What are motor neuron diseases?

Causes and Symptoms

In motor neuron diseases, certain nerves die, specifically those that allow any and all body movement. The actual cause of spontaneous motor neuron death is unknown, but genetic defects, neurotoxins, viruses, autoimmune disruptions, and metabolic disorders are contributing factors.

The predominant features of motor neuron disorders are muscular weakness, muscular wasting, and the presence of fasciculations. As a nerve dies, it can no longer effectively innervate its target muscle, but neighboring nerves may sprout to keep the muscle active. A consequence of nerve sprouting is the onset of brief, spontaneous contractions, or twitches. These visible twitches are called fasciculations. Eventually, as increasing numbers of nerves die, fewer healthy nerves are left to sprout until, finally, all muscles are denervated. Dead nerves cannot prompt muscle movement, nor can they release tropic factors as they do in health. This loss of tropic input from the neuron causes muscular
atrophy and renders the muscle useless.

Motor neuron diseases are usually first noticed in the hands or upper limbs, where muscle weakness and decreased ability to use arms or hands cause problems. Unlike some disorders, motor neuron diseases fail to show stages of exacerbation or remission. Rather they progress—either rapidly or slowly, but relentlessly—until death, usually as a result of respiratory complications.

Although there are childhood forms, motor neuron diseases are more likely to strike between the ages of fifty and fifty-five, and they are seen in males more than females by a ratio of 1.5 to 1. Motor neuron diseases seem to occur rarely in the obese person and tend to afflict otherwise healthy, thin, and perhaps athletic persons. A famed person afflicted by the debilitating motor neuron disease
amyotrophic lateral sclerosis (ALS) was baseball player Lou Gehrig, in whose memory it is often called Lou Gehrig’s disease.

Motor neuron diseases are often subgrouped into three categories: ALS, progressive spinal muscular atrophy, and progressive bulbar (brain-stem) palsy. In the plural form, motor neuron diseases refer to all forms of the affliction, whereas the singular form, motor neuron disease, is synonymous with ALS.

Amyotrophic lateral sclerosis is the most familiar of the motor neuron diseases primarily because it accounts for a full 60 percent of all such disorders. The name has clinical meaning: “Amyotrophy” refers to the loss of muscle bulk as a result of missing tropic factors from dying or dead neurons, “lateral” refers to the locations within the spinal cord that are affected, and “sclerosis” refers to the hardened quality of the lateral regions of the diseased spinal cord, which otherwise would be soft tissue. The brain stem may also be sclerotic (hardened). ALS has an incidence of 1 or 2 persons per 100,000, although some Pacific islands, such as Guam, seem to have a higher incidence attributable to undetermined genetic factors. In addition, some populations show an autosomal dominant genetic component. ALS is fatal, and death generally occurs as a result of respiratory failure within three to five years after the onset of symptoms.

ALS is characterized by upper and lower motor neuron signs of neural death; thus the presence of both fasciculations and spasticity is required for a diagnosis. Spasticity is a medical term that describes a certain kind of muscular resistance (stiffness) to movement. In particular, spastic means a resistance that increases the more rapidly a muscle is extended; tendon reflexes are also hyperactive and Babinski’s sign (abnormal reflexes of the toes) must be present. Babinski’s sign reveals the death of neurons in the corticospinal tracts, which signals the occurrence of upper motor neuron death. The presence of fasciculations reveals lower motor neuron death.

Progressive spinal muscular atrophy (SMA) will show only lower motor neuron signs—namely, muscular weakness, fasciculations, and atrophy. Babinski’s sign or spasticity is not found. The early symptoms may include increased clumsiness in using the fingers for fine movements (including writing or using kitchen utensils), stiffness of the fingers and hands, and cramping of the upper and lower limbs. Once the brain-stem nerves become involved, difficulty in speaking and swallowing occur. Of all persons afflicted with one of the motor neuron diseases, 7 to 15 percent will have lower motor neuron signs only and are presumed to have the progressive spinal muscular atrophy form.

Progressive bulbar palsy literally means progressive brain-stem paralysis. This form of motor neuron disease accounts for 20 to 25 percent of all cases. The tongue is usually the first place to show muscular wasting and fasciculations. As the nerves controlling the tongue die, the tongue shrivels and shrinks so that speaking, chewing, and moving solids or liquids to the back of the mouth for swallowing become difficult or impossible.

Children can be afflicted with spinal muscular atrophy. This disease is believed by many experts to be completely unique from the adult form. The childhood form seems to be more associated with environmental and genetic factors. (This concept is greatly debated, however, since the actual cause of any of the motor neuron diseases is unknown.) Three forms of childhood SMA have been identified: type 1, or acute infantile SMA (also known as Werdnig-Hoffman disease); type 2, or intermediate SMA; and type 3, or juvenile SMA (also known as Kugelberg-Welander disease).

