Section 6. Brain, Spinal Cord, and Nerve Disorders

Chapter 95. Peripheral Nerve Disorders

Topics: Introduction | Disorders of Muscle Stimulation | Disorders of the Neuromuscular Junction | Plexus Disorders | Thoracic Outlet Syndromes | Mononeuropathy | Mononeuritis Multiplex | Polyneuropathy | Hereditary Neuropathies | Spinal Muscular Atrophies

Hereditary Neuropathies

Hereditary neuropathies affect the peripheral nerves, causing subtle symptoms that worsen gradually.

Hereditary neuropathies may affect only motor nerves (motor neuropathies), only sensory nerves (sensory neuropathies), or both sensory and motor nerves (sensory-motor neuropathies). Some hereditary neuropathies are relatively common but often are not recognized. Hereditary sensory neuropathies are especially rare.

The genes responsible for many of these neuropathies have been identified. They include some forms of Charcot-Marie-Tooth disease, Refsum's disease (see Section 23, Chapter 282), porphyria (see Section 12, Chapter 160), Fabry's disease (see Section 23, Chapter 282), and hereditary neuropathy with liability to pressure palsies.

Charcot-Marie-Tooth Disease

Charcot-Marie-Tooth disease (peroneal muscular atrophy) is a hereditary neuropathy in which the muscles of the lower legs become weak and waste away (atrophy).

Charcot-Marie-Tooth disease is the most common hereditary neuropathy, affecting 1 of 2,500 people. There are 3 types and several subtypes of the disease. In some types, axons (the part of the nerve that sends messages) die because the myelin sheath surrounding them is damaged or destroyed (demyelinated). In other types, axons die even though the sheath is not damaged. Most types of the disease are inherited as an autosomal (not sex-linked) dominant trait: only one gene from one parent is required for the disease to develop.

Symptoms vary by type of the disease. In type 1, symptoms begin in middle childhood, adolescence, or later. Weakness begins in the lower legs, which causes an inability to flex the foot (footdrop) and wasting away of the calf muscles (stork leg deformity). Later, hand muscles begin to waste away. There is little loss of sensation. In milder subtypes of type 1, high arches and hammer toes may be the only symptoms. In one subtype of type 1, males have severe symptoms, and females have mild symptoms or may be unaffected. The disease progresses slowly and does not affect life span.

People with type 2 disease, which progresses more slowly, develop somewhat similar symptoms, often beginning in their teens.

Type 3 disease starts in infancy. Walking and running are delayed, and the peripheral nerves become enlarged. Muscle weakness in the legs progresses at a faster rate than in type 1. Sensation in the legs is lost.

Diagnosis and Treatment

The distribution of weakness, the age at which the disease began, the family history, the presence of foot deformities (high arches and hammer toes), and the results of nerve conduction studies help doctors identify the different types of Charcot-Marie-Tooth disease and distinguish them from other causes of neuropathy. Genetic testing and counseling for Charcot-Marie-Tooth disease is available.

No treatment can stop the progression of the disease. Wearing braces helps correct footdrop, and sometimes orthopedic surgery is needed.

Hereditary Neuropathy With Liability to Pressure Palsies

Hereditary neuropathy with liability to pressure palsies is malfunction or damage to one or more nerve resulting from slight pressure or injury.

People who have this neuropathy are susceptible to nerve damage resulting from relatively slight pressure or injury or from repetitive use. Usually, this neuropathy starts during adolescence or young adulthood, but it may start at any age. It affects both sexes equally. This neuropathy is inherited as an autosomal (not sex-linked) dominant trait (see Section 1, Chapter 2).

Peroneal nerve palsy with footdrop, ulnar nerve palsy, and carpal tunnel syndrome commonly develop. Numbness or weakness occurs periodically in the affected area. Symptoms vary from unnoticeable and mild to severe and incapacitating. Episodes may last several minutes to months.

Doctors may have difficulty diagnosing this neuropathy because the symptoms come and go. Genetic testing for this neuropathy is available.

About half of the people who have this neuropathy recover completely within days to months. In people who do not recover completely, symptoms are rarely severe.