Charcot-Marie-Tooth (CMT) disease is the most common inherited neuromuscular disorder. It is characterized by inherited neuropathies without known metabolic derangements.[1, 2, 3, 4]
In 1886, Professor Jean Martin Charcot of France (1825-1893) and his student Pierre Marie (1853-1940) published the first description of distal muscle weakness and wasting beginning in the legs, calling it peroneal muscular atrophy.
Howard Henry Tooth (1856-1926) described the same disease in his Cambridge dissertation in 1886, calling the condition peroneal progressive muscular atrophy. Tooth was the first to attribute symptoms correctly to neuropathy rather than to myelopathy, as physicians previously had done.
In 1912, Hoffman identified a case of peroneal muscular atrophy with thickened nerves. This disease was referred to as Hoffman disease and later was known as Charcot-Marie-Tooth-Hoffman disease.
In 1968, CMT disease was subdivided into two types, CMT 1 and CMT 2, on the basis of pathologic and physiologic criteria. It has been subdivided further on the basis of the genetic cause of the disease. With the advent of genetic testing, it is likely that all of the diseases currently falling under the heading of CMT syndrome will eventually become distinguishable.[5]
NextCMT disease is a heterogeneous group of genetically distinct disorders with similar clinical presentations.[1] Its genetic spectrum spans more than 80 genes.[6] Gene discovery has been revolutionized by new high-throughput molecular technologies.[7] CMT disease is divided into several types, as follows.
CMT type 1 is a disorder of peripheral myelination resulting from a mutation in the peripheral myelin protein-22 (PMP22) gene.[8, 9] Mutations in the gene encoding the major PNS myelin protein, myelin protein zero (MPZ), account for 5% of patients with CMT disease. The mutation results in abnormal myelin that is unstable and spontaneously breaks down.
This process results in demyelination, leading to uniform slowing of conduction velocity. Slowing of conduction in motor and sensory nerves was believed to cause weakness and numbness. However, a study by Krajewski and colleagues suggested that neurologic dysfunction and clinical disability in CMT 1A are caused by loss of or damage to large-diameter motor and sensory axons.[10, 11, 12]
Pain and temperature sensations usually are not affected because they are carried by unmyelinated (type C) nerve fibers. In response to demyelination, Schwann cells proliferate and form concentric arrays of remyelination. Repeated cycles ofdemyelination and remyelination result in a thick layer of abnormal myelin around the peripheral axons. These changes cause what is referred to as an onion bulb appearance.
CMT type 2 primarily is a neuronal (ie, axonal) disorder, not a demyelinating disorder.[9, 13, 14, 15] It results in peripheral neuropathy through direct axonal death and Wallerian degeneration.
Characterized by infantile onset, CMT type 3 (also known as Dejerine-Sottas disease) results in severe demyelination with delayed motor skills; it is much more severe than CMT type 1. Marked segmental demyelination with thinning of the myelin around the nerve is observed on histologic examination.
CMT X (X-linked CMT) and CMT 4 also are demyelinating neuropathies.[16, 17]
Hereditary motor and sensory neuropathies (HMSNs) are classified by Online Mendelian Inheritance in Man (OMIM). A broad division may be made between HMSNs with diffusely slow nerve conduction velocity and those with normal or borderline abnormal nerve conduction velocity.[1]
HMSN I (ie, CMT 1) includes the following subtypes[8, 9] :
HMSN III (Dejerine-Sottas disease, hypertrophic neuropathy of infancy, congenital hypomyelinated neuropathy) is inherited in an autosomal recessive manner.
HMSN IV (Refsum syndrome, phytanic acid excess) has an autosomal recessive inheritance and is characterized by a tetrad of peripheral neuropathy, retinitis pigmentosa, cerebellar signs, and increased cerebrospinal fluid (CSF) protein.
