Amyotrophic lateral sclerosis or ALS, also known as Lou Gehrig’s disease, is a progressive neurological disorder that damages nerve cells and causes disability.
The disease particularly affects motor neurons, the nerve cells that control voluntary movements, causing them to shrink and die. Upper motor neurons send messages from the brain to the spinal cord, and lower motor neurons send messages from the spinal cord to the muscles.
With the loss of these nerve cells, the brain is no longer able to move muscles or control muscle movement. As a result, the muscles become smaller and weaker. Within a few years of diagnosis, ALS patients lose the ability to perform everyday tasks like walking up steps, reaching for items, or dressing themselves.
Causes of ALS
The exact cause of ALS is largely unknown, but genetic, environmental, and lifestyle factors all are believed to play a role.
Mutations in several genes have been implicated in familial ALS and contribute to the development of sporadic ALS. About 25 to 40% of all familial cases are caused by a mutation in a gene called C9orf72. Other commonly mutated genes include SOD1, TARDBP, and FUS.
Other factors such as a history of military service or trauma, smoking, exposure to toxins such as heavy metals or pesticides, and viral infections also have been linked with ALS. The disease is more common in people around the ages of 40 to 70, and in men as compared with women.
In its earlier stages, ALS causes symptoms such as stiff muscles, muscle twitching, and a gradual loss of muscle strength. As the disease progresses, muscles become progressively paralyzed, and patients experience difficulty in speaking, swallowing, and eventually, breathing.
In about two-thirds of patients, ALS first affects the muscles in the arms, hands, and legs, with symptoms that include tripping, stumbling, or awkwardness when running. People with ALS also experience foot drop and a “slapping” gait. Foot drop is a general term to describe difficulty lifting the front part of the foot.
Other initial symptoms include reduced finger dexterity, cramps, stiffness, and weakness or wasting of intrinsic hand muscles, along with wrist drop that interferes with work performance. Slurred speech, hoarseness, or lower speech volume also are common signs of ALS. Some more concerning early symptoms are aspiration of food, which occurs when food is breathed into the airways, and choking during a meal.
People with ALS can experience emotional and cognitive difficulties, including bursts of involuntary and inappropriate laughing or crying, depression, impaired critical thinking, or abnormal social behaviors.
As the disease progresses, muscle atrophy (shrinkage) becomes more apparent and muscle cramps more common. In its advanced stages, patients often experience voice changes, and speech may eventually be lost completely. Patients may experience drooling and have difficulties swallowing.
There is no single test to diagnose ALS. Because it shares common symptoms with other neurological diseases, physicians diagnose the disorder based on a period of observation that is used to track the progression of symptoms. A series of neurological examinations can help eliminate other possible conditions and aid in the diagnosis.
These tests include nerve conduction studies to assess the speed at which nerve cells transmit electrical impulses to muscles in different areas of the body, and electromyography to measure the electrical activity of several muscles at rest and during contraction. An MRI may be done to study possible ALS-related changes in the brain or spinal cord.
In rare cases, patients also will undergo muscle or nerve biopsies, in which small samples of muscle or nerve tissue are collected. These samples are examined for changes in muscle morphology, loss of nerve supply, or other biomarkers of disease. A spinal tap also may be necessary to examine the proteins and cells in the cerebrospinal fluid, the liquid that surrounds the brain and spinal cord. Such testing may exclude inflammatory nerve conditions.
Blood and urine tests also help exclude other conditions whose symptoms are similar to those of ALS.
Sporadic ALS is the most common form of the disease in the U.S., accounting for 90 to 95% of all cases. It occurs in people with no apparent history of the disorder in their families, and patients usually first develop features of the condition in their late 50s or early 60s.
Familial, or inherited, ALS is caused by genetic mutations and occurs more than once in a family. It accounts for 5 to 10% of all cases in the U.S. The signs and symptoms of familial ALS typically first appear in the 40s or 50s.
Another type is called Guamanian ALS, and is especially common in people living in Guam and the Trust Territories of the Pacific. The disorder has signs and symptoms of ALS, in addition to dementia and a pattern of movement abnormalities that resemble Parkinson’s disease.
ALS can also be grouped into limb-onset and bulbar-onset disease, depending on the muscles it first affects.
Incidence and prevalence of ALS
The incidence of ALS, or the number of new cases per year, is estimated to be 2 per 100,000 people, meaning that every 90 minutes, someone is diagnosed with the disease. This incidence increases with age, typically starting in the 40s and increasing until around age 80. However, ALS can occur as early as in childhood.
The prevalence, or the number of people living with ALS, varies among geographic locations, but is estimated to be somewhere between 4.1 and 8.4 per 100,000 people. In the U.S., the prevalence is believed to be between 5 and 7 cases per 100,000 people, meaning that around 15,000-21,000 people in the U.S. live with the condition.
Last updated: Aug. 12, 2021
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