What to Know About Rare Motor Neuron Diseases

Motor neuron diseases (MNDs) are a class of diseases that affect certain important cells. These cells, called motor neurons, control the muscles that help with movement. Your body uses motor neurons to do many things like walk, breathe, and swallow.1

What causes motor neuron diseases?

MNDs destroy motor neurons. You have upper motor neurons in your brain and lower motor neurons in your brain stem and spinal cord. Typically, the upper motor neurons send messages to the lower motor neurons. These messages control your body's movement.1

MNDs can affect the upper or lower motor neurons. They can prevent the neurons from receiving messages from each other. They can also make it hard for the muscles to receive messages from the lower motor neurons. The specific effects depend on the disease.1

Experts understand how MNDs work. But they do not understand exactly what causes them. Some MNDs are inherited. Some may be linked to the environment, genetics, or viruses. But more research is needed to prove these links.1,2

What are some examples of rare motor neuron diseases?

There are many different types of MNDs. The most common MND is amyotrophic lateral sclerosis (ALS). ALS is also called Lou Gehrig’s disease.1,3

Other MNDs include:1,3

  • Spinal muscular atrophy (SMA) – SMA is the most common inherited cause of infant death.
  • Primary lateral sclerosis (PLS) – Some people consider PLS part of ALS. PLS by itself is not fatal. But it often evolves into ALS.
  • Progressive bulbar palsy (PBP) – This MND mainly affects the muscles of the mouth. PBP also often evolves into ALS.
  • Kennedy’s disease – This MND only affects men, but women are carriers of the gene. It is also called spinobulbar muscular atrophy.
  • Post-polio syndrome (PPS) – PPS affects people who previously had polio.

Some MNDs are much less common than those above. These rare MNDs include:1,2

  • ALS in children or young people.
  • PBP that does not evolve into ALS.
  • Progressive muscular atrophy (PMA) – This affects only the lower motor neurons. It is more common in men and can start at a younger age.
  • Flail-leg syndrome (FLS) – In FLS, symptoms begin in the lower legs. It is often misdiagnosed at first.
  • Flail-arm syndrome (FAS) – This is also called Vulpian-Bernhardt syndrome. It primarily affects the arms and may not affect any other areas.
  • O’Sullivan-McLeod syndrome – This condition primarily affects the hand and lower arm muscles.
  • Mills’ syndrome – This condition paralyzes the limbs. It typically starts in the legs.
  • Facial-onset sensory and motor neuronopathy (FOSMN) – This condition can affect brain function. It is very rare. Fewer than 100 cases have been reported in research.
  • Finger extension weakness and downbeat nystagmus (FEWDON-MND) – This is a new MND classification. It can start earlier but progresses slower than other MNDs.

What are the symptoms of motor neuron diseases?

The exact symptoms of an MND depend on the disease. But they all cause muscle weakness that gets worse over time. The most common symptom linked to MNDs is respiratory insufficiency. This means the lungs are not working properly.1,3

Respiratory insufficiency can cause problems like:1,3

  • Shortness of breath
  • Chest infections
  • Confusion
  • Headaches

Other possible symptoms of MNDs include:1-3

  • Difficulty swallowing, chewing, or eating
  • Difficulty walking or moving legs
  • Trouble speaking
  • Clumsiness
  • Slow, weak, or stiff arms and legs

Treating rare motor neuron diseases

Treatment depends on the specific disease. MNDs get worse with time, and there is no cure for them. Treatment is typically focused on comfort and quality of life.1-3

Treatment plans may include:1-3

  • Drugs that slow disease progression or protect the muscles
  • Muscle relaxers
  • Physical therapy
  • Nutrition and diet changes
  • Ventilators to help with breathing

If you think you might have an MND, talk to your doctor. They can make an official diagnosis and, if necessary, start you on a treatment plan that is right for you.

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