What is the likely diagnosis for a patient with chronic pain in the extremities and sensitivity to touch, without other abnormal findings on exam or labs?

Differential Diagnosis: Complex Regional Pain Syndrome vs. Fibromyalgia

Based on the clinical presentation of extremity pain with sensitivity to touch and no other abnormal findings, you should primarily consider Complex Regional Pain Syndrome (CRPS) if symptoms are unilateral or regional, or Fibromyalgia if symptoms are bilateral and widespread. 1

Key Diagnostic Distinctions

Complex Regional Pain Syndrome (CRPS)

CRPS is the most likely diagnosis if the pain is unilateral or affects a single extremity following any injury (even minor or forgotten trauma). 1

Clinical Features to Identify:

• Pain characteristics: Excruciating, burning quality that is disproportionate to any inciting event and worsens with touch or any stimulation 1, 2
• Allodynia: Increased painful response to light touch is the hallmark feature you're observing 1
• Progressive spread: Pain gradually increases in intensity and expands within the affected limb, potentially spreading to the contralateral limb 1, 2
• Autonomic changes: Look specifically for temperature dysregulation, skin color changes (mottling, cyanosis), abnormal sweating patterns, and edema 2
• Trophic changes: Hair loss, tissue changes, and skin discoloration at the pain site develop in chronic cases 1, 2
• Motor impairment: Functional limb weakness and decreased active range of motion without structural explanation 2

Diagnostic Workup for CRPS:

• Three-phase bone scintigraphy is the most useful imaging modality with 78% sensitivity and 88% specificity 2
• MRI has higher specificity (91%) but lower sensitivity (35%), making it unsuitable for screening 2
• Clinical diagnosis is primarily based on Budapest Criteria; symptoms must be present for at least 12 months to be considered chronic 2

Fibromyalgia

Consider fibromyalgia if pain is bilateral, widespread (affecting multiple body regions), and associated with other systemic symptoms. 1, 3, 4

Clinical Features to Identify:

• Pain distribution: Chronic widespread musculoskeletal pain affecting multiple body regions bilaterally, not just extremities 3, 4, 5
• Hyperalgesia and allodynia: Heightened responses to painful stimuli and painful responses to nonpainful stimuli throughout the body 3, 4
• Associated symptoms: Chronic fatigue, sleep disturbances, cognitive difficulties ("fibro fog"), and multiple functional somatic symptoms 1, 5
• Tender points: While the 2010 ACR criteria de-emphasized tender points, their presence supports the diagnosis 5
• Central pain processing disorder: Pain results from distorted sensory processing rather than local tissue abnormality 3, 4

Diagnostic Approach for Fibromyalgia:

• Clinical diagnosis using ACR 2010 criteria with Fibromyalgia Survey Questionnaire (FSQ) to assess patient symptoms 5
• Stepwise workup: Complete medical history including medications, complete physical examination, basic laboratory tests (CBC, ESR/CRP, thyroid function) to screen for inflammatory or endocrine diseases 5
• Rule out mimics: Exclude inflammatory rheumatic diseases, hypothyroidism, vitamin D deficiency, and mental disorders 5
• No specific diagnostic test: Laboratory and imaging studies are normal; diagnosis is based on symptom pattern and exclusion of other conditions 3, 4, 5

Critical Differentiating Features

Unilateral vs. Bilateral:

• CRPS: Typically unilateral or regional, affecting one extremity 1, 2
• Fibromyalgia: Always bilateral and widespread, affecting multiple body regions 3, 4, 5

Trauma History:

• CRPS: Usually follows an injury (often minor and may be forgotten), with pain disproportionate to the inciting event 1, 2
• Fibromyalgia: No specific trauma; develops from genetic and environmental factors 4

Autonomic Changes:

• CRPS: Prominent autonomic dysfunction with visible skin color changes, temperature differences, and abnormal sweating 2
• Fibromyalgia: No autonomic changes; primarily sensory processing abnormalities 3, 4

Response to Touch:

• CRPS: Touch makes pain excruciating and progressively worse 1, 2
• Fibromyalgia: Touch causes allodynia but not the same excruciating, progressive worsening 3, 4

Alternative Diagnoses to Consider

Peripheral Neuropathy:

• Look for: "Cold burning pain" in glove-and-stocking distribution starting distally, with lower extremities more affected than upper 1
• Risk factors: Diabetes, HIV, alcohol use, vitamin B12/B6 deficiency, thyroid dysfunction, certain medications (older antiretrovirals) 1
• Workup: Fasting glucose, HbA1c, vitamin B12, thyroid function, nerve conduction studies 1

Small Fiber Neuropathy:

• Presentation: Burning pain with allodynia but normal nerve conduction studies 1
• Diagnosis: Skin biopsy showing reduced intraepidermal nerve fiber density 1

Common Diagnostic Pitfalls

• Missing CRPS: Failing to ask about any preceding trauma (even minor) or assuming normal labs/imaging exclude CRPS 1, 2
• Overlooking fibromyalgia: Dismissing widespread pain as "psychosomatic" without proper assessment using validated criteria 5
• Unnecessary testing: Ordering extensive imaging or laboratory tests for fibromyalgia when clinical diagnosis is sufficient 5
• Delayed diagnosis: Both conditions benefit from early recognition and treatment; delayed diagnosis worsens outcomes 2, 5

Immediate Next Steps

For suspected CRPS: Order three-phase bone scintigraphy and refer to pain specialist or physiatrist within 2 weeks, as early physical therapy is critical 2

For suspected fibromyalgia: Use ACR 2010 criteria with FSQ, obtain basic screening labs (CBC, ESR/CRP, TSH, vitamin D), and initiate multimodal treatment including patient education, graded exercise, and consider duloxetine or pregabalin 5

For unclear cases: Document pain distribution (unilateral vs. bilateral), presence of autonomic changes, trauma history, and associated symptoms to guide further evaluation 1, 2, 5