Types of Motor Neuron Diseases


What are motor neuron diseases?

Motor neuron diseases (MNDs) are progressive neurological conditions that damage nerve cells known as motor neurons. Ordinarily, upper motor neurons in the brain transmit messages to lower motor neurons in the brainstem and spinal cord; the lower motor neurons then transmit these signals to the muscles to produce muscle movement. MNDs destroy motor neurons, so this process is disrupted, which affects skeletal muscle activity, such as walking, talking, swallowing or breathing.

Types of motor neuron diseases

Motor neuron diseases are primarily classified based on whether they affect the upper motor neurons, lower motor neurons, or both. Motor neuron diseases can also be classified based on whether they are hereditary or sporadic.

The most common MNDs include amyotrophic lateral sclerosis, progressive bulbar palsy, primary lateral sclerosis, progressive muscular atrophy, spinal muscular atrophy, and Kennedy’s disease

Amyotrophic lateral sclerosis (ALS)

ALS affects both the upper and lower motor neurons. This MND involves rapid loss of muscle control and eventually causes paralysis. Most cases of ALS are sporadic; however, approximately 5% to 10% are hereditary. ALS is a fatal disease. It eventually causes respiratory failure within three to five years of diagnosis; however, some individuals with ALS live ten or more years after diagnosis.

Progressive bulbar palsy (PBP)

PBP affects the lower motor neurons in the brainstem that control the muscles used for speaking, chewing and swallowing. Some medical professionals consider PBP to be a form of ALS because many individuals with PBP eventually develop ALS. Individuals with PBP may also deal with emotional changes, such as laughing or crying at inappropriate moments (pseudobulbar affect).

Primary lateral sclerosis (PLS)

PLS affects the upper motor neurons. Stiffness, weakness, and muscle spasms in one or both legs are typically the first symptoms. These symptoms then progress to the torso and arms and eventually affect the muscles in the mouth and throat. PLS progresses more slowly than ALS and, in most cases, is not fatal. However, some individuals with PLS eventually develop ALS.

Progressive muscular atrophy (PMA)

PMA is a rare motor neuron disease that affects the lower motor neurons. It can be inherited or sporadic. Muscle weakness typically begins in the hands and then spreads to the lower body. It can also affect the torso. Other symptoms of PMA include muscle cramps and muscle shrinking.

Spinal muscular atrophy (SMA)

SMA also affects the lower motor neurons. It is inherited and is caused by a defect in the SMN1 gene. It causes weakness in the muscles close to the center of the body, such as the torso, thighs, and upper arms. There are four types of SMA:

  • Type I (Werdnig-Hoffmann disease) develops in children around six months of age. Symptoms include poor muscle tone, an inability to sit independently, poor reflexes, and difficulty swallowing or breathing.
  • Type II develops in children between six and 18 months of age. These children typically cannot stand or walk independently and may have trouble breathing.
  • Type III (Kugelberg-Welander disease) develops in children between two and 17 years old. The muscles in the legs are most commonly affected, causing difficulties walking, running, or climbing stairs.
  • Type IV typically develops after age 30. Symptoms include muscle weakness, muscle twitches, and difficulty breathing.
Kennedy’s disease

Kennedy’s disease is an inherited MND and only affects males. However, females can be asymptomatic carriers of the disease. This type of MND affects the lower motor neurons. Symptoms tend to progress very slowly. Early symptoms of muscle tremors and cramps eventually progress to muscle weakness in the shoulders or pelvic region. Eventually, muscle weakness in the throat and mouth occurs.

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