What is muscular dystrophy?
Muscles, attached to bones through tendons, are responsible for movement in the human body. In muscular dystrophy, muscles become progressively weaker. As individual muscle fibers become so weak that they die, they are replaced by connective tissue, which is fibrous and fatty rather than muscular. These replacement fibers are commonly found in skin and scar tissue and are not capable of movement, and the muscles become progressively weaker. There are several different recognized types of muscular dystrophy. These have in common degeneration of muscle fibers and their replacement with connective tissue. They are distinguished from one another on the basis of the muscle group or groups involved and the age at which individuals are affected.
The most common type is Duchenne muscular dystrophy. In this disease, the muscles involved are in the upper thigh and pelvis. The disease strikes in early childhood, usually between the ages of four and seven. It is known to be genetic and occurs only in boys. Two-thirds of affected individuals are born to mothers who are known to carry a defective gene; one-third are simply new cases whose mothers are genetically normal. Individuals afflicted with Duchenne muscular dystrophy suffer from weakness in their hips and upper thighs. Initially, they may experience difficulty in sitting up or standing. The disease progresses to involve muscle groups in the shoulder and trunk. Patients lose the ability to walk during their early teens. As the disease progresses, portions of the brain become affected, and intelligence is reduced. Muscle fibers in the heart are also affected, and most individuals die by the age of twenty.
The dystrophin gene normally produces a very large protein called dystrophin that is an integral part of the muscle cell membrane. In Duchenne muscular dystrophy, a defect in the dystrophin gene causes no dystrophin or defective dystrophin to be produced, and the protein will be absent from the cell membrane. As a result, the muscle fiber membrane breaks down and leaks, allowing fluid from outside the cell to enter the muscle cell. In turn, the contents of affected cells are broken down by other chemicals called proteases that are normally stored in the muscle cell. The dead pieces of muscle fiber are removed by scavenging cells called macrophages. The result of this process is a virtually empty and greatly weakened muscle cell.
A second type is Becker’s muscular dystrophy, which is similar to the Duchenne form of the disease. Approximately three in two hundred thousand people are affected, and it too is found only among males. The major clinical difference is the age of onset. Becker’s muscular dystrophy typically first appears in the early teenage years. The muscles involved are similar to those of Duchenne muscular dystrophy, but the course of the disease is slower. Most individuals require the use of a wheelchair in their early thirties and eventually die in their forties.
Myotonic dystrophy is a form of muscular dystrophy that strikes approximately five out of one hundred thousand people in a population. Myotonia is the inability of a muscle group to relax after contracting. Individuals with myotonic dystrophy experience this difficulty in their hands and feet. On average, the disease first appears at the age of nineteen. The condition is benign, in that it does not shorten an affected person’s life span. Rather, it causes inconveniences to the victim. Affected persons also experience a variety of other problems, including baldness at the front of the head and malfunction of the ovaries and testes. The muscles of the stomach and intestines can become involved, leading to a slowing down of intestinal functions and diarrhea.
Another type is limb girdle muscular dystrophy. The muscles of both upper and lower limbs—the shoulders and the pelvis—are involved. The onset of this dystrophy form is variable, from childhood to middle age. While the disorder is not usually fatal, it does progress, and victims experience severe disability about twenty years after the disease first appears. While this variant is also genetically transmitted, men and women are about equally affected.
One type of muscular dystrophy found almost exclusively among individuals of Scandinavian descent is called distal dystrophy. It first appears relatively early in adult life, between the thirties and fifties. The muscles of the forearm and hand become progressively weaker and decrease in size. Eventually, the muscles of the lower leg and foot also become involved. This form of muscular dystrophy is not usually fatal.
Oculopharyngeal muscular dystrophy is a particularly serious form that involves the muscles of the eyes and throat. In this disease, victims are affected in their forties and fifties. There is progressive loss of control of the muscles that move the eyes and loss of the ability to swallow. Death usually results from starvation or from pneumonia acquired when the affected individual accidentally inhales food or drink.
