Charcot-Marie-Tooth Disease (CMT)

1. Clinical Overview

Summary

Charcot-Marie-Tooth Disease (CMT), also known as Hereditary Motor and Sensory Neuropathy (HMSN), represents the Most Common Inherited Peripheral Neuropathy Worldwide, with an estimated prevalence of 1 in 2,500 individuals (approximately 36-82 per 100,000 population). [1,2] Named after the three physicians who first described it in 1886 (Jean-Martin Charcot, Pierre Marie, and Howard Henry Tooth), CMT encompasses a Genetically and Clinically Heterogeneous Group of Disorders affecting peripheral nerve structure and function.

CMT is characterised by Progressive Distal Muscle Weakness and Atrophy, Sensory Loss, Foot Deformities (Particularly Pes Cavus and Hammer Toes), and Reduced or Absent Deep Tendon Reflexes. [3] The hallmark clinical presentation includes the characteristic "Champagne Bottle Legs" or "Inverted Champagne Bottle" Appearance—severe distal lower limb wasting with preserved proximal thigh musculature—combined with high-arched feet (pes cavus) and steppage gait due to foot drop. [4]

CMT is traditionally classified based on Electrophysiological Characteristics into:

• CMT1 (Demyelinating): Motor nerve conduction velocity (MCV) less than 38 m/s
• CMT2 (Axonal): MCV ≥38 m/s with reduced amplitudes
• Intermediate CMT: MCV 25-45 m/s
• CMTX (X-Linked): Variable, often intermediate MCV
• CMT4 (Autosomal Recessive Demyelinating): Often severe, early-onset

CMT1A, caused by Duplication of the PMP22 Gene on chromosome 17p11.2, is the Single Most Common Form, accounting for 50-60% of all CMT cases and approximately 70% of demyelinating CMT. [5,6] The second most common form is CMTX1 (X-linked CMT type 1), caused by mutations in the GJB1 gene encoding connexin-32, representing 10-15% of CMT cases and being the most common form after CMT1A. [7]

Onset is typically in the First or Second Decade of Life, although milder forms may present later. Progression is Slow and Variable, measured over decades rather than years, and Most Patients Maintain Ambulation Throughout Life (> 95% remain ambulatory). [8] Normal or near-normal lifespan is expected in most subtypes, although quality of life can be significantly impacted by disability, falls, and musculoskeletal pain.

No Disease-Modifying Therapies currently exist for CMT, but management focuses on Multidisciplinary Supportive Care including physiotherapy, ankle-foot orthoses (AFOs), occupational therapy, pain management, and selective orthopaedic surgery for severe deformities. [9] Genetic counselling is essential given the hereditary nature. Emerging therapies targeting PMP22 overexpression (gene silencing, antisense oligonucleotides) are in clinical development. [10]

Clinical Pearls

"Champagne Bottle Legs" / "Stork Legs" / "Inverted Champagne Bottle": The pathognomonic appearance of severe distal calf wasting with preserved proximal thigh musculature, reflecting length-dependent axonal degeneration affecting longest peripheral nerves first.

"Pes Cavus + Distal Weakness + Areflexia = Think CMT": This clinical triad is highly suggestive of hereditary neuropathy. Pes cavus (high-arched foot) is present in ~80-90% of CMT patients and is often the earliest skeletal manifestation. [11]

"CMT1A = PMP22 Duplication; HNPP = PMP22 Deletion": The reciprocal duplication/deletion of 17p11.2 produces two distinct phenotypes—CMT1A (demyelinating neuropathy) vs. HNPP (hereditary neuropathy with liability to pressure palsies, causing episodic focal neuropathies).

"Uniformly Slow NCS = CMT1 (Demyelinating); Reduced Amplitudes = CMT2 (Axonal)": Electrophysiology is critical for subtype classification and guides genetic testing strategy.

"CMTX1 Males >> Females": X-linked inheritance means affected males are typically more severely affected than carrier females, who may have mild symptoms or be asymptomatic.

"Avoid Vincristine in CMT": The chemotherapy agent vincristine can cause Severe, Potentially Irreversible Exacerbation of neuropathy in CMT patients. Patients should carry a "medications to avoid" card. [12]

"Slow but Not Stopping": CMT progression is measured in decades. Unlike acquired inflammatory neuropathies (CIDP, GBS), CMT does not exhibit rapid deterioration or respond to immunotherapy.

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2. Epidemiology and Genetics

Demographics

Genetic Architecture

CMT is one of the Most Genetically Heterogeneous Inherited Diseases, with Over 130 Genes associated with various CMT subtypes and related hereditary neuropathies. [14] Despite this genetic complexity, A Small Number of Genes Account for the Majority of Cases:

Genetic Diagnostic Yield

With comprehensive genetic testing (including whole genome sequencing), Diagnostic Yield Approaches 70-80% in well-characterized CMT cohorts. [14] The remaining 20-30% likely have mutations in Novel Genes Not Yet Discovered or Deep Intronic/Regulatory Variants not detected by standard sequencing panels.

Testing Strategy:

1. First-line: Test for PMP22 duplication/deletion (CMT1A/HNPP) using MLPA or array CGH
2. If demyelinating (MCV less than 38 m/s) and PMP22 negative: Sequence GJB1 (CMTX1), MPZ (CMT1B)
3. If axonal (MCV ≥38 m/s): CMT2 gene panel starting with MFN2 (CMT2A)
4. If recessive inheritance suspected: Test GDAP1, SH3TC2, SORD, FIG4
5. If negative: Whole Exome Sequencing (WES) or Whole Genome Sequencing (WGS) [14]

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3. Classification Systems

3.1 Electrophysiological Classification (Traditional)

CMT is classified based on Motor Nerve Conduction Velocity (MCV) of the median or ulnar nerve:

Key Point: The 38 m/s cutoff is based on median/ulnar nerve MCV. Uniformly slow velocities (all nerves similarly affected) suggest hereditary demyelinating neuropathy, whereas Non-uniform slowing (patchy, multifocal) suggests acquired demyelinating neuropathy (e.g., CIDP).

