Primary lateral sclerosis (PLS) is a type of motor neuron disease. A motor neuron disease affects the nerve cells in the brain that control movement. In PLS, the breakdown of nerve cells causes weakness in the muscles that control the legs, arms and tongue.

Symptoms typically begin with balance trouble. People with PLS may move slowly and are clumsy. Over time, symptoms evolve to include having trouble with their hands and arms, followed by problems with chewing, swallowing and speaking. Less commonly, the condition may begin with problems with speaking and swallowing.

This rare condition can develop at any age, but it usually occurs between ages 40 and 60. PLS is more common in men than in women. Rarely, PLS can begin in early childhood. This subtype is known as juvenile primary lateral sclerosis. Juvenile PLS is caused by a change in a gene that's passed from both parents to a child.

PLS is often mistaken for another, more common motor neuron disease called amyotrophic lateral sclerosis (ALS). While likely related to ALS, PLS gets worse more slowly than ALS. In most people, PLS isn't fatal.


Symptoms of primary lateral sclerosis usually take years to develop and get worse. Symptoms generally begin in the legs. Rarely, PLS begins with weakness in the tongue or hands. Then weakness gradually moves down the spinal cord to the legs. However, symptoms vary from person to person. Symptoms may include:

  • Stiffness, weakness and muscle spasms in the legs. This is known as spasticity. Rarely, spasticity starts in only one leg and moves to the arms, hands, tongue and jaw.
  • Slowed movement.
  • Tripping, being clumsy and having trouble with balance.
  • Hand clumsiness.
  • A hoarse voice, as well as slowed, slurred speech and drooling.
  • Trouble chewing and swallowing.
  • Sometimes, frequent, rapid and intense shifts in emotions.
  • Rarely, breathing problems and bladder problems late in the disease.

When to see a doctor

Make an appointment to see a health care professional if you have persistent problems with stiffness or weakness in your legs or arms. Also see a health care professional if you have trouble swallowing or speaking.

Take your child to a health care professional if your child develops muscle spasms or seems to be losing balance more often than usual.


In primary lateral sclerosis, the nerve cells in the brain that control movement slowly break down and stop working. These nerve cells are called upper motor neurons.

As a result, the nerves are not able to activate the motor neurons in the spinal cord, which control voluntary muscles. This loss causes movement problems. People with PLS may have problems with balance, weakness, slowed movement and clumsiness. PLS also may cause problems with speech and swallowing.

Adult-onset primary lateral sclerosis

The cause of PLS that begins in adulthood is not known. In most people, the disease is not inherited. It's not known why or how it begins.

Juvenile primary lateral sclerosis

Juvenile PLS is caused by changes in a gene called ALS2.

Researchers don't understand how this gene causes the disease. But they know that the ALS2 gene gives instructions for creating a protein called alsin. Alsin is present in motor neuron cells. When the instructions are changed in someone with juvenile PLS, the protein alsin becomes unstable and doesn't work properly. This affects the function of the muscle.

Juvenile PLS is an autosomal recessive inherited disease. This means both parents have to be carriers of the gene to pass it to their child. However, they don't need to have the disease themselves.

Risk factors

There are no established environmental risk factors for primary lateral sclerosis.


It can take as long as 20 years for primary lateral sclerosis to progress and become worse. Symptoms vary from person to person. Some people continue to walk, but others eventually need to use wheelchairs or other assistive devices.

For most people, adult-onset PLS isn't thought to shorten life expectancy. But it can gradually affect the quality of your life as more muscles become disabled. Weaker muscles can cause you to fall, which can result in injuries. Problems with chewing and swallowing may result in poor nutrition. Performing activities of daily living, such as bathing and dressing, may become hard.

People with PLS may develop problems with their thinking, known as cognitive decline. Or they may have changes in their behavior. For some people, their symptoms overlap with symptoms of frontotemporal dementia.

Sept. 20, 2023
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