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Pathology - Osteoarthritis
I. Definition & Epidemiology:
I. Definition & Epidemiology:
- Definition: Osteoarthritis (OA) is a group of diseases causing joint degradation. Crucially, it's not a single disease but a collection of similar conditions.
- Epidemiology: OA is the most common joint disease. In the UK, approximately 2 million people experience symptomatic OA. It primarily affects the elderly population.
- Primary OA: In most cases, the cause of OA is unknown (primary OA). This highlights the complexity of the disease and the challenges in pinpointing a single cause.
- Secondary OA: OA can also develop as a consequence of other joint disorders (secondary OA). Examples include rheumatoid arthritis and gout, where pre-existing joint damage sets the stage for OA development. Understanding this distinction is vital – treatment differs depending on the underlying cause.
- Cartilage Damage: The primary pathological change is damage to the articular cartilage, the smooth tissue covering joint surfaces. This damage is central to understanding OA's progression.
- Inflammation & Metalloproteinases: Low-grade inflammation within the joint plays a crucial role. This inflammation causes chondrocytes (cartilage cells) to release metalloproteinases, enzymes that break down the cartilage matrix, leading to further cartilage loss.
- Subchondral Bone Response: As cartilage is lost, the underlying bone becomes exposed. The bone responds by thickening (sclerosis) – a compensatory but ultimately unhelpful response. This bone thickening contributes to the pain and stiffness experienced by patients.
- Pain, Tenderness, Stiffness: The classic symptoms are pain, tenderness, and stiffness in the affected joint(s). It's important to note that symptom severity varies greatly between individuals.
- Activity-Related Worsening: Symptoms typically worsen throughout the day, particularly with increased activity. This contrasts with some other inflammatory arthritides where stiffness is worse in the morning.
- Commonly Affected Joints: OA most commonly affects the hip, knee, spine, and small joints of the hands, but it can affect any joint. Knowing the typical locations helps in differential diagnosis.
- Cartilage Loss & Thinning: Microscopic examination reveals thinned and lost articular cartilage. This confirms the central role of cartilage degeneration in the disease process.
- Subchondral Bone Sclerosis: Thickening and sclerosis (increased density) of the subchondral bone are also observed. This supports the clinical observation of bone changes in OA.
- Progressive Worsening: OA generally worsens over time. This emphasizes the need for early diagnosis and management to slow progression.
- Analgesics & Joint Replacement: Management typically involves analgesics (pain relievers) to manage symptoms. In severe cases, joint replacement surgery may be necessary. This highlights the potential long-term impact of the disease.
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Pathology - Rheumatoid Arthritis
I. Definition & Epidemiology
RA can affect multiple organ systems:
RA has a variable course:
I. Definition & Epidemiology
- Definition: RA is a systemic autoimmune disease primarily affecting synovial joints. It's characterized by chronic inflammation.
- Epidemiology: Affects approximately 1% of the population, with a significantly higher prevalence in young to middle-aged women.
- Etiology (Cause): The initial trigger for RA remains unknown. However, once inflammation starts, it becomes self-sustaining.
- Pathogenesis (Mechanism):
- Inflammation: The synovium (joint lining) is infiltrated by CD4+ T-cells, B-cells, plasma cells, and macrophages, leading to inflammation.
- Pannus Formation: The inflamed synovium proliferates, forming pannus – a hyperplastic mass of synovial tissue.
- Joint Destruction: Pannus erodes articular cartilage, resulting in joint damage.
- Joint Involvement: Symmetrical swelling, pain, and stiffness, predominantly affecting small joints of the hands and feet. Morning stiffness is a characteristic symptom.
- Rheumatoid Factor (RF): An autoantibody targeting the Fc portion of IgG. Positive in ~70% of RA patients, but also found in other autoimmune diseases and some healthy individuals, limiting its specificity.
- Anti-citrullinated protein antibodies (ACPAs): Newer, more specific antibodies for diagnosing RA, but not as widely available as RF.
RA can affect multiple organ systems:
- Cardiovascular: Ischemic heart disease, pericarditis
- Vascular: Accelerated atherosclerosis, vasculitis
- Hematological: Anemia, splenomegaly
- Pulmonary: Pulmonary fibrosis, pleuritis
- Skin: Rheumatoid nodules, erythema nodosum, pyoderma gangrenosum
- Neurological: Peripheral neuropathy, stroke
- Amyloidosis: Deposition of serum amyloid A, leading to AA amyloidosis in various organs.
- Synovial Hyperplasia: Marked increase in synovial tissue.