3.2 Genetic Classification (Modern)

Modern classification is based on Gene Mutation (e.g., CMT1A = PMP22 duplication; CMT2A = MFN2 mutation), which provides:

• Precise diagnosis
• Prognostic information
• Genetic counselling for family members
• Eligibility for gene-specific clinical trials

3.3 Clinical Severity Classification

Charcot-Marie-Tooth Neuropathy Score (CMTNS) or CMT Examination Score (CMTES): Validated clinical outcome measures ranging 0-36 (CMTNS) or 0-28 (CMTES), assessing:

• Sensory symptoms
• Motor symptoms (legs/arms)
• Pin sensibility
• Vibration
• Strength (ankle dorsiflexion, great toe extension, hand muscles)
• Reflexes

Used in Natural History Studies and Clinical Trials to quantify disease severity and progression. [25]

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4. Pathophysiology

4.1 CMT1 (Demyelinating Neuropathies)

Prototype: CMT1A (PMP22 Duplication)

1. Genetic Defect: 1.4 Mb tandem duplication at chromosome 17p11.2 containing the PMP22 gene → 1.5-fold overexpression of PMP22 protein (gene dosage effect). [5]

2. PMP22 Overexpression in Schwann Cells: PMP22 is a transmembrane glycoprotein constituting 2-5% of peripheral nerve myelin. Overexpression leads to:

   • Impaired Schwann Cell Myelination: Abnormal myelin compaction and instability
   • ER Stress and Unfolded Protein Response: Misfolded PMP22 accumulates in ER
   • Schwann Cell Dysfunction: Impaired axon-Schwann cell signalling

3. Demyelination: Progressive loss of myelin sheath → Slowed Nerve Conduction Velocity (less than 38 m/s, often 15-30 m/s in CMT1A)

4. Attempted Remyelination: Schwann cells proliferate and attempt remyelination → "Onion Bulb" Formations (concentric Schwann cell processes surrounding axons on nerve biopsy) → Nerve Hypertrophy (palpably thickened nerves in some patients)

5. Secondary Axonal Loss: Chronic demyelination eventually causes axonal degeneration → Progressive Weakness and Atrophy. Axonal loss is the primary determinant of disability, not degree of demyelination.

6. Length-Dependent Pattern: Longest axons affected first (feet > hands; distal > proximal) due to metabolic/transport vulnerability.

CMT1B (MPZ Mutations): Similar mechanism. MPZ (myelin protein zero) is the Most Abundant Myelin Protein in PNS (~50% of total myelin protein). Mutations cause dysmyelination with wide phenotypic variability. [16]

CMTX1 (GJB1/Connexin-32 Mutations): Connexin-32 forms Gap Junctions in non-compacted myelin (Schmidt-Lanterman incisures, paranodal loops), allowing transfer of ions and small molecules across myelin sheath. Mutations impair gap junction function → myelin instability → Mixed Demyelinating and Axonal Features on NCS. [7]

4.2 CMT2 (Axonal Neuropathies)

Prototype: CMT2A (MFN2 Mutations)

1. Genetic Defect: Missense mutations in MFN2 gene encoding Mitofusin-2, a mitochondrial outer membrane GTPase. [17]

2. Mitochondrial Dysfunction:

   • Impaired Mitochondrial Fusion: MFN2 mediates fusion of outer mitochondrial membranes. Loss-of-function → fragmented mitochondrial network
   • Defective Mitochondrial Transport: Impaired anterograde/retrograde transport along axons → Energy deficit in distal axon
   • ER-Mitochondria Tethering Defect: MFN2 also tethers ER to mitochondria; defect impairs calcium signalling
   • Impaired Mitophagy: Accumulation of damaged mitochondria

3. Axonal Energy Failure: Neurons are highly energy-dependent. Mitochondrial dysfunction → ATP Depletion particularly in distal axons (longest diffusion distance from cell body)

4. "Dying-Back" Axonopathy: Distal axon degeneration progresses proximally → Muscle Denervation → Weakness, Atrophy, Sensory Loss

5. Preservation of Myelin: Primary defect is axonal, so MCV Remains Normal or Near-Normal (≥38 m/s), but CMAP and SNAP Amplitudes Are Markedly Reduced (reflecting axonal loss)

Other CMT2 Genes: Diverse mechanisms affecting axonal transport (e.g., KIF5A, DYNC1H1), mitochondrial function (GDAP1), aminoacyl-tRNA synthetases (GARS1, YARS1), or cytoskeletal proteins (NEFL).

4.3 Pathophysiology of Pes Cavus

Pes cavus (high-arched foot) develops due to Muscle Imbalance:

1. Intrinsic Foot Muscle Weakness (supplied by tibial nerve): Weakness of plantar intrinsic muscles
2. Tibialis Anterior > Peroneus Brevis: Unopposed ankle dorsiflexion/inversion → forefoot plantarflexion
3. Tibialis Posterior Overactivity: Relative sparing of tibialis posterior → hindfoot varus
4. Peroneus Longus > Peroneus Brevis: Plantarflexion of first ray → high medial arch

Result: Forefoot Plantarflexion + Hindfoot Varus + High Longitudinal Arch = Cavovarus Foot [11]

Progression: Initially flexible (correctable with passive manipulation), but becomes rigid over time due to soft tissue contracture and bony remodelling → fixed deformity.

