Overview of muscular dystrophies

Patient information: Overview of muscular dystrophies

MUSCULAR DYSTROPHY OVERVIEW — Muscular dystrophy is an inherited disorder that causes progressive muscle weakness (myopathy) and atrophy (loss of muscle mass) due to defects in one or more genes required for normal muscle function. Some of the genes responsible for these conditions have been identified.

There are a number of different types of muscular dystrophy (table 1). The primary symptom for most types is muscle weakness, although some dystrophies also cause heart disease or reduced mental ability. The diseases are distinguished from one another by the type of symptoms and the nature of the genetic abnormality causing the disorder.

MUSCULAR DYSTROPHY GENETICS — The genetic defect that causes muscular dystrophy is passed from one or both parents to a child by a specific pattern of inheritance that varies from one type of muscular dystrophy to another. A brief explanation of how genes are inherited will assist in explaining how children develop muscular dystrophy.

In the process of reproduction, a male’s sperm and female’s egg each contributes one sex chromosome. Males have XY sex chromosomes while females have XX sex chromosomes; the male can contribute the X or Y chromosome, while the female must contribute one of their X chromosomes. A male infant results if the male contributes his Y chromosome while a female infant results if he contributes his X chromosome .

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Duchenne and Becker muscular dystrophy and some types of Emery-Dreifuss muscular dystrophy are caused by mutations on one of the X chromosomes carried by the female parent. These muscular dystrophies affect 50 percent of male infants of mothers who carry the genetic defect; this is called X-linked inheritance. Females who inherit their mother’s defective X chromosome are usually disease free, although mild symptoms can occur occasionally (figure 2) (see ‘Female DMD carriers’ below).
Myotonic muscular dystrophy, facioscapulohumeral muscular dystrophy, some types of congenital muscular dystrophy, one type of Emery-Dreifuss muscular dystrophy, and some types of limb-girdle muscular dystrophy can develop if either of the parent’s chromosomes carries the defect and is passed to the infant; this is called autosomal dominant inheritance.
Most types of congenital muscular dystrophy, a rare form of Emery-Dreifuss muscular dystrophy, and some types of limb-girdle muscular dystrophy develop if both the mother and father pass a defective chromosome to their infant; this is called autosomal recessive inheritance.
MUSCULAR DYSTROPHY MUSCLE FUNCTION — Normally, electrical signals travel from the brain through the spinal cord and into the nerves that lead to the muscles. From there, the signals are transmitted into the muscles, where they stimulate muscle tissue to contract. Along the way, the nerves must be in good working order, and the muscles must be able to accept the impulses and generate a response.

Problems can occur anywhere along this route. If the brain, spinal cord, or nerves are damaged or diseased, the electrical signal may not be generated or get through. If muscles are inflamed or abnormal, they may not be able to respond properly to a nerve impulse.

Diseases of muscles that cause weakness are called myopathies. With myopathy, nerve impulses usually reach the muscles without any problem, but the muscles are unable to respond in a normal way due to the disease.

Many things can cause myopathy to occur, a few of which include:

Inflammation of the muscles
Inherited conditions, particularly muscular dystrophies
Problems with certain hormones that affect muscle function
Chemical imbalances in the body
Drugs and toxic substances
Infections
MUSCULAR DYSTROPHY TESTS — A number of tools can be used to diagnose muscular dystrophy, including genetic testing, blood tests that identify the signs of muscle damage, electromyography (EMG), muscle biopsy, electrocardiogram (ECG), and/or echocardiogram (ECHO), which is an ultrasound of the heart. Tests are chosen based upon the type of disease that is suspected. This is particularly true of genetic tests, which must be tailored to search for specific abnormalities.

Electromyography — Electromyography (EMG) is a test that evaluates the electrical activity of muscles both at rest and during voluntary movement. Abnormalities in this activity can indicate the presence of a muscle or nerve disease.

A small needle is inserted through the skin into a muscle in several locations (usually the arms and legs). The needle is connected to a recording device that displays the muscle’s electrical activity at rest and in response to contraction. The electrical activity may also be heard as a static-like noise through a speaker.

The size and pattern of electrical activity recorded from different muscles is analyzed by the physician who supervises the EMG test to determine whether the muscles and nerves are functioning normally.

The test generally takes about 30 minutes. A patient may feel some discomfort as the needle is inserted, and the muscle may feel sore or bruised for several days. EMG is very useful in determining whether there is weakness due to muscle disease. However, it does not provide a specific diagnosis; genetic testing or muscle biopsy may be more useful if muscle weakness is obvious.

Muscle biopsy — A muscle biopsy is a procedure that removes one or more small piece(s) of muscle from either an arm or leg. Muscle biopsies can sometimes provide a specific diagnosis. However, muscle biopsies are done less frequently than previously because of the increased availability of genetic testing.

