproxen or other appropriate NSAIDs may be used if there are no contraindications.

2. Colchicine

Colchicine is another effective treatment for acute gout. It suppresses the inflammatory response to urate crystals by interfering with neutrophil activity. It is most effective when started early in the attack.

3. Corticosteroids

Corticosteroids may be used when NSAIDs or colchicine are unsuitable. They can be given systemically or, when one or a few joints are involved, by intra-articular injection after joint infection has been excluded.

Prevention of Recurrent Gout

Allopurinol

Allopurinol is a xanthine oxidase inhibitor that reduces the formation of uric acid. It is used as long-term urate-lowering therapy to prevent recurrent attacks and reduce tophus formation.

Allopurinol is generally introduced as part of a long-term prevention strategy rather than used solely to treat the pain of an acute attack. Serum urate levels are monitored and the dose is adjusted to achieve an appropriate target.

Modern prevention may also include other urate-lowering therapies, such as febuxostat, depending on the patient’s clinical situation, kidney function, drug tolerance, and treatment response.

Key Clinical Pattern

Gout is best remembered as an acute inflammatory arthritis caused by monosodium urate crystals, which are needle-shaped and negatively birefringent. The first MTP joint is the classic site, and the major mechanisms are either excess urate production or reduced renal excretion. Acute attacks are treated with NSAIDs, colchicine, or corticosteroids, while recurrent disease is prevented with urate-lowering therapy such as allopurinol.

Medicine – Pyrophosphate Arthropathy (Pseudogout)

Pyrophosphate arthropathy, commonly called pseudogout or calcium pyrophosphate deposition disease (CPPD), is an inflammatory joint disorder caused by the deposition of calcium pyrophosphate dihydrate crystals within articular cartilage and surrounding joint structures. These crystals can trigger acute episodes of intense joint inflammation that may resemble gout, although the crystal type and typical pattern of joint involvement are different.

1. Calcium Pyrophosphate Crystals

The underlying abnormality is the accumulation of calcium pyrophosphate crystals within the joint. These crystals may remain asymptomatic for long periods or provoke sudden inflammatory attacks when they are released into the joint space.

On polarised light microscopy, calcium pyrophosphate crystals are classically weakly positively birefringent. This helps distinguish them from monosodium urate crystals seen in gout, which are negatively birefringent.

2. Typical Joint Involvement

Pseudogout most commonly affects large joints, particularly the knee, although the wrist, shoulder, elbow, ankle, and other joints may also be involved. Acute attacks usually present with sudden onset of joint pain, swelling, warmth, and restricted movement, closely resembling septic arthritis or gout.

3. Chondrocalcinosis on Radiography

A characteristic radiographic finding is chondrocalcinosis, which refers to visible calcium deposition within articular cartilage or fibrocartilage. This appears as linear or punctate calcification on plain radiographs and is particularly common in structures such as the knee menisci or wrist cartilage.

Chondrocalcinosis supports a diagnosis of CPPD, although its absence does not completely exclude the condition.

Causes and Associations

1. Increasing Age

CPPD becomes much more common with advancing age. In older adults, it may occur without any specific underlying metabolic disorder.

2. Osteoarthritis

There is a strong association between CPPD and osteoarthritis. Calcium pyrophosphate deposition may coexist with degenerative joint disease and can alter the usual distribution or severity of osteoarthritic changes.

3. Familial Predisposition

Some patients have a familial form of CPPD, particularly when disease develops at a younger age. In such cases, inherited abnormalities in cartilage metabolism may promote early crystal deposition.

4. Diabetes Mellitus

Diabetes mellitus has historically been listed among disorders associated with CPPD, although the association is less strong than with some other metabolic conditions.

5. Acromegaly

Acromegaly may predispose to abnormal cartilage and joint changes and has been associated with calcium pyrophosphate crystal deposition.

6. Haemochromatosis

Haemochromatosis is one of the most important metabolic associations. Iron deposition within tissues can alter cartilage metabolism and promote CPPD. Pseudogout occurring in a younger patient should therefore raise consideration of haemochromatosis.

7. Hypothyroidism

Hypothyroidism has also been reported in association with CPPD, although the relationship is weaker and less consistent than with haemochromatosis or hyperparathyroidism.

8. Hyperparathyroidism

Hyperparathyroidism is a recognised metabolic cause of CPPD. Abnormal calcium and phosphate metabolism can promote crystal deposition within cartilage and increase the likelihood of pseudogout attacks.