4.4 Comparative Molecular Pathophysiology: CMT1A vs CMT1B vs CMT2A

Exam Detail: CMT1A (PMP22 Duplication):

• Molecular Defect: Gene dosage effect—1.5x PMP22 protein expression from 1.4 Mb tandem duplication at 17p11.2-p12
• Protein Function: PMP22 = 160 amino acid tetraspan transmembrane glycoprotein; constitutes 2-5% of compact peripheral myelin; role in myelin compaction, stability, and axon-Schwann cell adhesion
• Pathogenic Mechanism:
  • Overexpression → ER accumulation → Unfolded protein response (UPR) activation → Schwann cell stress
  • "Abnormal PMP22:MPZ ratio disrupts myelin stoichiometry → Unstable myelin"
  • Impaired Schwann cell differentiation and myelination
  • Secondary axonal loss (axon-myelin signalling disruption)
• NCS Pattern: Uniformly slow MCV (15-30 m/s median/ulnar nerve); preserved/mildly reduced amplitudes early; F-wave prolongation
• Histopathology: Onion bulb formations (proliferating Schwann cell processes); hypertrophic nerves; hypomyelination; secondary axonal loss
• Phenotype Severity: Moderate; slow progression; > 95% ambulatory; onset first-second decade
• Genotype-Phenotype: Duplication size invariant (1.4 Mb); phenotypic variability due to genetic modifiers, environmental factors

CMT1B (MPZ Mutations):

• Molecular Defect: Heterogeneous point mutations/deletions in MPZ gene (chromosome 1q23.3)
• Protein Function: MPZ (Myelin Protein Zero/P0) = 248 amino acid type I transmembrane glycoprotein; Most abundant peripheral myelin protein (~50% by mass); mediates myelin compaction via homophilic adhesion (extracellular Ig-like domain); essential for myelin integrity
• Pathogenic Mechanism:
  • Extracellular domain mutations → Loss of adhesion → Defective myelin compaction
  • Intracellular/transmembrane domain mutations → Protein misfolding → ER retention → UPR activation → Schwann cell apoptosis
  • Dominant-negative effect (mutant MPZ interferes with wild-type protein)
  • Some mutations cause complete loss of function → severe phenotype
• NCS Pattern: Highly variable depending on mutation:
  • "Severe mutations (e.g., p.Thr124Met): Very slow MCV (less than 10 m/s) → Dejerine-Sottas phenotype"
  • "Moderate mutations: MCV 15-35 m/s → CMT1B phenotype"
  • "Mild mutations (late-onset): MCV 25-40 m/s → Late-onset neuropathy"
• Histopathology: Severe hypomyelination (Dejerine-Sottas); onion bulbs; tomacula (focal myelin thickening) in some; axonal loss
• Phenotype Severity: Wide spectrum: Congenital hypomyelination (non-ambulatory) ↔ Late-onset mild neuropathy (ambulatory, minimal disability)
• Genotype-Phenotype: Strong correlation between mutation location/type and phenotype severity. Extracellular domain mutations often milder than intracellular domain mutations.

CMT2A (MFN2 Mutations):

• Molecular Defect: Heterogeneous missense mutations in MFN2 gene (chromosome 1p36.22)
• Protein Function: MFN2 (Mitofusin-2) = 757 amino acid GTPase; localized to mitochondrial outer membrane; mediates mitochondrial fusion (forms homo/heterodimers with MFN1); tethers ER to mitochondria (mitochondria-associated ER membranes, MAMs); regulates mitochondrial transport along axons
• Pathogenic Mechanism:
  • Loss of fusion → Fragmented mitochondrial network → Reduced oxidative phosphorylation capacity
  • Impaired mitochondrial transport → Distal axon energy deficit (longest axons most vulnerable)
  • ER-mitochondria tethering defect → Dysregulated Ca²⁺ homeostasis → Axonal dysfunction
  • Defective mitophagy → Accumulation of dysfunctional mitochondria → ROS production → Oxidative stress
  • Dominant-negative effect (mutant MFN2 disrupts wild-type function)
• NCS Pattern: Normal or mildly reduced MCV (40-55 m/s); markedly reduced CMAP amplitudes (median CMAP often less than 2 mV); absent or markedly reduced SNAP amplitudes (hallmark of axonal loss); normal F-wave latency (if recordable)
• Histopathology: Axonal degeneration (reduced myelinated fiber density); minimal demyelination; mitochondrial aggregates in axons (electron microscopy); no onion bulbs
• Phenotype Severity: Often severe; earlier onset (first decade common); faster progression than CMT1A; prominent hand involvement; ~10-20% wheelchair-dependent; some develop optic atrophy (HMSN6A)
• Genotype-Phenotype: > 100 pathogenic variants; mutation location correlates with severity (GTPase domain mutations often more severe); R94Q founder mutation (common in European populations) → classic severe phenotype

Summary Comparison:

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5. Clinical Presentation

5.1 Classic Features (CMT1A – Typical Phenotype)

5.2 Phenotypic Variability Between Subtypes

5.3 "Red Flags" Suggesting Alternative Diagnosis

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6. Investigations

6.1 Nerve Conduction Studies (NCS) and Electromyography (EMG)

Gold standard for electrophysiological classification. Guides genetic testing strategy.

