Lower Back Pain + Left Heel/Ankle Pain: Probable Unifying Causes

Most Likely Diagnoses (in order of probability)

1. Ankylosing Spondylitis (AS) - Top Suspect

• Primarily affects the axial skeleton and sacroiliac joints, causing chronic inflammatory lower back pain with insidious onset, typically worse in the morning and with rest, improving with activity.
• Up to 30% of AS patients have associated enthesopathies such as plantar fasciitis or Achilles tendinopathy - explaining the heel pain directly.
• Ankle involvement can occur as peripheral joint disease.
• Classic demographic: males under 40 years, HLA-B27 positive (~90% of cases).
• Radiographic "bamboo spine" and sacroiliitis are hallmarks on imaging.
• (Rosen's Emergency Medicine, p. 2640)

2. Reactive Arthritis - Strong Contender

• Triggered by prior infection (Chlamydia, Salmonella, Shigella, Yersinia, Campylobacter) 2-6 weeks before joint symptoms.
• Classic asymmetric oligoarthritis of lower extremities with inflammatory low back pain.
• Enthesitis at the Achilles tendon insertion and plantar fascia on the calcaneus is a defining feature - causes heel swelling.
• Can be indistinguishable from AS in its spinal involvement.
• Look for associated conjunctivitis, urethritis, oral ulcers, or a recent GI/STI episode.
• (Robbins Pathologic Basis of Disease, p. 1619)

3. Psoriatic Arthritis

• 20% of psoriasis patients develop arthropathy; may present as asymmetric oligoarthropathy or spondylitis.
• Can involve the sacroiliac joints asymmetrically (vs. symmetric in AS) + peripheral lower extremity joints.
• Heel enthesitis is also a feature. Check for skin/nail changes (psoriatic plaques, nail pitting).
• (Rosen's Emergency Medicine, p. 2657)

4. Enteropathic Arthritis

• Associated with Crohn's disease or ulcerative colitis.
• Peripheral arthritis follows gut disease activity; axial disease is independent of bowel disease.
• Ask about any GI symptoms (chronic diarrhea, blood in stool).

Why the Left Side Specifically?

Other Causes to Consider (Non-SpA)

Key Distinguishing Features to Assess

Recommended Workup

• HLA-B27 typing
• CRP and ESR (elevated in active SpA)
• Pelvic X-ray / MRI sacroiliac joints - sacroiliitis is the hallmark
• Rheumatoid factor and anti-CCP (to exclude RA)
• ANA panel if needed
• STI screen and stool cultures if reactive arthritis suspected
• Skin/nail exam for psoriasis

Sources: Goldman-Cecil Medicine (Chapter 244 - Spondyloarthritis); Rosen's Emergency Medicine (Seronegative Spondyloarthropathies, p. 2638-2657); Robbins Pathologic Basis of Disease (Reactive Arthritis, p. 1619)

HLA-B27 and Associated Diseases

Primary HLA-B27 Associated Diseases (Seronegative Spondyloarthropathies)

Other Notable Associations

Important Points

1. Not all B27+ people get disease - only ~1-5% of HLA-B27 positive individuals develop AS. It is a susceptibility marker, not a diagnostic test.
2. B27 negativity does not exclude SpA - 10% of AS patients are B27-negative. Do not use a negative result to rule out the diagnosis.
3. Mechanism - The three leading theories are:
   • Molecular mimicry (cross-reactivity between B27 and bacterial antigens)
   • Abnormal peptide presentation to CD8+ T cells triggering auto-inflammation
   • B27 misfolding in the ER causing an unfolded protein response (Cellular and Molecular Immunology, p. 1011)
4. Subtypes matter - B*2705 and B*2702 confer the highest risk; B*2706 and B*2709 do NOT increase spondyloarthritis risk.

Quick Summary Table

Strongest Association:    Ankylosing Spondylitis       (~90%, OR >100)
Strong:                   Reactive Arthritis           (~67%)
                          IBD Arthritis                (~72%)
Moderate/Variable:        Psoriatic Arthritis          (24-90%)
                          Acute Anterior Uveitis       (19-88%)
                          Juvenile Spondylitis         (60-80%)

Sources: Cellular and Molecular Immunology (Table 15.3, p. 1011-1012); Emery's Medical Genetics (Table 13.3, p. 188); Wills Eye Manual (p. 3457); Goldman-Cecil Medicine (Chapter 244)

Diagnostic Criteria for Ankylosing Spondylitis

1. Modified New York Criteria (1984) - Gold Standard for AS

Clinical Criteria

Radiographic Criterion (Sacroiliitis Grading)

• Bilateral sacroiliitis Grade ≥2, OR
• Unilateral or bilateral sacroiliitis Grade 3 or 4

Harrison's Principles of Internal Medicine 22E, Table 374-2 and 374-3

2. ASAS Criteria for Axial Spondyloarthritis (2009) - Captures Early Disease

Arm 1 - Imaging Arm

Arm 2 - Clinical Arm

SpA Features (used in both arms)

Diagnostic Flowchart (Harrison's 22E)

Inflammatory vs. Mechanical Back Pain - Key Distinguisher

Key Practical Points

• Diagnostic delay of 5-14 years is common because chronic back pain is so common in the general population
• ESR and CRP are elevated in only 30-40% of active axSpA - normal labs do NOT exclude the diagnosis
• A single AP or Ferguson view X-ray of the pelvis is sufficient; multiple views add nothing
• Only ~10% of AS patients progress to "bamboo spine"; most have more limited disease
• Risk factors for progression: male sex, persistent MRI inflammation, high CRP, syndesmophytes, HLA-B27+, smoking

Sources: Harrison's Principles of Internal Medicine 22E (Chapter 374, Tables 374-2, 374-3, 374-4, Fig. 374-2); ASAS Classification Criteria 2009 (van der Linden et al.)