44.1 Acute Inflammatory Arthritis & Spondyloarthropathies

Clinical Triage & Differential Diagnosis of Acute Monoarthritis

Acute monoarthritis—defined as acute-onset inflammation, swelling, erythema, and severe pain localized to a single synovial joint developing over hours to a few days—is one of the most common and high-stakes musculoskeletal presentations in primary care and emergency medicine. The primary clinical imperative is to rapidly distinguish between a true infectious emergency (septic arthritis) and crystal-induced arthropathies (gout or pseudogout), as untreated septic arthritis causes irreversible chondrocyte death, subchondral bone necrosis, and permanent joint destruction within 24 to 48 hours.

The Cardinal Rule of Monoarthritis: Diagnostic Arthrocentesis

[!IMPORTANT] URGENT ARTHROCENTESIS MANDATE Every patient presenting with an unexplained acute monoarticular joint effusion or severe joint inflammation must undergo urgent diagnostic arthrocentesis prior to the initiation of antimicrobial therapy, unless an absolute contraindication exists (e.g., severe overlying cellulitis at the puncture site or severe uncorrected coagulopathy with active bleeding).

Clinical examination, plain radiography, and serum inflammatory markers (ESR, CRP, leukocytosis) CANNOT reliably differentiate septic arthritis from an acute crystal flare. Even if a patient has a documented history of chronic gout, crystal deposition and bacterial infection can, and frequently do, coexist in the same joint.

Synovial Fluid Analysis & Classification

Synovial fluid obtained via arthrocentesis should immediately be analyzed for four cardinal parameters: macroscopic appearance/clarity, total leukocyte count with differential, polarized light microscopy for crystals, and microbiological staining (Gram stain) and aerobic/anaerobic bacterial cultures.

                    SYNOVIAL FLUID CLASSIFICATION MATRIX

   Category        Clarity & Color      WBC Count (/μL)   % PMNs     Glucose (% Serum)  Common Etiologies
   ═══════════════════════════════════════════════════════════════════════════════════════════════════════════
   Normal          Transparent, clear   <200              <25%       ~100%              Normal joint

   Non-            Transparent, yellow  200 to 2,000      <25%       ~100%              Osteoarthritis,
   Inflammatory                                                                         trauma, meniscal tear

   Inflammatory    Translucent to       2,000 to 50,000   50% to     50% to 80%         Gout, Pseudogout,
                   opaque, yellow                         80%                           Rheumatoid arthritis,
                                                                                        Spondyloarthropathies

   Septic          Opaque, turbid,      >50,000*          >75% to    <50%               Bacterial infection
   (Infectious)    purulent yellow-                       90%        (often marked drop) (S. aureus, N. gonorrhoeae,
                   green                                                                Streptococcus spp.)

   Hemorrhagic     Opaque, bloody/red   200 to 10,000     Variable   ~100%              Trauma, ACL rupture,
                                                                                        coagulopathy, tumor
   ═══════════════════════════════════════════════════════════════════════════════════════════════════════════

Note on WBC Overlap: While a synovial WBC count >50,000/μL is the classical threshold for septic arthritis, bacterial infection (especially low-virulence organisms, partially treated infections, or disseminated gonococcal infection) can present with counts between 20,000 and 50,000/μL. Conversely, severe acute gout flares can occasionally exceed 50,000 to 100,000 WBC/μL. Therefore, Gram stain and culture remain the definitive gold standard.

