Charcot-Marie-Tooth disease (CMT), also called hereditary motor sensory neuropathy and peroneal muscular atrophy, is an inherited disorder that causes nerve damage, or neuropathy. This nerve damage worsens over time.
The disease affects the myelin sheath, which is the fatty substance that insulates the nerves. Myelin is needed to transmit impulses along nerve cells. In CMT, genetic mutations cause the peripheral nerves to become damaged or the myelin to become dysfunctional, which makes the nerves vulnerable to damage.
The disorder typically causes muscle weakness and sometimes numbness in the arms, legs, hands, and feet. Symptoms usually develop during adolescence or early adulthood, but they may occur at any time from early childhood into middle age.
CMT is the most common hereditary form of neuropathy, affecting about one in 2,500 people worldwide. It is estimated that 150,000 people in the United States have CMT.
There are five main variations of CMT. Each type is caused by different genetic mutations, or abnormal changes in a person's biological makeup. Each is inherited, or passed down among families, in a different pattern. In addition, the age of onset, symptoms, severity, and progression of the disease varies, depending on the specific variation of CMT a person has.
The most common form of CMT is called CMT1. People with CMT1 are born with abnormal genes that are involved in the development and function of the myelin sheath. These defective genes cause the breakdown of myelin called demyelination. When the protective myelin deteriorates, the peripheral nerves are exposed and are vulnerable to damage. Other, less common, forms include CMT2 (which is predominantly axonal), and also, CMT3, CMT4, and CMTX.
Although this condition is not considered life-threatening, symptoms worsen over time. There is currently no cure for CMT, but treatments are available to help manage the symptoms of the disease. Physical and occupational therapy or surgery can help people with CMT to lead active, productive lives.
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types of the disease
CMT1: About 60 to 80% of patients with Charcot-Marie-Tooth disease (CMT) have CMT1. People with CMT1 are born with abnormal genes that are involved in the development and function of the myelin sheath. These defective genes result in the breakdown of myelin, a process called demyelination. When the protective myelin deteriorates, the peripheral nerves are exposed and may become damaged, causing the characteristic sensory and motor skill symptoms.
CMT2: CMT2 occurs when there are abnormalities in the peripheral nerves. An estimated one-third of patients with dominant CMT have CMT2.
CMT3: CMT3, also called Dejerine-Sottas disease, is a severe and rare form of CMT that affects the myelin sheath. Symptoms of CMT3 generally develop before the age of three. This type is caused by a mutation in the PMP-22 gene.
CMT4: CMT4 is a complex form of CMT. There are many different subtypes of CMT4, which are caused by different genetic mutations. Symptoms may be severe and affect many parts of the body.
CMTX: Patients with this form of the disease have a mutation on the connexin-32 gene on the X chromosome. Because males have only one X chromosome, they tend to have more severe symptoms of the disease than females.
Females, on the other hand, usually have milder symptoms. This is because females have two copies of the X chromosome. Most females with X-linked CMT have one normal X chromosome and one mutated X chromosome. As a result, they usually have mild symptoms that develop during adolescence or later in life. Some may not develop any symptoms at all.