Acute Monoarthritis

Synonym: Hot Swollen Joint.

Patients presenting with a single painful and/or inflamed joint require thorough and rapid assessment. The principal diagnosis to consider is septic arthritis, as a failure to rapidly diagnose this condition can lead to irreversible severe joint damage in a very short time. Where there is a question of this diagnosis, immediate inpatient assessment and management is the safest course.¹

A combination of clinical assessment, synovial fluid aspiration/analysis and other investigations may be needed to reach a diagnosis. Where the cause is uncertain and infection remains a possibility, management should be directed to treating this as the default diagnosis. It is possible for conditions that normally present with acute polyarthritis to begin by affecting only one joint and evolve into the classical pattern over time, but be wary of assuming this to be the case. Oligoarthritis (<5 joints involved) is less likely to be due to sepsis but it is not unheard of for this to be the case. Where a small number of joints are involved in an active inflammatory process, the differential diagnosis is very similar to a monoarthritis, but evolving causes of an acute polyarthritis must be considered.
For a detailed discussion of the assessment of the swollen knee, see our record Knees that Swell.

See our Rheumatological History and Examination record:

• Characterise the speed of onset of the symptoms
• Establish whether this is a first episode or has occurred previously
• Enquire about symptoms of infection such as recent fever, rigors, focal symptoms of infection
• Enquire about any extra-articular manifestations of rheumatological disease, eg ocular symptoms, urethritis, diarrhoea, nodules, dyspnoea
• Are the symptoms intra- or peri-articular?
• Past history of psoriasis, other arthropathy, inflammatory bowel disease, sexually transmitted infections?
• Any recent trauma to affected area?
• Getting better or worse?
• Any other symptoms of systemic illness? eg rash, myalgia, headache, visual disturbance
• Any previous surgery to joint/prosthesis

The history will give an indication of the likely cause. For instance, pain coming on very suddenly over seconds or minutes suggests a mechanical cause, whereas that coming on over the course of several hours to a day or so suggests sepsis, crystal arthropathies or an inflammatory condition. Onset over days to weeks suggests atypical infection, osteoarthritis or synovial infiltration. Septic arthritis is likely in the immunosuppressed (remember steroids) or in injecting drug users. Steroid use is also associated with avascular necrosis. Haemarthrosis is more likely in those with a bleeding disorder or taking anticoagulants. Previous attacks of arthritis suggests a diagnosis of gout or other crystal arthropathy, as does the use of diuretics or a history of renal colic/stones. Associated symptoms like eye irritation, diarrhoea or rash suggest an inflammatory, reactive or vasculitic cause. A rash on the shins suggests erythema nodosum and sarcoidosis. Psoriatic pattern rash suggests a psoriatic arthropathy. It is worth enquiring about alcohol and recreational drug use where this could be a possible factor. In younger patients consider taking a sexual history, particularly if there is a history of rash and migratory arthralgia, suggesting gonococcal arthritis. A history of a recent sore throat may suggest a diagnosis of rheumatic fever.

• General: Check temperature, pulse and blood pressure. Does the patient appear to be suffering from sepsis. Is there pharyngitis? Look at the nail folds and listen to the heart if there is possible rheumatic fever.
• Eyes: Check for any inflammation there.
• Skin: Any rash? Examine the extensor aspects of the forearms for nodules and the shins for evidence of erythema nodosum. Are there gouty tophi?
• Joint examination:
  • When examining the affected joint, first inspect it for evidence of any deformity, swelling, erythema, periarticular muscle wasting or evidence of overlying bursitis.
  • Palpate to discern if swelling is due to bony enlargement, synovial thickening (firmness without fluctuance at joint margin) or effusion. If effusion is suspected confirm it by testing for fluctuance or patellar tap in the knee joint.
  • Test the active and passive movements of the joint. Note if there is pain or crepitus for each.
  • If the affected joint is prosthetic examine the skin carefully for evidence of abscess or sinus formation.
  • Don't forget to examine other joints that may be the cause of symptoms, eg hip causing knee symptoms. If the painful and surrounding joints are normal on examination, consider referral from another pain source, eg shoulder pain caused by cardiac/gallbladder pathology.

• Aspiration - If a single joint is acutely hot, red and painful then the most important investigation is to aspirate and analyse synovial fluid. This should be performed only by those with appropriate training and clinical experience of aspiration of the relevant joint. See joint injection and aspiration. Overlying cellulitis is a contraindication to the procedure. Intra-articular steroids should not be given unless it is certain that the diagnosis of septic arthritis is excluded. Do not aspirate a prosthetic joint without first seeking an orthopaedic opinion. Anticoagulated patients with INR in the therapeutic range can have the procedure in expert hands and using the smallest possible needle size. The table below shows the findings in the more common causes of monoarthritis.
  
  

  Synovial fluid changes in common causes of monoarthritis3
  Normal	Appearance: Clear, viscous fluid WBC (cells per 10-6l): <200 Crystals: Nil Culture: Sterile
  Septic arthritis	Appearance: Turbid, low viscosity WBC (cells per 10-6l): 100,000 neutrophils Crystals: Nil Culture: Positive (in some cases)
  Gout (uric acid)	Appearance: Clear, low viscosity WBC (cells per 10-6l): 10,000 neutrophils Crystals: Needle-shaped and negatively birefringent Culture: Sterile
  Pseudogout (pyrophosphate)	Appearance: Clear, low viscosity WBC (cells per 10-6l): 10,000 neutrophils Crystals: Block-shaped and positively-birefringent Culture: Sterile
  Inflammatory, eg RA	Appearance: Turbid, yellowish-green (chicken soup), low viscosity WBC (cells per 10-6l): 30,000 neutrophils Crystals: Nil Culture: Sterile
  Osteoarthritis/injury	Appearance: Large volume, normal viscosity, may be blood stained if trauma/haemarthrosis WBC (cells per 10-6l): 3,000 mononuclear Crystals: usually none (5% have pyrophosphate crystals) Culture: Sterile

  
  
  There is little evidence that analysis of other parameters of synovial fluid is useful in diagnosis. It must be remembered that there is variable sensitivity and specificity for each of the tests so diagnosis must be made in the context of all available information, including the clinical context.⁴

• Urinalysis - Dipstick for microscopic haematuria/protein indicating inflammatory condition. Consider microscopy/culture.
• Bloods:
  • Blood culture if suspected sepsis.
  • FBC, ESR, CRP, urate, U and E, may aid in diagnosis. Low serum urate does not exclude gout.
  • Consider Rheumatoid Factor and other autoantibodies if inflammatory arthritis suspected.
  • ASO titre and throat swab should be considered if rheumatic fever is possible.⁵
• X-rays - These are often unhelpful, but may reveal evidence of gout or other underlying pathology.⁶ Usually normal in septic arthritis.

This is determined by the cause. See the links to the various possible causes for detail.

• Giving intra-articular steroids before sepsis is definitively excluded.
• Ruling out sepsis due to presence of crystals; the two may co-exist.
• Attributing fever purely to sepsis when it may occur in crystal arthropathy and other conditions.
• Discounting gout when serum urate is normal; it is often low in an acute attack.
• Excluding sepsis on basis of initial Gram staining and culture; repeated culture of synovial fluid, blood and other sources of sepsis may be needed.