Overview

Primary lateral sclerosis (PLS) is a type of motor neuron disease that causes nerves within the brain to slowly break down. This makes the nerves unable to activate the motor neurons in the spinal cord, which control muscles. PLS causes weakness in your voluntary muscles, such as those you use to control your legs, arms and tongue.

This condition can develop at any age, but it usually occurs between ages 40 and 60. A subtype of primary lateral sclerosis, known as juvenile primary lateral sclerosis, begins in early childhood and is caused by an abnormal gene passed from parents to children.

Primary lateral sclerosis is often mistaken for another, more common motor neuron disease called amyotrophic lateral sclerosis (ALS). However, primary lateral sclerosis progresses more slowly than ALS, and in most cases isn't fatal.

Symptoms

Signs and symptoms of primary lateral sclerosis (PLS) usually take years to progress. They include:

  • Stiffness, weakness and muscle spasms (spasticity) in your legs, often starting in one leg
  • Tripping, difficulty with balance and clumsiness as the leg muscles weaken
  • Weakness and stiffness progressing to your trunk, then your arms, hands, tongue and jaw
  • Hoarseness, as well as slowed, slurred speech and drooling as the facial muscles weaken
  • Difficulties with swallowing and occasionally breathing late in the disease

Less commonly, PLS begins in your tongue or hands and then progresses down your spinal cord to your legs.

When to see a doctor

Make an appointment to see your doctor if you have persistent problems with stiffness or weakness in your legs, or with swallowing or speaking.

If your child develops involuntary muscle spasms or seems to be losing balance more often than usual, make an appointment with a pediatrician for an evaluation.

Causes

In primary lateral sclerosis (PLS), the nerve cells in the brain that control movement fail over time. This loss causes movement problems, such as slow movements, balance problems and clumsiness.

Adult primary lateral sclerosis

The cause of adult primary lateral sclerosis is unknown. In most cases, it's not an inherited disease, and it's not known why or how it begins.

Juvenile primary lateral sclerosis

Juvenile primary lateral sclerosis is caused by mutations in a gene called ALS2.

Although researchers don't understand how this gene causes the disease, they know that the ALS2 gene gives instructions for creating a protein called alsin, which 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, which in turn impairs normal muscle function.

Juvenile primary lateral sclerosis is an autosomal recessive disease, meaning that both parents have to be carriers of the gene to pass it to their child, even though they don't have the disease themselves.

Complications

Although the average progression of primary lateral sclerosis (PLS) lasts around 20 years, the disease has highly variable effects from person to person. Some people continue to walk, but others eventually need to use wheelchairs or other assistive devices.

Adult 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.