Key Points:

• Uniform slowing (all nerves affected similarly) → Hereditary demyelinating neuropathy (CMT1)
• Non-uniform/Patchy slowing → Acquired demyelinating neuropathy (CIDP)
• Conduction Block/Temporal Dispersion → Acquired neuropathy (CIDP, MMN), NOT typical of CMT
• Sural-Sparing Pattern (Sural sensory preserved, median/ulnar sensory absent) → Non-specific but can occur in CMT

Exam Detail: Detailed NCS Criteria for CMT Subtyping:

CMT1 (Demyelinating) – Diagnostic Criteria:

• Median or ulnar motor NCV less than 38 m/s (critical threshold)
• Uniform slowing: All tested nerves (median, ulnar, peroneal, tibial) show similar degree of slowing (coefficient of variation typically less than 15%)
• CMAP amplitude: Mildly to moderately reduced (median CMAP 2-5 mV typical in CMT1A; less than 1 mV suggests severe axonal loss)
• Distal motor latency (DML): Prolonged (> 150% of upper limit of normal)
• F-wave latency: Markedly prolonged (often > 100 ms for median/ulnar; > 150 ms for tibial)
• Sensory NCS: Absent or severely reduced SNAP amplitudes; slow sensory velocities (less than 30 m/s)
• Temporal dispersion/conduction block: Absent (if present, consider CIDP)

CMT2 (Axonal) – Diagnostic Criteria:

• Median or ulnar motor NCV ≥38 m/s (preserves myelin function)
• CMAP amplitude: Markedly reduced (median CMAP often less than 2 mV; less than 0.5 mV indicates severe axonal loss)
• SNAP amplitude: Markedly reduced or absent (most sensitive marker of axonal loss)
• Distal motor latency: Normal or mildly prolonged (less than 150% ULN)
• F-wave latency: Normal or mildly prolonged
• Conduction velocity: May be mildly reduced (40-50 m/s) due to selective loss of fastest fibers, but remains > 38 m/s
• EMG: Chronic neurogenic changes (large polyphasic motor units, reduced recruitment); active denervation (fibrillations, positive sharp waves) if ongoing axonal loss

Intermediate CMT – Diagnostic Criteria:

• Median or ulnar motor NCV 25-45 m/s (overlaps demyelinating and axonal ranges)
• Mixed features: Some nerves may show MCV less than 38 m/s, others > 38 m/s
• Amplitudes: Variable reduction
• Genes: Often CMTX1 (GJB1), DNM2, MPZ (some variants), NEFL

Pitfalls in NCS Interpretation:

• Age-related slowing: Normal MCV decreases with age (reduce by ~1 m/s per decade after age 20). Adjust thresholds accordingly.
• Temperature: Cold limbs slow conduction velocity. Ensure limb temperature > 32°C.
• Secondary axonal loss in CMT1: Long-standing demyelinating CMT may develop significant axonal loss → CMAP amplitude reduction. MCV remains slow.
• Mild CMT2: Early axonal CMT may have only subtle amplitude reduction. Serial studies may be needed.
• CIDP mimicking CMT: Acquired CIDP can present with chronic progressive neuropathy. Key discriminators: CMT has uniform slowing, no conduction block, normal/mildly elevated CSF protein; CIDP has non-uniform slowing, conduction block, CSF protein > 1.0 g/L, response to immunotherapy.

6.2 Genetic Testing

Most Important Investigation for Definitive Diagnosis

Indications:

• Confirmed or suspected hereditary neuropathy (clinical + NCS features)
• Family history of neuropathy
• Young-onset neuropathy (less than 40 years) with atypical acquired causes
• Chronic neuropathy with pes cavus/skeletal deformities
• Genetic counselling request

Testing Algorithm:

Step 1: PMP22 Testing (First-Line)

• Multiplex Ligation-Dependent Probe Amplification (MLPA) or Array CGH to detect PMP22 duplication (CMT1A) or deletion (HNPP)
• Detects 50-60% of all CMT cases
• Cost-effective first step

Step 2: Targeted Testing Based on Phenotype

• If Demyelinating (MCV less than 38 m/s) + PMP22 negative:
  • Sequence GJB1 (CMTX1) – especially if X-linked inheritance or male predominance
  • Sequence MPZ (CMT1B)
  • Sequence PMP22 for point mutations (CMT1E)
• If Axonal (MCV ≥38 m/s):
  • Sequence MFN2 (CMT2A) – most common axonal form
  • CMT2 gene panel (MFN2, GDAP1, NEFL, HSPB1, MPZ, etc.)
• If Recessive Inheritance:
  • Test GDAP1, SH3TC2, SORD, FIG4, NEFL

Step 3: Comprehensive Genomic Testing

• CMT Gene Panel (50-100+ genes)
• Whole Exome Sequencing (WES)
• Whole Genome Sequencing (WGS) – increasing diagnostic yield by detecting deep intronic variants, structural variants, repeat expansions [14]

Diagnostic Yield: 70-80% with comprehensive testing [14]

6.3 Nerve Biopsy

Rarely Performed in modern practice (genetic testing has largely replaced it).