- Inflammatory Infiltrate: A significant presence of lymphocytes and plasma cells.
- Germinal Centers: Formation of germinal centers within lymphoid aggregates is a typical finding.
RA has a variable course:
- Remission: Approximately 25% experience long-term remission.
- Mild-to-Moderate Disability: About 50% have chronic disease with mild-to-moderate disability.
- Severe Disability: Approximately 25% experience progressive disease leading to severe disability.
- Autoimmune nature: RA is driven by the body's immune system attacking its own tissues.
- Synovitis: Inflammation of the synovial membrane is central to the disease process.
- Pannus formation & joint destruction: Understand the sequence of events leading to joint damage.
- Diagnostic markers: Compare the sensitivity and specificity of RF and ACPAs.
- Systemic involvement: Be aware of the diverse extra-articular manifestations.
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Pathology - -Septic Arthritis
I. Definition:
I. Definition:
- Infection within a joint. This is the core concept – remember this first.
- Any age: Septic arthritis can affect anyone.
- Increased Risk: Individuals with pre-existing joint conditions are significantly more vulnerable. Think of this as a weakened defense system making the joint more susceptible.
- Hematogenous Spread (Most Common): The infection travels through the bloodstream to the joint. This is the primary pathway.
- Penetrating Trauma (Less Common): Infection can be introduced directly into the joint via injury (e.g., a puncture wound).
- Staphylococcus aureus (S. aureus): This bacterium is the primary culprit in almost all cases. Memorize this.
- Phagocyte Limitation: The joint space has limited access for phagocytes (immune cells that engulf and destroy pathogens). This makes it difficult for the body to fight the infection naturally. This is a key factor in why the infection spreads so rapidly.
- Rapid Destruction: Septic arthritis progresses swiftly, causing irreversible joint damage if antibiotics aren't administered promptly. Early intervention is critical.
- Classic Signs of Inflammation: The affected joint is intensely painful, hot, red, and swollen. This is a hallmark of septic arthritis; remember these four key characteristics (pain, heat, redness, swelling).
- Microscopy: Joint fluid analysis reveals neutrophils (a type of white blood cell indicating infection) but no crystals (ruling out gout or pseudogout).
- Culture: Essential for identifying the specific bacteria (usually S. aureus) and determining its antibiotic susceptibility. Culturing both joint fluid and blood is crucial for accurate diagnosis and treatment.
- Irreversible Joint Damage: Without appropriate antibiotic treatment, permanent joint destruction is inevitable. This highlights the urgency of prompt diagnosis and treatment.
- S. aureus is the most common cause.
- Early antibiotic treatment is crucial to prevent irreversible joint damage.
- The four cardinal signs of inflammation (pain, heat, redness, swelling) are present.
- Joint fluid analysis is essential for diagnosis and guiding treatment.
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Pathology - Spondyloarthropathies
I. Definition & Epidemiology:
A. Ankylosing Spondylitis:
I. Definition & Epidemiology:
- Definition: Spondyloarthropathies are inflammatory joint diseases affecting the spine and peripheral joints, characterized by arthritis and enthesitis (inflammation at tendon/ligament-bone insertion sites).
- Epidemiology: These are common diseases (affecting ~1% of the population), predominantly affecting young adults (20-40 years old) with a slight male bias.
- HLA-B27 Association: A strong genetic association exists with the HLA-B27 allele. This is a crucial risk factor.
- Older Theories: Initially, researchers hypothesized that an unidentified "arthritogenic peptide" presented by HLA-B27 to CD8+ T cells triggered inflammation.
- Current Understanding: Recent focus is on HLA-B27 protein itself. The HLA-B27 heavy chain is prone to misfolding, forming abnormal homodimers. Studies (e.g., in transgenic rats) show that this misfolding leads to endoplasmic reticulum stress and IL-23 production via the T-helper 17 pathway, driving inflammation.
A. Ankylosing Spondylitis:
- Prevalence: Affects approximately 0.5% of the population, typically presenting in young adults (20-40 years).
- Presentation: Lower back pain due to sacroiliitis (inflammation of the sacroiliac joints) is the hallmark symptom.
- Extra-articular Manifestations: Inflammation can extend beyond the joints, affecting structures like the eyes (iritis), lungs (pulmonary fibrosis), and aorta (aortitis).
- Onset: Develops within one month of an infection elsewhere in the body.
- Infections: Commonly associated with genitourinary infections (e.g., Chlamydia) or gastrointestinal infections (e.g., Shigella, Salmonella, Campylobacter).