Septic Arthritis: Microbiology, Workup & Emergent Management

1. Microbiology

• Staphylococcus aureus: The most common causative pathogen across all age groups in adults (>50% of non-gonococcal cases). Methicillin-resistant S. aureus (MRSA) accounts for a substantial fraction in patients with healthcare exposure, intravenous drug use (IVDU), indwelling catheters, or chronic skin ulcers.
• Streptococcus Species: Streptococcus pneumoniae, Group A streptococci, and Group B streptococci account for 15% to 20% of cases.
• Neisseria gonorrhoeae: The leading pathogen in healthy, sexually active adolescents and young adults. Gonococcal arthritis manifests in two distinct clinical syndromes:
  1. Disseminated Gonococcal Infection (DGI) Triad: Migratory asymmetric tenosynovitis (wrists, ankles), dermatitis (painless pustular, petechial, or necrotic papules on distal extremities), and asymmetric polyarthralgias without purulent monoarthritis.
  2. Purulent Gonococcal Monoarthritis: Frank purulent monoarthritis (knee, wrist, ankle) without prominent skin rash.
  • Clinical Pearl: Synovial fluid cultures are positive in fewer than 25% to 50% of DGI cases because the organism is fastidious. Clinicians must obtain nucleic acid amplification testing (NAAT) from urogenital (cervical, urethral), pharyngeal, and rectal sites, alongside blood cultures.
• Gram-Negative Bacilli (Escherichia coli, Pseudomonas aeruginosa): Encountered in older adults with urinary tract infections, immunocompromised hosts, and individuals who use intravenous drugs (where Pseudomonas has a predilection for axial fibrocartilaginous joints: sternoclavicular, sacroiliac, and pubic symphysis).

2. Clinical Presentation & Physical Examination

• Acute monoarticular pain, swelling, warmth, and profound functional limitation.
• The joint is held in slight flexion (maximizing intracapsular volume).
• Exquisite pain with both active AND passive range of motion is a cardinal sign distinguishing true articular pathology from periarticular inflammation (such as prepatellar bursitis or tenosynovitis, where passive motion is relatively preserved).

3. Emergent Treatment Protocol

1. Urgent Orthopedic Surgical Consultation: Closed needle joint aspiration, arthroscopic debridement, or open arthrotomy (mandatory for hip and shoulder infections, or refractory knee infections) to evacuate pus, decompress intra-articular pressure, and prevent avascular necrosis.
2. Immediate Empiric Intravenous Antimicrobials (initiated immediately after synovial fluid aspiration):
   • Standard empiric regimen: Intravenous Vancomycin (15-20 mg/kg IV q8-12h; targeting trough 15-20 mcg/mL) PLUS a third-generation cephalosporin such as Intravenous Ceftriaxone (1-2 g IV q24h) to cover MRSA, Gram-negative bacilli, and N. gonorrhoeae.
   • In patients with severe beta-lactam anaphylaxis: Vancomycin plus an aminoglycoside (tobramycin) or aztreonam/fluoroquinolone.
   • Tailor antibiotics based on definitive Gram stain and culture susceptibility data; total antibiotic duration is typically 3 to 4 weeks (minimum 2 weeks intravenous followed by oral step-down for susceptible organisms).

Crystal-Induced Arthropathies: Gout

Gout is the most common inflammatory arthritis in adult males, resulting from the tissue deposition of monosodium urate (MSU) crystals into synovial joints and soft tissues following sustained hyperuricemia (serum urate concentration exceeding the physiologic saturation threshold of approximately 6.8 mg/dL).

Pathophysiology

Hyperuricemia arises predominantly from renal underexcretion (>90% of patients) due to genetic variants in renal urate transporters (URAT1, GLUT9) or acquired renal impairment, whereas metabolic overproduction accounts for <10% (e.g., myeloproliferative disorders, tumor lysis syndrome, high-purine diets, inborn errors of metabolism). MSU crystals precipitate within the joint space and are phagocytosed by synovial macrophages, activating the intracellular NLRP3 inflammasome complex. This triggers caspase-1-dependent activation and massive secretion of interleukin-1 beta (IL-1β), recruiting waves of neutrophils that release reactive oxygen species and lysosomal proteases, creating an explosive inflammatory flare.

Polarized Light Microscopy: The Definitive Diagnostic Hallmark

                  POLARIZED MICROSCOPY: GOUT VS. PSEUDOGOUT

   Feature                 Gout (Monosodium Urate)         Pseudogout (CPPD)
   ═════════════════════════════════════════════════════════════════════════════════════════
   Crystal Morphology      Needle-shaped or needle-like;   Rhomboid-shaped, rod-like,
                           sharp pointed ends              parallelepiped; blunt ends

   Birefringence           STRONGLY NEGATIVE               WEAKLY POSITIVE
                           birefringence                   birefringence

   Compensated Color:      YELLOW                          BLUE
   Parallel to Axis        ("Yellow Parallel")             ("Blue Parallel")

   Compensated Color:      BLUE                            YELLOW
   Perpendicular to Axis   ("Blue Perpendicular")          ("Yellow Perpendicular")
   ═════════════════════════════════════════════════════════════════════════════════════════

High-Yield Exam Memory Aid: Remember the mnemonic "YENP": Yellow is Elongated (needle) and Negative when Parallel (Gout). For pseudogout: "PBP": Pseudogout is Blue when Parallel.

