Neuromuscular Disorders | CMT

INTRODUCTION

Charcot-Marie-Tooth disease (CMT) is a broad term used to describe a group of genetic disorders that affects the peripheral and sensory nervous system. CMT most often affects the lower legs, feet and hands, resulting in weakness and atrophy, as well as causing a mild degree of loss of sensation. Motor problems tend to be much more significant than sensory problems.

About one in 2,500 people has a form of CMT, which makes it of the most common of the genetic (hereditary) disorders.

CMT is usually divided into types 1 and 2, a classification that is made according to the manner in which the peripheral nerves are affected. About two-thirds of people with CMT have type 1, which affects the myelin sheath, the insulating covering that surrounds nerve fibers. Approximately one-third of patients have type 2, which affects the nerve fibers themselves.

A clinician can determine the integrity and function of the myelin sheath by measuring the speed with which nerve impulses travel along the length of the nerve. In type 1 CMT, the nerve impulses travel more slowly than is normal. In type 2 CMT, the speed of nerve impulses is normal, but the size or amount of the impulses is smaller than normal, indicating a problem with the nerve fibers.

CMT can be further classified by the genetic defect that underlies the disorder. For example, type 1 CMT, caused by a defect on chromosome 17, is called CMT1A, and type 1 CMT, caused by a defect on chromosome 1, is CMT1B.

Although the onset is often difficult to determine, the initial symptoms of CMT are usually foot abnormalities, such as a high arch or flexed toes. It can be difficult to hold up the foot, which may result in the individual tripping on curbs, having to take a higher than normal step (picking up the foot by bending the knee), and walking with deliberation.

CMT varies highly in severity of its symptoms, even among members of the same family. Some individuals' symptoms are so mild that they escape notice. For those with symptoms, foot abnormalities and walking difficulties pose the most serious problems.

CMT has no effect on the cardiovascular system and no effect on cognitive skills. Individuals with CMT do not experience a decrease in life expectancy.