Motor neuron diseases are characterized by weakness and wasting of affected muscles, without accompanying sensory changes. Werdnig-Hoffman disease and Kugelberg-Welander syndrome occur in infants or children. Motor neuron disease in adults generally commences between 30 and 60 years of age. There is degeneration of the anterior horn cells in the spinal cord, the motor nuclei of the lower cranial nerves, and the corticospinal and corticobulbar pathways. The disorder
is usually sporadic, but familial cases may occur.
Five varieties have been distinguished on clinical grounds.
A. Progressive Bulbar Palsy: Bulbar involvement predominates owing to disease processes affecting primarily the motor nuclei of the cranial nerves.
B. Pseudobulbar Palsy: Bulbar involvement predominates in this variety also, but it is due to bilateral corticobulbar disease and thus reflects upper motor neuron dysfunction.
C. Progressive Spinal Muscular Atrophy: This is characterized primarily by a lower motor neuron deficit in the limbs due to degeneration of the anterior horn cells in the spinal cord.
D. Primary Lateral Sclerosis: There is a purely upper motor neuron deficit in the limbs.
E. Amyotrophic Lateral Sclerosis (Lou Gehrig’s Disease): A mixed upper and lower motor neuron deficit is found in the limbs. This disorder is sometimes associated with dementia or parkinsonism.
A. Symptoms and Signs: Difficulty in swallowing, chewing, coughing, breathing, and talking (dysarthria) occur with bulbar involvement. In progressive bulbar palsy, there is drooping of the palate, a depressed gag reflex, pooling of saliva in the pharynx, a weak cough, and a wasted, fasciculating tongue. In pseudobulbar palsy, the tongue is contracted and spastic and cannot be moved rapidly from side to side. Limb involvement is characterized by motor disturbances
(weakness, stiffness, wasting, fasciculations) reflecting lower or upper motor neuron dysfunction; there are no objective changes on sensory examination, though there may be vague sensory complaints. The sphincters are generally spared.
The disorder is progressive and usually fatal within 3–5 years; death usually results from pulmonary infections. Patients with bulbar involvement generally have the poorest prognosis.