Treatment

Treatment is primarily symptomatic, with the goal of reducing pain and inflammation during acute attacks. Management may include NSAIDs, provided there are no contraindications, as well as colchicine in selected patients. If a single large joint is involved, aspiration of the joint followed by intra-articular corticosteroid injection can be effective after infection has been excluded.

For patients who cannot receive local therapy or who have multiple inflamed joints, a short course of systemic corticosteroids may be considered. When recurrent CPPD occurs, any associated metabolic disorder such as hyperparathyroidism or haemochromatosis should also be identified and managed appropriately.

Key Clinical Pattern

Pseudogout is best remembered as an acute large-joint inflammatory arthritis caused by calcium pyrophosphate crystals, typically involving the knee, with weakly positive birefringence on crystal analysis and chondrocalcinosis on radiographs. Important associations include advanced age, osteoarthritis, haemochromatosis, hyperparathyroidism, acromegaly, and familial disease.

Medicine – Rheumatoid Arthritis

Rheumatoid arthritis (RA) is a chronic, systemic autoimmune inflammatory disease that predominantly affects the synovial joints. It typically produces a symmetrical inflammatory polyarthritis, particularly involving the small joints of the hands and feet. Persistent synovial inflammation can progressively damage cartilage, bone, tendons, and ligaments, eventually resulting in characteristic joint deformities and functional impairment. Although joint disease is the dominant manifestation, RA is a systemic condition and may also affect the skin, lungs, heart, eyes, nervous system, and other organs.

Revised American College of Rheumatology Criteria for Rheumatoid Arthritis

The 1987 revised American College of Rheumatology (ACR) criteria historically classified a patient as having rheumatoid arthritis when at least four of seven criteria were present. For the first four criteria, the abnormalities were required to have been present for at least six weeks. These are historical classification criteria; modern practice generally uses the 2010 ACR/EULAR classification criteria.

1. Morning Stiffness

Patients characteristically experience morning stiffness lasting for more than one hour. The stiffness usually improves gradually with movement and activity. Under the 1987 criteria, it needed to persist for at least six weeks.

2. Arthritis of Three or More Joint Areas

There should be objective evidence of arthritis affecting at least three joint areas simultaneously. Affected joints demonstrate soft-tissue swelling or synovial fluid accumulation rather than enlargement caused solely by bony changes. This pattern should persist for at least six weeks.

3. Arthritis of the Hand Joints

RA has a strong tendency to involve the hands. At least one area involving the wrist, metacarpophalangeal (MCP), or proximal interphalangeal (PIP) joints should demonstrate inflammatory arthritis for at least six weeks.

4. Symmetrical Arthritis

Joint involvement is typically symmetrical, meaning that corresponding joint areas on both sides of the body are affected. Perfect symmetry is not always present, particularly early in the disease, but a bilateral pattern becomes characteristic as established RA develops.

5. Rheumatoid Nodules

Rheumatoid nodules are firm subcutaneous nodules that commonly develop over pressure points and extensor surfaces, such as around the elbows. They are particularly associated with seropositive and more severe rheumatoid arthritis.

6. Serum Rheumatoid Factor

The presence of rheumatoid factor (RF) supports the diagnosis. Rheumatoid factors are autoantibodies, most commonly IgM antibodies, directed against the Fc portion of IgG. However, RF is not specific to RA and may occur in other diseases and in healthy individuals.

7. Radiological Changes

Characteristic radiographic abnormalities include marginal erosions and periarticular osteopenia, particularly in the hands and wrists. More advanced disease produces joint-space narrowing, deformity, subluxation, and progressive joint destruction.

Joint Involvement in Rheumatoid Arthritis

RA typically causes a symmetrical polyarthropathy. The small joints of the hands and feet are particularly frequently affected, although larger joints and the cervical spine may become involved as the disease progresses.

The MCP, PIP, and metatarsophalangeal (MTP) joints are among the most frequently affected, traditionally reported in approximately 90% of patients with established disease. The wrists, knees, ankles, and subtalar joints have historically been reported as affected in around 80%. Shoulder involvement may occur in approximately 60%, while the hips and elbows may be affected in around 50%. The cervical spine can also become involved, particularly in longstanding disease, and older sources report this in approximately 40% of patients.

Features of the Rheumatoid Hand

1. Symmetrical Deforming Polyarthropathy

Established RA produces a characteristic symmetrical deforming polyarthropathy involving predominantly the MCP, PIP, and wrist joints. Chronic synovitis damages cartilage, bone, tendons, and supporting ligaments, gradually altering the normal alignment and stability of the hand.

