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INHERITED DISEASES — MUSCULAR DYSTROPHIES

By Dr. T. VISHNU MURTHY
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Muscular dystrophies are inherited myopathies, characterised primarily by progressively severe weakness.

Although it is generally believed that inherited diseases must be due to a missing or structurally abnormal protein either an enzyme or a structural protein, this abnormality has not been identified in any form of dystrophy. It is conceivable, however that the fundamental abnormality is in another organ such as the liver or bowel and that the muscular abnormalities are secondary. According to the classification of human muscular dystrophies are there 1) Duchenne dystrophy 2) Facioscapulohumerial dystrophy 3) Limb- girdle dystrophy 4) Myotonic muscular dystrophy.

None of the muscular dystrophies are common. Incidence rates vary from 5 per million births for facioscapulohumerial dystrophy to about 250 per million births for duchenne dystrophy. Many cases seem to be sporadic; the mutation rate of Duchenne dystrophy is high and about two thirds of the cases appear sporadically, with no other affected individual in the family.

The abnormalities in muscle seem to involve all fibers in random fashion. Early, there is scattered evidence of necrosis and regeneration, with prominent variation in fiber size, including many fibers much larger than normal. Later, fibers disappear, to be replaced by fibrous connective tissue and fat. Pseudo hypertrophy is probably due to both true hypertrophy and increased accumulations of fat and connective tissues. In myotonic dystrophy, unusual figures from ring fibers and sarcoplasmic masses or accumulations of sarcoplasam that are free of myofilaments.

The manifestations according to classification of muscular dystrophy. Becker dystrophy has manifestations similar to those Duchenne dystrophy, but the onset is later in childhood or in adolescence, and the tempo is slower and more variable. This form may also be devastating, but some patients are able to function, albeit with limitations, well into adult life. The clinical similarities include pseudo hypertrophy of calf muscles. How ever, it is clear that although the two forms are similar, they are genetically separate: there are no mildly affected individuals in typical Duchenne families, nor are children affected severely in Becker families.

Fasioscapulohumeral dystrophy is distinct. Symptoms vary in severity so that some affected individuals never have any disability where as others become incapacitated early: there are all grades in between. The first symptoms are apt to be related to difficulty in raising the arms or to prominence of the scapulae. Weakness of the legs may affect pelvic girdle muscles, are equally prominent scoliosis. The face is always involved examination; the peri oral muscles may be more affected than those of upper face but ultimately patients develop difficulty in closing eyes. The sternal head of the pectoral muscle is affected earlier than the clavicular head, a selectivity that can be detected on testing individual muscles, and leads to a peculiar appearance of the axillary fold of normal people is formed by the sternal portion of the pectoral muscle.

The manifestations of Limb-girdle dystrophy are also similar because weakness of muscle of the pelvic girdle usually initiates the syndrome,but symptoms start in late childhood and adolescence. Girls are affected as often as boys, and pseudo hypertrophy is rare . Waddling gait, difficulty in walking and climbing and frequently falls are common.Occasionally, symptoms may begin in the shoulder girdle. In either case, there is usually weakness in all four limbs by the time patient seeks medical attention.

Myotonic muscular dystrophy diverges from the preceding types in several respects.

  1. The distribution of weakness differs in that cranial muscles are often affected and limb weakness is initially more marked in distal muscles. Thus weakness of the hands precedes shoulder weakness and foot drop or steppage gait precedes symptoms of pelvic muscle weakness, Ptosis, Facial weakness, and dysarthria are signs not seen in the other forms of dystrophy.
  2. Myotonia or difficulty relaxation may be symptomatic and after a firm grip the patient may have difficulty letting go. Myotonia may cause other symptoms in patients with myotonia congenita but in the dystrophy only the hands are affected by this kind of stiffness. Myotonia of grip may be evident of examination, and can also be elicited by percussing the thenareminence.
  3. Other systems are involved in the pleomorphic disorder, cataracts appear sooner or later in all patients, and are sometimes the only sign of the disease most of the men ( but not women ) have frontal baldness, testicular atrophy affects most of the men but often after they have sired children to perpetuate the disease. The incidence of diabetes mellitus may be increased.
  4. Conduction defects are common in the electrocardiogram, and may be lead to clinically significant arrhythmia or congestive heart failure.

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