Indications:

• Atypical features raising concern for acquired/inflammatory neuropathy (CIDP, vasculitis, amyloidosis)
• Negative genetic testing despite strong suspicion
• Research purposes

Findings in CMT1:

• Onion Bulb Formations: Concentric Schwann cell processes surrounding thinly myelinated axons (pathognomonic of chronic demyelination/remyelination)
• Hypomyelination or Demyelination
• Axonal Loss (variable, correlates with severity)
• Nerve Hypertrophy: Increased endoneurial collagen

Findings in CMT2:

• Axonal Loss (reduced myelinated fiber density)
• Minimal Demyelination (secondary)
• Wallerian Degeneration (axonal degeneration and myelin debris)

6.4 Other Investigations

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7. Differential Diagnosis

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8. Management

No Disease-Modifying Therapy Currently Available. Management is Supportive and Multidisciplinary focused on maintaining function, preventing complications, and optimizing quality of life. [9]

8.1 Multidisciplinary Team (MDT) Approach

┌─────────────────────────────────────────────────────────────────┐
│                   CMT DIAGNOSIS CONFIRMED                        │
│           (Clinical + NCS + Genetic Testing)                     │
└─────────────────────────────────────────────────────────────────┘
                              ↓
┌─────────────────────────────────────────────────────────────────┐
│                 MULTIDISCIPLINARY TEAM (MDT)                     │
├─────────────────────────────────────────────────────────────────┤
│  • Neurologist (Lead clinician)                                 │
│  • Physiotherapist                                              │
│  • Occupational Therapist                                       │
│  • Orthotist (AFO specialist)                                   │
│  • Orthopaedic Surgeon (foot/spine)                             │
│  • Genetic Counsellor                                           │
│  • Podiatrist                                                   │
│  • Pain Specialist (if needed)                                  │
│  • Respiratory Physician (CMT4C/4J)                             │
│  • Psychologist/Support Groups                                  │
└─────────────────────────────────────────────────────────────────┘

8.2 Physiotherapy (CORE INTERVENTION)

Goal: Maintain strength, flexibility, balance, and function. Prevent contractures.

Evidence-Based Interventions: [28,29]

Caution: "Overwork Weakness": High-intensity or exhaustive exercise may cause irreversible weakness in hereditary neuropathies. Moderate-intensity exercise is safe and beneficial; avoid excessive eccentric loading or fatigue. [29]

8.3 Orthoses (Ankle-Foot Orthosis - AFO)

Most Common and Effective Intervention for Foot Drop and Ankle Instability

Outcomes: AFOs improve gait speed, reduce falls, improve balance, reduce energy expenditure. [30] High patient satisfaction with custom AFOs. [31]

Hand Splints: For severe hand weakness/deformity (e.g., wrist extension splint, thumb opposition splint).

8.4 Occupational Therapy (OT)

Goal: Maximize independence in activities of daily living (ADLs)

8.5 Orthopaedic Surgery

Indication: Severe Fixed Deformities unresponsive to conservative measures causing pain, instability, difficulty with bracing.

Timing: Ideally after skeletal maturity (if pediatric patient) unless severe progressive deformity.

Foot and Ankle Procedures: [32,33]

Outcomes: Moderate-to-good pain relief, improved brace tolerance, improved gait. [33] Recurrence is possible if muscle imbalance persists.

Scoliosis Surgery:

• Indication: Progressive scoliosis > 40-50° with significant symptoms or cosmetic concern
• Procedure: Spinal fusion (posterior instrumentation)

8.6 Pain Management

Musculoskeletal Pain (Most Common):

• Related to foot deformity, abnormal gait mechanics, joint stress
• Management: Physiotherapy, orthoses (AFO improves biomechanics), NSAIDs, paracetamol, weight management

Neuropathic Pain (Less Common):

• Burning, shooting pain in feet/hands
• Management:
  • "First-line: Gabapentin 300-1200 mg TDS or Pregabalin 75-300 mg BD"
  • "Second-line: Duloxetine 60 mg OD, Amitriptyline 10-75 mg nocte"
  • Tramadol, opioids (use sparingly due to addiction risk)

8.7 Genetic Counselling

Essential Component of CMT Management

Exam Detail: Detailed Genetic Counseling Scenarios:

Scenario 1: Newly Diagnosed CMT1A Patient (35-year-old woman planning pregnancy)

• Diagnosis: CMT1A (PMP22 duplication) confirmed by MLPA
• Inheritance: Autosomal dominant
• Risk to offspring: 50% risk for each child to inherit the PMP22 duplication
• Phenotype variability: Variable expressivity—child may be asymptomatic, mildly affected, or moderately affected. Anticipation is NOT typical in CMT1A (unlike trinucleotide repeat disorders).
• Reproductive options:
  1. Natural conception with 50% risk
  2. Prenatal diagnosis: Chorionic villus sampling (CVS) at 10-13 weeks or amniocentesis at 15-20 weeks → MLPA for PMP22 duplication → Option to continue or terminate pregnancy based on results and patient values
  3. Preimplantation Genetic Testing (PGT): In vitro fertilization (IVF) + embryo biopsy → Select unaffected embryos for implantation → Avoids affected offspring
  4. Adoption, egg/sperm donation, no children
• Counseling points:
  • Emphasize CMT1A typically has favorable prognosis (> 95% ambulatory, normal lifespan)
  • Discuss patient's own experience with CMT (severity, impact on life) to inform decision
  • Ethical considerations (disability rights, quality of life, burden vs. value)
  • Support decision-making without coercion

Scenario 2: CMTX1 (X-linked) – Male Patient with Affected Daughters

• Diagnosis: CMTX1 (GJB1 mutation)
• Inheritance: X-linked dominant
• Risk to offspring (from affected male):
  • "Sons: 0% risk (father passes Y chromosome to sons, not X)"
  • "Daughters: 100% risk (father passes affected X chromosome to all daughters)"
• Phenotype in carrier daughters: Typically milder than affected males (due to X-inactivation/lyonization); may be asymptomatic or have mild neuropathy; ~10% may be as severely affected as males
• Reproductive options: PGT to select male embryos (100% unaffected); prenatal diagnosis with option to terminate affected female pregnancies (ethically complex)