Acute Gout Flare Triggers

• Dietary Purines: Organ meats (liver, kidneys), red meat, shellfish, sardines, and anchovies.
• Alcohol: Beer contains high levels of purines (guanosine) and alcohol metabolizes to lactic acid, which competitively inhibits renal tubular urate secretion. Spirits and wine also increase risk.
• High-Fructose Corn Syrup: Fructose phosphorylation in the liver consumes ATP, generating AMP that enters the purine degradation pathway, rapidly elevating uric acid.
• Medications: Thiazide and loop diuretics (enhance proximal tubular urate reabsorption), low-dose aspirin (<2 g/day), cyclosporine, tacrolimus, and niacin.
• Physiologic Stressors: Surgery, systemic infection, starvation, dehydration, trauma, and rapid fluctuations in serum urate levels (such as commencing urate-lowering therapy).

Clinical Presentation: Podagra

Over 50% of initial acute flares involve the first metatarsophalangeal (MTP) joint (podagra); over a lifetime, >80% of gout patients experience podagra. Other common sites include the midfoot, ankle, knee, wrist, and olecranon bursa. Onset is characteristically explosive, often beginning overnight, reaching maximum pain intensity within 12 to 24 hours. The joint displays dusky erythema, dramatic swelling, warmth, and exquisite tenderness (patients describe agony even from a bedsheet resting on the toe), followed by desquamation of overlying skin as inflammation resolves.

Evidence-Based Pharmacotherapy for Acute Gout Flares

Three equally effective first-line pharmacologic classes exist; the selection depends entirely on patient comorbidities:

1. Oral Non-Steroidal Anti-Inflammatory Drugs (NSAIDs):
   • Regimens: Indomethacin (50 mg PO TID) or Naproxen (500 mg PO BID) or Ibuprofen (800 mg PO TID) initiated at maximum doses and tapered over 5 to 7 days after symptom resolution.
   • Contraindications: Chronic kidney disease (eGFR <60 mL/min), active peptic ulcer disease or GI bleeding, decompensated heart failure, cirrhosis, and concurrent therapeutic anticoagulation.
2. Oral Colchicine:
   • Regimen: 1.2 mg PO stat, followed by 0.6 mg PO 1 hour later (total 1.8 mg). A maintenance dose of 0.6 mg once or twice daily may be continued starting 12 hours later until the flare resolves.
   • Window of Efficacy: Highly effective if started within 24 to 36 hours of symptom onset; efficacy drops dramatically after 36 hours.
   • Safety & Interactions: Substrate for CYP3A4 and P-glycoprotein (P-gp). Dose reduction required in renal impairment (eGFR <30 mL/min); contraindicated in combined renal and hepatic failure or with strong CYP3A4 inhibitors (clarithromycin, ketoconazole) due to fatal neuromyopathy.
3. Oral Systemic Corticosteroids:
   • Regimen: Prednisone 35 to 40 mg PO daily for 5 to 7 days, then stop (or taper over 7 to 10 days).
   • Preferred First-Line Choice: Patients with chronic kidney disease (CKD), heart failure, peptic ulcer disease, or anticoagulation where NSAIDs and colchicine are hazardous.
   • Intra-articular Corticosteroid Injection: Triamcinolone acetonide (10-40 mg depending on joint size); ideal for an acute flare in 1 or 2 accessible joints once septic arthritis is definitively excluded.