2. Sparing of the DIP Joints

The distal interphalangeal (DIP) joints are usually spared in rheumatoid arthritis. Significant DIP involvement should therefore raise consideration of other conditions, particularly osteoarthritis or psoriatic arthritis.

3. Active Synovitis

During active inflammation, affected joints are typically warm, swollen, tender, and painful on movement. The swelling results primarily from synovial inflammation and effusion.

4. Ulnar Deviation

Chronic inflammation and ligamentous damage can cause ulnar deviation of the fingers at the MCP joints. The fingers progressively deviate toward the ulnar side of the hand, producing one of the classic deformities of longstanding RA.

5. Subluxation

Progressive destruction of the supporting structures can lead to subluxation of the MCP joints and wrists. This contributes to instability, deformity, reduced grip strength, and impaired hand function.

6. Swan-Neck Deformity

A swan-neck deformity consists of hyperextension of the PIP joint with flexion of the DIP joint. It develops because chronic inflammatory damage disrupts the normal balance between the tendons and ligaments controlling the finger.

7. Boutonnière Deformity

A boutonnière deformity has the opposite configuration, with flexion of the PIP joint and hyperextension of the DIP joint. It results from damage to the central portion of the extensor tendon mechanism.

8. Z-Deformity of the Thumb

The thumb may develop a characteristic Z-shaped deformity, resulting from abnormal flexion and extension at different thumb joints. This can interfere significantly with pinch and grip function.

9. Wasting of the Dorsal Interossei

Chronic disease may cause wasting of the dorsal interosseous muscles, producing visible hollowing between the metacarpal bones. Disuse, joint dysfunction, and neurological involvement can contribute to this finding.

10. Absence of Psoriatic Nail Changes

Typical psoriatic nail abnormalities, such as nail pitting and onycholysis, are not characteristic of rheumatoid arthritis. Their presence may suggest psoriatic arthritis rather than RA.

11. Nail-Fold Infarcts and Vasculitic Lesions

Severe rheumatoid vasculitis can compromise small blood vessels and produce nail-fold infarcts or other vasculitic skin lesions. These manifestations are more likely in longstanding, severe, seropositive disease.

12. Carpal Tunnel Syndrome

Inflammatory swelling around the wrist may compress the median nerve within the carpal tunnel, producing numbness, tingling, pain, and sometimes weakness in the median nerve distribution. Carpal tunnel syndrome is therefore a recognised neurological complication of RA.

13. Palmar Erythema

Some patients develop palmar erythema, producing diffuse redness of the palms. Although nonspecific, it may accompany active systemic inflammatory disease.

Extra-Articular Features of Rheumatoid Arthritis

Extra-articular disease is more likely in patients with severe, longstanding, and seropositive RA.

Non-Organ-Specific Features

Systemic inflammation may cause constitutional manifestations such as weight loss and fever, while lymphoid activation may result in lymphadenopathy. Rheumatoid nodules are particularly associated with RF-positive disease.

Felty syndrome is an important complication characterised classically by the combination of rheumatoid arthritis, splenomegaly, and neutropenia. Patients may consequently have an increased susceptibility to infection.

Longstanding inflammatory disease may rarely lead to secondary AA amyloidosis, which can particularly affect the kidneys. Patients with RA also have an increased susceptibility to infections, resulting from the disease itself and from immunosuppressive therapy. Osteoporosis may develop because of chronic inflammation, reduced physical activity, and corticosteroid exposure.

Organ-Specific Features

1. Cardiac Manifestations

Cardiac involvement may include pericarditis and pericardial effusion. Valvular abnormalities can also occur, although they are less characteristic. Chronic RA additionally increases the risk of atherosclerotic cardiovascular disease.

2. Pulmonary Manifestations

RA can produce several pulmonary complications, including pleurisy and pleural effusions. Interstitial lung disease may cause pulmonary fibrosis, resulting in progressive breathlessness and impaired gas exchange.

Rheumatoid pulmonary nodules may also occur. Bronchiectasis is another recognised association. Caplan syndrome refers to the development of rheumatoid pulmonary nodules in individuals with pneumoconiosis, classically coal workers exposed to mineral dust.

3. Neurological Manifestations

Neurological disease may result from nerve compression, vasculitis, or cervical spine involvement. Carpal tunnel syndrome is a common example of a compressive neuropathy.

Rheumatoid vasculitis may cause mononeuritis multiplex, in which several individual peripheral nerves become damaged. In the cervical spine, particularly at the atlantoaxial joint, instability or subluxation can compress the spinal cord and produce cervical myelopathy.

