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Motor neuron disease
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.
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.
Overview
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.
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History
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.
Incidence
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.
Causes
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.
Diagnosis
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.
Treatment
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.
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