[!CAUTION] THE CRITICAL BOARD EXAM RULE: ALLOPURINOL MANAGEMENT IN ACUTE FLARES

1. DO NOT INITIATE allopurinol (or other urate-lowering therapy) during an acute gout flare! Rapidly reducing serum urate promotes crystal dissolution, destabilizing articular tophi and prolonging or intensifying the acute flare. Wait until the acute flare has fully resolved (typically 2 to 4 weeks) before starting urate-lowering therapy.
2. DO NOT DISCONTINUE allopurinol if the patient is ALREADY taking it chronically at the time of the flare! Abrupt discontinuation causes a rebound spike in serum urate, triggering secondary crystal deposition and worsening the flare. Continue the baseline allopurinol dose unchanged and treat the flare aggressively with NSAIDs, colchicine, or steroids.

Long-Term Management of Hyperuricemia & Gout Prophylaxis

Indications for Urate-Lowering Therapy (ULT)

According to American College of Rheumatology (ACR) guidelines, ULT is strongly indicated for patients with:

• >=2 acute gout flares per year
• Presence of tophi on physical examination or imaging
• Radiographic evidence of gouty joint damage / erosions (e.g., "punched-out" erosions with overhanging sclerotic margins / Martel sign)
• Gout with concurrent stage 3 or higher chronic kidney disease (CKD eGFR <60 mL/min)
• History of urolithiasis (uric acid nephrolithiasis)

First-Line Urate-Lowering Therapy: Allopurinol

• Mechanism: Xanthine oxidase inhibitor that blocks the sequential conversion of hypoxanthine to xanthine and xanthine to uric acid.
• Dosing Strategy: Start at a low dose (100 mg PO daily; or 50 mg daily in CKD stage 3-5) to avoid precipitously dropping serum urate. Titrate every 2 to 4 weeks by 100 mg increments based on serial serum urate measurements, up to a maximum dose of 800 mg/day.
• Target Serum Urate Level:
  • Standard target: Serum urate <6.0 mg/dL (below the 6.8 mg/dL solubility threshold to promote crystal dissolution).
  • Severe tophaceous gout: Serum urate <5.0 mg/dL for accelerated tophus resolution.
• HLA-B*5801 Genetic Screening:
  • Mandatory testing prior to allopurinol initiation in high-risk populations: patients of Southeast Asian descent (Han Chinese, Korean, Thai) and African American descent.
  • Possession of the HLA-B*5801 allele confers an astronomical risk of Allopurinol Hypersensitivity Syndrome (AHS), characterized by Stevens-Johnson syndrome (SJS), toxic epidermal necrolysis (TEN), eosinophilia, systemic vasculitis, hepatitis, and acute renal failure with a 25% mortality rate.

Mandatory Flare Prophylaxis During ULT Initiation

When initiating urate-lowering therapy, mobilizing crystals frequently triggers secondary gout flares. Clinicians must co-prescribe prophylactic anti-inflammatory therapy:

• First-line prophylaxis: Low-dose oral Colchicine 0.6 mg once or twice daily (0.3 mg daily in CKD), OR low-dose oral NSAID (e.g., naproxen 250 mg PO BID with a PPI).
• Duration: Continue prophylaxis for at least 3 to 6 months after achieving the target serum urate level without active flares.

Calcium Pyrophosphate Dihydrate (CPPD) Crystal Deposition Disease (Pseudogout)

CPPD disease results from the precipitation of calcium pyrophosphate dihydrate crystals in articular and fibrocartilaginous structures.

Clinical Presentation

• Acute CPPD Crystal Arthritis (Pseudogout): Acute, self-limiting monoarthritis or oligoarthritis resembling gout. However, unlike gout's predilection for the 1st MTP, pseudogout most commonly involves the knee (>50% of cases), followed by the wrist, shoulder, ankle, and elbow.
• Chronic CPPD Arthropathy: Resembles severe osteoarthritis with prominent joint destruction, but characteristically affects atypical joints (radiocarpal joint, metacarpophalangeal joints) with prominent morning stiffness.

Radiographic Hallmark: Chondrocalcinosis

Plain radiographs demonstrate pathognomonic chondrocalcinosis—punctate and linear radiodense calcifications within hyaline cartilage and fibrocartilage. Classic anatomic sites to image:

1. Knees: Calcification of the medial and lateral menisci and articular hyaline cartilage.
2. Wrists: Calcification of the triangular fibrocartilage complex (TFCC).
3. Pelvis: Calcification of the pubic symphysis.