4. Renal Manifestations

The kidneys are not usually directly affected by classic rheumatoid synovitis, but renal complications can occur. Longstanding uncontrolled inflammation may rarely result in AA amyloidosis, while medications historically used for RA can cause glomerulonephritis or interstitial nephritis.

5. Ocular Manifestations

Eye involvement may include episcleritis, which is generally relatively mild, and scleritis, which is more painful and potentially serious. Severe destructive scleral inflammation may produce scleromalacia perforans, particularly in longstanding RA.

RA may also coexist with secondary Sjögren’s syndrome, producing reduced lacrimal and salivary secretion and resulting in dry eyes and dry mouth.

Laboratory Findings in Rheumatoid Arthritis

1. Anaemia

Anaemia is common and frequently has the pattern of anaemia of chronic inflammation, usually producing a normocytic, normochromic picture. Iron deficiency may coexist and can produce hypochromic or microcytic changes.

2. Thrombocytosis

Active systemic inflammation may produce reactive thrombocytosis, with an elevated platelet count. The degree of thrombocytosis may broadly reflect inflammatory activity.

3. Raised ESR and CRP

Both the erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) are commonly elevated during active disease. They are nonspecific inflammatory markers but are useful for assessing and monitoring disease activity.

4. Iron and Ferritin Changes

The inflammatory response alters iron metabolism. Serum iron concentration and total iron-binding capacity may be reduced, while ferritin may be normal or elevated because it acts as an acute-phase reactant. These findings are typical of anaemia associated with chronic inflammation.

5. Raised Globulins

Chronic immune activation may result in increased immunoglobulin production and therefore raised serum globulin concentrations.

6. Alkaline Phosphatase

Alkaline phosphatase (ALP) may occasionally be elevated, particularly in active systemic disease, although it is not a specific diagnostic marker for RA.

Rheumatoid Factor

Rheumatoid factor refers to autoantibodies directed against the Fc portion of IgG. The classic rheumatoid factor is an IgM autoantibody, although other immunoglobulin classes may also occur.

Older sources report RF positivity in approximately 70–80% of patients with established RA. However, it is not specific for the disease and can occur in other autoimmune disorders, chronic infections, and healthy individuals. The frequency of positive RF also increases with age.

Modern evaluation commonly includes anti-cyclic citrullinated peptide (anti-CCP/ACPA) antibodies, which have greater specificity for RA and can help predict a greater likelihood of erosive disease.

Radiological Features of Rheumatoid Arthritis

1. Soft-Tissue Swelling

An early radiographic finding is soft-tissue swelling around affected joints, reflecting active synovitis and joint effusion.

2. Joint-Space Narrowing

Progressive destruction and loss of articular cartilage cause narrowing of the joint space. In RA, this narrowing is often relatively uniform across the affected joint.

3. Juxta-Articular Osteopenia

Inflammation around affected joints may produce juxta-articular or periarticular osteopenia, causing the bone close to the joint to appear less dense on radiographs.

4. Marginal Bone Erosions

Persistent synovitis produces invasive inflammatory tissue that damages the bone at the joint margins. This leads to characteristic marginal erosions, which are an important radiological sign of established RA.

5. Joint Deformities

Advanced disease can result in subluxation, malalignment, and fixed deformities because of progressive destruction of cartilage, bone, tendons, and ligaments.

Drug Treatment of Rheumatoid Arthritis

Treatment aims not only to relieve symptoms but also to suppress inflammation, achieve remission or low disease activity, prevent structural joint damage, and preserve physical function. Modern management emphasises early use of disease-modifying therapy rather than relying solely on symptomatic treatment.

1. Symptom-Modifying Treatment

Analgesics may be used to relieve pain but do not control the underlying autoimmune disease or prevent joint destruction.

NSAIDs can reduce inflammatory pain and stiffness and may improve short-term function. However, they do not prevent progression of joint damage, so they should not replace disease-modifying treatment.

2. Disease-Modifying Antirheumatic Drugs

Methotrexate is generally the preferred first-line conventional synthetic disease-modifying antirheumatic drug (DMARD) for patients with active RA unless contraindicated. Other conventional DMARDs include sulfasalazine, hydroxychloroquine, and leflunomide, either alone or in combination depending on disease severity and response.

Corticosteroids may provide rapid suppression of inflammation and are sometimes used as short-term bridging therapy or for significant flares. Because of their numerous long-term adverse effects, prolonged corticosteroid exposure should be minimised whenever possible.

Other immunosuppressive drugs, including azathioprine and ciclosporin, have historically been used in selected cases but are much less central to routine modern RA treatment. Similarly, gold salts and penicillamine are older DMARDs that are now rarely used because more effective and better-tolerated alternatives are available.

