In a 48-year-old HLA-B27-positive male who does not meet criteria for ankylosing spondylitis, presenting with chronic diffuse muscle pain, fatigue, dorsal spine pain, a normal spinal MRI (only prominent paraspinal muscles), and a family history of dry eye without dry mouth, what is the most likely diagnosis and what are the differential diagnoses?

Most Likely Diagnosis: Fibromyalgia

The most likely diagnosis in this 48-year-old HLA-B27-positive male with chronic diffuse muscle pain, fatigue, dorsal spine pain without joint inflammation, normal spinal MRI showing only prominent paraspinal muscles, and family history of dry eye is fibromyalgia. While his HLA-B27 positivity raises consideration of axial spondyloarthritis (axSpA), he explicitly does not meet criteria for ankylosing spondylitis and lacks the cardinal features of inflammatory spinal disease.

Key Clinical Reasoning

Why Fibromyalgia is Most Likely:

• Diffuse muscle pain with fatigue lasting months to years is the hallmark presentation of fibromyalgia
• Absence of joint swelling or inflammation argues strongly against inflammatory arthritis
• Normal MRI excludes active sacroiliitis or spondylitis—the imaging shows only prominent paraspinal muscles, which likely reflects muscle tension or deconditioning rather than inflammation 1
• Chronic pain pattern without inflammatory features (no morning stiffness improvement with exercise, no night pain pattern described)
• The prominent paraspinal muscles on MRI may represent chronic muscle tension or myofascial pain, common in fibromyalgia

Why NOT Axial Spondyloarthritis:

Despite HLA-B27 positivity, several critical features exclude axSpA:

• Does not meet classification criteria for AS (as stated in the question)
• Normal spinal MRI rules out active inflammatory changes—axSpA requires evidence of sacroiliitis on imaging or radiographs 1
• No inflammatory back pain features: True inflammatory back pain includes insidious onset before age 40, improvement with exercise, no improvement with rest, night pain, and morning stiffness >30 minutes 1
• No sacroiliac joint involvement documented
• Absence of enthesitis (no mention of heel pain, Achilles tendinitis, or other entheseal sites)

The guidelines are clear: HLA-B27 alone has only 32% post-test probability for axSpA in chronic back pain patients and should not be used as a sole diagnostic criterion 2. Only 10% of HLA-B27-positive individuals develop spondyloarthritis 3.

Differential Diagnoses (in order of likelihood):

1. Fibromyalgia (Most Likely)

• Chronic widespread pain >3 months
• Fatigue as prominent feature
• Normal inflammatory markers and imaging
• Muscle tenderness without inflammation

2. Myofascial Pain Syndrome

• Localized to dorsal spine region
• Prominent paraspinal muscles on MRI suggest chronic muscle tension
• Trigger points in affected muscles
• No systemic inflammation

3. Non-radiographic Axial Spondyloarthritis (nr-axSpA) (Less Likely but Consider)

• HLA-B27 positivity is present
• Family history of autoimmune features (dry eye—possible Sjögren's connection)
• However: Normal MRI argues strongly against this—nr-axSpA requires MRI evidence of sacroiliitis or spinal inflammation per ASAS criteria 1
• The absence of inflammatory back pain features makes this unlikely
• If considering this, would need MRI of sacroiliac joints specifically, not just spine 1, 4

4. Incomplete/Undifferentiated Connective Tissue Disease

• Family history of dry eye (possible Sjögren's syndrome in family)
• HLA-B27 positive but doesn't meet SpA criteria
• Chronic pain and fatigue
• Would need additional serologic testing (ANA, anti-Ro/La, RF)

5. Chronic Fatigue Syndrome/Myalgic Encephalomyelitis

• Prominent fatigue with muscle pain
• Chronic duration
• Normal objective findings

6. Mechanical/Degenerative Spine Disease with Chronic Pain Syndrome

• Dorsal spine pain
• Prominent paraspinal muscles suggest compensatory muscle spasm
• Age-appropriate degenerative changes possible

Critical Diagnostic Pitfalls to Avoid:

1. Do not diagnose axSpA based on HLA-B27 alone—sensitivity is 90% but specificity is only 90%, giving post-test probability of only 32% in chronic back pain 2

2. Normal inflammatory markers do not exclude axSpA—ESR/CRP are elevated in only 50% of AS patients 2—but combined with normal MRI, this makes inflammatory disease very unlikely

3. HLA-B27 positivity does not equal disease—74-89% of axSpA patients are HLA-B27 positive, but only a small fraction of HLA-B27-positive individuals develop disease 1, 5

4. The family history of dry eye is intriguing but isolated—not the acute anterior uveitis typically seen with HLA-B27-associated disease 6

Recommended Diagnostic Approach:

1. Assess for fibromyalgia criteria: Widespread pain index and symptom severity score

2. Evaluate for inflammatory back pain features specifically 1:

   • Age of onset <40 years
   • Insidious onset
   • Improvement with exercise
   • No improvement with rest
   • Pain at night (second half)
   • Morning stiffness >30 minutes

3. If any inflammatory features present, obtain MRI of sacroiliac joints (not just spine) with STIR sequences to evaluate for sacroiliitis 1, 4

4. Check inflammatory markers (ESR, CRP) though normal values don't exclude disease 2

5. Consider rheumatology referral if inflammatory features emerge or if fibromyalgia diagnosis is uncertain

6. Screen for associated autoimmune conditions given family history: ANA, anti-Ro/La antibodies, Schirmer test for dry eyes

The absence of inflammatory features, normal imaging, and predominance of diffuse muscle pain with fatigue strongly favor fibromyalgia over inflammatory spondyloarthritis, despite HLA-B27 positivity.