Scenario 3: CMT4 (Autosomal Recessive) – Carrier Parents

• Diagnosis: Both parents are heterozygous carriers for pathogenic GDAP1 variant (identified through cascade testing after affected child diagnosed)
• Inheritance: Autosomal recessive
• Risk to offspring:
  • 25% chance of affected child (homozygous or compound heterozygous)
  • 50% chance of carrier child (heterozygous, typically asymptomatic)
  • 25% chance of unaffected, non-carrier child
• Phenotype: CMT4A (GDAP1) is severe with early onset, rapid progression, often wheelchair-dependent by adolescence
• Reproductive options: PGT to select unaffected/carrier embryos; prenatal diagnosis; high-resolution ultrasound (may detect polyhydramnios/fetal akinesia in severe cases, though unreliable)

Scenario 4: Cascade Testing in Extended Family

• Proband: 45-year-old man diagnosed with CMT1A (PMP22 duplication)
• At-risk relatives:
  • Mother (70 years, no symptoms) → Genetic testing reveals PMP22 duplication → Asymptomatic carrier with very mild phenotype (subclinical)
  • Sister (42 years, mild foot problems, never diagnosed) → Genetic testing reveals PMP22 duplication → Affected, now referred for AFOs, physiotherapy
  • Daughter (18 years, no symptoms) → Genetic testing reveals PMP22 duplication → Presymptomatic carrier → Early intervention (physiotherapy, monitoring, avoid vincristine, genetic counseling for future reproduction)
• Benefits of cascade testing: Early diagnosis → proactive management; informed reproductive decisions; avoidance of neurotoxic medications

Scenario 5: Negative Genetic Testing – Management and Counseling

• Clinical diagnosis: Hereditary neuropathy (pes cavus, distal weakness, areflexia, slow MCV 25 m/s, family history suggestive of AD inheritance)
• Genetic testing: PMP22 duplication negative; GJB1, MPZ, PMP22 sequencing negative; CMT gene panel (80 genes) negative
• Options:
  1. Whole exome sequencing (WES) or whole genome sequencing (WGS) → Diagnostic yield increases to 70-80% [14]
  2. Research-based sequencing (novel gene discovery studies)
  3. Clinical diagnosis without genetic confirmation
• Genetic counseling:
  • Inheritance pattern still likely AD based on pedigree
  • Empiric recurrence risk ~50% for offspring (if AD) even without identified gene
  • Prenatal testing not available without identified variant
  • Re-test in future as new genes discovered

8.8 Avoidance of Neurotoxic Medications

CRITICAL: Certain medications can cause severe, potentially irreversible exacerbation of neuropathy in CMT patients. [12]

Recommendation: Patients should carry a "CMT Medications to Avoid" Card or alert bracelet, especially if considering chemotherapy.

8.9 Monitoring and Follow-Up

8.10 Emerging Therapies (Research/Clinical Trials)

None currently approved. Patients should be counselled about realistic expectations. Enroll in clinical trials if eligible (ClinicalTrials.gov).

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9. Prognosis and Outcomes

9.1 Natural History

9.2 Prognostic Factors

9.3 Quality of Life

• Functional Limitations: Difficulty with walking (uneven ground, stairs, prolonged distances), running (most cannot run by adulthood), fine motor tasks (buttoning, writing, manipulating small objects).
• Pain: Chronic musculoskeletal pain (feet, ankles, knees, back) in ~50%. [34]
• Fatigue: Common complaint (~60-80%), multifactorial (muscle weakness, gait inefficiency, sleep disturbance). [34]
• Psychosocial Impact: Depression, anxiety, social isolation, reduced employment opportunities. Psychological support and patient support groups beneficial.
• Employment: Most patients with CMT1A are employed. Occupations requiring manual dexterity or prolonged standing/walking may be challenging.

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10. Complications

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11. Special Populations

11.1 Pregnancy and CMT

• Effect of Pregnancy on CMT: Pregnancy does not significantly worsen long-term neuropathy in most patients. Some report transient worsening during third trimester (weight gain, fluid retention, mechanical stress), which typically improves postpartum. [35]
• Effect of CMT on Pregnancy: Most women with CMT have uncomplicated pregnancies and deliveries. Foot drop/balance issues may increase fall risk. Pelvic floor weakness rare.
• Obstetric Management: Standard obstetric care. Anaesthetic considerations if scoliosis (may affect epidural placement).
• Neonatal Considerations: Test infant if autosomal dominant inheritance (50% risk). Early physiotherapy if affected.

11.2 Pediatric CMT

• Delayed Motor Milestones: Walking delay in severe early-onset forms (CMT4, DSS)
• Orthotic Use: Early AFO use can improve gait development
• School Accommodations: Extra time for written work, use of laptop/tablet, elevator access, modified PE
• Psychosocial Support: Peer support, counseling re: body image, bullying
• Growth Monitoring: Monitor for scoliosis (peaks during adolescence)

11.3 CMT and Anesthesia

• General Anesthesia: Generally safe. No specific contraindications to common anesthetic agents.
• Regional Anesthesia (Spinal/Epidural): Controversial. Theoretical concern of exacerbating neuropathy. Most evidence suggests safe if performed carefully. Discuss risks/benefits with anesthetist.
• Neuromuscular Blockers: Use with caution (may have prolonged effect). Avoid succinylcholine if severe muscle atrophy (risk of hyperkalemia).
• Postoperative: Higher risk of respiratory complications if baseline respiratory muscle weakness.

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12. Evidence and Guidelines

Key Clinical Practice Guidelines

Level of Evidence

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13. Patient and Layperson Explanation

What is Charcot-Marie-Tooth Disease?