The biopsy may be done by making a small incision and removing a piece of muscle or by using a thick needle (called a punch biopsy), which removes a small cylindrical piece of muscle. The type of biopsy used will depend upon the physician’s preference and need to examine the muscle directly before biopsy. Both types of biopsy can be done in an office or clinic setting, usually with local anesthesia (medication is injected to numb the area).

DUCHENNE MUSCULAR DYSTROPHY — Duchenne muscular dystrophy (DMD) is caused by a defective gene located on the X chromosome. This gene is responsible for producing a protein called dystrophin, which normally functions to protect muscle fibers. Without dystrophin, muscles are broken down by enzymes, which cause degeneration and ultimately weakness of muscles.

DMD occurs in 1 in 3500 (0.02 percent) of all males. It is primarily seen in boys, and occurs in all ethnic groups.

Female DMD carriers — Most female carriers of the abnormal dystrophin gene have few or no symptoms of their disease. Muscle weakness develops in 2.5 to 20 percent of females. Symptoms may present early in life and become progressive (worsen) in certain situations.

Women who have no symptoms but who have a family history of muscular dystrophy and want to become pregnant can consider specialized testing; this can help to determine if the woman is a carrier of the defective gene. However, testing is not always accurate in identifying female carriers. Any woman with a family history of muscular dystrophy should consider genetic counseling to discuss the probability of having an affected child.

If testing confirms that the woman is a carrier, she can consider preimplantation genetic diagnosis to avoid passing the defective gene to her child. (See “Preimplantation genetic diagnosis”.)

Symptoms — Symptoms of DMD usually appear in children between age two and three years. Children are usually wheelchair bound by age 12 to 15 years. These ages represent the average for all patients with DMD; individual patients may develop symptoms earlier or later in life.

Weakness starts near the trunk, and spreads to the extremities, affecting the legs before the arms. A child may have difficulty running, jumping, and walking up steps, and may use their hands to push upright from squatting or sitting (called Gower’s sign).

DMD may also cause heart problems, including enlargement of the heart tissue (dilated cardiomyopathy) and irregular heartbeat. Abnormal curvature of the spine (scoliosis) combined with muscle weakness can lead to problems with lung function. Fractures involving the arms and legs are frequent, usually caused by falling. In many cases, affected children have some degree of mental impairment, although some children have average or above-average intelligence. (See “Patient information: Dilated cardiomyopathy”.)

Management of DMD — Early identification of children who are affected by DMD is important to prevent complications and prolong life. Recommendations for managing and preventing problems in these children include:

Pneumococcal vaccine (given once) to help prevent pneumonia, and annual influenza vaccine for children six months of age and older. (See “Patient information: Vaccines for infants and children age 0 to 6 years”.)
Screening (usually by echocardiography or cardiac MRI) and early treatment for cardiomyopathy, starting at around age 10 years. Female carriers may be screened beginning in the teens. (See “Patient information: Dilated cardiomyopathy”.)
Lung treatment while sleeping (nocturnal noninvasive ventilation) or respiratory assistance during periods of lung infection is recommended. Lung function testing should begin around age 9 or 10 years, before the child becomes wheelchair bound, and should be repeated several times per year when lung function worsens or the child requires a wheelchair.
Bone density should be maintained to reduce the risk of fractures by ensuring that the child has a diet rich in calcium and vitamin D. Parents should monitor their child’s weight to avoid excessive weight gain and obesity; a nutritionist can assist with food planning.
Physical therapy can help to maintain muscle function and prevent joint stiffening (contractures). Leg braces can assist with standing and walking. X-rays are taken periodically to monitor for changes in the bones.
Surgery can help correct joint contractures, especially at the ankle. Lengthening and transferring tendons can return the joint to its normal position and prolong walking with, and sometimes even without, braces. Spinal fusion operations have been very effective in correcting scoliotic deformities and maintaining upright sitting position in a wheelchair.
Scoliosis, or curvature of the spine, is a common complication of DMD. When severe, it can cause breathing problems, so that surgical treatment may be necessary. In less severe cases, braces may be effective.
Treatment of DMD — Steroids (such as prednisone or deflazacort) are the primary treatment for DMD and are generally offered to boys who are over the age of five years. Steroids have been proven to significantly increase strength, muscle function, and lung function.

Regular visits with a healthcare provider are needed to monitor the benefits and potential side effects of prednisone treatment. Side effects can include weight gain, swelling in the face, decrease in height, osteoporosis, long bone and vertebral bone fractures, acne, excessive hair growth, gastrointestinal symptoms, and behavioral changes. The dosage can be adjusted if excessive weight gain occurs.

