Sam Shepard, the Pulitzer Prize-winning playwright and actor, died Monday from complications related to amyotrophic lateral sclerosis, or ALS, a progressive nervous system disorder, a family spokesman told the New York Times . Shepard was 73.

What is ALS?

ALS is often referred to as Lou Gehrigs disease after the famous baseball player who was diagnosed with the condition in the 1930s. The nervous system disorder is characterized by nerve cells slowly breaking down and dying over time , according to the Mayo Clinic.

The U.S. Centers for Disease Control and Prevention doesnt require doctors to report ALS cases, so theres no precise record of how many Americans are currently living with the disease. But the agency estimates that about 5,000 people are diagnosed with ALS each year in the United States.

Early symptoms of ALS include tripping or falling, difficulty walking, slurred speech, and muscle cramps or twitching of the arms, shoulders or tongue. The disease often starts in the limbs, hands or feet and destroys nerve cells as it progresses, weakening the bodys muscles and eventually hindering chewing, swallowing, speaking and breathing.

The disease is genetic in about about 5 to 10 percent of cases, and children in families with familial ALS have about a 50 percent chance of developing the disease.

Experts dont know what causes ALS, but risk of developing the disease increases with age, and is most common between ages 40 and 60. Theres also the possibility that environmental factors, such as smoking, lead exposure or military service could trigger ALS, but much more study needs to be done to confirm those links.

What are ALS complications?

While its not clear which ALS-related health complications Shepard was experiencing, common health problems related to ALS include difficulty breathing, eating, sleeping and speaking, as well as a form of dementia called frontotemporal dementia. The most common cause of death associated with ALS is respiratory failure , and death from ALS typically occurs about three to five years after symptoms begin.

There are two U.S. Food and Drug Administration-approved medications for treating ALS, which can slow the progression of symptoms in some people, but those medications dont reverse nerve damage thats already occurred.

For now, the disease has no known cure.

CLARIFICATION: An earlier version of this article stated that ALS symptoms start in the limbs. While this is often the case, people with bulbar onset ALS experience difficulty speaking and swallowing first.