Types of Motor Neuron Diseases

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Motor neuron diseases are uncommon, and amyotrophic lateral sclerosis (ALS), also often called "Lou Gehrig's disease" is the most well-known motor neuron disease. There are several other types of motor neuron diseases as well.

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Overview

Conditions that affect the motor neurons are described as motor neuron diseases. Motor neurons are located throughout the length of the spinal cord in columns described as anterior horns. They have this name because the columns of motor neurons in the spinal cord appear like "horns" when viewed with imaging studies, and they run along the front of the spinal cord (towards the front of the body).

Motor nerves follow a path from the brain to the muscle, and they fire very quickly, within milliseconds.

Motor neuron diseases are caused by certain defects in the motor nerve pathway:

  • Before you move any of your muscles, electrical signals are sent from your brain to your spinal cord through upper motor neuron nerve cells.
  • The nerve cells synapse in the anterior horn of the spinal cord and then are sent out along lower motor neurons in peripheral nerves.
  • Electrical signals traveling along these neurons signal for a muscle to contract, resulting in movement.

Neurologists can identify the problem with motor neuron functioning and diagnose a specific motor neuron disease based on symptom history, neurological examination, and diagnostic testing.

General Signs and Symptoms

Motor neuron diseases may affect upper motor neurons, lower motor neurons, or both. Some motor neuron diseases affect only the upper motor neurons, whereas others affect primarily the lower motor neurons. Some, like ALS, affect both.

Symptoms of upper motor neuron disease include:

  • Spasticity: A combination of muscle stiffness, tightness, rigidity, and muscle inflexibility. With mild spasticity, you may be able to move your muscles, but they move in an unexpected or jerky way. With severe spasticity, your muscles may feel "stuck."
  • Rigidity: An involuntary "stiffness" of the muscles.
  • Increased deep tendon reflexes: For example, your knee jerk may have more of a reaction than normal.

Symptoms of lower motor neuron disease include:

  • Atrophy: Thinning muscles and loss of strength.
  • Fasciculations: A spontaneous and involuntary contraction of muscles that may be visible as twitching under the skin.

Types of Motor Neuron Diseases

There are several different motor neuron diseases. They differ with respect to whether they affect upper or lower motor neurons, their initial symptoms, the age group they affect, progression, and their prognosis.

Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a progressive motor neuron disease that affects roughly about 16,000 Americans. It begins with muscle weakness, usually on only one side of the body. The condition begins in the hands more often than the feet.

Early on, the primary signs may be weakness and fasciculations, but eventually, it progresses with both upper and motor neuron signs and symptoms. When muscles of the diaphragm are affected, breathing becomes difficult or impossible, and mechanical ventilation may be needed.

The disease usually does not affect cognition (thinking skills), and most people remain alert, without dementia, even when the disease is very advanced. The average life expectancy with ALS is roughly two to five years after diagnosis, but it can vary widely, with 10% of people alive after 10 years.

A few treatments, Rilutek (riluzole) and Radicava (edaravone) may be beneficial for some people.

Primary Lateral Sclerosis

Primary lateral sclerosis (PLS) is a disease of upper motor neurons, disrupting signals from the brain to the spinal cord. The result is a slowly progressive weakness associated with upper motor neuron signs, such as spasticity, rigidity, and increased deep tendon reflexes. Unlike amyotrophic lateral sclerosis, lower motor neuron findings, such as atrophy and fasciculations, are not as prominent.

It is not certain just how common PLS is, but it is less common than ALS. PLS tends to progress more slowly than ALS, with patients commonly living about a decade with their symptoms.

Early in the course of the disease, it is difficult to distinguish PLS from ALS:

  • Since ALS can begin with just upper motor neuron signs, it may be years before a diagnosis of PLS becomes apparent.
  • It may be difficult to tell which of the conditions is causing the symptoms since some people with a diagnosis of PLS will later develop lower motor neuron findings and become diagnosed with ALS.

It may be impossible to know if a condition is really ALS or PLS for several years after the onset of symptoms. Other conditions, such as hereditary spastic paraparesis, will also need to be ruled out.

Progressive Bulbar Palsy

Progressive bulbar palsy involves a slow degeneration of certain cells in the brainstem, which contains the cranial nerves that control the face, tongue, and throat. As a result, someone with progressive bulbar palsy will start having difficulty speaking, swallowing and chewing. Limb weakness may also become evident as the disease progresses, with both upper and lower motor neuron signs.