Systemic Metabolic Associations: High-Yield Screening

CPPD occurring in patients under age 60 mandates laboratory screening for secondary metabolic and endocrine disorders:

• Hereditary Hemochromatosis: Iron overload; check serum ferritin and transferrin saturation (>45%). CPPD characteristically produces squaring of bone ends and hook-like osteophytes at the 2nd and 3rd MCP joints.
• Primary Hyperparathyroidism: Check serum calcium and intact parathyroid hormone (PTH).
• Hypomagnesemia: Low magnesium levels impair pyrophosphatase enzyme activity, accelerating crystal deposition.
• Hypophosphatasia: Check serum alkaline phosphatase (characteristically low).

Acute Management

Identical to acute gout flares: oral NSAIDs, low-dose colchicine, oral systemic corticosteroids, or intra-articular corticosteroid injection. Long-term urate-lowering therapies (such as allopurinol) have no efficacy in CPPD.

Spondyloarthropathies (SpA)

The spondyloarthropathies represent a family of interconnected chronic inflammatory rheumatic diseases characterized by genetic association with HLA-B27, seronegativity for rheumatoid factor (RF) and anti-CCP, prominent axial skeletal involvement (sacroiliitis and spondylitis), asymmetrical peripheral oligoarthritis, enthesitis (inflammation at tendon/ligament insertions into bone), and dactylitis.

                  COMPARISON OF THE SPONDYLOARTHROPATHIES

   Disease                 Demographics    Key Clinical Features                 Radiographic & Lab Findings
   ═══════════════════════════════════════════════════════════════════════════════════════════════════════════
   Ankylosing              Males > Females Insidious inflammatory back pain;     Bilateral sacroiliitis;
   Spondylitis (AS)        (2-3:1);        improves with exercise, not rest;     syndesmophytes; "bamboo spine";
                           Age 20-30       morning stiffness >30 min; uveitis    HLA-B27 positive (>90%)

   Psoriatic               Males = Females Dactylitis ("sausage digits");        "Pencil-in-cup" deformity;
   Arthritis (PsA)         Age 30-50;      DIP joint predilection; nail          asymmetric sacroiliitis;
                           Psoriasis (85%) pitting; plaque psoriasis             HLA-B27 in axial disease (50%)

   Reactive                Young adults;   Triad: arthritis, urethritis,         Asymmetric sacroiliitis;
   Arthritis (Reiter)      Post-infection  conjunctivitis; keratoderma           HLA-B27 positive (65-80%);
                           (GU or enteric) blennorrhagica; circinate balanitis   sterile synovial fluid
   ═══════════════════════════════════════════════════════════════════════════════════════════════════════════

1. Ankylosing Spondylitis (AS)

• Inflammatory Back Pain Criteria: Chronic axial pain present for >3 months, age at onset <45 years, insidious onset, improves with physical exercise and activity, does NOT improve with rest, and prominent morning stiffness lasting >30 to 60 minutes with frequent nocturnal awakenings during the second half of the night.
• Physical Examination Maneuvers:
  • Modified Schober Test: Assesses restricted lumbar flexion. Mark the lumbosacral junction (dimples of Venus / L5 level) and a point 10 cm superiorly while standing upright. Have the patient bend fully forward with knees straight. Normal lumbar expansion increases the distance by >=5 cm (to >=15 cm). An increase of <5 cm (<15 cm total) indicates restricted lumbar mobility.
  • Chest Expansion: Measured at the 4th intercostal space; expansion <2.5 cm reflects costovertebral and costosternal joint ankylosis.
  • Occiput-to-Wall Distance: Normally 0 cm; an inability to touch the occiput to the wall while standing against it signifies fixed thoracic kyphosis and loss of cervical extension.
• Diagnostic Imaging:
  • Pelvic plain radiograph: Demonstrates bilateral, symmetric sacroiliitis (blurring of joint margins, sclerosis, erosions, and ankylosis).
  • Lumbar spine radiograph: Vertebral body squaring, calcification of the annulus fibrosus forming vertically oriented marginal syndesmophytes, and late continuous ossification producing the classic "bamboo spine".
  • Pelvic MRI (STIR sequence): Most sensitive early diagnostic tool for non-radiographic axial SpA, detecting subchondral bone marrow edema years before plain radiographic changes emerge.
• Extraskeletal Manifestations:
  • Acute Anterior Uveitis (Iritis): Most common extraskeletal feature (25% to 30% of patients); presents as acute unilateral eye pain, photophobia, ciliary flush, and blurred vision. Requires urgent ophthalmologic referral and topical glucocorticoid/cycloplegic drops.
  • Cardiovascular: Aortic root dilation, aortic valve regurgitation, and cardiac conduction defects (AV blocks).
  • Pulmonary: Apical pulmonary fibrocystic changes and restrictive ventilatory defect from chest wall rigidity.
• Treatment Protocol:
  • First-line: Continuous, scheduled oral NSAIDs (e.g., naproxen, celecoxib) plus structured physical therapy (aerobic exercise and swimming to preserve spinal mobility).
  • Second-line for active axial disease refractory to NSAIDs: Biologic DMARDs—Tumor Necrosis Factor (TNF) inhibitors (adalimumab, infliximab, etanercept) or Interleukin-17 (IL-17) inhibitors (secukinumab, ixekizumab). Oral systemic corticosteroids are ineffective for axial disease.