A type of muscular dystrophy for which the location of the genetic abnormality is known is facioscapulohumeral muscular dystrophy; the defect is confined to the tip of the fourth chromosome. This disease initially involves the muscles of the face and later spreads to the muscles of the posterior or back of the shoulder. Eventually, muscles in the upper thigh are involved. The affected person loses the ability to make facial expressions and assumes a permanent pout as a result of loss of muscle function. As the condition advances, the shoulder blades protrude when the arms are raised. Weakness and difficulty walking are eventually experienced. As with other forms of muscular dystrophy, there is some variability in the degree to which individuals are affected. Occasionally, a variety of deafness occurs involving the nerves that connect the inner ear and the brain. Less commonly, victims become blind.
There are other variants of muscular dystrophy that have been recognized and described. These forms of the disease, however, are rare. The main problem facing physicians is differentiating accurately the variety of muscular dystrophy seen in a particular patient so as to arrive at a correct diagnosis.
The diagnosis of muscular dystrophy is initially made through observation. Typically, parents notice changes in their affected children and bring these concerns to the attention of a physician. The physician takes a careful family history and then examines a suspected victim to make a tentative or working diagnosis. Frequently, knowledge of other family members with the condition and observations are sufficient to establish a firm diagnosis. Occasionally, a physician may elect to order physiological or genetic tests to confirm the tentative diagnosis. As Duchenne muscular dystrophy is the most common form of muscular dystrophy, it provides a convenient example of this process.
A diagnosis of Duchenne or any other form of muscular dystrophy is rarely made before the age of three. This form of the disease almost always occurs in boys. (Variants, rather than true Duchenne muscular dystrophy, are seen in girls, but this situation is extremely rare.) The reason for this finding is that the genetic defect occurs on the X chromosome, of which males only possess one. Approximately two-thirds of all victims inherit the defective chromosome from their mothers, who are asymptomatic carriers; thus, the condition is recessive and said to be X-linked. The disease occurs in the remaining one-third of victims as a result of a fresh mutation, in which there is no family history of the disease and the parents are not carriers.
Victims usually begin to sit, walk, and run at an older age than normally would be expected. Parents describe walking as waddling rather than the usual upright posture. Victims have difficulty climbing stairs. They also have apparently enlarged calf muscles, a finding called muscular hypertrophy. While the muscles are initially strong, they lose their strength when connective and fatty tissues replace muscle fibers. The weakness of muscles in the pelvis is responsible for difficulties in sitting and the unusual way of walking. Normal children are able to go directly from a sitting position to standing erect. Victims of Duchenne muscular dystrophy first roll onto their stomachs, then kneel and raise themselves up by pushing their hands against their shins, knees, and thighs; they literally climb up themselves in order to stand. These children also have a pronounced curvature of their lower backs, an attempt by the body to compensate for the weakness in the muscles of the hips and pelvis.
There is frequently some weakness in the muscles of the shoulder. This finding can be demonstrated by a physician, but it is not usually seen by parents and is not an early problem for the victim. A physician tests for this weakness by lifting the child under the armpits. Normal children will be able to support themselves using the muscles of the shoulder. Individuals with Duchenne muscular dystrophy are unable to hold themselves up and will slip through the physician’s hands. Eventually, these children will be unable to lift their arms over their heads. Most victims of Duchenne muscular dystrophy are unable to walk by their teen years. The majority die before the age of twenty, although about one-quarter live for a few more years. Most victims also have an abnormality in the muscles of the heart that leads to decreased efficiency of the heart and decreased ability to be physically active; in some cases, it also causes sudden death. Most victims of Duchenne muscular dystrophy suffer mental impairment. As their muscles deteriorate, their measured intelligence quotient (IQ) drops approximately twenty points below the level that it was at the onset of the disease. Serious mental handicaps are experienced by about one-quarter of victims.
Other forms of muscular dystrophy are similar to Duchenne muscular dystrophy. Their clinical courses are also similar, as are the methods of diagnosis. The critical differences are the muscles involved and the age of onset.
Laboratory procedures used to confirm the diagnosis of muscular dystrophy include microscopic analysis of muscle tissue, measurement of enzymes found in the blood, and measurement of the speed and efficiency of nerve conduction, a process called electromyography. Some cases have been diagnosed at birth by measuring a particular enzyme called creatinine kinase. It is possible to diagnose some types of muscular dystrophy before birth with chorionic villus sampling or amniocentesis.