- Mechanism: Possibly due to bacterial antigens or DNA deposition in joints, although this isn't definitively proven.
- Presentation: Pain and stiffness in the lower back, knees, ankles, and feet; enthesitis is frequent.
- Prevalence: Occurs in approximately 5% of patients with psoriasis.
- Joint Involvement: Primarily affects distal interphalangeal joints (joints at the tips of the fingers and toes), potentially causing significant deformity.
- Cause: Currently unknown.
- Prevalence: Found in about 10% of individuals with inflammatory bowel disease.
- Joint Involvement: Typically affects sacroiliac and lower limb joints asymmetrically.
- Cause: Unknown.
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Pathology - Crystal Arthropathies
I. Definition & Pathogenesis
A. Gout
Microscopic examination of joint fluid is crucial for differentiating gout and pseudogout:
I. Definition & Pathogenesis
- Crystal Arthropathies: A family of joint diseases stemming from crystal deposition within the joints.
- Pathogenic Mechanism: Crystals trigger an inflammatory response. Neutrophils attempt to phagocytose (engulf) the crystals, leading to degranulation and the release of damaging enzymes. This enzymatic action causes joint damage.
A. Gout
- Etiology: Deposition of urate crystals in joints. Primarily caused by hyperuricemia (high uric acid levels in the blood), often due to impaired renal (kidney) excretion of urate.
- Acute Gout: Presents as an acutely painful, swollen, and red joint. While any joint can be affected, the first metatarsophalangeal joint (base of the big toe) is highly characteristic.
- Chronic Tophaceous Gout: In individuals with persistently high urate levels, large urate deposits (tophi) can form in the skin and around joints.
- Etiology: Deposition of calcium pyrophosphate crystals in joints. Pyrophosphate is a byproduct of nucleotide triphosphate hydrolysis within cartilage chondrocytes.
- Pathogenesis: Shedding of these crystals into the joint space initiates an acute arthritis closely resembling gout.
- Clinical Presentation: Typically affects older women, commonly involving the knee and wrist joints.
Microscopic examination of joint fluid is crucial for differentiating gout and pseudogout:
- Joint Fluid: Contains neutrophils and crystals.
- Urate Crystals (Gout): Needle-shaped; exhibit negative birefringence under polarized light.
- Pyrophosphate Crystals (Pseudogout): Rhomboid or rod-shaped; exhibit positive birefringence under polarized light. The difference in birefringence is key for distinguishing between the two conditions.
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Pathology - Chronic myelogenous leukemia
A myeloproliferative neoplasm mostly affecting the granulocytic lineage, consistently related with the BCR-ABL1 fusion gene on the Philadelphia chromosome.
Epidemiology • Incidence ranges from 1 to 2 per 100,000 individuals annually. • The peak age of onset is between 50 and 70 years old.
Aetiology: Unknown. Genetics • Chronic myelogenous leukaemia (CML) is typically characterized by the t(9;22) translocation, which produces the Philadelphia chromosome. • This translocation fuses the BCR gene on chromosome 22 with the ABL1 gene on chromosome 9. • The resulting BCR-ABL1 protein exhibits increased tyrosine kinase activity, resulting in the persistent activation of signal transduction pathways and uncontrolled proliferation of myeloid cells.
Presentation • The majority of patients receive a diagnosis during the chronic phase of the disease, characterized by an elevated white cell count. • Hepatosplenomegaly frequently accompanies the diagnosis.
Peripheral blood • Leukocytosis resulting from elevated neutrophil counts at various maturation phases. • Basophilia and eosinophilia are prevalent. No dysplasia is observed.
Bone marrow • Bone marrow trephines exhibit hypercellularity attributed to elevated quantities of neutrophils and their precursors. • Megakaryocytes are generally diminutive and hypolobated. • Blasts constitute less than 5% of marrow cells throughout the chronic phase.
Prognosis • The outcome has significantly improved following the introduction of the tyrosine kinase inhibitor, imatinib, resulting in 5-year survival rates of 80–90%. • Disease progression is typically indicated by an elevation in circulating blasts to over 10% (accelerated phase) and culminates in acute leukemia when blasts constitute more than 20% of circulating cells.
A myeloproliferative neoplasm mostly affecting the granulocytic lineage, consistently related with the BCR-ABL1 fusion gene on the Philadelphia chromosome.
Epidemiology • Incidence ranges from 1 to 2 per 100,000 individuals annually. • The peak age of onset is between 50 and 70 years old.
