Amyotrophic Lateral Sclerosis (ALS)/Lou Gehrig's Disease

Home » Amyotrophic Lateral Sclerosis (ALS)/Lou Gehrig's Disease » Overview, Risk Factors, Causes

Overview

Amyotrophic lateral sclerosis (ALS), also called Lou Gehrig's disease, is a progressive neuromuscular disease that weakens and eventually destroys motor neurons (components of the nervous system that connect the brain with the skeletal muscles).

Upper motor neurons are located in the cerebral cortex of the brain and conduct impulses from the motor cortex to nerves in the brain or the spinal cord. Lower motor neurons are peripheral neurons that originate in the spinal cord and conduct impulses to the skeletal muscles. Skeletal muscles are involved with voluntary movements, such as walking and talking. The lower motor neurons transmit the command to move from the brain to the skeletal muscles, which respond by contracting.

A person with ALS usually presents with problems in dexterity or gait resulting from muscle weakness, or with difficulty speaking or swallowing. Sphincter control, sensory function, intellectual ability, and skin integrity are preserved. Patients become paralyzed and often require ventilation and surgery to provide a new opening in the stomach (gastrostomy). Loss of respiratory function is ultimately the cause of death.

Incidence and Prevalence

Approximately 30,000 patients in the United States currently have ALS. The disease has no racial, socioeconomic, or ethnic boundaries. The life expectancy of ALS patients usually ranges from about 3 to 5 years after diagnosis. ALS is most commonly diagnosed in middle age and affects men more often than women.

Risk Factors

Risk factors include an inherited genetic defect, which accounts for 5–10% of cases of familial ALS (FALS) in the United States. FALS is linked to a genetic defect on chromosome 21. This gene codes for an enzyme called superoxide dismutase (SOD), an antioxidant that protects motor neurons from free radical damage (i.e., molecules introduced to the body, or produced by body processes that interact and cause cellular damage). More than 60 different mutations that cause SOD to lose its antioxidant properties have been found. However, only 40% of familial ALS cases are linked to SOD mutations, so there may be other unknown genetic defects involved.

In the United States, 90–95% of ALS cases are sporadic. Sporadic ALS appears to be increasing worldwide. The causes are not clear, yet some evidence suggests that the immune system may be involved. Excessive levels of glutamate can overstimulate motor neurons and cause them to die. Glutamate is one of the most important neurotransmitters for healthy brain function. Neurotransmitters are chemicals that transmit signals from one nerve to another.

In Guamanian ALS, a dietary neurotoxin is the risk factor. The suspected neurotoxin is an amino acid (BMAA) found in the seed of the cycad Cyas cirinalis, a tropical plant found in Guam, which was used to make flour and was a major dietary component during the 1950s and the early 1960s, when this type of ALS had an exceptionally high incidence.

Causes

The cause of ALS is not completely understood. Researchers and physicians suspect viruses, neurotoxins (especially in Guamanian ALS), heavy metals, DNA defects (especially in familial ALS), immune system abnormalities, and enzyme abnormalities.

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