In a patient with severe knee pain and longstanding allodynia (hypersensitivity to light touch) since adolescence, should we consider evaluation for Charcot‑Marie‑Tooth disease rather than, or in addition to, Complex Regional Pain Syndrome?

Yes, Charcot-Marie-Tooth Disease Should Be Evaluated in This Patient

This patient with severe knee pain and longstanding allodynia since adolescence warrants evaluation for Charcot-Marie-Tooth (CMT) disease in addition to, or potentially instead of, CRPS. The chronic nature since teen years and the presence of hypersensitivity to touch are consistent with hereditary neuropathy, which can present with prominent sensory symptoms that are often overlooked.

Why CMT Should Be Considered

Sensory Symptoms Are Common but Underrecognized in CMT

While CMT is classically thought of as a motor neuropathy, sensory manifestations occur in 54-75% of patients and can be a presenting feature 1. Positive sensory symptoms including:

• Hypersensitivity to touch (allodynia)
• Neuropathic pain (present in up to 40-66% of CMT patients)
• Paresthesias

These symptoms are particularly prominent in CMT type 2 (axonal forms), where 75% report positive sensory symptoms compared to only 20% in CMT1 (demyelinating forms) 1. Importantly, sensory symptoms can be the main or presenting feature, especially in patients with later onset disease 1.

Pain Is a Major Feature Often Requiring Treatment

Pain affects 56-66% of CMT patients and is frequently undertreated 2, 3. The pain can be:

• Neuropathic (DN4 positive in 40-50% of painful CMT patients)
• Located predominantly in distal extremities (feet affected in 80% of cases)
• Symmetric and peripheral in distribution (64.7%)
• Moderate to severe intensity (VAS >4 in 79% of cases) 3

The chronic nature since adolescence in your patient fits this pattern perfectly.

Diagnostic Approach

Step 1: Clinical Assessment and Family History

Elicit a comprehensive three-generation family history 4. Key points:

• 30% of CMT mutations are de novo, so negative family history does not exclude CMT 4
• Look for subtle signs in family members (high arches, hammer toes, distal weakness)
• Ask about foot deformities, ankle sprains, difficulty with balance since childhood

Step 2: Neurophysiological Testing (EMG/NCS)

Electrodiagnostic studies are essential to determine if neuropathy is present and classify it as demyelinating vs. axonal 4, 5. This guides genetic testing strategy:

• Demyelinating pattern (motor conduction velocity <38 m/s): Test for CMT1A (PMP22 duplication) first
• Axonal pattern: Consider CMT2 (MFN2 mutations account for 33% of CMT2) or CMTX
• Intermediate or mixed patterns: Consider CMTX (GJB1 mutations)

Step 3: Genetic Testing Strategy

Follow a stepwise approach based on EDX results 4:

For demyelinating neuropathy:

1. Test for PMP22 duplication (CMT1A) first - accounts for 70% of CMT1 and 76-90% of sporadic cases
2. If negative and pedigree suggests possible X-linked inheritance, test GJB1 (CMTX - 12% of all CMT)
3. Consider next-generation sequencing panel if above negative

For axonal neuropathy:

1. Test MFN2 (accounts for 33% of CMT2)
2. If female or X-linked pattern possible, test GJB1
3. Consider comprehensive genetic panel

Genetic testing has 100% specificity and high sensitivity for established pathogenic mutations 4.

Step 4: Consider Skin Biopsy if Small Fiber Involvement Suspected

If EMG/NCS is normal but clinical suspicion remains high, skin biopsy with intraepidermal nerve fiber (IENF) density assessment may be considered to diagnose small fiber sensory neuropathy 6. This can identify neuropathy when standard nerve conduction studies are normal.

Clinical Pitfalls to Avoid

1. Don't dismiss CMT based on negative family history - 30% are de novo mutations 4
2. Don't assume CMT is purely motor - sensory symptoms dominate in many patients, especially CMT2 1
3. Don't skip EMG/NCS - this is critical for directing appropriate genetic testing and has much higher yield than random genetic panels 4
4. Don't overlook pain management - pain is undertreated in CMT and significantly impacts quality of life 2, 3

Why This Matters for Your Patient

The longstanding nature since adolescence is atypical for CRPS, which usually follows trauma or surgery. CMT commonly presents in childhood or adolescence with:

• Gradual onset of symptoms
• Distal sensory symptoms (including allodynia and pain)
• Often subtle motor findings that may be missed initially

If CMT is diagnosed:

• No disease-modifying pharmacological treatment currently exists 5, 7
• Management focuses on symptomatic treatment of pain, rehabilitation, orthotic support, and multidisciplinary care 5, 8
• Genetic counseling becomes important
• Prognosis and management expectations differ significantly from CRPS

Concurrent Evaluation Strategy

Pursue both evaluations in parallel:

• Complete the CRPS evaluation as planned by sports medicine
• Simultaneously obtain EMG/NCS and initiate CMT workup
• The two conditions have different diagnostic criteria and treatment approaches, and distinguishing between them (or identifying CMT as the underlying cause) will fundamentally change management

The key distinguishing feature is the chronic progressive nature since adolescence, which strongly suggests an inherited neuropathy rather than CRPS, making CMT evaluation not just reasonable but clinically indicated.