Patients who remain inadequately controlled despite conventional DMARD therapy may receive biologic or targeted synthetic DMARDs. These include TNF inhibitors such as infliximab and other therapies directed against specific inflammatory pathways or immune cells. Treatment selection is individualised according to disease activity, previous therapy, comorbidities, infection risk, and other clinical factors.

Key Clinical Pattern

Rheumatoid arthritis is characterised by chronic symmetrical inflammatory polyarthritis, especially involving the MCP, PIP, wrist, and MTP joints while usually sparing the DIP joints. Persistent inflammation may produce characteristic deformities such as ulnar deviation, swan-neck deformity, boutonnière deformity, and Z-deformity of the thumb. Important investigations include RF, anti-CCP antibodies, ESR, CRP, and radiographs, while early treatment with DMARDs, particularly methotrexate, is central to preventing irreversible joint damage.

Medicine – Seronegative Spondyloarthritides and Comparison with Seropositive Rheumatoid Arthritis

The seronegative spondyloarthritides, now commonly referred to collectively as spondyloarthritis, are a group of related inflammatory rheumatic diseases that share several clinical, genetic, and radiological characteristics. They are traditionally called seronegative because rheumatoid factor is usually absent. These disorders have an important association with HLA-B27 and frequently involve the sacroiliac joints, spine, peripheral joints, and entheses. Extra-articular manifestations involving the eyes, skin, gastrointestinal tract, cardiovascular system, and genitourinary tract may also occur.

Main Seronegative Spondyloarthritides

1. Ankylosing Spondylitis

Ankylosing spondylitis is the classic axial form of spondyloarthritis and predominantly affects the sacroiliac joints and spine. Patients typically develop inflammatory back pain, prolonged morning stiffness, and progressive restriction of spinal movement. Advanced disease may lead to ankylosis and the characteristic radiological appearance of a bamboo spine. Extra-articular manifestations include anterior uveitis, aortic regurgitation, conduction abnormalities, and, rarely, apical pulmonary fibrosis.

2. Psoriatic Arthritis

Psoriatic arthritis is an inflammatory arthritis associated with psoriasis. Its presentation is highly variable and may include asymmetrical oligoarthritis, symmetrical polyarthritis, distal interphalangeal joint disease, spondylitis, or the severe destructive form known as arthritis mutilans. Dactylitis, enthesitis, and psoriatic nail changes are particularly characteristic and can help distinguish the disease from rheumatoid arthritis.

3. Enteropathic Arthritis

Enteropathic arthritis refers to inflammatory arthritis associated with inflammatory bowel disease, particularly Crohn’s disease and ulcerative colitis. Patients may develop peripheral arthritis affecting predominantly the lower limbs or axial disease involving the sacroiliac joints and spine. The severity of peripheral arthritis may sometimes parallel intestinal disease activity, whereas axial disease can progress relatively independently of bowel inflammation.

4. Reactive Arthritis

Reactive arthritis, formerly called Reiter’s syndrome, develops after an infection elsewhere in the body, particularly a gastrointestinal or genitourinary infection. It classically presents with the combination of arthritis, conjunctivitis, and urethritis, although the complete triad is not present in every patient. Other manifestations include enthesitis, sacroiliitis, oral ulceration, circinate balanitis, and keratoderma blennorrhagicum.

Comparison with Seropositive Rheumatoid Arthritis

1. Pattern of Peripheral Arthritis

Peripheral arthritis in the seronegative spondyloarthritides is commonly asymmetrical, often presenting as an oligoarthritis predominantly involving the lower limbs. By contrast, seropositive rheumatoid arthritis typically causes a symmetrical inflammatory polyarthritis, particularly affecting the small joints of the hands and feet.

2. Spinal Involvement

Spinal disease is characteristic of the spondyloarthritis group. Chronic inflammation may cause sacroiliitis, spondylitis, syndesmophyte formation, and eventually ankylosis. Rheumatoid arthritis generally does not produce this pattern of axial ankylosis. When the spine is involved in rheumatoid arthritis, the cervical spine is particularly important, with atlantoaxial instability or subluxation being a recognised complication.

3. Sacroiliac and Cartilaginous Joints

The sacroiliac joints are commonly affected in spondyloarthritis, and sacroiliitis is one of the characteristic features of axial disease. Progressive inflammation can lead to erosions, sclerosis, narrowing, and eventually fusion. Significant sacroiliac involvement is much less characteristic of rheumatoid arthritis.