Myasthenia gravis is an autoimmune neuromuscular disorder that may cause similar symptoms at early stages.

Post-Polio Syndrome

Polio is a virus that attacks motor neurons in the anterior horn of the spinal cord, resulting in paralysis. Thankfully, due to aggressive vaccinations, this virus has largely been eradicated.

Some people who have had the disease may later develop a type of weakness known as post-polio syndrome. This may be due to aging or injury causing the relatively few surviving motor neurons to die. The disorder only affects older people who have had polio in the past. It is usually not life-threatening.

Kennedy’s Disease

Kennedy's disease is due to an X-linked genetic mutation that affects the androgen receptor. The disorder causes slowly progressive weakness and pain of the muscles closest to the torso. The face, jaw, and tongue are also involved.

Because it is X-linked, Kennedy’s disease generally affects men. Women with the genetic mutation are carriers, with a 50 percent chance of passing the gene on to their children. Women who carry the mutation may also experience minor symptoms, such as finger cramps, instead of more profound weakness.

Because the disease affects the androgen receptor (the receptor to which estrogen and testosterone attach), men with the disorder may also have symptoms such as gynecomastia (breast enlargement), testicular atrophy, and erectile dysfunction. The lifespan of people with Kennedy’s disease is usually normal, though as their weakness progresses they may require a wheelchair.

Spinal Muscular Atrophy

Spinal muscular atrophy is an inherited disease that predominantly begins during childhood. It is caused by defects in the SMN1 gene and is inherited in an autosomal recessive pattern. Due to this defective gene, not enough SMN protein is made, and this leads to degeneration of lower motor neurons. This leads to weakness and muscle wasting.

There are three main types of SMA, and they differ by severity and the age at which they start.

  • SMA type 1, also called Werdnig-Hoffman disease, becomes obvious by the time a child is six months old. The child will have hypotonia (floppy muscles) and will not often move spontaneously. They will not be able to sit up on their own at the expected time. Due to difficulty with the airway and maintaining strength enough to breathe, most of these children die by the age of two.
  • SMA type II starts a little later, becoming apparent between the ages of 6 to 18 months. These children will not be able to stand or walk without assistance, and will also have difficulties with breathing. However, children with SMA type II typically live longer than those with Werdnig-Hoffman, sometimes living into young adulthood.
  • SMA type IIII also called Kugelberg-Welander disease, becomes apparent between the ages of 2 and 17 years. Children with this disorder may have some difficulty running or climbing steps. They may also have back problems, such as scoliosis. However, children with this disorder may have a normal lifespan.

Treatments can improve outcomes, and include Zolgensma, a type of gene therapy, and Spinraza, which is injected into the spinal fluid to help the body make SMN protein.

Progressive Muscular Atrophy

Progressive muscular atrophy (PMA) affects only the lower motor neurons. Progressive weakness is a common symptom. Progressive muscular atrophy is less common than ALS and has a better prognosis.

Since upper motor neurons are not affected, upper motor neuron signs such as rigidity do not occur. The symptoms of PMA are similar to those of other conditions. In particular, diseases such as ALS, multifocal motor neuropathy (a form of peripheral neuropathy), and spinal muscular atrophy need to first be ruled out first before a conclusive diagnosis can be made.

Diagnosis and Treatment

There is no curative treatment for motor neuron disease. Medical therapy focuses on controlling symptoms of the disease and preventing complications. In order to know what symptoms to anticipate, as well as rule out other more treatable diseases, it is important to get the right diagnosis.

Using their physical exam and other techniques such as electromyography, nerve conduction studies, and genetic testing when appropriate, neurologists can help define the correct diagnosis. Having the right diagnosis allows your neurologist to manage your symptoms as much as possible and to prepare for any expected complications.

Coping

Motor neuron diseases are uncommon, which can mean you might be the only person you know who has been diagnosed with this type of condition. You may feel alone if you've been diagnosed with a motor neuron disease.

People with motor neuron diseases need support. If you don't have a support group in your community, there are support communities online where people with specific motor neuron conditions can meet and communicate with others who are facing some of the same challenges.

Awareness of motor neuron diseases is rising. Though we don't have a medication or surgery to cure these diseases, there is much that can be done to help people live well ​with the disease, and current research offers hope that advances will be made in the not so far off future. Measures such as the Orphan Drug Act are directing more attention to these less common conditions.

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7 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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