Motor neuron disease is a group of chronic disorders affecting the anterior horn cells of the spinal cord and lower brain stem plus in many instances the large motor neurons of the cerebral cortex that give rise to the corticospinal tract.
Characteristically, sensory changes and cerebellar dysfunction are absent.
Motor Neurone Disease (MND) is a term used to cover a number of illnesses of the motor neurone. Amyotrophic Lateral Sclerosis (ALS), Progressive Muscular Atrophy (PMA), Progressive Bulbar Palsy (PBP) and Progessive Lateral Sclerosis (PLS) are all types of MND. MND is the term used internationally whilst ALS is often used in the USA (where it is also known as Lou Gehrig’s disease) to cover all forms of MND. It was first described by Jean-Martin Charcot, a French neurologist, in 1869 and in France the disease is also known as Maladie de Charcot.
The disorder is characterized by the progressive loss of voluntary muscle contraction due to the destruction of nerve cells in the brain and the spinal cord that are responsible for the stimulation of the voluntary muscles.
The muscles are simply stimulated by a group of neurons located on the frontal portion of the spinal cord projecting to the muscle cells (second motor neurons) and these nerve cells are stimulated by a group of nerve cells that project from a specific region called the motor area, located on the frontal lobe (first motor neurons). The latter projection is called the corticospinal tract. The nerve cells of both pathways shrink and die for some unknown reason giving rise to muscle weakness, muscle cramps, speech impairment, difficulty swallowing and breathing. The international support organization uses the abbreviation ALS/MND to refer to the disease.
Lou Gehrig brought national and international attention to the disease in 1939 when he abruptly retired from baseball after being diagnosed with ALS/MND. Theoretical physicist Steven Hawking also suffers from the disease.
ALS/MND is indeed the most frequently observed member of a family of disorders called motor neuron diseases. This family has three major subgroups called primary lateral sclerosis (only the first motor neurons are affected), spinal muscular atrophy (only the second motor neurons are affected) and ALS (both are affected).
The incidence of ALS/MND is approximately 1 out of 100,000 people, and men are affected slightly more than women. The onset of symptoms is usually after the 5th or 6th decade and there are also familial forms of the disease.
Some cases with familial forms of the disease were shown to have a mutation on their superoxide dismutase (SOD) 1 genes, which produces an enzyme that reduces the oxidative stress of the nerve cells. Similar findings led the researchers to assume that the nerve cell death was caused due an excess increase of free radicals in the cell. This hypothesis is one of many others developed to describe the etiology of ALS/MND and is waiting to be reliably proven. Meanwhile, some experimental drugs are used to reduce the oxidative stress of the cells with very limited success.
The diagnosis is established on both clinical grounds and an electromyography (EMG) examination, which is obligatory to demonstrate the diffuse loss of nervous stimulation of muscles of extremities, face and abdomen. Neuroimaging examinations can be performed for some occasional cases especially for a mass lesion of upper parts of spinal cord to exclude some disorders that may mimic the symptoms of ALS/MND. The disease has always a grave prognosis and the patients usually die within 2 to 10 years. There is no cure and the treatment is usually supportive and symptomatic.
Some current promising research towards a cure has focused on gene therapy and the use of stem cells, though the ethical and legal difficulties surrounding the harvesting of stem cells have slowed progress, particularly in the United States.