4. Genetic Associations

The major genetic association of the seronegative spondyloarthritides is HLA-B27, particularly in ankylosing spondylitis and axial spondyloarthritis. In comparison, rheumatoid arthritis has important associations with certain HLA-DRB1 alleles, historically described in simplified teaching tables as an association with HLA-DR4.

5. Eye Manifestations

Ocular inflammation may occur in both groups but tends to take different forms. Acute anterior uveitis is particularly characteristic of spondyloarthritis and presents with a painful red eye, photophobia, and blurred vision. Conjunctivitis is especially associated with reactive arthritis. Rheumatoid arthritis, in contrast, can produce scleritis or episcleritis, with scleritis representing the more serious inflammatory manifestation.

6. Skin and Mucosal Manifestations

The spondyloarthritis group has several characteristic mucocutaneous manifestations depending on the particular disease. These include psoriasis in psoriatic arthritis, keratoderma blennorrhagicum and mucosal ulceration in reactive arthritis, and erythema nodosum in association with inflammatory bowel disease.

Seropositive rheumatoid arthritis may instead produce rheumatoid nodules, particularly in patients with longstanding seropositive disease. More severe systemic rheumatoid arthritis can also be associated with cutaneous vasculitis.

7. Cardiovascular Manifestations

Cardiovascular involvement in spondyloarthritis, particularly longstanding ankylosing spondylitis, may include aortitis, aortic regurgitation, and cardiac conduction abnormalities. These complications result from inflammation and fibrosis involving the aortic root and nearby cardiac structures.

In rheumatoid arthritis, pericarditis is a recognised extra-articular cardiac manifestation. Patients with rheumatoid arthritis also have an increased long-term risk of cardiovascular disease related to chronic systemic inflammation.

8. Pulmonary Manifestations

Advanced ankylosing spondylitis may cause restriction of chest wall movement because of ankylosis of the thoracic and costovertebral joints. Apical pulmonary fibrosis is another recognised but uncommon complication.

Rheumatoid arthritis can produce several pulmonary abnormalities, including rheumatoid lung nodules, pleural effusions, and interstitial lung disease with pulmonary fibrosis. Lung involvement can therefore represent an important extra-articular complication of rheumatoid disease.

9. Gastrointestinal Manifestations

Gastrointestinal disease has a particularly strong relationship with the spondyloarthritis group because Crohn’s disease and ulcerative colitis are associated with enteropathic arthritis. Intestinal ulceration and inflammation may therefore coexist directly with the musculoskeletal manifestations.

Gastrointestinal problems in rheumatoid arthritis are less characteristic of the disease itself and may instead arise from medications, including NSAIDs, corticosteroids, and other antirheumatic treatments.

10. Genitourinary Manifestations

Genitourinary involvement is particularly relevant to reactive arthritis, in which urethritis may occur following a sexually acquired infection. Genital mucosal lesions, including circinate balanitis, may also develop.

These manifestations are not characteristic features of uncomplicated rheumatoid arthritis and therefore may provide useful clues when distinguishing reactive arthritis from rheumatoid disease.

Key Differences

The seronegative spondyloarthritides can be distinguished from seropositive rheumatoid arthritis by their tendency toward asymmetrical peripheral arthritis, sacroiliitis, axial inflammation, enthesitis, and HLA-B27 association. They are also associated with characteristic extra-articular manifestations such as anterior uveitis, psoriasis, inflammatory bowel disease, and urethritis. In contrast, rheumatoid arthritis more typically produces symmetrical peripheral polyarthritis, is associated with rheumatoid factor and anti-CCP antibodies, and may cause complications such as rheumatoid nodules, cervical spine instability, scleritis, pericarditis, and rheumatoid lung disease.

Medicine – Common Features of Seronegative Spondyloarthritides

The seronegative spondyloarthritides, now more commonly grouped under the term spondyloarthritis, are a family of inflammatory rheumatic disorders that share a number of clinical, radiological, and genetic features. This group includes conditions such as ankylosing spondylitis, psoriatic arthritis, reactive arthritis, and arthritis associated with inflammatory bowel disease. Although each disease has its own characteristic presentation, there is substantial overlap between them.

1. Negative Rheumatoid Factor

A defining laboratory feature is the usual absence of rheumatoid factor (RF), which is why these conditions are traditionally described as seronegative. This helps distinguish them from rheumatoid arthritis, although a negative RF alone is not sufficient to make the diagnosis.

2. Asymmetrical Inflammatory Peripheral Arthritis

Peripheral joint involvement often takes the form of an asymmetrical inflammatory oligoarthritis, meaning that only a few joints are affected and the distribution is uneven between the two sides of the body. The lower limbs are commonly involved, particularly the knees and ankles. Affected joints may become painful, swollen, warm, and stiff.

