Primary lateral sclerosis (PLS) and amyotrophic lateral sclerosis (ALS) are neurodegenerative diseases. PLS typically develops slowly, while ALS rapidly affects mobility and can become fatal.

Neurodegenerative diseases cause neurons in the brain and spinal cord to slowly die, leading to movement problems, disability, and in advanced cases, death.

PLS and ALS are two types of neurodegenerative diseases that share several features. They are motor neuron diseases, which are rare and severe neurodegenerative diseases. Some are life-threatening, while others are not, depending on the type. ALS always progresses to death, but PLS does not.

PLS is a motor neuron disease that primarily affects the upper motor neurons to cause progressive weakness in the legs. This weakness can spread to the arms and other muscles as the disease progresses. The disease typically occurs during adulthood and progresses slowly.

Lou Gehrig’s disease is another name for ALS, which affects the upper and lower motor neurons. It causes various muscle problems, such as twitching, cramping, and tightness. The disease progresses quickly and can become fatal within 3–5 years.

In this article, we will discuss the differences and similarities between ALS and PLS.

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PLS and ALS are both motor neuron diseases that cause progressive neuron death. It affects motor neurons, which control voluntary movements. Damage to these neurons affects how someone controls their muscles and movement. For example, these diseases can cause muscle spasms, weakness, and stiffness.

PLS is a rare form of motor neuron disease that primarily affects the upper motor neurons. In contrast, ALS is the most common form of motor neuron disease that affects both the upper and lower motor neurons.

One main difference between PLS and ALS is how quickly the diseases progress. Another main difference is that ALS affects both upper and lower motor neurons.

PLS progresses slowly and may take years to affect mobility. However, ALS rapidly progresses to cause severe disability and death within 3–5 years of symptom onset.

The life expectancy of someone with PLS is usually not substantially affected by the disease, and most people are expected to experience significant disability, with near normal life expectancy.

There could be delays in the treatment and diagnosis of PLS as the symptoms develop slowly and appear similar to other neurological conditions, such as multiple sclerosis or Parkinson’s disease.

Sometimes ALS and PLS can cause similar symptoms in the early stages, especially in cases when ALS starts with a higher predominance of upper motor neuron signs and symptoms. ALS also causes lower motor neuron symptoms like decreased muscle tone and twitches, where PLS only causes upper motor neuron symptoms.

Both diseases cause symptoms that include:

  • muscle weakness
  • muscle spasms, cramps, and stiffness
  • movement and coordination problems
  • problems with balance

However, the symptoms of PLS typically affect the lower body first, before affecting the upper body, and people with PLS typically have trouble walking.

Conversely, ALS causes symptoms that affect the whole body, including the legs. This means that people with ALS may eventually have additional problems standing or walking.

As with many diseases, sometimes people have a disease course that doesn’t exactly follow this pattern, and you might experience a different sequence of symptoms with PLS.

The diseases can cause severe symptoms as they progress, including:

  • difficulty chewing and swallowing
  • breathing problems
  • an inability to maintain weight
  • anxiety and depression
  • difficulty speaking or forming words

These symptoms will occur much quicker in people with ALS than in PLS.

The exact causes of PLS and ALS are still unclear. Motor neuron diseases typically develop due to genetic alterations that are present at birth. Both conditions are likely due to a combination of genetic and environmental factors.

For example, PLS may run in families, and variations of ALS2 genes could cause the disease. The disease can also occur without a family history of the disease. Genes likely play a role in causing PLS, but it may generally be a result of other environmental factors.

However, only around 5–10% of ALS cases are due to genetics. In these cases, the disease can result from alterations in certain genes, such as SOD1 and C9ORF72. Most cases occur at random without any genetic influences.

Doctors diagnose PLS and ALS by assessing symptoms, neurological examinations, and diagnostic tests, such as electromyography and an MRI. The former involves using thin needles to measure muscle activity. Doctors will first rule out other conditions that might cause similar symptoms, such as multiple sclerosis.

If you have the unusual combination of upper motor neuron and lower motor neuron signs and symptoms, your doctors will consider ALS as a possible cause. Other possibilities that might be considered include myasthenia gravis, cervical spine disease, or Guillan-Barre.

If you have upper motor neuron signs and symptoms, doctors might initially consider MS or Parkinson’s because they are more common than PLS.

Healthcare professionals will also check the person’s medical and family history for signs of disease.

There are currently no cures for PLS or ALS, but several treatments could slow their progression and help people manage their symptoms.

Doctors may prescribe medications that reduce symptoms. For example, they may recommend riluzole to treat ALS by reducing the damage to motor neurons. Additionally, edaravone can slow the decline of daily functioning in people with the condition.

Doctors may also prescribe muscle relaxants, such as baclofen and tizanidine, to reduce stiffness from PLS. Other medications can help with pain or depression, such as antidepressants.

People with these diseases will receive various types of support alongside medication. For example, they might receive physical or occupational therapy for certain symptoms, such as muscle weakness or movement problems. Speech and language therapists can help people experiencing difficulties with speech.

PLS and ALS are types of motor neuron disease that can cause a progressive decline in neuron function. Both diseases cause problems with voluntary control of the muscles and lead to disability. ALS progresses faster than PLS and can quickly become fatal.

While PLS typically only affects the upper body, ALS affects the whole body. Both diseases share symptoms with other neurological diseases, such as multiple sclerosis. Treatment for these conditions includes a combination of medication, therapy, and lifestyle changes.