There is no specific treatment for any of the muscular dystrophies. Physical therapy is frequently ordered and used to prevent the remaining unaffected muscles from losing their tone and mass. In some stages of the disease, braces, appliances, and orthopedic surgery may be used. These measures do not reverse the underlying pathology, but they may improve the quality of life for a victim. The cardiac difficulties associated with myotonic dystrophy may require treatment with a pacemaker. For victims of myotonic dystrophy, some relief is obtained by using drugs; the most commonly used pharmaceuticals are phenytoin and quinine. The inability to relax muscles once they are contracted does not usually present a major problem for sufferers of myotonic dystrophy.
More useful and successful is prevention, which involves screening individuals in families or kinship groups who are potential carriers. Carriers are persons who have some genetic material for a disease or condition but lack sufficient genes to cause an apparent case of a disease or condition; in short, they appear normal. When an individual who is a carrier conceives a child, however, there is an increased risk of the offspring having the disease. Genetic counseling should be provided after screening, so that individuals who have the gene for a disease can make more informed decisions about having children.
Chemical tests are available for use in diagnosing some forms of muscular dystrophy. Carriers of the gene for Duchenne muscular dystrophy can be detected by staining a muscle sample for dystrophin; a cell that is positive for Duchenne muscular dystrophy will have no stained dystrophin molecules. The dystrophin stain test is also used to diagnose Becker’s muscular dystrophy, but the results are not quite as consistent or reliable. Approximately two-thirds of carriers and fetuses at risk for both forms of muscular dystrophy can be identified by analyzing DNA. Among individuals at risk for myotonic dystrophy, nine out of ten who carry the gene can be identified with DNA analysis before they experience actual symptoms of the disease.
Muscular dystrophy has been recognized as a medical entity for several centuries. Initially, it was considered to be a degenerative disease only of adults, and it was not until the nineteenth century that the disease was addressed in children with Guillaume-Benjamin-Amand Duchenne’s description of progressive weakness of the hips and upper thighs. An accurate classification of the various forms of muscular dystrophy depended on accurate observation and on the collection of sets of cases. Correct diagnosis had to wait for the development of accurate laboratory methods for staining muscle fibers. The interpretation of laboratory findings depended on the development of biochemical knowledge. Thus, much of the integration of knowledge concerning muscular dystrophy is relatively recent.
Genes play an important role in the understanding of muscular dystrophy. All forms of muscular dystrophy are hereditary, although different chromosomes are involved in different forms of the disease. The development of techniques for routine testing and diagnosis has also occurred relatively recently. Specific chromosomes for all forms of muscular dystrophy have not yet been discovered. Considering initial successes of the Human Genome Project, an effort to identify all human genes, it seems likely that more precise genetic information related to muscular dystrophy will emerge.
There still are no cures for muscular dystrophies, and many forms are relentlessly fatal. Cures for many communicable diseases caused by bacteria or viruses have been discovered, and advances have been made in the treatment of cancer and other degenerative diseases by identifying chemicals that cause the conditions or by persuading people to change their lifestyles. Muscular dystrophy, however, is a group of purely genetic conditions. Many of the particular chromosomes involved are known, but no techniques are yet available to cure the disease once it is identified.
The availability of both a mouse model and a dog model of Duchenne muscular dystrophy, however, has facilitated the testing of gene therapy for this disease. Dystrophic mouse early embryos have been cured by injection of a functional copy of the dystrophin gene; however, this technique must be performed in embryos and is not useful for human therapy. Two avenues of research under way in these animal models are the introduction of normal muscle-precursor cells into dystrophic muscle cells and the direct delivery of a functional dystrophin gene into dystrophic muscle cells. It is hoped that these studies will lead to a cure for the disease.
In the meantime, muscular dystrophy continues to cause human suffering and to cost victims, their families, and society large sums of money. The disease is publicized on an annual basis via efforts to raise money for research and treatment, but there is little publicity on an ongoing basis. For these reasons, muscular dystrophy remains an important medical problem in contemporary society.
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