Muscular dystrophies of latter onset
What is muscular dystrophy?
Muscular dystrophy (MD) is a general designation of a group of chronic, hereditary diseases characterised by the progressive degeneration and weakness of voluntary muscles.
At what age do signs of dystrophy appear?
Contrary to the widespread notion that muscular dystrophy is exclusively a childhood disorder, clinical onset may occur at any point in the life span. The different types of the disease vary in the age at which muscle wasting becomes manifest and in the muscle groups first affected. Muscular dystrophies of latter onset include limb-girdle MD, congenital MD, ophthalmoplegic MD and distal MD.
Does the rate of progression vary?
Degeneration of muscle in muscular dystrophy is a continuing process, with considerable variation in its rate and severity among the different forms of the disease. As a rule, it can be said that the earlier the clinical signs appear, the more rapid the progression and the more widespread and disabling the deterioration.
As muscles deteriorate, patients become weaker. In the severe forms of the disease, patients lose the power of ambulation and are confined to wheelchairs, and eventually to bed. In such cases, they are finally unable to carry out the simplest activities of everyday life. They cannot combat intercurrent infections, and death usually results from respiratory disease; it also may be precipitated by involvement of heart muscle.
How is a diagnosis of muscular dystrophy established?
The age of onset, distribution and severity of muscle weakness, and the pattern of inheritance indicated by a family history provide essential information in the diagnosis of muscular dystrophy. Examination of a muscle biopsy is the definitive procedure for confirming the presence of degeneration. Electromyography is also a valuable diagnostic tool, as is the measurement of various serum enzymes.
Is muscular dystrophy always inherited?
It is now well established that all forms of muscular dystrophy are hereditary conditions with the genetic defect transmitted by one parent in some types of the disease and by both parents in other types. However, there are many cases of muscular dystrophy in families with no known history of the disease. This is explained, in Duchenne dystrophy, by a high spontaneous mutation rate.
Is there any treatment for muscular dystrophy?
At this time, there is no known treatment that will arrest or reverse the dystrophic process, but medical management can increase mobility, maximise independence in daily activities, and ease the patient's discomfort. The use of orthopaedic devices and physiotherapy, for example, can keep patients ambulatory longer, minimise crippling contractures, and prevent or delay scoliosis (curvature of the spine).
What are the specific forms of muscular dystrophy?
The following descriptions summarise the major characteristics of the various types of muscular dystrophy.
Limb-girdle muscular dystrophy
Clinical onset of the disease occurs anywhere from the first to the third decade of life. The initial muscles affected are the proximal muscles of the pelvic and shoulder girdles. The progression of limb-girdle muscular dystrophy varies considerably, as does the degree of disability. Progression is sometimes quite slow and sometimes fairly rapid although never as rapid as in Duchenne dystrophy. When progression is slow, patients may have a normal life span.
The hereditary pattern in limb-girdle muscular dystrophy is autosomal recessive. Unless both parents carry the defective gene, none of their children will manifest the disease. When both parents carry the gene, each offspring has a 25 per cent probability of being clinically affected, a 50 per cent probability of being normal but carrying the defective gene, and a 25 per cent probability of being completely free of the hereditary defect. Sons and daughters are equally at risk.
Congenital muscular dystrophy
In this less familiar form of muscular dystrophy, the most progressive and active phase of muscle degeneration takes place during the fetal period, and the disease is already manifest at birth. Its essential features include hypotonia, muscle weakness, and contractures—all present at birth—and some functional improvement during childhood, with little or no progression thereafter. The pattern of inheritance is probably autosomal recessive.
Ophthalmoplegic muscular dystrophy
Ophthalmoplegic muscular dystrophy usually becomes manifest in adulthood. Extraocular muscles are involved initially and muscles used in swallowing tend to become affected. Typical facial appearance, especially drooping of the upper eyelids, resembles that found in myasthenia gravis. The inheritance pattern often then follows as autosomal dominant pattern; however, occasional sporadic cases and cases with autosomal recessive inheritance have also been described.
Distal muscular dystrophy
The chief distinguishing characteristic of distal muscular dystrophy is the initial and primary involvement of the small muscles of the extremities. The resulting impairment is frequently confused with Charcot-Marie-Tooth disease, a disorder of peripheral nerves. Distal muscular dystrophy is the rarest subgroup of dystrophies. In Sweden, however, its incidence is comparatively high and as yet unexplained. The pattern of inheritance is autosomal dominant.
Last Reviewed: 13 June 2007
