Neuromuscular diseases are a group of disorders affecting the motor unit—the functional system that controls muscle movement. The motor unit consists of four parts: the nerve cell body in the spinal cord, the axon that extends through a peripheral nerve, the neuromuscular junction (motor end plate), and the muscle fibers that the nerve activates.
The specific characteristics of each disease depend on which part of the motor unit is damaged. Some diseases attack the nerve cell bodies (motor neuron disease); others affect the nerve fibers themselves (peripheral neuropathy); still others damage the muscles directly (myopathy). These injuries can be acquired—from toxins or inflammation—or congenital, caused by inherited genetic defects. Many motor unit disorders stem from a genetic abnormality affecting how cells function or are structured. The altered gene is inherited from parents through patterns determined by the disease's genetic expression (autosomal or X-linked inheritance).
Inherited neuromuscular diseases encompass a wide spectrum of conditions. They may appear in childhood or adulthood, progress at varying speeds, but are always progressive. Each primarily affects one component of the motor unit: the spinal nerve cell, the nerve itself, or the muscle.
Medical and therapeutic advances have significantly improved quality of life and life expectancy. Mechanical ventilation, medications, orthopedic surgery, orthotic devices (braces, wheelchairs), and physical therapy all help—while we await curative treatments.
Let us examine several inherited neuromuscular diseases, describing their clinical features.
Spinal Muscular Atrophy (SMA)
SMA is a group of disorders caused by degeneration of the spinal motor neurons, marked by reduced muscle tone and weakness. It affects males and females equally. Usually both parents are carriers—a state that produces no symptoms—and the recurrence risk is 25% with each pregnancy. SMA is generally classified into three types:
SMA I (Werdnig-Hoffmann Disease): Severe generalized weakness is present at birth. The infant lies motionless and has serious difficulty breathing. The brightness in the child's face contrasts sharply with complete immobility. This form occurs in approximately 1 of every 400,000 live births.
SMA II (intermediate form): Onset typically occurs after six months of life. Weakness is severe but less extreme than in SMA I. The incidence is about 1 in 200,000 live births. The child eventually learns to sit upright, though often late or imperfectly, but does not walk independently. Life expectancy is often normal, though it depends on whether serious respiratory problems develop. Other complications include joint contractures, scoliosis, and swallowing difficulties (dysphagia).
SMA III (Kugelberg-Welander Disease): Symptoms appear after the child is already walking. Muscle weakness produces a characteristic waddling gait and difficulty rising from the floor or climbing stairs. Age of onset varies widely. The disease progresses slowly, though independent walking may eventually be lost. Life expectancy is generally normal. This form occurs in roughly 1 of 100,000 live births.
Inherited Sensory-Motor Neuropathies
These are the most common neuromuscular diseases (30 new cases per 100,000 births). They are complex, genetically diverse disorders affecting both motor and sensory nerves.
The most prominent is Charcot-Marie-Tooth disease (CMT), named after the three physicians who first described it. Within the CMT category, the 1A form is by far the most widespread.
CMT 1A typically begins between ages 20 and 30, though asymptomatic cases do occur. It causes weakness in the hands and feet, making it difficult to grip objects or walk normally. Skeletal deformities are common—cavus foot (high arch), spinal curvature. The disease usually progresses slowly, with long periods of stability. Life expectancy is normal.
Muscular Dystrophies
These are diseases characterized by primary, progressive damage to skeletal muscle and, in some forms, the heart as well.
Duchenne Muscular Dystrophy (DMD) is the most well-known. It results from the absence of dystrophin, a structural protein in muscle cells. The gene for dystrophin is located on the X chromosome. An affected mother who carries the altered gene can transmit the disease only to her sons; daughters typically have no symptoms because they possess two X chromosomes.
Clinically, the disease may first appear as delayed walking (in 50% of cases) or delayed speech and mental development (in 30%). Symptoms become noticeable around ages 2 or 3: the child walks on tiptoe, falls frequently, struggles to rise from the floor or climb stairs. Within a few years, the gait becomes consistently tiptoeing and waddling, with an exaggerated forward curve of the lower spine. The child develops distinctive muscle-lifting strategies to stand. A characteristic feature is pseudohypertrophy—enlargement of the calf muscles despite overall muscle wasting.
Complications include joint contractures, scoliosis (sometimes requiring surgery), progressive respiratory failure requiring mechanical ventilation, involvement of the heart muscle, and side effects from chronic corticosteroid use (obesity, bone loss)—drugs given to slow disease progression.
Antonello Damiani Neurologist, Medical Director