Charcot-Marie-Tooth Disease (CMT) is an inherited condition that affects the nerves in your arms and legs, particularly the feet, lower legs, hands, and forearms. It causes weakness, muscle wasting, and numbness in these areas. CMT is not a muscle disease—it is a nerve disease (neuropathy) that affects how your nerves control your muscles and sense touch, vibration, and position.

CMT is named after the three doctors who first described it in 1886: Charcot, Marie, and Tooth. It is also called Hereditary Motor and Sensory Neuropathy (HMSN).

What are the symptoms?

Early Symptoms (often in childhood or teenage years):

• Clumsiness and frequent tripping
• Difficulty running or keeping up with peers
• Weak ankles and recurrent ankle sprains
• High-arched feet (pes cavus)
• Hammer toes (toes that curl downward)

Later Symptoms (in adulthood):

• Foot drop: Difficulty lifting the front of the foot, causing a "slapping" gait or high-stepping walk (steppage gait)
• Thin calves: The lower legs become very thin (sometimes called "stork legs" or "champagne bottle legs"), while the thighs remain normal
• Hand weakness: Difficulty with buttoning shirts, opening jars, writing, or using keys
• Numbness or tingling in the feet and hands (usually mild)
• Balance problems and falls

What causes it?

CMT is caused by faulty genes (genetic mutations) that are inherited from your parents. The most common type, CMT1A, is caused by having an extra copy of a gene called PMP22 on chromosome 17. This leads to problems with the insulation (myelin sheath) around your nerves, causing them to work poorly.

Inheritance:

• Autosomal dominant (most types, including CMT1A): If one parent has CMT, there is a 50% chance of passing it to each child.
• X-linked (CMTX1): Passed on the X chromosome. Males are more severely affected than females.
• Autosomal recessive (CMT4): Both parents must carry the gene; there is a 25% chance if both are carriers.

How is it diagnosed?

1. Clinical Examination: A neurologist will examine your strength, sensation, reflexes, and look for typical features (high-arched feet, thin calves, weak ankles).
2. Nerve Conduction Studies (NCS): Tests that measure how fast electrical signals travel along your nerves. CMT causes slow nerve signals (demyelinating type) or weak signals (axonal type).
3. Genetic Testing: A blood test to identify the specific gene mutation. This confirms the diagnosis and helps with family planning.

Is there a cure?

No, there is currently no cure for CMT. However, there are many treatments to help manage symptoms and maintain your quality of life:

Physiotherapy:

• Stretching exercises to prevent your ankles and toes from becoming stiff
• Strengthening exercises to keep your muscles as strong as possible
• Balance training to reduce falls

Ankle-Foot Orthoses (AFOs):

• Braces that support your ankle and foot, helping you walk more safely and preventing foot drop
• Custom-made to fit your foot shape

Occupational Therapy:

• Tools and techniques to help with daily tasks (e.g., button hooks, jar openers, adapted cutlery)
• Modifications to your home or workplace

Surgery (in severe cases):

• Operations to correct severe foot deformities (e.g., high arches, curled toes, ankle stiffness)
• Usually only needed if braces don't help and the deformity causes pain or difficulty walking

Pain Management:

• Painkillers (paracetamol, ibuprofen) for musculoskeletal pain
• Medications for nerve pain if needed (e.g., gabapentin, pregabalin)

Genetic Counselling:

• Advice on the risk of passing CMT to your children
• Information about prenatal testing or IVF with genetic selection (if desired)

What should I avoid?

• Vincristine (a chemotherapy drug): This can make CMT much worse, sometimes permanently. Always tell your doctors you have CMT before any treatment.
• Very high doses of Vitamin B6 (pyridoxine): Can damage nerves.
• Excessive or exhausting exercise: Moderate exercise is good, but overdoing it can cause "overwork weakness."

What is the outlook?

• CMT progresses slowly over many years or decades.
• Most people with CMT remain able to walk throughout their lives (more than 95%).
• Lifespan is usually normal or near-normal.
• Quality of life varies: Some people have very mild symptoms and live active lives; others have more significant disability and need walking aids, braces, or wheelchairs.
• With the right support and treatment, many people with CMT work, have families, and lead fulfilling lives.

Where can I get support?

• Patient Organizations:
  • "CMT Association (USA): https://www.cmtausa.org"
  • "CMT UK: https://www.cmtuk.org.uk"
  • "Charcot-Marie-Tooth Australia: https://www.cmt.org.au"
• Genetic Counselling services at major hospitals
• Online Support Groups and forums for people with CMT and their families

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14. Examination Focus (Medical Students and Trainees)

High-Yield Exam Questions

1. What is the most common genetic cause of Charcot-Marie-Tooth Disease?

Answer: CMT1A, caused by duplication of the PMP22 gene on chromosome 17p11.2. This accounts for ~50-60% of all CMT cases.

2. What is the classic foot deformity associated with CMT?

Answer: Pes cavus (high-arched foot). Present in ~80-90% of CMT patients. Often accompanied by hammer toes and cavovarus deformity (high arch + hindfoot varus).

3. How do you differentiate CMT1 (demyelinating) from CMT2 (axonal) using nerve conduction studies?

Answer:

• CMT1 (Demyelinating): Motor conduction velocity (MCV) less than 38 m/s (uniformly slow). Mildly reduced amplitudes. Prolonged F-wave latencies.
• CMT2 (Axonal): MCV ≥38 m/s (normal or near-normal). Markedly reduced CMAP and SNAP amplitudes (reflecting axonal loss).

4. What medication should be strictly avoided in patients with CMT?

Answer: Vincristine (a chemotherapy agent). It can cause severe, potentially irreversible exacerbation of neuropathy in CMT patients. Other medications to use with caution include high-dose pyridoxine (vitamin B6), nitrofurantoin, metronidazole.

