In a 48-year-old man with chronic generalized muscle pain (predominantly posterior trunk), normal spinal MRI, normal creatine kinase, normal thyroid and renal function, elevated triglycerides, positive antinuclear antibody, dry eyes and occasional dry mouth, sleep disturbance and fatigue that worsen after viral infection or long‑haul flights, what is the most likely diagnosis and what are the key differential diagnoses?

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Last updated: September 26, 2026 • View editorial policy

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Most Likely Diagnosis: Fibromyalgia Syndrome

The most likely diagnosis in this patient is fibromyalgia syndrome (FMS), with secondary Sjögren's syndrome as a key differential diagnosis that requires further evaluation.

Clinical Reasoning

This 48-year-old man presents with the classic triad of fibromyalgia: chronic widespread pain (particularly posterior trunk/dorsal spine region), sleep disturbance, and fatigue that worsens throughout the day 1. The characteristic features strongly supporting FMS include:

  • Chronic generalized muscle pain with normal CK levels (elevated CK would suggest true myopathy) 2
  • Fatigue that worsens as the day progresses - a hallmark of FMS 1
  • Sleep disturbance - one of the core associated features of FMS 1
  • Symptom exacerbation after viral infections - consistent with central sensitization in FMS 1
  • Improvement with stretching/exercise - typical of FMS, whereas true myopathies worsen with activity
  • Normal CK, normal creatinine, normal thyroid function - effectively ruling out inflammatory myopathy, metabolic myopathy, and hypothyroidism 2
  • Social withdrawal and lack of interest in new people - consistent with the depression/anxiety commonly associated with FMS 1

The normal MRI (except minimal age-appropriate osteophytes) and normal laboratory markers (CK, creatinine) are critical in excluding structural and inflammatory muscle diseases 2, 3.

Key Differential Diagnoses to Consider

1. Primary Sjögren's Syndrome

This requires serious consideration given:

  • Positive ANA
  • Dry eyes and occasional dry mouth
  • Family history of dry eyes in mother
  • The overlap between Sjögren's and fibromyalgia is well-documented 4

Critical next step: The patient needs formal evaluation for Sjögren's syndrome including:

  • Anti-SSA/Ro and anti-SSB/La antibodies
  • Schirmer's test and/or Ocular Surface Disease Index
  • Minor salivary gland biopsy if serologies are negative but clinical suspicion remains high

Important caveat: Sjögren's syndrome frequently coexists with fibromyalgia in middle-aged patients, and the somatic symptoms (pain, fatigue) may be predominantly driven by fibromyalgia rather than the autoimmune process itself 4. The guideline specifically notes that "no studies have confirmed a solid aetiopathogenic autoimmune link between SjS and chronic fatigue syndrome/fibromyalgia beyond the evident epidemiological overlap" 4.

2. Seronegative Inflammatory Myopathy (Lower probability but must exclude)

While less likely given normal CK, recent literature shows that:

  • Some inflammatory myopathies can present with normal or minimally elevated CK 5, 6
  • However, these typically show progressive weakness rather than pain as the primary symptom
  • The improvement with stretching argues strongly against this diagnosis

If clinical suspicion persists: Consider MRI of thigh muscles looking for muscle edema and potentially muscle biopsy, though the normal CK makes this diagnosis unlikely 2, 3.

3. ANCA-Associated Vasculitis with Muscle Involvement (Unlikely but serious)

Recent case series describe AAV presenting with truncal muscle weakness and normal CK 7. However, this patient lacks:

  • Progressive weakness (he has pain, not weakness)
  • Other organ involvement typical of AAV (renal, ENT)
  • The clinical picture doesn't fit

4. Hypertriglyceridemia-Related Myopathy

With triglycerides at 300 mg/dL, this warrants consideration, but:

  • Typically causes episodic rather than chronic symptoms
  • Would expect CK elevation during symptomatic periods
  • The chronic, persistent nature argues against this

Common Diagnostic Pitfalls to Avoid

  1. Over-investigating the positive ANA: ANA positivity occurs in 10-15% of healthy individuals and does not automatically indicate systemic autoimmune disease. The key is whether the patient meets classification criteria for a specific disease 4.

  2. Ordering routine CK and TSH in suspected fibromyalgia: A study of 373 patients with suspected FMS found that routine CK testing identified abnormal values in only 7.5% using local reference ranges, and 0.5% using stricter criteria - with none receiving an alternative diagnosis 2. The final diagnosis was FMS in 89% of patients with abnormal CK.

  3. Misinterpreting "muscle pain" as "muscle weakness": This patient describes pain and fatigue, not true weakness. He can perform activities and feels better with stretching - incompatible with true myopathy.

  4. Attributing all symptoms to fibromyalgia without excluding Sjögren's: The sicca symptoms and positive ANA mandate formal Sjögren's evaluation, as this is a treatable condition with specific complications that require monitoring 4.

Recommended Diagnostic Approach

Immediate workup:

  • Anti-SSA/Ro and anti-SSB/La antibodies
  • Schirmer's test or formal ophthalmologic evaluation with Ocular Surface Disease Index
  • Fasting lipid panel confirmation (already shows elevated triglycerides)

If Sjögren's antibodies negative but sicca symptoms persist:

  • Consider minor salivary gland biopsy
  • Formal rheumatology evaluation

Do NOT routinely order:

  • Repeat CK testing (already normal, low yield) 2
  • Muscle MRI or biopsy (not indicated with normal CK and no true weakness) 2, 3
  • Extensive myositis panels (low pretest probability)

Management should focus on:

  • Confirming FMS diagnosis using ACR criteria 1
  • Addressing sleep disturbance (critical for FMS management)
  • Graded exercise program (evidence-based for FMS) 1
  • Considering pharmacologic options: antidepressants (amitriptyline, duloxetine) or pregabalin based on EULAR recommendations 1
  • Cognitive behavioral therapy if available 1
  • Treating hypertriglyceridemia
  • Specific management of sicca symptoms if Sjögren's confirmed 4

The post-flight symptom exacerbation likely reflects the combination of immobility, dehydration, and sleep disruption during travel - all known to worsen fibromyalgia symptoms 1.

References

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