ATROPHY, a medical term implying wasting from some interference with healthy nutrition. Waste and repair are constant processes in the animal body ; when they balance, the volume of the body and of its constituent parts remains constant; when repair is greater than waste there is growth or hypertrophy; when waste is greater than repair there is atrophy. The chief causes of atrophy are (I) disuse, (2) pressure, (3) interference with nervous supply. Thus the diminution in size of the pregnant uterus after child birth, the shrivelling of thymus and umbilical vessels in the infant, are atrophies from disuse ; the disappearance of soft tissues and even bone in the neighbourhood of tumours, or an aneurysm, is an example of pressure-atrophy ; the wasting of limbs in disease affecting the large motor ganglia of the ventral horns of the spinal cord, or the anterior nerve roots or efferent nerves, is atrophy from interference with nervous supply.
These causes may act singly or in conjunction, and may them selves be dependent upon antecedent conditions. A limb may atrophy from disuse because disease of bone, or of joint, renders movement painful, and pressure-atrophy depends upon interfer ence of blood supply which the pressure induces. For interference with nervous supply to cause atrophy of muscle—other than dis use-atrophy—it is necessary for the efferent nerve to be disor ganized in some part of its course. Hence in paralysis affecting the leg, when the lesion is in the brain, such atrophy as occurs is due to disuse, whereas in infantile paralysis and progressive muscular atrophy, the lesion is in the large multipolar cells of the ventral horns of the spinal cord, and the atrophy of muscle is truly of nervous origin and far more conspicuous. Interference with afferent or sensory nerves causes atrophy of skin and subcutaneous tissue. This is seen in certain injuries to nerves. Thus, severance of the radial nerve (or musculo-spinal from which it arises) occa sions a glossy condition of the skin covering the thumb, index, middle and contiguous half of the ring finger, and the skin becomes as thin as paper.
The intimate structure of an atrophied part may be little or greatly altered. In the natural atrophy accompanying old age the microscope shows little difference from normal adult conditions. The biceps of a man aged 8o is visibly smaller than that of a pugi list aged 25, but the two could not easily be distinguished micro scopically. The same is true of the muscle of an undersized and ill-developed factory worker. For this reason it is customary in pathology to distinguish between true atrophy and hypoplasia, the former signifying that the part regressed after having reached a normal size, the latter that it never reached the normal size. It is clear, too, that a part may be hypoplastic either because it is composed of fewer elements each of which is normal size, or be cause it contains a normal number of elements each of which is smaller than usual. Unknown factors of inheritance may control these two conditions.
In cases of true atrophy, particularly those moderately rapid in onset, the diminution in size is accompanied by other changes, such as fatty degeneration and fibrosis, which are recognizable microscopically. Even in the atrophy of muscle in the aged the muscle nuclei are packed more closely and appear somewhat shrivelled, but in the atrophy of muscle caused by pressure of an aneurysm these changes are well marked, and in the region where atrophy is greatest the muscle fibres contain numerous minute fat globules due to fatty degeneration of the protein substance. In the renal atrophy characteristic of chronic granular kidney, fatty changes and fibrosis co-exist with actual disappearance of true renal elements, but it is uncertain which is the primary change.
Sometimes these associated changes are so pronounced that their bulk more than counterbalances the diminished bulk due to atrophy itself, with the result that the part, though atrophied in respect of its essential elements, is actually larger than normal. Thus a fatty heart is atrophied so far as concerns its muscular elements, but may measure, and even weigh, more than the normal organ. In hypertrophic muscular paralysis, too, the muscles of calf and buttock are so large and firm that they give the impression of magnificent development, and yet they are totally inefficient for their function, and the microscope shows that their bulk is composed of fat and fibrous tissue almost to the exclusion of muscular fibres. In spite of .appearances the condition is essentially atrophic. This disease does not come into the classes of atrophy mentioned above, for the cause is unknown; it seems to depend upon a hereditary factor and more than one member of a family may suffer from it.
Atrophy may be very rapid in onset. In acute yellow atrophy of the liver, and in allied conditions caused by some poisons, the organ may diminish in size to one-third or half within a few days. Under these circumstances the destructive changes are so intense that it may be impossible to recognize hepatic substance in the fatty, blood-impregnated and pigmented material revealed by the microscope. But as a rule atrophy is a gradual and slow process.
Since an atrophied part is abnormal and depends upon abnormal nutrition, it is sensitive to conditions which the normal part can resist. The small injuries and adverse circumstances of life pro duce disproportionately great results in the first instance, and reparative processes are correspondingly slow and unsatisfactory. Sometimes it becomes necessary to consider surgical removal of the affected part. In cases due to severance of a nerve, suture of the divided ends may effect great improvement or even a cure. In the case of muscles and skin suffering from disuse-atrophy, mas sage and surgical treatment of any underlying condition are indi cated. But in many forms no treatment modifies the atrophy it self and treatment of symptoms as they arise alone is possible.
(W. S. L.-B.)