Charcot–Marie–Tooth disease (CMT), also known as Charcot–Marie–Tooth neuropathy, hereditary motor and sensory neuropathy (HMSN) and peroneal muscular atrophy (PMA) — is a genetically and clinically heterogeneous group of inherited disorders of the peripheral nervous system characterised by progressive loss of muscle tissue and touch sensation across various parts of the body.
Signs and symptoms
Symptoms of CMT usually begin in late childhood or early adulthood. Some people do not experience symptoms until their early thirties or forties. Usually, the initial symptom is foot drop early in the course of the disease. This can also cause claw toe, where the toes are always curled. Wasting of muscle tissue of the lower parts of the legs may give rise to a “stork leg” or “inverted bottle” appearance. Weakness in the hands and forearms occurs in many people later in life as the disease progresses.
Loss of touch sensation in the feet, ankles and legs, as well as in the hands, wrists and arms is characteristic in various types of the disease. Early and late onset forms occur with ‘on and off’ painful spasmodic muscular contractions that can be disabling when the disease activates. High arched feet (pes cavus) are classically associated with the disorder. Sensory and proprioceptive nerves in the hands and feet are often damaged, while pain nerves are left intact. Overuse of an affected hand or limb can activate symptoms including numbness, spasm, and painful cramping.
Symptoms and progression of the disease can vary. Breathing can be affected in some; so can hearing, vision, as well as the neck and shoulder muscles. Scoliosis is common. Hip sockets can be malformed. Gastrointestinal problems can be part of CMT, as can chew, swallowing, and speaking (due to atrophy of vocal cords). A tremor can develop as muscles waste. Pregnancy has been known to exacerbate CMT, as well as extreme emotional stress. Patients with CMT must avoid periods of prolonged immobility such as when recovering from a secondary injury as prolonged periods of limited mobility can drastically accelerate symptoms of CMT.
Neuropathic pain is often a symptom of CMT, though, like other symptoms of CMT, its presence and severity varies from case to case. For some people, pain can be significant to severe and interfere with daily life activities. However, pain is not experienced by all people with CMT. When pain is present as a symptom of CMT, it is comparable to that seen in other peripheral neuropathies, as well as Postherpetic neuralgia and Complex regional pain syndrome, among other diseases.
Charcot–Marie–Tooth disease is caused by mutations that cause defects in neuronal proteins. Nerve signals are conducted by an axon with a myelin sheath wrapped around it. Most mutations in CMT affect the myelin sheath, but some affect the axon.
The most common cause of CMT (70-80% of the cases) is the duplication of a large region on the short arm of chromosome 17 that includes the gene PMP22. Some mutations affect the gene MFN2, which codes for a mitochondrial protein. Cells contain separate sets of genes in their nucleus and in their mitochondria. In nerve cells, the mitochondria travel down the long axons. In some forms of CMT, mutated MFN2 causes the mitochondria to form large clusters, or clots, which are unable to travel down the axon towards the synapses. This prevents the synapses from functioning.
CMT is divided into the primary demyelinating neuropathies (CMT1, CMT3, and CMT4) and the primary axonal neuropathies (CMT2), with frequent overlap. Another cell involved in CMT is the Schwann cell, which creates the myelin sheath, by wrapping its plasma membrane around the axon in a structure that is sometimes compared to a Swiss roll.
Neurons, Schwann cells, and fibroblasts work together to create a working nerve. Schwann cells and neurons exchange molecular signals that regulate survival and differentiation. These signals are disrupted in CMT.
Demyelinating Schwann cells causes abnormal axon structure and function. They may cause axon degeneration, or they may simply cause axons to malfunction.
The myelin sheath allows nerve cells to conduct signals faster. When the myelin sheath is damaged nerve signals are slower, and this can be measured by a common neurological test, electromyography. When the axon is damaged, on the other hand, this results in a reduced compound muscle action potential (CMAP).
Although there is no current standard treatment, the use of ascorbic acid has been proposed, and has shown some benefit in animal models. A clinical trial to determine the effectiveness of high doses of ascorbic acid (vitamin C) in treating humans with CMT type 1A has been conducted. The results of the trial upon children have shown that a high dosage intake of ascorbic acid is safe but the efficacy endpoints expected were not met. In 2010, a study published in the Journal Science indicated that scientists had identified those proteins that control the thickness of myelin sheath. This discovery is expected to open the avenue to new treatments in the coming years.
The most important activity for patients with CMT is to maintain what movement, muscle strength, and flexibility they have. Therefore, physical therapy and moderate activity are recommended but overexertion should be avoided. A physiotherapist should be involved in designing an exercise program that fits a patient’s personal strengths and flexibility. Bracing can also be used to correct problems caused by CMT. Gait abnormalities can be corrected by the use of either articulated (hinged) or unarticulated, braces called AFOs (ankle-foot orthoses). These braces 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 help in trimming nails or removing calluses that develop on the pads of the feet. A final decision a patient can make is to have surgery. Using a podiatrist or an orthopedic surgeon, patients can choose to stabilize their 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 because fractures take longer to heal in someone with an underlying disease process. Additionally, the resulting inactivity may cause the CMT to worsen.