What is Charcot-Marie-Tooth Disease?
Charcot-Marie-Tooth (CMT) disease is a genetically heterogeneous group of inherited peripheral neuropathies that cause progressive degeneration or abnormal development of peripheral nerves, resulting in distal muscle weakness, atrophy, sensory loss, and skeletal deformities. 1
Disease Characteristics
CMT represents the most common inherited neurological disorder affecting the peripheral nervous system, with a prevalence of approximately 1 in 2,500 people globally. 1, 2 The disease affects both motor and sensory nerves and exhibits various patterns of genetic transmission including autosomal dominant, autosomal recessive, and X-linked inheritance. 3, 4
Clinical Presentation
The disease typically manifests in infancy or childhood with:
- Distal limb muscle weakness and atrophy (predominantly affecting legs more than arms)
- Foot deformities, particularly pes cavus (high-arched feet) and foot drop
- Awkward, high-stepping gait with frequent tripping or falls
- Distal sensory loss in a length-dependent pattern
- Reduced or absent deep tendon reflexes
- Hand deformities in advanced cases
- Progressive skeletal deformities 4, 5, 2
Classification Systems
CMT can be classified in three complementary ways:
By Electrophysiological Pattern:
- CMT1 (Demyelinating): The most prevalent form with slow nerve conduction velocities; approximately 70% have PMP22 gene duplication (CMT1A) 1
- CMT2 (Axonal): Characterized by axonal degeneration; most commonly caused by MFN2 mutations (approximately 33% of CMT2 cases) 1
- CMTX (X-linked): Caused by Cx32(GJB1) mutations, accounting for approximately 12% of all CMT cases; can present with either demyelinating or axonal features 1
- Intermediate forms: With conduction velocities between demyelinating and axonal ranges 5
By Inheritance Pattern:
- Autosomal dominant (most common)
- Autosomal recessive (CMT4 - typically more severe, early onset)
- X-linked 3, 6
By Genetic Mutation:
The CMT phenotype has been linked to 36 loci with mutations identified in 28 different genes, though more recent data suggests even greater genetic heterogeneity. 1, 2
Genetic Architecture
CMT1A (PMP22 duplication) is the single most common genetic cause, accounting for 76-90% of sporadic CMT1 cases and approximately 70% of all demyelinating CMT. 1 This is critical because it should be the first genetic test ordered when demyelinating CMT is suspected on nerve conduction studies.
Other important genetic causes include:
- PMP22 point mutations: 2.5% of CMT population 1
- MPZ mutations: 5% of CMT population 1
- MFN2 mutations: 33% of CMT2 (axonal) cases 1
- GJB1 (Cx32) mutations: 12% of all CMT cases 1
Pathophysiology
Despite diverse genetic causes, all forms of CMT ultimately result in length-dependent progressive axonal loss, which explains the characteristic distal predominance of symptoms. 5, 2 Mutations affect various cellular functions including:
- Myelin sheath proteins
- Mitochondrial biology
- Aminoacyl-tRNA synthetases
- Axonal cytoskeleton
- Endosomal signaling and recycling 2
Diagnostic Approach
A stepwise evaluation is essential: Begin with detailed family history (noting that 30% of mutations are de novo), followed by nerve conduction studies to classify as demyelinating versus axonal, then proceed with targeted genetic testing based on the electrophysiological pattern. 1
For demyelinating neuropathy with autosomal dominant inheritance, test for PMP22 duplication first (yield 54-80% in this population). 1 If negative and pedigree is uninformative for X-linked versus autosomal dominant inheritance, test for GJB1 mutations. For axonal forms, consider MFN2 testing. 1
Impact on Quality of Life
Compared to the general population, both children and adults with CMT have significantly reduced quality of life across physical, emotional, and social domains, with the physical domain being the most disabling. 2 Progressive disability correlates with clinical impairment scores. 7
Current Treatment Landscape
No disease-modifying pharmacological therapies are currently available for any form of CMT. 2, 8 Management is entirely supportive and includes:
- Muscle strength training and rehabilitation
- Orthotic devices for foot drop and gait abnormalities
- Surgical interventions for severe deformities (including tendon transfers)
- Symptom management
- Monitoring for complications 2, 8
Important Clinical Caveat
While CMT typically presents with the classic distal symmetric phenotype, atypical presentations can occur, including vocal cord paralysis, prominent sensory manifestations, autonomic dysfunction, or cranial nerve involvement. 5, 9 These atypical features may aid in identifying specific genetic subtypes but can also delay diagnosis if CMT is not considered in the differential.