HMSN II (ie, CMT 2) includes the following subtypes[9, 13, 15] :
HMSN V (ie, spastic paraplegia) is characterized by normal upper limbs and the absence of sensory symptoms. Roussy-Levy syndrome has an autosomal dominant inheritance and is characterized by essential tremor. HMSN VI is characterized by optic atrophy. HMSN VII is associated with retinitis pigmentosa. Prednisone-responsive hereditary neuropathy is the final HMSN of this type.
Genetic and clinical features of CMT disorders are listed in Table 1 below.
Table 1. Charcot-Marie-Tooth Disorders: Genetic and Clinical Feature Comparison (Open Table in a new window)
CMT Type Chromosome; Inheritance Pattern Age of Onset Clinical Features Average NCVs§ CMT 1A (PMP-22¶ dupl.) 17p11; AD* First decade Distal weakness 15-20 m/s CMT 1B (P0 -MPZ)** 1q22; AD First decade Distal weakness <20 m/s CMT 1C (non A, non B) 16p13;AD Second decade Distal weakness 26-42 m/s CMT 1D (early growth response [EGR]–2)#[[22] 24] 10q21; AD First decade Distal weakness 15-20 m/s CMT 1E 17p11; AD First decade Distal weakness, deafness 15-20 m/s CMT 1F 8p21; AD First decade Distal weakness 15-20 m/s CMT X (Connexin-32)[23, 24, 25, 26, 27] Xq13; XD‡ Second decade Distal weakness 25-40 m/s CMT 2A 1p36; AD 10 y Distal weakness >38 m/s CMT 2B 3q; AD Second decade Distal weakness,The prevalence of CMT disease is 1 person per 2500 population, or about 125,000 people in the United States. The incidence of CMT 1 is 15 persons per 100,000 population; the incidence of CMT 1A is 10.5 persons per 100,000 population, or 70% of CMT 1. The incidence of CMT 2 is 7 persons per 100,000 population. Persons with CMT X represent at least 10-20% of people with the CMT syndrome.
In Japan, the prevalence is reported to be 10.8 cases per 100,000 population; in Italy, it is reported to be 17.5 cases per 100,000 population; and in Spain, it is 28.2 cases per 100,000 population.[28, 29]
According to a Norwegian genetic epidemiologic study, CMT disease is the most common inherited disorder of the peripheral nervous system, with an estimated prevalence of 1 in 1214. CMT 1 and CMT 2 are equally frequent in the general population. The prevalence of PMP22 duplication and of mutations in Cx32, MPZ, and MFN2 is 19.6%, 4.8%, 1.1% and 3.2%, respectively. The ratio of probable de novo mutations in CMT families was estimated to be 22.7%. Genotype-phenotype correlations for seven novel mutations in the genes Cx32 (2), MFN2 (3) and MPZ (2) are described.[30]
Prognoses for the different types of CMT disease vary and depend on clinical severity. Generally, CMT disease is a slowly progressive neuropathy that causes eventual disability secondary to distal muscle weakness and deformities. In rare cases, phrenic nerve involvement of the diaphragm can cause ventilatory difficulties. CMT disease does not usually shorten the expected life span.
Shy et al developed the CMT neuropathy score, which is a modification of the total neuropathy score.[31] This has been shown to be a validated measure of length-dependent axonal and demyelinating CMT disability and can be investigated as an end point for longitudinal studies of and clinical trials related to CMT disease.[20]
Genetic counseling is the process of providing individuals and families with information on the nature, inheritance patterns, and implications of genetic disorders in order to help them make informed medical and personal decisions. Offer patients with CMT disease genetic counseling so that they can make informed decisions regarding the potential risk of passing the disease to their children.[19, 32]
Drugs and medications, such as vincristine, isoniazid, paclitaxel, cisplatin, and nitrofurantoin, are known to cause nerve damage and should be avoided.
Routine exercise within the individual's capability is encouraged; many individuals remain physically active.[33] No specific activity limitation is recommended.
Obesity should be avoided, because it makes walking more difficult.
Daily heel-cord stretching exercises are warranted to prevent Achilles tendon shortening.
Clinical Presentation
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