3. Radiological Sacroiliitis

Sacroiliitis is an important radiological feature, especially in axial forms of spondyloarthritis. Imaging may show inflammation, erosions, sclerosis, joint-space narrowing, and eventually fusion of the sacroiliac joints. In early disease, MRI can demonstrate active inflammation before definite abnormalities become visible on plain radiographs.

4. Spondylitis

Inflammation of the spine, or spondylitis, is another characteristic feature. Patients may experience inflammatory back pain, typically with morning stiffness and improvement after activity rather than rest. Progressive spinal inflammation can eventually lead to reduced mobility and structural changes.

5. Enthesitis

Enthesitis refers to inflammation at the sites where tendons, ligaments, or joint capsules attach to bone. It is a hallmark of the spondyloarthritis group and commonly affects the Achilles tendon insertion and plantar fascia. Patients may present with heel pain or tenderness at other entheses.

6. Association with HLA-B27

There is a strong genetic association with HLA-B27, although the frequency varies between the different spondyloarthritides and between populations. The association is particularly strong in ankylosing spondylitis, where older teaching sources often quote a prevalence of around 90–96% in affected individuals of certain ethnic backgrounds.

7. Anterior Uveitis

Anterior uveitis is an important extra-articular manifestation and may occur in several forms of spondyloarthritis. It usually presents with an acutely painful red eye, photophobia, and blurred vision. The inflammation often affects one eye at a time and may recur, especially in HLA-B27-associated disease.

8. Clinical Overlap Between the Disorders

A significant feature of the seronegative spondyloarthritides is the clinical overlap between the individual diseases. For example, a patient may have axial inflammation together with peripheral arthritis, enthesitis, uveitis, psoriasis, bowel inflammation, or a history of preceding infection. Because of this overlap, classification is often based on the predominant pattern of disease rather than completely separate and rigid categories.

Key Clinical Pattern

The common pattern can be remembered as RF negativity, asymmetrical inflammatory arthritis, sacroiliitis, spondylitis, enthesitis, HLA-B27 association, and anterior uveitis, with considerable overlap between the different disorders in the spondyloarthritis spectrum.

Medicine – Ankylosing Spondylitis (The ‘A’ Disease)

Ankylosing spondylitis (AS) is a chronic inflammatory disease that mainly affects the sacroiliac joints and axial skeleton. It belongs to the seronegative spondyloarthritis group and is strongly associated with HLA-B27. The disease typically begins in younger adults and is characterised by inflammatory back pain, progressive spinal stiffness, and sacroiliitis. It is sometimes remembered as the “A disease” because several important extra-articular complications begin with the letter A.

Clinical Features

1. Arthritis

Arthritis is a major feature of ankylosing spondylitis. The disease primarily affects the sacroiliac joints and spine, producing inflammatory back pain and morning stiffness that tend to improve with activity. Peripheral joints may also be involved, particularly the hips and shoulders, and this may contribute to pain and reduced mobility.

2. Anterior Uveitis

Anterior uveitis is one of the most common extra-articular manifestations of ankylosing spondylitis. It usually presents acutely with painful red eye, photophobia, blurred vision, and excessive tearing. The condition often affects one eye at a time and may recur. Prompt ophthalmological treatment is important to prevent complications.

3. Apical Pulmonary Fibrosis

Longstanding ankylosing spondylitis may rarely cause fibrosis of the upper lobes of the lungs, particularly at the apices. This can lead to reduced respiratory function and may predispose to secondary infection. Restriction of chest wall movement due to involvement of the costovertebral joints may further impair ventilation.

4. Amyloidosis

Chronic inflammation can occasionally result in secondary amyloidosis, in which amyloid proteins are deposited in organs such as the kidneys. This may lead to proteinuria and progressive renal dysfunction, although it is much less common with modern treatment.

5. Aortic Regurgitation

Ankylosing spondylitis can involve the aortic root and aortic valve, leading to aortic regurgitation. Patients may eventually develop exertional breathlessness, palpitations, or other signs of valvular heart disease if the regurgitation becomes significant.

6. Aortitis

Aortitis, particularly inflammation involving the aortic root, is another recognised cardiovascular complication. Chronic inflammation and fibrosis may cause dilation of the aortic root and contribute to the development of aortic regurgitation or conduction abnormalities.