5. What is the characteristic appearance of the lower legs in CMT?

Answer: "Champagne bottle legs" / "Inverted champagne bottle" / "Stork legs"—severe distal lower limb wasting (thin calves) with preserved proximal thigh musculature.

6. What histological finding on nerve biopsy is characteristic of CMT1 (demyelinating CMT)?

Answer: "Onion bulb" formations—concentric layers of Schwann cell processes surrounding thinly myelinated axons, representing chronic demyelination and remyelination.

7. What is the difference between CMT1A and HNPP?

Answer:

• CMT1A: PMP22 duplication → chronic progressive demyelinating neuropathy (distal weakness, pes cavus, slow MCV)
• HNPP (Hereditary Neuropathy with Liability to Pressure Palsies): PMP22 deletion (reciprocal) → recurrent focal neuropathies triggered by pressure/trauma (e.g., peroneal palsy, ulnar palsy). Episodes usually recover. Mild baseline neuropathy.

8. What is the prognosis for ambulation in CMT1A?

Answer: Excellent. More than 95% of patients remain ambulatory throughout life. Only ~5% require wheelchair, usually in severe cases or very late stages.

9. What is the inheritance pattern of CMTX1?

Answer: X-linked dominant. Caused by mutations in the GJB1 gene (encoding connexin-32). Males are more severely affected than females (heterozygous females may be mildly affected or asymptomatic).

10. What are the core components of CMT management?

Answer: CMT management is multidisciplinary and supportive (no cure):

1. Physiotherapy (stretching, strengthening, balance training)
2. Ankle-Foot Orthoses (AFOs) for foot drop and ankle instability
3. Occupational Therapy (adaptive devices, home modifications)
4. Orthopaedic Surgery for severe fixed deformities
5. Genetic Counselling
6. Avoidance of neurotoxic drugs (vincristine, high-dose pyridoxine)

Viva Voce Pearls

• "Why is CMT also called HMSN?": Hereditary Motor and Sensory Neuropathy—reflects inherited nature and involvement of both motor and sensory nerves.
• "Why does pes cavus develop in CMT?": Muscle imbalance—intrinsic foot muscle weakness + relative sparing of tibialis posterior and peroneus longus → forefoot plantarflexion and high arch. Initially flexible, becomes rigid with contracture.
• "What is the 38 m/s cutoff?": Median or ulnar nerve motor conduction velocity less than 38 m/s defines demyelinating neuropathy (CMT1); ≥38 m/s defines axonal neuropathy (CMT2).
• "What is 'overwork weakness'?": Excessive or high-intensity exercise in CMT may cause irreversible muscle weakness. Moderate-intensity exercise is safe and beneficial; avoid exhaustive eccentric loading.
• "What is the most common CMT after CMT1A?": CMTX1 (GJB1 mutations), accounting for ~10-15% of CMT. X-linked inheritance; males more severe; intermediate MCV.
• "Can CMT patients have children?": Yes. Genetic counselling essential. Risk depends on inheritance pattern: 50% (autosomal dominant), 25% (autosomal recessive if both parents carriers), variable (X-linked). Prenatal testing and PGD available.
• "What is Dejerine-Sottas Syndrome?": Severe, early-onset demyelinating neuropathy (subset of CMT1 or CMT4). Delayed motor milestones, very slow MCV (less than 10 m/s), nerve hypertrophy. Severe MPZ, PMP22, EGR2, or PRX mutations.

OSCE/Clinical Examination Approach

Scenario: "Examine this patient's lower limbs and peripheral nervous system."

On Inspection:

• Pes cavus (high-arched feet)
• Hammer toes or claw toes
• Distal lower limb wasting ("champagne bottle legs")—contrast with preserved thigh bulk
• Scars (previous foot surgery)
• AFOs (ankle-foot orthoses) or walking aids

On Gait Assessment:

• Steppage gait: High-stepping due to foot drop
• Foot slap on heel strike
• Difficulty walking on heels (foot dorsiflexion weakness)

On Palpation:

• Assess for palpable nerve thickening (greater auricular, ulnar, peroneal)—suggests CMT1

On Power Testing (MRC Scale):

• Ankle dorsiflexion (tibialis anterior): Weak (grade 3-4)
• Foot eversion (peroneus longus/brevis): Weak
• Plantarflexion (gastrocnemius): Relatively preserved (until late)
• Toe extension: Weak
• Hand grip and intrinsic hand muscles: Assess for weakness/wasting

On Sensory Testing:

• Reduced vibration and proprioception (large fiber loss)—distal > proximal
• Mild reduction in pin-prick and temperature (small fibers)—"glove and stocking" distribution

On Reflexes:

• Ankle jerks: Absent
• Knee jerks: Reduced or absent
• Upper limb reflexes: Reduced or absent
• Plantars: Flexor (or mute)—NO UPPER MOTOR NEURON SIGNS

Complete Examination:

• Examine upper limbs (hand wasting, weakness)
• Examine spine (scoliosis)
• Assess functional status (stand from chair, walk, write)

Present: "This patient has bilateral pes cavus, hammer toes, distal lower limb wasting with preserved proximal musculature consistent with 'champagne bottle legs,' bilateral foot drop with steppage gait, distal weakness, distal sensory loss in a glove-and-stocking distribution, and global areflexia. There are no upper motor neuron signs. These findings are consistent with a hereditary peripheral neuropathy, most likely Charcot-Marie-Tooth Disease. I would like to perform nerve conduction studies to classify as demyelinating (CMT1) or axonal (CMT2), followed by genetic testing starting with PMP22 duplication for CMT1A."

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15. References

Primary Sources

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