Acute Monoarthritis

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oPatientPlus articles are written by UK doctors and are based on research evidence, UK and European Guidelines. They are designed for health professionals to use, so you may find the language more technical than the condition leaflets.

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 rapidly to 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 are 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 small numbers 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 the separate article on Knees That Swell.

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See the separate article on Rheumatological History, Examination and Investigations.

  • 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.
  • Establish whether symptoms are intra- or peri-articular.
  • Ask whether there is any history of psoriasis, other arthropathy, inflammatory bowel disease, and sexually transmitted infections.
  • Ask whether there has been any recent trauma to the affected area.
  • Establish whether symptoms are getting better or worse.
  • Note any other symptoms of systemic illness - eg, rash, myalgia, headache, and visual disturbance.
  • Establish whether there has been any previous joint/prosthesis surgery.

The history will give an indication of the likely cause:

  • 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 suggest 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 either 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. Establish whether the patient appears to be suffering from sepsis. Check to see if there is pharyngitis. Look at the nail folds and listen to the heart if there is possible rheumatic fever.
  • Eyes: check for any inflammation there.
  • Skin: check to see if there is any rash. Examine the extensor aspects of the forearms for nodules and the shins for evidence of erythema nodosum. Check whether there are  gouty tophi.
  • Joint examination:
    • When examining the affected joint, first inspect it for evidence of any deformity, swelling, erythema, peri-articular 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, a 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.
Septic arthropathy
  • Bacterial - eg, streptococcal, staphylococcal
  • Viral arthritis - eg, mumps, parvovirus, Epstein-Barr virus (EBV), hepatitis B virus (HBV), enteroviruses. May cause synovial infection or reactive arthritis
  • Fungal infection
  • Mycobacteria
  • Lyme disease
  • Brucellosis
  • Leptospirosis
Crystal arthropathy
  • Gout (uric acid)
  • Pseudogout (calcium pyrophosphate)
  • Apatite arthropathy (this may be associated with secondary septic arthritis)
  • Calcium oxalate arthritis
Bony or cartilaginous disease
Inflammatory arthritis
Manifestation of systemic illness
Trauma or haemorrhage
  • Peri-articular/intra-articular fracture
  • Traumatic effusion
  • Haemarthrosis
  • Associated with haemoglobinopathy
  • Neuropathic joint (painless)

Drugs may cause arthritis due to their metabolic effects or as part of an idiosyncratic reaction. Intermittent hydrarthrosis is an unusual and rare benign condition which does not fit into the above classification. It causes regular and recurrent joint effusions, usually of the knee. It often affects peri-pubertal girls.

Diagnosis is by exclusion and no definitive treatment except symptomatic relief is indicated. In children consider Osgood-Schlatter disease if there is tenderness over the tibial tuberosity, or slipped capital femoral epiphysis if there is pain in one hip or knee (referred symptoms).

  • 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 the separate article on Joint Injection and Aspiration.
  • Overlying cellulitis is a contra-indication 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 monoarthritis[1] 
Normal
  • Appearance: clear, viscous fluid
  • WBC count (cells per 10-6/L): 0-200
  • Crystals: nil
  • Culture: sterile
Septic arthritis
  • Appearance: turbid, low viscosity
  • WBC count (cells per 10-6/L): 50,000-200,000 neutrophils
  • Crystals: nil
  • Culture: positive (in some cases)
Gout (uric acid)
  • Appearance: clear, low viscosity
  • WBC count (cells per 10-6/L): 500-200,000 neutrophils
  • Crystals: needle-shaped and negatively birefringent
  • Culture: sterile
Pseudogout (pyrophosphate)
  • Appearance: clear, low viscosity
  • WBC count (cells per 10-6/L): 500-10,000 neutrophils
  • Crystals: block-shaped and positively birefringent
  • Culture: sterile
Inflammatory - eg, rheumatoid arthritis
  • Appearance: turbid, yellowish-green (chicken soup), low viscosity
  • WBC count (cells per 10-6/L): 2,000-100,000 neutrophils
  • Crystals: nil
  • Culture: sterile
Osteoarthritis/injury
  • Appearance: large volume, normal viscosity, may be blood-stained if trauma/haemarthrosis
  • WBC count (cells per 10-6/L): 0-2,000 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.[3] 

  • Urinalysis - dipstick for microscopic haematuria/protein indicating an inflammatory condition. Consider microscopy/culture.
  • Blood tests:
    • Blood culture if there is suspected sepsis.
    • FBC, ESR, CRP, urate, and U&E may aid in diagnosis. Low serum urate does not exclude gout.
    • Consider rheumatoid factor and other autoantibodies if inflammatory arthritis is suspected.
    • Anti-streptolysin O (ASO) titre and throat swab should be considered if rheumatic fever is possible.
  • X-rays - these are often unhelpful, but may reveal evidence of gout or other underlying pathology.[4] Usually normal in septic arthritis.

The management will depend on the underlying cause of acute monoarthritis.

  • Giving intra-articular steroids before sepsis is definitively excluded.
  • Ruling out sepsis due to the 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 the basis of initial Gram staining and culture; repeated culture of synovial fluid, blood and other sources of sepsis may be needed.

Further reading & references

  1. Siva C, Velazquez C, Mody A, et al; Diagnosing acute monoarthritis in adults: a practical approach for the family physician. Am Fam Physician. 2003 Jul 1;68(1):83; Excellent overview with useful clinical algorithms and full differential diagnoses.
  2. Ma L, Cranney A, Holroyd-Leduc JM; Acute monoarthritis: what is the cause of my patient's painful swollen joint? CMAJ. 2009 Jan 6;180(1):59-65. doi: 10.1503/cmaj.080183.
  3. Courtney P, Doherty M; Joint aspiration and injection and synovial fluid analysis. Best Pract Res Clin Rheumatol. 2013 Apr;27(2):137-69. doi: 10.1016/j.berh.2013.02.005.
  4. Llauger J, Palmer J, Roson N, et al; Nonseptic monoarthritis: imaging features with clinical and histopathologic correlation. Radiographics. 2000 Oct;20 Spec No:S263; Excellent images of conditions diagnosable on radiolgical grounds in monoarthritis.

Disclaimer: This article is for information only and should not be used for the diagnosis or treatment of medical conditions. EMIS has used all reasonable care in compiling the information but make no warranty as to its accuracy. Consult a doctor or other health care professional for diagnosis and treatment of medical conditions. For details see our conditions.

Original Author:
Dr Sean Kavanagh
Current Version:
Peer Reviewer:
Dr John Cox
Document ID:
1759 (v23)
Last Checked:
13/01/2014
Next Review:
12/01/2019