Radiological Features

1. Sacroiliac Joints

Irregular Joint Margins

One of the earliest radiographic abnormalities is irregularity of the sacroiliac joint margins due to inflammatory erosions. These changes may initially appear patchy and can affect both sides.

Sclerosis and Fusion

As the disease progresses, subchondral sclerosis develops around the sacroiliac joints. Continued inflammation and repair can eventually result in partial or complete fusion of the joints, producing ankylosis.

2. Spine

Loss of Lumbar Lordosis

Progressive spinal inflammation and stiffness may lead to loss of the normal lumbar lordosis. The lower back becomes straighter and less flexible, contributing to the characteristic postural changes of advanced disease.

Vertebral Squaring

Inflammation at the vertebral corners may cause erosion followed by new bone formation, producing squaring of the vertebral bodies on radiographs.

Syndesmophyte Formation

Syndesmophytes are thin, vertical bony bridges that form between adjacent vertebral bodies. They develop as a result of ossification at the outer fibres of the annulus fibrosus and related spinal ligament attachments. Progressive syndesmophyte formation gradually limits spinal movement.

Bamboo Spine

In advanced ankylosing spondylitis, extensive syndesmophyte formation together with ossification of spinal ligaments can produce the classic “bamboo spine” appearance. This reflects continuous bony bridging between vertebrae and indicates advanced spinal ankylosis.

3. Peripheral Joints

Erosive Arthropathy

Although axial involvement is dominant, peripheral joints can develop erosive inflammatory arthropathy, especially the hips and shoulders. Severe peripheral joint disease may result in pain, loss of movement, and functional impairment.

Key Clinical Pattern

Ankylosing spondylitis can be remembered by the “A” features: arthritis, anterior uveitis, apical pulmonary fibrosis, amyloidosis, aortic regurgitation, and aortitis. Radiologically, the disease is characterised by sacroiliitis, vertebral squaring, syndesmophyte formation, and ultimately a bamboo spine.

Medicine – Triggers of Reactive Arthritis (Reiter’s Syndrome)

Reactive arthritis, previously called Reiter’s syndrome, is a sterile inflammatory arthritis that develops after an infection elsewhere in the body. The triggering infection most commonly involves the genitourinary or gastrointestinal tract. Although the joints become inflamed, the causative organism is generally not present within the joint itself. Symptoms usually begin days to weeks after the initial infection.

1. Chlamydia trachomatis

Chlamydia trachomatis is one of the most important genitourinary triggers of reactive arthritis. The preceding infection may cause urethritis or cervicitis, although it can also be asymptomatic. Reactive arthritis following Chlamydia infection may present with lower-limb arthritis, enthesitis, conjunctivitis, or other mucocutaneous features.

2. Campylobacter jejuni

Campylobacter jejuni is a common gastrointestinal cause of reactive arthritis. It typically produces an episode of acute gastroenteritis with diarrhoea, abdominal pain, and sometimes fever. Joint inflammation may develop after the intestinal symptoms have improved or resolved.

3. Salmonella Species

Salmonella species can also trigger reactive arthritis after gastrointestinal infection. Patients may initially experience diarrhoea, abdominal cramps, and systemic symptoms, followed later by inflammatory joint symptoms. The arthritis is usually asymmetric and tends to involve the lower limbs.

4. Shigella flexneri

Shigella flexneri is another recognised enteric trigger. It causes an acute diarrhoeal illness, often with abdominal cramps and sometimes blood or mucus in the stool. Reactive arthritis can develop afterward as an immune-mediated complication of the infection.

5. Neisseria gonorrhoeae

Neisseria gonorrhoeae has historically been listed among infections associated with post-infectious arthritis. However, an important distinction is that gonococcal infection can also cause true septic arthritis through disseminated gonococcal infection, in which the organism directly infects the joint. Therefore, acute arthritis in the setting of gonorrhoea requires careful assessment to exclude septic arthritis rather than assuming a reactive process.

6. Borrelia burgdorferi

Borrelia burgdorferi, the organism responsible for Lyme disease, has also historically appeared in some lists of infectious triggers. However, Lyme arthritis is generally considered a distinct infectious or post-infectious manifestation of Lyme disease rather than a classic cause of reactive arthritis. It should therefore be differentiated from the more typical reactive arthritis triggered by Chlamydia, Campylobacter, Salmonella, or Shigella.

Key Clinical Concept

The classic infectious triggers of reactive arthritis are Chlamydia trachomatis, Campylobacter jejuni, Salmonella species, and Shigella species. These infections usually affect the genitourinary or gastrointestinal tract, and the subsequent arthritis represents an immune-mediated inflammatory response rather than direct infection of the joint.