Treatments other than steroids have not been extensively studied or proven to be effective and safe.

Prognosis — In early studies, most patients with DMD died in their late teens or twenties as a result of respiratory infections or cardiomyopathy. However, later studies have reported a median survival to age 35 years. Thus, survival appears to be improving with advances in respiratory care and cardiac care. Again, these ages represent the average for all patients with DMD; individual patients may have complications earlier or later in life. (See ‘Life with muscular dystrophy’ below.)

BECKER MUSCULAR DYSTROPHY — The pattern of inheritance of Becker muscular dystrophy (BMD) is the same as DMD, with the abnormal gene carried by the mother. It also occurs mainly in boys. BMD is also caused by problems with the dystrophin gene, although people with BMD often have partial production of dystrophin.

Female BMD carriers — Female carriers of the abnormal dystrophin gene that causes BMD are similar to those that carry the DMD gene (see ‘Female DMD carriers’ above).

Symptoms — Compared with DMD, the age of onset for children with BMD is usually later and symptoms are usually milder. Children can usually walk until they are approximately 15 years old, with some people continuing to walk as adults. Mental impairment is less common than with DMD. Similarly, heart problems are not common; when they occur, they can be more severe in patients with BMD than in those with DMD.

Management of BMD — Management of patients with BMD is similar to that of DMD (see ‘Management of DMD’ above).

Treatment of BMD — Prednisone may be used for treatment of BMD, although little is known about its effect on the symptoms and disease progression (see ‘Treatment of DMD’ above).

Prognosis — Patients with BMD usually survive into their mid 40s. The most common cause of death is heart failure from cardiomyopathy.

BMD/DMD OUTLIERS — Some patients are found to have the genetic abnormalities of the dystrophin gene, but, clinically, have symptoms between those of DMD or BMD. These so-called “outliers” may be thought of as having mild DMD or severe BMD. They usually become confined to a wheelchair between the ages of 12 and 16 years.

EMERY-DREIFUSS MUSCULAR DYSTROPHY — Emery-Dreifuss muscular dystrophy (EDMD), also known as humeroperoneal muscular dystrophy, can be caused by a number of inheritance patterns. It can affect both boys and girls.

Symptoms of muscle weakness and loss typically begin in the arms between 10 and 20 years of age. Leg muscle weakness follows and, in some cases, mild facial weakness is also seen. The disease tends to progress slowly. Contractures of the elbows are sometimes the first sign of disease. Involvement of the heart is also common, and can lead to serious problems with the regulation of heartbeat.

EDMD is usually diagnosed based upon signs and symptoms, family history, and genetic testing. Treatment is focused on preventing death from heart disease. A pacemaker can be lifesaving in patients with arrhythmia. (See “Patient information: Pacemakers”.) Physical therapy is useful in preventing contractures.

MYOTONIC DYSTROPHY — Myotonic dystrophy is the most common form of adult-onset muscular dystrophy. It can affect both boys and girls. Symptoms typically occur in adolescence or adulthood, although a newborn form does occur. There are two genetic forms of myotonic dystrophy, type 1 and type 2.

Myotonic dystrophy affects multiple body systems, causing muscle loss and weakness, especially in the facial muscles, arms, and legs. In addition, myotonic dystrophy can cause heart problems, cataracts, problems with glands, and abnormal intellectual functioning. In some cases, there are joint problems or difficulty swallowing. Excessive daytime sleepiness is found in about one-third of patients with myotonic dystrophy.

Myotonia, or delayed muscle relaxation after contraction, is also seen, but this problem typically does not require treatment. Pulmonary function tests, swallowing studies, and heart tests are often used in the management of myotonic dystrophy. People with myotonic dystrophy are treated as symptoms develop. Leg braces are often used as weakness in the legs and feet worsen.

Life expectancy appears to be reduced for people with myotonic dystrophy. Respiratory and cardiac diseases are the most common causes of death.

LIMB-GIRDLE MUSCULAR DYSTROPHY — Limb-girdle muscular dystrophy (LGMD) is a group of disorders that have a number of inheritance patterns. It refers to dystrophies that affect the shoulder girdle (or area surrounding the shoulder), the pelvic girdle (the area surrounding the hips), or both. Low back pain may be a prominent symptom.

The age of onset varies from early childhood to adulthood. The course is usually slowly progressive. Cognitive (brain) functioning is usually normal and cardiac involvement is rare.

Treatment of LGMD focuses on the prevention of contractures. Stretching exercises are often recommended after diagnosis, and are useful to prevent disabling symptoms.

