The resemblance of this disease to progressive muscular atrophy strongly suggests that it is primarily due to a lesion in the spinal cord. But repeated examinations of the cord have shown no constant lesion there, and in most cases the anterior cornua have been found intact. The muscular nerves are also apparently unaffected, and the general opinion is now that it is a primary disease of the muscular system. It occurs mostly in childhood.
The disease is due to a local constitutional fault which, like most constitutional peculiarities, is hereditary. Its hereditary nature is shown by its occurrence, in many instances, in several members of the same family. There is not apparently an inheritance of the disease, but there seems to be some condition in the parents which leads to the development of the disease in the offspring. Considering the close relationship in function and even in nutrition between the nervous system and the muscles, as shown by the affections already described, it is perhaps justifiable to consider this disease along with those of the spinal cord.
In many forms of disease in which the muscles are disused or paralyzed there is an overgrowth of the interstitial connective tissue, sometimes with excessive development of fat. We have noted this in some of the affections considered above, and it occurs also when the muscles are disused by reason of fixation of joints, etc. (See under Fatty Infiltration).
In the present disease there is an apparently spontaneous atrophy of the muscular substance with a new-formation of connective and adipose tissue between the muscular fibres. The new-formation, at least in some situations, usually more than compensates in bulk for the atrophy, and there is an apparent hypertrophy of the muscle as a whole. < By the time the patient dies there is generally a very extraordinary and widely distributed affection of the muscular system. Many of the muscles on being cut into look like pieces of fat, in which to the naked eye no trace of muscular tissue is visible. Under the microscope occasional narrowed muscular fibres may be visible in the midst of the adipose tissue(see Fig. 319), usually accompanied by strands of connective tissue. The muscular fibres, although varying in size and in general greatly narrowed, for the most part preserve their transverse striation. In some situations the fat may have, to a large extent, disappeared, so that fibrous tissue replaces the muscle.
Fig. 319. - Muscle in pseudo-hypertrophic paralysis. There is chiefly adipose tissue with a few atrophied muscular fibres.
An idiopathic muscular atrophy has been described in which the wasting of the muscles has not been associated with increase of connective tissue or fat. This condition shows many analogies with that described above, occurring like it in several members of the same family, but not generally beginning in childhood.
In addition to the forms mentioned there is still another, which has been recently separated and described by Charcot and Marie in France, and in this country by Howard Tooth. It begins symmetrically, commonly in the peroneal muscles of the legs, or in the extensor muscles of the toes, and usually results in a deformity like club-foot. After the leg muscles the thigh muscles are involved, and only after years the arms, the small muscles of the hands being the first to suffer.
It is sometimes hereditary, and occurs mostly in males about the time of puberty. It holds an intermediate position between the forms due to changes in the central nervous system and those of purely muscular origin, and it is probably due to a neuritis.