Motor neuron diseases are a group of conditions that cause the nerves in the spine and brain to lose function over time. They are a rare but severe form of neurodegenerative disease. Motor neurons are nerve cells that send electrical output signals to the muscles, affecting the muscles' ability to function.

Motor neuron disease (MND) can appear at any age, but symptoms usually appear after the age of 40 years. It affects more men than women.

The most common type of MND, amyotrophic lateral sclerosis (ALS), probably affects up to 30,000 Americans at any given time, with over 5,600 diagnoses each year, according to the ALS Association.

The renowned English physicist Stephen Hawking lived with ALS for many decades until his death in March 2018. Guitar virtuoso Jason Becker is another example of someone who has been living with ALS for several years.

Types

There are several types of MND. Doctors classify them according to whether they are hereditary or not, and which neurons they affect.

— ALS, or Lou Gehrig's disease, is the most common type, affecting both the upper and lower motor neurons (neurons in the brain and spinal cord). It affects the muscles of the arms, legs, mouth, and respiratory system. A person with ALS will live, on average, another 3–5 years, but, with supportive care, some people live 10 years or more.

— Primary lateral sclerosis affects the neurons in the brain. It is a rare form of MND that advances more slowly than ALS. It is not fatal, but it can affect a person's quality of life. Juvenile primary lateral sclerosis can affect children.

— Progressive bulbar palsy (PBP) involves the brain stem. People with ALS often have PBP too. The condition causes frequent choking spells, difficulty speaking, eating, and swallowing.

— Progressive muscular atrophy (PMA) is a rare condition that affects the lower motor neurons in the spinal cord. It causes slow but progressive muscle wasting, especially in the arms, legs, and mouth.

— Spinal muscular atrophy (SMA) is an inherited MND that affects children. There are three types, all caused by a genetic change known as SMA1. It tends to affect the trunk, legs, and arms. The long-term outlook depends on the type.

— The different types of MND share similar symptoms, but they progress at different speeds and vary in severity.

Symptoms

MND has three stages — early, middle, and advanced.
Early stage signs and symptoms
In the early stage, symptoms develop slowly and can resemble those of other conditions. The symptoms will depend on the type of MND a person has and which part of the body it affects.

Typical symptoms begin in one of the following areas:
the arms and legs
the mouth
the respiratory system

They include:
a weakening grip, which makes it hard to pick up and hold things
fatigue
muscle pains, cramps, and twitches
slurred speech
weakness in the arms and legs
clumsiness and stumbling
difficulty swallowing
trouble breathing or shortness of breath
inappropriate emotional responses, such as laughing or crying
weight loss, as muscles lose their mass
Middle stage signs and symptoms

Prognosis

The table below lists life expectancy for patients who are diagnosed with MND. Please refer to individual articles for more detail.

Type                                                          Median survival time From start of symptoms

Amyotrophic lateral sclerosis (ALS)          2–5 years
Primary lateral sclerosis (PLS)                  8–10 years
Progressive muscular atrophy (PMA)       2–4 year
Progressive bulbar palsy (PBP)                6 months – 3 years
Pseudobulbar palsy                                  No change in survival