Charcot-Marie-Tooth disease

Other Names: CMT; Hereditary motor and sensory neuropathy; HMSN; Charcot Marie Tooth disease

Charcot-Marie-Tooth disease encompasses a group of disorders called hereditary sensory and motor neuropathies that damage the peripheral nerves.

Peripheral nerves connect the brain and spinal cord to muscles and to sensory cells that detect sensations such as touch, pain, heat, and sound. Damage to the peripheral nerves that worsens over time can result in alteration or loss of sensation and wasting (atrophy) of muscles in the feet, legs, and hands.

Sometimes other, historical names are used to refer to particular forms of Charcot-Marie-Tooth disease. For example, Roussy-Levy syndrome is a form of CMT11 with the additional feature of rhythmic shaking (tremors). Dejerine-Sottas syndrome is a term sometimes used to describe a severe, early childhood form of Charcot-Marie-Tooth disease; it is also sometimes called type 3 (CMT3). Depending on the specific gene that is altered, this severe, early-onset form of the disorder may also be classified as CMT1 or CMT4. CMTX5 is also known as Rosenberg-Chutorian syndrome.

File:Impaired-protein-translation-in-Drosophila-models-for-Charcot–Marie–Tooth-neuropathy-caused-by-ncomms8520-s2.ogv

Onset

Charcot-Marie-Tooth disease usually becomes apparent in adolescence or early adulthood, but onset may occur anytime from early childhood through late adulthood.

Types

There are several types of Charcot-Marie-Tooth disease, which are differentiated by their effects on nerve cells and patterns of inheritance.

Type 1 (CMT1) is characterized by abnormalities in myelin, the fatty substance that covers nerve cells, protecting them and helping to transmit nerve impulses. These abnormalities slow the transmission of nerve impulses and can affect the health of the nerve fiber.

Type 2 (CMT2) is characterized by abnormalities in the fiber, or axon, that extends from a nerve cell body to muscles or to sense organs. These abnormalities reduce the strength of the nerve impulse. In forms of Charcot-Marie-Tooth disease classified as intermediate type, the nerve impulses are both slowed and reduced in strength, probably due to abnormalities in both myelin and axons.

Type 4 (CMT4) is distinguished from the other types by its pattern of inheritance; it can affect either the axons or the myelin.

Type X Charcot-Marie-Tooth disease (CMTX) is caused by mutations in genes on the X chromosome, one of the two sex chromosomes.

Within the various types of Charcot-Marie-Tooth disease, subtypes (such as CMT1A, CMT1B, CMT2A, CMT4A, and CMTX1) indicate different genetic causes.

Epidemiology

Charcot-Marie-Tooth disease is the most common inherited disorder that involves the peripheral nerves, affecting an estimated 150,000 people in the United States. It occurs in populations worldwide with a prevalence of about 1 in 3,300 individuals.

Cause

Charcot-Marie-Tooth disease can be caused by mutations in many different genes. These genes provide instructions for making proteins that are involved in the function of peripheral nerves in the feet, legs, and hands. The gene mutations that cause Charcot-Marie-Tooth disease affect the function of the proteins in ways that are not fully understood; however, they likely impair axons, which transmit nerve impulses, or affect the specialized cells that produce myelin.

In most cases, longer nerves that transmit impulses to the appendages of the body are more likely to be affected. As a result, peripheral nerve cells slowly lose the ability to stimulate the muscles in the feet, legs, and eventually the hands, and to transmit sensory signals from these appendages to the brain. Different mutations within a single gene may cause signs and symptoms of differing severities or lead to different types of Charcot-Marie-Tooth disease.

Between 70 and 80 percent of individuals with CMT1 have mutations affecting the PMP22 gene. Most of these cases occur when there is an extra copy of the gene resulting from a small duplication of genetic material on chromosome 17.

Another 10 to 12 percent of individuals with CMT1 have mutations in the MPZ gene. MPZ gene mutations are also occasionally identified in people with other forms of the disorder.

The most common cause of CMT2 is mutations in the MFN2 gene, which accounts for about 20 percent of cases.

Approximately 90 percent of people with CMTX have GJB1 gene mutations. Mutations in dozens of other genes have been identified in smaller numbers of people with these and the other types. The list of genes associated with Charcot-Marie-Tooth disease continues to grow as researchers study this disorder.

Inheritance

The pattern of inheritance varies with the type of Charcot-Marie-Tooth disease. CMT1, most cases of CMT2, and most intermediate forms are inherited in an autosomal dominant pattern. This pattern of inheritance means that one copy of the altered gene in each cell is sufficient to cause the disorder. In most cases, an affected person has one affected parent. Each of the children of an affected parent has a 50 percent chance of inheriting the disorder.

CMT4, a few CMT2 subtypes, and some intermediate forms are inherited in an autosomal recessive pattern, which means both copies of the gene in each cell have mutations. Most often, the parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but do not show signs and symptoms of the condition. Children of affected individuals are not affected unless the other parent also passes down a mutation in the same gene.

CMTX is inherited in an X-linked dominant pattern. A condition is considered X-linked if the mutated gene that causes the disorder is located on the X chromosome. The inheritance is dominant if one copy of the altered gene is sufficient to cause the condition. In most cases, affected males, who have the alteration on their only copy of the X chromosome, experience more severe symptoms of the disorder than affected females, who have the alteration on one of their two X chromosomes. A characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons. All daughters of affected men will have one altered X chromosome, but they may have only mild symptoms of the disorder.