2. Psoriatic Arthritis (PsA)

• Develops in 15% to 30% of patients with cutaneous psoriasis; cutaneous lesions precede arthritis in 85% of cases.
• Cardinal Clinical Hallmarks:
  • Dactylitis ("Sausage Digits"): Diffuse, uniform inflammatory swelling of an entire digit resulting from concurrent tenosynovitis and synovitis of MCP, PIP, and DIP joints.
  • Distal Interphalangeal (DIP) Joint Predilection: Characteristic joint involvement that distinguishes PsA from rheumatoid arthritis (which strictly spares the DIP joints).
  • Psoriatic Nail Dystrophy: Deep punctate nail pitting, onycholysis (separation of nail plate from bed), subungual hyperkeratosis, and "oil-drop" discoloration.
  • Radiographic "Pencil-in-Cup" Deformity: Severe osteolysis tapering the head of the proximal phalanx ("pencil") fitting into the expanded, hollowed base of the adjacent distal phalanx ("cup").
• Treatment: NSAIDs for mild disease; conventional synthetic DMARDs (Methotrexate) for peripheral arthritis; TNF inhibitors or IL-17/IL-23 inhibitors for erosive disease, dactylitis, or axial involvement.

3. Reactive Arthritis (Reiter Syndrome)

• Pathophysiology: Autoimmune sterile synovitis developing 1 to 4 weeks following an antecedent mucosal genitourinary or gastrointestinal infection in an HLA-B27-positive individual.
• Triggering Pathogens:
  • Genitourinary: Chlamydia trachomatis (most common) or Mycoplasma genitalium.
  • Gastrointestinal: Enteric Gram-negative bacilli—Campylobacter jejuni, Salmonella enterica, Shigella flexneri, Yersinia enterocolitica, and Clostridioides difficile.
• Classic Clinical Triad: Mnemonic "Can't see, can't pee, can't climb a tree":
  1. Asymmetric Oligoarthritis: Predominantly lower extremities (knees, ankles, subtalar joints, dactylitis).
  2. Non-Gonococcal Urethritis / Cervicitis: Dysuria, mucopurulent discharge, or aseptic sterile pyuria.
  3. Ocular Inflammation: Bilateral sterile conjunctivitis or acute anterior uveitis.
• Pathognomonic Mucocutaneous Signs:
  • Keratoderma Blennorrhagica: Waxy, hyperkeratotic papules and pustules on the palms and soles that coalesce into crusting plaques resembling pustular psoriasis.
  • Circinate Balanitis: Painless, superficial, serpiginous erythematous erosions on the coronal sulcus or glans penis.
  • Painless oral aphthous ulcerations.
• Treatment Protocol:
  • First-line: High-dose oral NSAIDs (indomethacin 50 mg PO TID or naproxen 500 mg PO BID).
  • Antimicrobials: Treat active genitourinary Chlamydia infection with Doxycycline 100 mg PO BID for 7 days (including sexual partners); note that antibiotics do NOT alter or shorten the joint arthritis once established.
  • Chronic refractory disease: Intra-articular steroid injections or oral Sulfasalazine.