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 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.
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.
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
- a weakening grip, which makes it hard to pick up and hold things
- 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
As the condition progresses, the early symptoms remain and become more severe.
People may also experience:
- muscle shrinkage
- difficulty moving
- joint pain
- drooling due to problems with swallowing
- uncontrollable yawning, which can lead to jaw pain
- changes in personality and emotional state
- difficulty breathing
Advanced stage signs and symptoms
Eventually, a person in the advanced stage of ALS will need help to move, eat, or breathe, and the condition can become life-threatening.
Breathing problems are the most common cause of death.
Motor neurons instruct the muscles to move by sending signals from the brain. They play a role in both conscious and automatic movements, such as swallowing and breathing.
Experts believe that around 10% of MNDs are hereditary. The other 90% happen randomly.
The exact causes are unclear, but the National Institute of Neurological Diseases and Stroke (NINDS) note that genetic, toxic, viral, and other environmental factors may play a role.
MNDs can occur in adults or children, depending on the type. They are more likely to affect men than women. Inherited forms of the condition may be present at birth. They are most likely to appear after the age of 40 years.
The various types may have different risk factors. SMA is always hereditary, but this is not true for all forms of MND.
According to NINDS, around 10% of ALS cases in the United States are hereditary. It is most likely to appear at the age of 55–75 years.
They also note that veterans appear to have a 1.5–2 times higher chance of developing ALS than non-veterans. This may indicate that exposure to certain toxins increases the risk of having ALS.
Doctors often find it difficult to diagnose MND in the early stages as it can resemble other conditions, such as multiple sclerosis (MS).
If a doctor suspects someone has MND, they will refer them to a neurologist, who will take a medical history, do a thorough examination, and may suggest other tests, such as:
Blood and urine tests: These can help a doctor rule out other conditions and detect any rise in creatinine kinase, a substance that muscles produce when they break down.
Electromyography (EMG) and nerve conduction study (NCS): An EMG tests the amount of electrical activity within muscles, while a NCS tests the speed at which electricity moves through muscles.
Spinal tap, or lumbar puncture: A doctor will look for changes in the cerebrospinal fluid, which surrounds the brain and spinal cord. It can help rule out other conditions.
Muscle biopsy: This can help detect or rule out a muscle disease.
The doctor will normally monitor the individual for some time after the tests before confirming that they have MND.
There is no cure for MND, but treatment may slow progression and maximize the individual's independence and comfort.
Techniques include the use of supportive devices and physical therapy.
The correct choice will depend on factors such as:
- the type of MND a person has
- the type and severity of symptoms
- personal choice
- the availability and affordability of drugs
Slowing disease progression
Drugs appear to be effective at slowing the progress of some types of MND.
Muscle cramps and stiffness
Medications, such as botulinum toxin (Botox) injections. Botox blocks the signals from the brain to the stiff muscles for about 3 months.
Baclofen, a muscle relaxer, may help relieve muscle stiffness, spasms, and yawning. A doctor can surgically implant a small pump outside the body to deliver regular doses to the space around the spinal cord, from where it can reach the nervous system.
Some people may find physical therapy helps alleviate cramps and stiffness.
A non-steroidal anti-inflammatory drug (NSAID), such as ibuprofen, will help with mild to moderate pain from muscle cramping as spasms.
A doctor may prescribe stronger pain relief medication for severe joint and muscle pain in the advanced stages.
Scopolamine, worn as a patch, can help manage drooling.
Antidepressants may help with episodes of uncontrollable laughter or crying, which doctors call emotional lability.
Assistive devices and therapies
In time, a person may need special devices for:
- moving around
- communicating with others
- feeding and swallowing
Speech and language therapy can help with communication and swallowing.
Physical and occupational therapy can help maintain mobility and function and encourage people to find new ways to do specific tasks.
There are different types of MND, and the outlook varies widely, depending on the type of condition.
People with spinobulbar muscular atrophy, for example, can expect to have a normal lifespan. For other types, people may have a reduced life expectancy.
New medications are emerging that may help slow the progression of MND. Researchers are also investigating how to use stem cells to repair the damage that occurs with MND, in the hope of one day producing a cure.
A better knowledge of genetic factors might also make it possible to prevent MND in those with inherited traits.