Some cases of autosomal dominant or type X Charcot-Marie-Tooth disease result from a new mutation and occur in people with no history of the disorder in their family.

Signs and symptoms

Symptoms of Charcot-Marie-Tooth disease vary in severity and age of onset even among members of the same family. Some people never realize they have the disorder because their symptoms are so mild, but most have a moderate amount of physical disability. A small percentage of people experience severe weakness or other problems which, in very rare cases, can be life-threatening. In most affected individuals, however, Charcot-Marie-Tooth disease does not affect life expectancy. Typically, the earliest symptoms of Charcot-Marie-Tooth disease result from muscle atrophy in the feet. Affected individuals may have foot abnormalities such as high arches (pes cavus), flat feet (pes planus), or curled toes (hammer toes). They often have difficulty flexing the foot or walking on the heel of the foot. These difficulties may cause a higher than normal step (steppage gait) and increase the risk of ankle injuries and tripping. As the disease worsens, muscles in the lower legs usually weaken, but leg and foot problems rarely require the use of a wheelchair. Affected individuals may also develop weakness in the hands, causing difficulty with daily activities such as writing, fastening buttons, and turning doorknobs. People with Charcot-Marie-Tooth disease typically experience a decreased sensitivity to touch, heat, and cold in the feet and lower legs, but occasionally feel aching or burning sensations. In rare cases, affected individuals have loss of vision or gradual hearing loss that sometimes leads to deafness.

For most diseases, symptoms will vary from person to person. People with the same disease may not have all the symptoms listed. 80%-99% of people have these symptoms

• Abnormality of the pharynx
• Abnormality of the voic(Voice abnormality)
• Ataxia
• Decreased nerve conduction velocity
• Distal amyotrophy(Distal muscle wasting)
• EMG abnormality
• Gait disturbance(Abnormal gait)
• Impaired pain sensation(Decreased pain sensation)
• Kyphosis(Hunched back)
• Laryngomalacia(Softening of voice box tissue)
• Paralysis(Inability to move)
• Reduced tendon reflexes
• Scoliosis

Diagnosis

CMT can be diagnosed through three different forms of tests: measurement of the speed of nerve impulses (nerve conduction studies), a biopsy of the nerve, and DNA testing. DNA testing can give a definitive diagnosis, but not all the genetic markers for CMT are known. CMT is first most noticed when someone develops lower leg weakness, such as foot drop, or foot deformities, including hammertoes and high arches, but signs alone do not lead to diagnosis.

Patients must be referred to a physician specialising in neurology or rehabilitation medicine. To see signs of muscle weakness, the neurologist may ask patients to walk on their heels or to move part of their leg against an opposing force. To identify sensory loss, the neurologist tests for deep-tendon reflexes, such as the knee jerk, which are reduced or absent in CMT. The doctor may also ask the patient's family history since CMT is hereditary. The lack of family history does not rule out CMT, but is helpful to rule out other causes of neuropathy, such as diabetes or exposure to certain chemicals or drugs.

In 2010, CMT was one of the first diseases where the genetic cause of a particular patient's disease was precisely determined by sequencing the whole genome of an affected individual. This was done by the scientists employed by the Charcot Marie Tooth Association (CMTA) Two mutations were identified in a gene, SH3TC2, known to cause CMT. Researchers then compared the affected patient's genome to the genomes of the patient's mother, father, and seven siblings with and without the disease. The mother and father each had one normal and one mutant copy of this gene, and had mild or no symptoms. The offspring who inherited two mutant genes presented fully with the disease.

Treatment

Often, the most important goal for patients with CMT is to maintain movement, muscle strength, and flexibility. Therefore, an interprofessional team approach with occupational therapy (OT), physical therapy (PT), orthotist, podiatrist, and or orthopedic surgeon is recommended.

PT typically focuses on muscle-strength training, muscle stretching, and aerobic exercise, while OT can provide education on energy conservation strategies and activities of daily living.

Physical therapy should be involved in designing an exercise program that fits a person's personal strengths and flexibility. Bracing can also be used to correct problems caused by CMT. An orthotist may address gait abnormalities by prescribing the use of ankle-foot orthoses. These orthoses help control foot drop and ankle instability and often provide a better sense of balance for patients.

Appropriate footwear is also very important for people with CMT, but they often have difficulty finding well-fitting shoes because of their high arched feet and hammer toes. Due to the lack of good sensory reception in the feet, CMT patients may also need to see a podiatrist for assistance in trimming nails or removing calluses that develop on the pads of the feet. Lastly, patients can also decide to have surgery performed by a podiatrist or an orthopedic surgeon. Surgery may help to stabilize the patients' feet or correct progressive problems. These procedures include straightening and pinning the toes, lowering the arch, and sometimes, fusing the ankle joint to provide stability. CMT patients must take extra care to avoid falling as fractures take longer to heal in someone with an underlying disease process. Additionally, the resulting inactivity may cause the CMT to worsen.

The Charcot–Marie–Tooth Association classifies the chemotherapy drug vincristine as a "definite high risk" and states, "vincristine has been proven hazardous and should be avoided by all CMT patients, including those with no symptoms." Several corrective surgical procedures can be done to improve the physical condition of the affected individuals.

NIH genetic and rare disease info

• NIH info on Charcot-Marie-Tooth disease

Charcot-Marie-Tooth disease is a rare disease.

Charcot-Marie-Tooth disease Resources
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