Primary Lateral Sclerosis


What is primary lateral sclerosis?

Primary lateral sclerosis (PLS) is a rare neuromuscular disorder affecting motor neurons (nerve cells) in your brain and their projections (fibers) to the spinal cord, which together make up the upper motor neuron (UMN).

PLS results in the slow degeneration (breakdown) of the UMN, which ends in the center of the spinal cord on a second set of motor neurons. Normally, messages are then relayed from these spinal cord neurons along nerve fibers to the voluntary muscles (muscles under your direct control), which together make up the lower motor neuron (LMN). In PLS, the LMN is unaffected.

Symptoms of PLS, like muscle stiffness and weakness, usually begin in the legs but over time, spread throughout your body. Because there is no cure for PLS, treatment centers on managing your symptoms, providing support, and improving the quality of your life.

Who is likely to have primary lateral sclerosis (PLS)?

Anyone can develop primary lateral sclerosis (PLS). The condition is most likely to affect individuals between the ages of 35 and 66, and usually by about 50 years old.

Symptoms and Causes

What causes primary lateral sclerosis (PLS)?

The cause of primary lateral sclerosis (PLS) is unknown. A very rare form of PLS that affects young children and teens has been linked to a specific genetic mutation (abnormal code).

What are the symptoms of primary lateral sclerosis (PLS)?

Primary lateral sclerosis (PLS) develops slowly and, as a result, symptoms may be relatively mild at first.

The first symptoms of PLS typically include:

  • Stiffness (from spasticity) and weakness in your leg muscles.
  • Difficulty walking or maintaining your balance.
  • Intermittent muscle spasms (sudden marked stiffening) of voluntary muscles, which can be painful due to the spasms. Muscle (‘Charley Horse’) cramps are different and occur with LMN injury or degeneration, as in amyotrophic lateral sclerosis (ALS).

Rarely, muscles of speaking and swallowing are affected first, causing slurred speech (dysarthria) and trouble swallowing (dysphagia).

As the disease progresses, other common symptoms include:

  • Finger, hand, and arm stiffness and weakness.
  • Bladder control problems (urinary urgency and even incontinence).
  • Pain in your lower back, neck and other muscles.

Breathing problems are rare or nonexistent, in contrast to ALS.

How is primary lateral sclerosis (PLS) different from amyotrophic lateral sclerosis?

Both conditions are motor neuron diseases, although degeneration in primary lateral sclerosis (PLS) is of only the UMN whereas in ALS it is of both the UMN and LMN. Muscle wasting, Charley Horse cramps, and significant weakness seen in ALS is the result of LMN degeneration. In general, both the speed of progression and extent of disease are greater in ALS possibly because both levels of motor neurons degenerate.

Importantly, some forms of ALS mainly affect the UMN (at least at first), and progress more slowly, making it difficult to distinguish from PLS, especially in early stages. Most cases that look like PLS are actually early stages of UMN-predominant ALS that eventually develop into ALS. For this reason, a diagnosis of PLS should only be made after symptoms have been present for at least three to four years. PLS is much less common than ALS.

Diagnosis and Tests

How is primary lateral sclerosis (PLS) diagnosed?

Your healthcare provider diagnoses primary lateral sclerosis (PLS) by a physical examination, excluding look-alike conditions (for example, multiple sclerosis), and observing the condition over time.

Diagnostic tests include:

  • Complete neurologic exam.
  • Blood work.
  • Electrodiagnostic examination (nerve conduction and needle electrode studies), which measures how well your nerves and muscles (LMN) are working.
  • Magnetic resonance imaging (MRI) of brain and spinal cord.
  • Cerebrospinal fluid (CSF) analysis done after lumbar puncture to look for abnormalities that would suggest conditions resembling it.

As mentioned above, because UMN-predominant forms of ALS can look like PLS in early stages, a diagnosis of PLS is most certain when you are four or more years from symptom onset.

Management and Treatment

How is primary lateral sclerosis (PLS) treated?

Unfortunately, primary lateral sclerosis (PLS) does not have a cure. Treatment is aimed at managing symptoms and may include:

  • Medications to reduce muscle spasticity (stiffness), spasms, and difficulties swallowing saliva or controlling emotions (pseudobulbar affect [PBA]).
  • Physical therapy to improve muscle flexibility and joint range of motion.
  • Devices to assist with moving around, such as a cane, walker or wheelchair (eventually).
  • Speech therapy although devices to assist with speaking may be more useful.


Can primary lateral sclerosis (PLS) be prevented?

Because we do not know the cause of primary lateral sclerosis (PLS), there is no way to prevent it.

Outlook / Prognosis

What is the prognosis (outlook) for people with primary lateral sclerosis (PLS)?

The rate at which your symptoms appear and progress varies from person to person. It is usually very slow and occurs over years or decades. There may be periods of time with little or no decline; if progression becomes more rapid, revaluation by your healthcare provider is important to determine whether another condition is responsible. Many factors appear to influence the severity and progression of your PLS, including your general health, nutrition, and other factors such as your genetic code. If you have PLS, you can live up to a normal lifespan of someone without the disease.

Living With

What questions should I ask my doctor?

If you have primary lateral sclerosis (PLS), you probably have many questions for your healthcare provider, such as:

  • Which treatments will help me control my symptoms as much as possible?
  • How will I know if my condition is worsening over time?
  • What types of exercises can I do to help maintain muscle function?
  • Are there any research studies or clinical trials that I can participate in to help further our understanding of PLS?

Last reviewed by a Cleveland Clinic medical professional on 08/08/2020.


  • Amyotrophic Lateral Sclerosis Association (DC/MND/VA Chapter). About PLS. ( Accessed 08/08/2020.
  • Amyotrophic Lateral Sclerosis Society of Canada. Primary Lateral Sclerosis (PLS). ( Accessed 08/08/2020.
  • National Institute of Neurological Disorders and Stroke. Primary Lateral Sclerosis Information Page. ( Accessed 08/08/2020.
  • National Organization for Rare Disorders. Primary Lateral Sclerosis. ( Accessed 08/08/2020.
  • Spastic Paraplegia Foundation. About PLS (Primary Lateral Sclerosis). ( Accessed 08/08/2020.

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