Monoarticular arthritis

Background

• Monoarticular arthritis (monoarthritis) refers to inflammation of a single joint
• The critical EM question is: Is this septic arthritis?
• Septic arthritis is a joint emergency requiring urgent drainage — delay increases risk of joint destruction
• Other common causes include crystal arthropathy (gout, pseudogout) and traumatic hemarthrosis
• Arthrocentesis is the key diagnostic procedure and should be performed on any hot, swollen joint without clear alternative diagnosis

Clinical Features

History

• Onset (acute vs. subacute), joint involved, trauma history
• Prior episodes (recurrent suggests crystal disease)
• Fever, chills, constitutional symptoms
• Recent infection, skin break, surgery, or injection
• Sexual history (disseminated gonococcal infection)
• History of gout, pseudogout, or autoimmune disease
• Immunosuppression, IV drug use, prosthetic joint

Physical Exam

• Joint warmth, erythema, effusion, decreased range of motion
• Pain with passive range of motion (highly suggestive of intra-articular process)
• Overlying skin: cellulitis, track marks, surgical scars, tophi
• Assess for signs of systemic infection
• Examine other joints (polyarticular process may present initially as monoarticular)

Red Flags for Septic Arthritis

• Fever with acute monoarthritis
• Recent bacteremia, skin infection, or surgical procedure
• Prosthetic joint with new pain/swelling
• Immunosuppressed patient
• IV drug use
• Non-weight-bearing or unable to flex joint

Differential Diagnosis

Monoarticular arthritis

Algorithm for Monoarticular arthralgia

• Acute osteoarthritis
• Avascular necrosis
• Crystal-induced (Gout, Pseudogout)
• Gonococcal arthritis, arthritis-dermatitis syndrome
• Nongonococcal septic arthritis
• Lyme disease
• Malignancy (metastases, osteochondroma, osteoid osteoma)
• Reactive poststreptococcal arthritis
• Trauma-induced arthritis
  • Fracture
  • Ligamentous injury
  • Overuse
• Avascular necrosis
• Decompression sickness
• Spontaneous osteonecrosis
• Hemorrhagic (e.g. hemophilia, systemic anticoagulation
• Seronegative spondyloarthropathies (ankylosing spondylitis, IBD, psoriatic arthritis, reactive arthritis
• RA, SLE
• Sarcoidosis, amyloidosis
• Periarticular pathology
  • Cellulitis
  • Tendonitis
  • Bursitis
• Transient (Toxic) Synovitis (Hip)
• Slipped Capital Femoral Epiphysis (SCFE)
• Legg Calve Perthes Disease

Septic Arthritis

• Gonococcal arthritis
• Nongonococcal Arthritis
• Arthritis-Dermatitis Syndrome

Crystal-Induced Monoarthritis

• Gout
• Pseudogout

Traumatic

• Fracture
• Ligamentous injury
• Overuse

Ischemic

• Avascular necrosis
• Decompression sickness
• Spontaneous osteonecrosis
  • Pain in absence of trauma
  • Femoral head, medial condyle of knee

Hemorrhagic

• Posttraumatic
• Hemophilia
• Systemic anticoagulation

Neoplastic

• Metastases
• Osteochondroma
• Osteoid osteoma
• Pigmented villonodular synovitis

Systemic Disease

• Remote infection, infectious endocarditis
• Rheumatic fever
• Seronegative (no RF) spondyloarthropathies (ankylosing spondylitis, IBD, psoriatic, reactive arthritis)
• Rheumatoid arthritis, SLE

Periarticular (Non-Articular)

• Bursitis
• Tendinitis
• Cellulitis

Evaluation

Arthrocentesis (Key Diagnostic Study)

• Perform arthrocentesis on any acute hot, swollen joint unless clear alternative diagnosis
• Do not delay arthrocentesis for imaging
• Overlying cellulitis is a relative contraindication — consult orthopedics

Arthrocentesis of synoval fluid

Synovium	Normal	Noninflammatory	Inflammatory	Septic
Clarity	Transparent	Transparent	Cloudy	Cloudy
Color	Clear	Yellow	Yellow	Yellow
WBC	<200	<200-2000	200-50,000	>1,100 (prosthetic joint) >25,000; LR=2.9 >50,000; LR=7.7 >100,000; LR=28
PMN	<25%	<25%	>50%	>64% (prosthetic joint) >90%
Culture	Neg	Neg	Neg	>50% positive
Lactate	<5.6 mmol/L	<5.6 mmol/L	<5.6 mmol/L	>5.6 mmol/L
LDH	<250	<250	<250	>250
Crystals	None	None	Multiple or none	None

• Viscosity of synovial fluid may actually be decreased in inflammatory or infectious etiologies, as hyaluronic acid concentrations decrease
• The presence of crystals does not rule out septic arthritis; however, the diagnosis is highly unlikely with synovial WBC < 50,000
• WBC >50,000/mm³ with >90% PMNs: highly suggestive of septic arthritis
• Crystals: negatively birefringent (gout), positively birefringent (pseudogout)
• Note: crystals and infection can coexist — always send culture even if crystals present
• Gram stain: positive in ~50% of non-gonococcal septic arthritis
• Culture: gold standard; also send blood cultures

Laboratory

• CBC with differential, ESR, CRP
• Blood cultures (positive in ~50% of septic arthritis)
• Uric acid (may be normal during acute gout flare)
• GC/CT NAAT if disseminated gonococcal infection suspected

Imaging

• X-ray of affected joint: chondrocalcinosis (pseudogout), fracture, joint destruction
• Ultrasound: confirm effusion, guide arthrocentesis
• MRI if osteomyelitis or periarticular abscess suspected

Management

Septic Arthritis

• Emergent orthopedic consultation for surgical drainage/washout
• Empiric IV antibiotics after arthrocentesis:
  • Vancomycin (MRSA coverage) for most patients
  • Add gram-negative coverage (ceftriaxone or cefepime) for immunocompromised, elderly, or IV drug users
  • Ceftriaxone alone if gonococcal arthritis suspected
• Prosthetic joint infection: orthopedic consultation for operative management

Crystal Arthropathy

• Gout: NSAIDs (indomethacin, naproxen), colchicine, or corticosteroids (PO or intra-articular)
• Pseudogout: NSAIDs, colchicine, or intra-articular/systemic corticosteroids
• Avoid allopurinol initiation or changes during acute flare

Traumatic

• Splinting, pain management
• Orthopedic follow-up for hemarthrosis or fracture

Disposition

Admit

• Septic arthritis (for surgical drainage and IV antibiotics)
• Prosthetic joint infection
• Sepsis from joint source
• Unable to rule out septic arthritis with pending cultures in high-risk patient

Discharge

• Crystal arthropathy with adequate pain control
• Traumatic arthritis/hemarthrosis with orthopedic follow-up arranged
• Provide return precautions: fever, worsening pain/swelling, inability to bear weight
• Primary care or rheumatology follow-up for gout/pseudogout management

See Also

• Septic arthritis
• Gout
• Pseudogout
• Arthrocentesis
• Gonococcal arthritis