FSH muscular dystrophy
The term muscular dystrophy refers to a group of conditions characterized by progressive muscle weakness and atrophy (deterioration). Many different types of muscular dystrophy have been described, each of which have unique features and usually a unique underlying genetic cause. Facioscapulohumeral (FSH) muscular dystrophy affects the muscles of the face and shoulders first. Usually the first signs of weakness appear before the age of 20 years. The symptoms of FSH muscular dystrophy are variable and are not fatal. One in five people who are affected require a wheelchair after the age of 40 years.
Facio refers to the face, scapulo to the shoulder blades, and humeral to the bone of the upper arm. The symptoms of FSH muscular dystrophy are quite variable, even within the same family. Some individuals who have the altered DNA sequence never develop noticeable symptoms. Most people with the condition first notice weakness in their teenage years. Muscles of the shoulders and face are usually the first to be affected. These may remain the only parts of the body that are affected, or the weakness may progress to include the pelvic muscles, the lower limbs, and the hands. Intelligence and life expectancy are not affected.
FSH muscular dystrophy has autosomal dominant inheritance . This means that an affected person has a 50% chance, with each pregnancy, to pass the altered gene on to the child. Every person has two copies of every DNA sequence, one inherited maternally and the other inherited paternally. The altered DNA sequence that causes FSH muscular dystrophy is on chromosome 4. If a person has one normal sequence and one altered sequence, he or she will probably develop FSH muscular dystrophy.
When an autosomal dominant condition is present in multiple generations of a family, usually someone from each generation is affected. If a person is the first in his or her family to have an autosomal dominant condition, doctors often assume that the gene mutated for the first time in the egg or sperm that came together to make that person. (This is called a new mutation.) However, when the physical symptoms associated with an altered gene are highly variable, the distinction between these two scenarios is less obvious.
The term non-penetrance refers to altered genes that do not always cause a person to have the typical associated symptoms. FSH muscular dystrophy is non-penetrant in some individuals. Therefore, an individual who appears to be the first person affected in his or her family may have actually inherited the mutated DNA sequence from his or her mother or father. If so, his or her siblings would be at a 50% risk to also have inherited the altered sequence. Similarly, a mildly affected individual may have a child who is severely affected. Occasionally, two affected siblings are born to unaffected parents because of a genetic process called germline mosaicism.
Describing the genetics of FSH muscular dystrophy is slightly complicated by an interesting phenomenon. Genes are the DNA sequences that give the body instructions for growth, development, and functioning. Usually a mutation that causes a disease occurs in the gene associated with that disease. The above description refers to the mutation in FSH muscular dystrophy as an altered DNA sequence because it does not appear that this sequence is actually part of a gene. The mutated sequence affects the gene for FSH muscular dystrophy, but probably is not part of the gene itself.
The incidence of FSH muscular dystrophy is approximately 1/20,000. Some references report a lower incidence. Individuals from all ethnic groups are affected.
Signs and symptoms
The severity of the symptoms of FSH muscular dystrophy is highly variable. Some people are debilitated while others are minimally affected. Symptoms of progressive muscle weakness are usually first noticed in the teenage years, but may be noticed much later. For unknown reasons, more males than females with FSH muscular dystrophy develop symptoms by the age of 30 years. Specific muscle groups are affected. FSH muscular dystrophy does not lead to reduced sensation, nor does it affect intelligence.
Progressive muscle weakness of the shoulders/upper arms and face muscles are usually noticed first. The facial muscle weakness may be noticed as difficulty puckering the lips, smiling, sucking a straw, and closing the eyes while sleeping. Weakness may be asymmetrical, i.e., one shoulder may be weaker than the other shoulder. As the condition progresses, the muscles of the lower legs, abdomen, and hips may also become weak. The muscle weakness leads to abnormal positioning such as forward-sloping shoulders and exaggerated curvature of the spine. Although the weakness progresses continuously, the affected individual may perceive it as progressing rapidly at times and slowly at other times. This is because he or she notices the weakness when it results in loss of function. Reflexes are often weaker than normal. Twenty percent of affected individuals eventually require wheelchairs.
