The motor neuron diseases (MNDs) are a group of progressive neurological disorders that destroy cells that control essential muscle activity such as speaking, walking, breathing, and swallowing.
Normally, messages from nerve cells in the brain (called upper motor neurons) are transmitted to nerve cells in the brain stem and spinal cord (called lower motor neurons) and from them to particular muscles. When there are disruptions in these signals, the result can be gradual muscle weakening, wasting away, and uncontrollable twitching (called fasciculations). Eventually, the ability to control voluntary movement can be lost. MNDs may be inherited or acquired, and they occur in all age groups.
MNDs occur more commonly in men than in women, and symptoms may appear after age 40. In children, particularly in inherited or familial forms of the disease, symptoms can be present at birth or appear before the child learns to walk.
The causes of sporadic (non-inherited) MNDs are not known, but environmental, toxic, viral, or genetic factors may be implicated. Common MNDs include amyotrophic lateral sclerosis (ALS), progressive bulbar palsy, primary lateral sclerosis, and progressive muscular atrophy. Other MNDs include the many inherited forms of spinal muscular atrophy and post-polio syndrome, a condition that can strike polio survivors decades after their recovery from poliomyelitis.
Prognosis varies depending on the type of MND and the age of onset. Some MNDs, such as primary lateral sclerosis and Kennedy disease, are not fatal and progress slowly. Patients with spinal muscular atrophy may appear to be stable for long periods, but improvement should not be expected. Some MNDs, such as ALS and some forms of spinal muscular atrophy, are fatal.
There is no cure or standard treatment for the MNDs. Symptomatic and supportive treatment can help patients be more comfortable while maintaining their quality of life. The drug riluzole (Rilutek®), which has approved by the U.S. Food and Drug Administration (FDA) to treat ALS, prolongs life by 2-3 months but does not relieve symptoms. The FDA has also approved the use of edarvarone to reduce the clinical decline seen in ALS.
Other medicines that may help reduce symptoms include muscle relaxants such as baclofen, tizanidine, and the benzodiazepines for spasticity; glycopyrrolate and atropine to treat excessive saliva; and anticonvulsants and nonsteroidal anti-inflammatory drugs to relieve pain.
Panic attacks can be treated with benzodiazepines. Some individuals may require stronger medicines such as morphine to cope with musculoskeletal abnormalities or pain in later stages of the disorders, and opiates are used to provide comfort care in terminal stages of the disease.
The FDA has approved nusinersen (Spinraza ™) as the first drug approved to treat children and adults with spinal muscular atrophy. The drug is administered by intrathecal injection into the fluid surrounding the spinal cord. It is designed to increase production of the full-length SMN protein, which is critical for the maintenance of motor neurons.
Physical and speech therapy, occupational therapy, and rehabilitation may help to improve posture, prevent joint immobility, slow muscle weakness and atrophy, and cope with swallowing difficulties. Applying heat may relieve muscle pain.
Assistive devices such as supports or braces, orthotics, speech synthesizers, and wheelchairs help some patients retain independence. Proper nutrition and a balanced diet are essential to maintaining weight and strength.