Motor neuron diseases destroy the nerves that control muscles. Muscle control originates from the brain, and consists of nerve impulses that travel through two neurons to reach the muscles. The upper motor neuron starts in the brain and sends signals to the lower motor neurons, which are located in the brain stem and spinal cord. The lower motor neuron then sends a nerve impulse to the muscle. Motor neuron disease can affect the upper motor neuron, the lower one, or both. Symptoms depend on which type of neuron is affected, the location of the affected neurons and which muscles they innervate. Motor neuron diseases are progressive, though in the different diseases symptoms progress at different rates.
Amyotrophic Lateral Sclerosis
According to "Harrison's Principles of Internal Medicine," amyotrophic lateral sclerosis, or ALS, is the most common motor neuron disease. ALS affects both upper and lower motor neurons. The first symptom is often weakness in one limb, accompanied by cramping. The muscles atrophy and appear wasted. Twitching of the muscles, which is called fasciculations, is another sign of muscles losing their input from the nerves. Speech and swallowing are affected. As the upper motor neurons become involved, the patient develops stiffness and increased reflexes. Eventually, the patient becomes paralyzed. According to Mayoclinic.com, respiratory failure, caused by paralysis of the respiratory muscles, is the most common cause of death. Most patients live only a few years after symptoms begin.
Spinal Muscular Atrophy
Spinal Muscular Atrophies, or SMAs, are a group of genetic conditions that begin during infancy or childhood. Werdnig Hoffman disease, or infantile SMA, can be symptomatic even before birth, as evidenced by decreased fetal movements. The infant has a low muscle tone, is floppy and weak. Cognitive development is not affected. The child will usually die around age one or two from complications of paralysis. Other forms of SMA start later in childhood and progress more slowly. The main symptom is weakness.
Secondary Motor Neuron Diseases
According to "Harrison's Principles of Internal Medicine," motor neuron disease is often secondary to other causes, including infections, toxins and cancer. The symptoms vary greatly depending on the causative agent, which include lyme, tetanus, lead poisoning and poliomyelitis. Polio affects the motor neurons in the spine and brain stem. Symptoms include weakness, paralysis, difficulty swallowing and speaking. Respiratory paralysis leads to death, unless the patient is treated with mechanical respiration. Some people who have survived polio develop a recurrence of symptoms many years later, known as the post polio syndrome.
References
- "Harrison's Principles of Internal Medicine, 17th edition"; Anthony Fauci et al.; 2008
- Mayoclinic.com: ALS
- National Institutes of Neurological Disorders and Stroke: Motor Neuron DiseasesTardive Dyskinesia
- Mayoclinic.com: Poliomyelitis
