Medicine – Common Causes of an Acute Monoarthritis

Acute monoarthritis refers to the sudden onset of pain, swelling, and inflammation affecting a single joint. It is an important clinical presentation because some causes, particularly septic arthritis, require urgent diagnosis and treatment to prevent irreversible joint damage. The differential diagnosis includes crystal arthropathies, trauma, infection, inflammatory arthritis, and bleeding into the joint.

1. Gout

Gout is a common cause of acute monoarthritis and results from deposition of monosodium urate crystals within the joint. The classic site is the first metatarsophalangeal joint, producing podagra, although the ankle, knee, wrist, and other joints may also be affected. The joint is typically extremely painful, red, swollen, and tender. Diagnosis is confirmed by identifying needle-shaped, negatively birefringent crystals in synovial fluid.

2. Pseudogout

Pseudogout, or calcium pyrophosphate deposition disease, can also present as an acute monoarthritis. It most commonly affects large joints, especially the knee, although the wrist and other joints may be involved. Synovial fluid examination shows calcium pyrophosphate crystals that are weakly positively birefringent, and radiographs may demonstrate chondrocalcinosis.

3. Trauma

Traumatic injury can produce acute pain and swelling in a single joint. Causes include ligament injury, meniscal damage, fracture, dislocation, or direct soft-tissue injury. A careful history of recent injury, examination for instability or deformity, and appropriate imaging can help distinguish trauma from inflammatory or infectious causes.

4. Acute Septic Arthritis

Septic arthritis is one of the most important causes of acute monoarthritis because it can rapidly destroy articular cartilage. Patients usually present with a severely painful, hot, swollen joint with markedly restricted movement, often accompanied by fever or systemic illness. The knee is commonly affected, although any joint can be involved. Urgent joint aspiration, Gram stain, culture, and prompt antimicrobial treatment are essential.

5. Seronegative Spondyloarthritides

The seronegative spondyloarthritides may occasionally present with acute inflammation of a single peripheral joint. This is more likely in disorders such as reactive arthritis or psoriatic arthritis. Associated features such as enthesitis, sacroiliitis, psoriasis, uveitis, or a recent gastrointestinal or genitourinary infection may help identify the underlying diagnosis.

6. Rheumatoid Arthritis

Although rheumatoid arthritis usually presents as a symmetrical polyarthritis, it can occasionally begin with inflammation of a single joint, particularly early in the disease. Persistent synovitis, later involvement of additional joints, and serological findings such as rheumatoid factor or anti-CCP antibodies may support the diagnosis.

7. Haemarthrosis

Haemarthrosis refers to bleeding into a joint and can produce sudden pain, swelling, and restricted movement. It may result from trauma, anticoagulant therapy, or inherited bleeding disorders such as haemophilia. Joint aspiration may reveal bloody synovial fluid, and management depends on the underlying cause of bleeding.

Key Clinical Point

The main causes of acute monoarthritis are gout, pseudogout, trauma, septic arthritis, seronegative spondyloarthritis, rheumatoid arthritis, and haemarthrosis. Among these, septic arthritis must always be considered urgently, especially when the joint is hot, swollen, and very painful or the patient is systemically unwell.