Primary lateral sclerosis (PLS) is a rare neurological disorder that affects the motor neurons in the brain and spinal cord, which control movement. It is a form of motor neuron disease (MND).

PLS leads to weakness and stiffness in the limbs and body. Doctors may find it challenging to diagnose the condition, as it shares symptoms with various neurological disorders.

While PLS is not typically fatal, it can significantly affect a person’s quality of life, making it difficult to perform daily activities such as walking, dressing, and eating.

This article explores the causes, symptoms, diagnosis, and management of PLS. It also provides resources for information and support for those living with this condition.

A person walking by a river.Share on Pinterest
Westend61/Getty Images

Primary lateral sclerosis, or PLS, is a rare neurodegenerative disorder that causes gradual degeneration of neurons in the motor cortex of the brain and sections of the spinal cord.

These neurons carry signals to the lower motor neurons, initiating movement and coordinating muscle activity. Damage to these neurons leads to weakness and stiffness in the arms, legs, and body.

PLS belongs to the group of conditions known as motor neuron disease (MND). MND affects around 4 to 6 people per 100,000, while the PLS subtype affects fewer than 2 people per 100,000.

Although PLS is a progressive condition, it does not usually cause death, but it does significantly affect quality of life.


PLS and amyotrophic lateral sclerosis (ALS) are both types of MND, but they affect different parts of the nervous system.

While PLS only affects upper motor neurons, ALS affects both upper and lower motor neurons.

The upper motor neurons are the nerve pathways in the brain and spinal cord. They send messages to the lower motor neurons, which are the motor spinal nerve roots and peripheral nerves. These run along the limbs to the muscles.

The symptoms of PLS include:

  • weakness
  • muscle stiffness
  • clumsiness
  • slowing of movement
  • balance and speech problems
  • fatigue

PLS often affects the legs first, then the body, trunk, arms, and hands. Eventually, it affects the muscles that control speech, swallowing, and chewing.

Experts do not know the exact cause of PLS. It appears to occur randomly. One risk factor may be age, as PLS tends to develop during the fifth decade of life. Doctors believe there may also be a familial form of the disease that affects children.

PLS can be challenging to diagnose, as it has similar symptoms to other neurological disorders. A doctor will request a thorough medical history, and perform a physical examination assessing muscle tone, strength, and reflexes.

Imaging studies, such as MRI or CT scans, can help the doctor rule out other conditions. Electromyography (EMG) and nerve conduction studies also help doctors evaluate signal transmission along the nerves. In PLS, these tests may appear typical.

A doctor may diagnose PLS if:

  • the person is aged 25 years or older
  • they have gradual onset and progression of upper motor neuron signs, such as spasticity (muscle stiffness), hyperreflexia (overactive reflexes), and weakness for at least the past 2 years
  • they have no lower motor neuron signs, such as muscle wasting or muscle twitching
  • their senses are working as usual
  • they have no family history of ALS

If a person does not meet these criteria yet, a doctor may consider other causes for the symptoms, such as ALS, multiple sclerosis, or Guillain-Barré syndrome. As the symptoms evolve, they will be able to provide a firm diagnosis.

PLS has no cure, but medications can help manage specific symptoms. These include:

  • baclofen and tizanidine for spasticity
  • quinine for cramps
  • diazepam for muscular contractions

Other therapies, such as physical and speech therapy, can help the person maintain the ability to move and communicate.

Despite PLS being a progressive condition, many people who have it live a typical lifespan following their diagnosis.

PLS tends to develop slowly, with little change in motor function over many years. However, this can vary significantly from person to person. A person’s symptoms and speed of disease progression may become more evident over time.

People may find living with PLS challenging and require support from their family, friends, and healthcare team. This multidisciplinary care team could include various medical professionals, such as:

  • primary care physicians
  • neurologists
  • physical therapists
  • occupational therapists
  • speech-language therapists
  • dietitians
  • psychologists
  • social workers

These professionals work closely with the person, their family, and their primary doctor to provide individualized treatment and support.

For example, a dietitian may help with finding nutrient-dense foods that are easy to swallow, while a psychologist may support a person’s mental health.

A person may also benefit by joining an online or in-person support group. This allows people to share personal experiences and gain emotional support from others living with MND. People can reach out to the following organizations for assistance:

Primary lateral sclerosis (PLS) is a rare neurological disorder that causes muscle weakness and stiffness due to the degeneration of upper motor neurons.

Although no cure exists, medications and therapies can help manage symptoms. Various organizations provide information, resources, and emotional support to those living with PLS.

With assistance from family, friends, and healthcare professionals, people with PLS can expect to live a typical lifespan.