PLS and ALS are two types of motor neuron disease. PLS affects only upper motor neurons and progresses slowly. ALS affects both upper and lower motor neurons and progresses more quickly.

Motor neuron diseases affect the nerve cells that control muscles involved in movement, breathing, and speaking. An estimated 268,674 people around the world were living with a motor neuron disease in 2019.

There are several types of motor neuron disease. Keep reading to learn more about two types — primary lateral sclerosis (PLS) and amyotrophic lateral sclerosis (ALS). We’ll explore their causes, symptoms, diagnosis, treatment, and more.

It’s still unknown what causes PLS and ALS. In both diseases, motor neurons in the body stop working and begin to die off. This leads to progressive muscle weakness.

PLS and ALS affect motor neurons differently. You have two types of motor neurons that work together to send signals from your brain to the muscles in your body:

  • Upper motor neurons are the nerves in your brain and spinal cord.
  • Lower motor neurons are the peripheral nerves that run through your body.

PLS affects only upper motor neurons. Meanwhile, ALS causes both upper and lower motor neuron signs.

There are few known risk factors for PLS. Most people develop it between the ages of 40 and 60. It’s also more common in people assigned male at birth.

There’s also a form of PLS that starts in childhood or adolescence. This type of PLS is associated with mutations in the ALS2 gene.

ALS typically comes on at a slightly older age, typically between the ages of 55 and 75. It’s also slightly more common in people assigned male at birth.

Several known genetic mutations are linked to ALS. There are several other potential risk factors for ALS, including:

  • smoking
  • military service
  • exposure to environmental toxins
  • exposure to electromagnetic fields
  • viral infections
  • excessive exercise

Below are some differences between the symptoms of PLS and ALS.

PLS symptoms

The symptoms of PLS may include:

ALS symptoms

The symptoms of ALS are typically more severe and can include:

No specific tests can diagnose motor neuron diseases like PLS and ALS. Instead, a doctor will make a diagnosis based on your medical history and symptoms after ruling out other conditions.

The diagnostic process starts with a medical history and physical exam. A doctor will also perform a neurological exam, during which they’ll test things like your movement, balance and coordination, and speech and hearing capabilities.

Further tests they may order include:

PLS causes degeneration of motor neurons in the brain, with visible changes on brain MRI imaging.

ALS causes degeneration of motor neurons in the spinal cord. People with ALS often have specific changes in EMG and nerve conduction studies. In late stages, sometimes ALS can cause visible changes on MRI images of the spinal cord.

Conditions that mimic PLS or ALS

PLS and ALS can be mistaken for each other, as well as other motor neuron diseases. Examples of other conditions that can mimic either PLS or ALS include:

There’s no cure for PLS or ALS. Instead, treatment of these conditions focuses on managing symptoms.

There are several FDA-approved medications doctors can use to slow the progression of ALS, but not PLS. Other medications may also help with PLS or ALS symptoms. For example, muscle relaxants can help with muscle spasticity.

People with PLS and ALS can also benefit from physical, occupational, or speech therapy. Assistive devices can help with mobility and speech difficulties.

Psychotherapy, with or without medications for anxiety or depression, can help you cope with the effects of these diseases.

PLS typically progresses more slowly than ALS. It initially affects the legs before progressing to other areas like the torso, arms, and face.

ALS can present in many different ways. Generally, symptoms begin in a limb or in the muscles associated with the mouth and throat. Over time, it affects additional muscles throughout the body.

Because PLS progresses more slowly, it’s typically not fatal. Depending on your age and overall health, you can live with PLS for at least a decade and often longer, with the average duration of the disease ranging between 7.2 to 14.5 years.

While the rate of progression of ALS can vary greatly, it generally progresses more rapidly than PLS. Most people with ALS live 3–5 years after symptoms come on. Death is typically due to respiratory failure.

Can PLS be misdiagnosed as ALS?

Yes. The initial symptoms of PLS can resemble those of ALS, which is more common than PLS.

Is ALS or PLS more common?

ALS is more common than PLS. It makes up an estimated 85% of motor neuron diseases, while PLS only accounts for 2–3% of all motor neuron diseases.

Does PLS turn into ALS?

Some people with a PLS diagnosis may eventually receive an ALS diagnosis. This is because a small number of people with ALS initially present with upper motor neuron disease and don’t have signs of lower motor neuron disease until much later.

PLS and ALS are two types of motor neuron disease. While they can be mistaken for each other, key differences exist.

PLS only affects the upper motor neurons and typically starts in the legs before progressing to the torso, arms, and face. Overall, it progresses much more slowly than ALS and is usually not fatal.

ALS affects both the upper and lower motor neurons. It often begins in a limb or in the face before progressing to other muscles throughout the body. It progresses much more rapidly than PLS and is typically fatal after a few years.