Describing the weakness as shoulder weakness or facial weakness is an oversimplification. In FSH muscular dystrophy, very specific muscles are affected. Not all of the facial muscles are affected, and not all of the muscles of the shoulder are affected. For example, the biceps and triceps of the upper arm are affected before the deltoids, and the forearm is relatively unaffected.
Some researchers report that more males than females with FSH muscular dystrophy develop symptoms by the age of 30 years. The reasons for this are unknown. Other researchers report that men and women are equally affected. Autosomal dominant conditions such as FSH muscular dystrophy usually affect men and women equally.
Many individuals with early–onset FSH muscular dystrophy develop hearing loss of the high tones. Some individuals have more significant hearing loss. Slight changes of the retina are also a symptom of FSH muscular dystrophy. These changes usually do not affect vision.
A subset of FSH muscular dystrophy patients are severely affected. Individuals with severe infantile FSH muscular dystrophy are symptomatic at birth.
The diagnosis of FSH muscular dystrophy is based on clinical history (symptoms), family history, and genetic testing . Many evaluations may be necessary to confirm the diagnosis. A thorough physical examination will be performed. Additional testing may include measuring the level of creatine kinase (CK) in the blood, special analysis of tissue obtained by muscle biopsy, and electromyogram (EMG). Sometimes it is difficult to rule out other possible causes of the muscle weakness.
Genetic testing is available for FSH muscular dystrophy, but it is complicated. Not everyone who is shown to have the associated abnormality of chromosome 4 develops symptoms of FSH muscular dystrophy. Alternately, not everyone who has FSH muscular dystrophy shows the typical genetic abnormality. Therefore, the test is helpful, but it must be interpreted in the context of the individual's medical history. A small subset of people tested will have inconclusive results. This is not due to lab error; some people have a genetic change that is midway between normal and abnormal.
Genetic testing can be performed on fetal cells that are obtained by amniocentesis , performed after the sixteenth week of pregnancy, or chorionic villus sampling (CVS). CVS is usually performed between 10 and 12 weeks of pregnancy.
Researchers have shown some correlation between the type of mutation in the FSH region of chromosome 4 and the severity of the disease. Abnormal genetic results fall into a range from nearly normal or far from normal. People with certain abnormal genetic testing results tend to have earlier onset of symptoms and more rapidly progressive muscle weakness. Although many researchers have observed this correlation, the cause and effect relationship is not clear.
Because of the variable severity of symptoms, assumptions should not be made about the family history. A thorough clinical examination by an experienced physician may show that a person believed to be unaffected actually has mild symptoms.
Treatment and management
As of 2001, there is no effective treatment, prevention, or cure for FSH muscular dystrophy. Available treatments help affected persons with the effects of the disease but do not treat the disease itself. Supportive therapies include orthodic devices such as splints and braces, and sometimes surgery. Physical and occupational therapy may be helpful to ease discomfort and adjust to physical changes. Researchers continue to study various medications. Previous studies indicated that prednisone may improve muscle strength. However, this was not confirmed in more recent studies. Another medication, albuterol, was shown to be beneficial in early studies. Preliminary results of follow–up studies will be available in 2001 or 2002.
The prognosis for FSH muscular dystrophy is extremely variable. Prognosis cannot be predicted based on family history. Most people remain ambulatory, but some do not. Progression is usually slow. One third of affected individuals over 40 years of age have mild symptoms. A few people with FSH muscular dystrophy never develop muscle weakness. The typical course is weakness that becomes noticeable before the age of 20 years and progresses slowly but continuously throughout life.
Although FSH muscular dystrophy is rare in the general population, it is a relatively common neuromuscular disorder. Identification of the altered DNA sequence associated with FSH muscular dystrophy has stimulated research efforts. If the mechanism underlying the disease practice is discovered, researchers can better study possible treatments.
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Michelle Queneau Bosworth, MS, CGC