Aetiology: Unknown. Genetics • Chronic myelogenous leukaemia (CML) is typically characterized by the t(9;22) translocation, which produces the Philadelphia chromosome. • This translocation fuses the BCR gene on chromosome 22 with the ABL1 gene on chromosome 9. • The resulting BCR-ABL1 protein exhibits increased tyrosine kinase activity, resulting in the persistent activation of signal transduction pathways and uncontrolled proliferation of myeloid cells.
Presentation • The majority of patients receive a diagnosis during the chronic phase of the disease, characterized by an elevated white cell count. • Hepatosplenomegaly frequently accompanies the diagnosis.
Peripheral blood • Leukocytosis resulting from elevated neutrophil counts at various maturation phases. • Basophilia and eosinophilia are prevalent. No dysplasia is observed.
Bone marrow • Bone marrow trephines exhibit hypercellularity attributed to elevated quantities of neutrophils and their precursors. • Megakaryocytes are generally diminutive and hypolobated. • Blasts constitute less than 5% of marrow cells throughout the chronic phase.
Prognosis • The outcome has significantly improved following the introduction of the tyrosine kinase inhibitor, imatinib, resulting in 5-year survival rates of 80–90%. • Disease progression is typically indicated by an elevation in circulating blasts to over 10% (accelerated phase) and culminates in acute leukemia when blasts constitute more than 20% of circulating cells.
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Pathology - Acute myeloid leukemia
Definition: A collection of hematological neoplasms consisting of malignant myeloid blasts.
Epidemiology The global incidence is 3 cases per 100,000 individuals annually. • Primarily, a condition affecting adults (average age at diagnosis is 65 years).
Aetiology • Acute myeloid leukaemias (AML) can be either spontaneous or arise as a consequence of prior treatment or as a final manifestation of an existing myeloproliferative or myelodysplastic disorder.
Pathogenesis • Mutations in a hematopoietic stem cell result in the clonal proliferation of immature myeloid blasts. • Rapidly proliferating myeloid blasts saturate the bone marrow and enter the peripheral blood. • Infiltration of organs by myeloid blasts may occur in acute myeloid leukemia (AML), although this is less frequent than in acute B-lymphoblastic leukemia.
Presentation • The majority of patients manifest as bone marrow failure, resulting in anaemia, thrombocytopenia, and neutropenia. Leukocytosis may be present.
Microscopy • By definition, over 20% of the cells in peripheral blood or bone marrow are myeloid blasts. • The blasts are medium to large cells characterized by a high nuclear-to-cytoplasmic ratio. Certain myeloid blasts possess cytoplasmic granules or Auer rods. Immunophenotype • Myeloid blasts typically exhibit expression of CD13, CD117, CD33, and CD34. • They lack expression of B-lymphoid markers, including CD79a.
The prognosis is contingent upon the specific subtype of AML; still, the majority are aggressive conditions necessitating rigorous ablative treatments to attain remission. Acute myeloid leukemia (AML) linked to prior treatment or an existing myeloid disease typically exhibits a dismal prognosis.
Definition: A collection of hematological neoplasms consisting of malignant myeloid blasts.
Epidemiology The global incidence is 3 cases per 100,000 individuals annually. • Primarily, a condition affecting adults (average age at diagnosis is 65 years).
Aetiology • Acute myeloid leukaemias (AML) can be either spontaneous or arise as a consequence of prior treatment or as a final manifestation of an existing myeloproliferative or myelodysplastic disorder.
Pathogenesis • Mutations in a hematopoietic stem cell result in the clonal proliferation of immature myeloid blasts. • Rapidly proliferating myeloid blasts saturate the bone marrow and enter the peripheral blood. • Infiltration of organs by myeloid blasts may occur in acute myeloid leukemia (AML), although this is less frequent than in acute B-lymphoblastic leukemia.
Presentation • The majority of patients manifest as bone marrow failure, resulting in anaemia, thrombocytopenia, and neutropenia. Leukocytosis may be present.
Microscopy • By definition, over 20% of the cells in peripheral blood or bone marrow are myeloid blasts. • The blasts are medium to large cells characterized by a high nuclear-to-cytoplasmic ratio. Certain myeloid blasts possess cytoplasmic granules or Auer rods. Immunophenotype • Myeloid blasts typically exhibit expression of CD13, CD117, CD33, and CD34. • They lack expression of B-lymphoid markers, including CD79a.
The prognosis is contingent upon the specific subtype of AML; still, the majority are aggressive conditions necessitating rigorous ablative treatments to attain remission. Acute myeloid leukemia (AML) linked to prior treatment or an existing myeloid disease typically exhibits a dismal prognosis.