FACIOSCAPULOHUMERAL MUSCULAR DYSTROPHY — Facioscapulohumeral muscular dystrophy (FSHD) can affect boys and girls. It is usually progresses slowly, but is extremely variable in its severity and age of onset. It is usually inherited in an autosomal dominant pattern, which means that a parent with the disease has a 50 percent chance of passing it to their child.

In the infant form, symptoms appear within the first few years of life. Most children require a wheelchair by the age of 9 or 10 years. There is profound facial weakness, with an inability to close the eyes in sleep, to smile, or to show other facial expressions. The weakness rapidly progresses to include the shoulder and hips. There may be marked weakness of the wrist. Children with early onset FSHD may have epilepsy, reduced mental ability, and severe hearing loss.

In the classic form, symptoms usually begin between 20 and 30 years of age. Progression is slow, and the life span is near normal. The facial muscles are involved initially, with inability to close the eyes tightly, smile, or whistle. There may be a pouting appearance to the face. The facial weakness can be mild and may remain mild for many years. The muscles of the shoulders and upper arms are usually involved.

In some of cases, the disease progresses rapidly in middle age, leading to significant disability. There may be problems with vision or hearing. The heart’s rhythm can be affected in some patients.

CONGENITAL MUSCULAR DYSTROPHY — Congenital muscular dystrophy (CMD) refers to cases of muscular dystrophy that are apparent at birth. There are several types, including a classic form, Fukuyama type (seen primarily in Japan), Ullrich congenital muscular dystrophy, Bethlem myopathy, Walker-Warburg syndrome, and muscle-eye-brain disease.

Symptoms of CMD are usually seen at birth, and include lack of muscle strength (“floppy baby”) and multiple joint contractures (arthrogryposis). No definitive treatment is available for CMD.

LIFE WITH MUSCULAR DYSTROPHY — Many children and adults with muscular dystrophy can lead active lives. However, the diagnosis of muscular dystrophy can be overwhelming for a parent and a child. Support and education from healthcare providers and community organizations can help a family to provide their child with the best possible care. Organizations such as the Muscular Dystrophy Association (www.mdausa.org) have a large network of support services and information available. A week-long summer camp is available in many locations throughout the United States.

Parents are often unsure about how to discuss muscular dystrophy with their child. Because it is a slowly progressing disease in most cases, parents are not obligated to discuss their child’s long term prognosis until the child is mature enough to understand the implications of their diagnosis. Most children have an understanding of death by the age of 7 years, but even before this point, children want and deserve to know basic information about their disease and treatments that may be needed. In the terminal stages of illness, adolescents and young adults may want to participate in the decision making processes about end of life care, resuscitation options, and the location of their care.

WHERE TO GET MORE INFORMATION — Your healthcare provider is the best source of information for questions and concerns related to your medical problem.

 

Related topics for patients, as well as selected articles written for healthcare professionals, are also available. Some of the most relevant are listed below.

Patient Level Information:

Patient information: Dilated cardiomyopathy
Patient information: Vaccines for infants and children age 0 to 6 years
Patient information: Pacemakers

Professional Level Information:

Clinical features and diagnosis of Duchenne and Becker muscular dystrophy
Emery-Dreifuss muscular dystrophy
Facioscapulohumeral, oculopharyngeal, distal, and congenital muscular dystrophies
Limb-girdle muscular dystrophy
Myotonic dystrophy: Etiology, clinical features, and diagnosis
Treatment of Duchenne and Becker muscular dystrophy
Preimplantation genetic diagnosis

The following organizations also provide reliable health information.

National Library of Medicine
(www.nlm.nih.gov/medlineplus/musculardystrophy.html)

National Institute of Neurological Disorders and Stroke
(www.ninds.nih.gov/disorders/md/md.htm)

Muscular Dystrophy Association
(www.mdausa.org)

Gene Reviews (discusses genetic testing)
(www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=gene&part=dbmd)

REFERENCES
Emery AE. The muscular dystrophies. Lancet 2002; 359:687.
Finder JD, Birnkrant D, Carl J, et al. Respiratory care of the patient with Duchenne muscular dystrophy: ATS consensus statement. Am J Respir Crit Care Med 2004; 170:456.
Moxley RT 3rd, Ashwal S, Pandya S, et al. Practice parameter: corticosteroid treatment of Duchenne dystrophy: report of the Quality Standards Subcommittee of the American Academy of Neurology and the Practice Committee of the Child Neurology Society. Neurology 2005; 64:13.
Darras BT, Korf BR, Urion DK. Dystrophinopathies. In: GeneReviews at GeneTests: Medical Genetics Information Resource [database online]. Copyright, University of Washington, Seattle. 1997-2008. Available at file://www.genetests.org.

Author
Basil T Darras, MD
Section Editor
Jeremy M Shefner, MD, PhD
Deputy Editor
John F Dashe, MD, PhD