Of children afflicted with SMA, 25 percent have type 1. This form of the disease is an autosomal recessive genetic disorder that occurs in 1 of 15,000 to 25,000 births. In an experienced mother, there may be awareness of minimal fetal movement in the last trimester of pregnancy; the fetus tends to stay still as a result of muscular weakness. Upon birth, the newborn may be a “floppy” baby of great weakness and may immediately have trouble with nursing and breathing. In other cases, it may take three to six months before symptoms begin. Because of the eventual weakening of the muscles of respiration, the child becomes prone to respiratory infections that cannot be cleared because of a lost cough reflex. Death usually occurs at two to three years of age.

When a child fails to stand or walk between six to twelve months of age, the physician considers the possibility that the child has type 2 SMA. An abnormal curvature of the spine to the forward and sideways position (kyphoscoliosis) is often seen, but rarely is there any problem with feeding or breathing. It is generally the case that very fine tremors of the child’s hands can be noticed, and sometimes contractures of the hips and knees can occur. There is no delay in terms of mental health or intellect for these children.

Type 3 SMA is most often seen in the adolescent, but this disease can be observed in some children as early as five years of age. The predominant feature is weakness of the hip muscles. Since these children have been walking for some time, a change in their walking gait to a waddle can be seen over the course of years. Most people with type 3 SMA must use wheelchairs in their mid-thirties, but some may lose their ability to walk earlier. Type 3 SMA has been shown to be an autosomal recessive disorder in many cases, but there are also reported cases of sporadic occurrences within families that have previously been unaffected. Clearly, there are unanswered questions about this disease.

It should be noted that controversy abounds on the assigned classifications of motor neuron diseases. This controversy arises from the fact that the origins of the diseases are not known. Since cause has not been established for any form of motor neuron disease, physicians must use clusters of symptoms to sort the differences in disease manifestation. This sorting is used to plan the best possible treatment programs for the circumstances; nevertheless, these distinctions may seem arbitrary once more is known about the causes of motor neuron death.

Treatment and Therapy

Perhaps one of the most frustrating attributes of motor neuron diseases is that neither prevention nor effective treatment and cures are available. For a person living with motor neuron disease, physicians and health care professionals must work as a team to manage the symptoms of the diseases and offer palliative care.

In general, patients are encouraged to use and exercise their muscles cautiously in order to avoid disuse atrophy, but activity to the point of fatigue is forbidden since it is believed to aggravate the progression of muscular wasting. In addition, exposure to cold may worsen muscular contractures. Physical therapy facilitates a delay in the total loss of willed body movement by allowing the use of braces, walkers, and wheelchairs as modes of locomotion. Adults are encouraged to continue nonexertive work for as long as possible; it aids both the body and the mind to maintain independence and a sense of wholeness, well-being, and dignity.

As muscular control of the voice wanes, sketch pads, word boards, and computers can aid the ill person in communicating with loved ones, doctors, nurses, and colleagues. In addition, respiratory therapy aids in maintaining healthy breathing in spite of ever-weakening respiratory muscles. Prophylactic immunizations for influenza and pneumococci are given, especially to those who are wheelchair-dependent or bedridden. Forced deep breathing and coughing are needed at least once every four hours to bring up any congestion that may otherwise lead to grave consequences. Almost all persons with motor neuron diseases die from respiratory insufficiency. For this reason, it is imperative that the patient and physician discuss respiratory care early after diagnosis to determine whether the patient wants to be placed on mechanical ventilators in the later stages of the disease. Other issues such as tube feedings should be discussed while the patient is still able to voice an opinion and express any concerns about the dying process associated with the disease .

The past decade has seen a significant increase in the offering of fake treatments for motor neuron diseases by unaccredited organizations and physicians online. Many of these treatements offer patients relief from symptoms using human or animal stem cells. There remains no credible scientific evidence that stem cells of any kind are beneficial in the treatment of motor neuron disease. Reputable medical organization and law enforcement officials in the United States have stepped up efforts to combat scams and hoaxes offering false promises to patients and their familes.

Perspective and Prospects

Life can be socially difficult for people with motor neuron diseases. Others tend to assume that persons who must use wheelchairs and are unable to control mouth movements (so that speech and swallowing are lost and drooling may occur) are not intelligent, thinking, or aware. This is a sad misperception.

Many persons suffering from a motor neuron disease rise above its physical challenges to conquer in spirit that which the body cannot. For example, former United States senator Jacob Javits labored hard to improve the awareness of and funding for ALS in spite of being on a ventilator and completely immobile because of his battle with the disease. Another example of how well the intellect is preserved in this physically tragic disease can be seen in the life and work of the world-renowned astrophysicist Stephen Hawking.

Until there is an established cause or causes for these diseases, effective treatments or cures are likely to remain hidden. The research continues in the hope of pinning down the ever-elusive motor neuron diseases.

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