Monoarthritis

Monoarthritis is inflammation confined to a single joint. It always requires urgent exclusion of septic arthritis, because missed or delayed treatment of bacterial infection can cause rapid, irreversible joint destruction.

The differential diagnosis extends beyond infection to crystal arthropathy, haemarthrosis, and monoarticular presentation of an underlying systemic inflammatory arthritis.

Aetiology

• Septic arthritis must always be actively considered and excluded first, given the time critical nature of appropriate management.
• Crystal arthropathy is a common cause of acute monoarthritis, particularly in older adults. It includes gout (monosodium urate crystals) and calcium pyrophosphate deposition disease (pseudogout).
• Haemarthrosis, reflecting bleeding into the joint space, occurs in the context of trauma, anticoagulant therapy, or an underlying bleeding disorder such as haemophilia.
• An underlying systemic inflammatory arthritis (rheumatoid arthritis, psoriatic arthritis, reactive arthritis, or one of the seronegative spondyloarthropathies) can present as monoarthritis. This is particularly likely early in the disease course, before the more typical polyarticular pattern becomes established.
• Osteoarthritis typically presents as chronic rather than acute monoarthritis. It can produce acute symptomatic flares, particularly following minor trauma or overuse.

• Pain, swelling, warmth, erythema and restricted range of movement affecting a single joint characterise the general presentation.
• The specific pattern, speed of onset and associated features provide important clues toward the underlying cause.
• Rapid onset (hours) of severe pain, marked swelling and inability to bear weight or use the joint, together with fever and systemic upset, raises particular concern for septic arthritis or acute crystal arthropathy.
• Both can present with a very similar acute clinical picture, so synovial fluid analysis is essential to distinguish between them reliably.
• A history of relevant trauma, anticoagulant use, or known bleeding disorder points toward haemarthrosis.
• A history of preceding gastrointestinal or genitourinary infection, or known psoriasis, points toward a reactive or psoriatic arthritis presentation.
• Actively identify risk factors for septic arthritis, given their relevance to both diagnostic suspicion and, where confirmed, management approach. They include recent joint injection or surgery, prosthetic joint, intravenous drug use, immunocompromise, diabetes, and pre-existing joint damage from another cause such as rheumatoid arthritis.
• Examination should assess the specific joint involved, the presence of an effusion, range of movement (both active and passive), and evidence of overlying skin changes or a portal of entry for infection (cellulitis, a wound, or a recent injection site).

• Joint aspiration (arthrocentesis) is the essential investigation in any patient presenting with acute monoarthritis where septic arthritis is a genuine clinical possibility.
• Perform it promptly, before antibiotic administration wherever feasible. Treatment can sterilise the synovial fluid and obscure the diagnosis.
• Send synovial fluid for white cell count and differential, Gram stain and culture, and polarised light microscopy for crystal identification. Together these results are the cornerstone of distinguishing between the major causes of acute monoarthritis.
• A markedly elevated synovial white cell count (generally above 50,000 per microlitre, with a high proportion of neutrophils) is highly suggestive of septic arthritis.
• Lower counts do not reliably exclude infection, particularly in partially treated infection or infection with a less virulent organism. Gram stain and culture results, together with the overall clinical picture, remain essential for a confident diagnosis rather than relying on cell count alone.
• Blood cultures, full blood count, inflammatory markers and, where relevant, serum urate support the overall assessment. Serum urate may be normal during an acute gout flare and is therefore of limited diagnostic value in the acute setting.
• Plain radiography is frequently unremarkable in acute presentations. It can identify chondrocalcinosis (supporting calcium pyrophosphate deposition disease) or evidence of pre-existing joint damage.
• Where trauma or a bleeding tendency is suspected as the cause, coagulation studies and specific factor assays (where haemophilia or another inherited bleeding disorder is a relevant consideration) are appropriate additional investigations.

• Manage any acute monoarthritis where septic arthritis cannot be confidently excluded as septic arthritis until proven otherwise. This means prompt joint aspiration, empirical intravenous antibiotic therapy, and orthopaedic involvement for consideration of surgical washout, following the detailed management approach outlined under septic arthritis, given the severe consequences of delayed treatment in genuine infection.
• Confirmed crystal arthropathy (gout or calcium pyrophosphate deposition disease) is managed with nonsteroidal anti inflammatory drugs, colchicine, or corticosteroids (oral, or intra articular once infection has been confidently excluded). The choice depends on the specific condition and patient factors, following the management principles detailed under those specific conditions.
• Haemarthrosis is managed with appropriate correction of any underlying coagulopathy (reversal of anticoagulation where relevant, or specific factor replacement in haemophilia).
• Joint aspiration gives symptomatic relief where the effusion is large and painful. Physiotherapy supports subsequent joint function recovery.
• Monoarticular presentation of an underlying systemic inflammatory arthritis is managed according to the specific diagnosis once established. Rheumatology referral is appropriate for further characterisation and initiation of disease specific treatment where a systemic inflammatory arthropathy is confirmed or strongly suspected.
• Analgesia, rest and, where appropriate once septic arthritis has been excluded, ice application form general supportive measures alongside specific treatment of the underlying identified cause.
• Physiotherapy input supports functional recovery, particularly following a significant effusion or period of reduced joint use.