Cases occasionally occur of a limited transverse myelitis, manifested in a softening of the cord, without any sufficient cause being apparent. These cases, which are usually designated spontaneous or primary, are assigned to exposure to cold, especially when the body is overheated or during menstruation. The nature of the irritant in such cases, and its source, are quite obscure, as well as the reason for its localization at a particular part of the cord. Syphilis and certain of the acute specific fevers, such as small-pox, typhus fever, and measles, are sometimes assigned as causes of softening of the cord.

Direct injury to the cord, as by fractures and dislocations, or acute curvature of the spine, produces myelitis, especially where haemorrhage has been induced. In this connection the great vascularity of the grey substance as compared with the white, is to be taken into account. An injury to the cord, as by an acute curvature or even a concussion, may produce a haemorrhage, which is often limited to the grey substance. The haemorrhage may itself be the efficient cause of the softening. Inflammations extending to the meninges may, by the action of the inflammatory exudation, induce a softening. Tumours have sometimes a similar effect, although their action is liable to be more gradual, so as to produce a chronic inflammation.

Characters Of The Lesion

The myelitis is limited in longitudinal extent, and as the usual result is softening of the cord, the expression Acute softening of the cord is almost equivalent to myelitis. The softening is usually most manifest in the grey substance, and may even be apparently confined to it, but it involves the white substance as well, and usually the whole thickness of the cord.

The softened nervous tissue presents, in different cases, considerable variations in colour, so that red, yellow, grey, white, and even green softening have been described. If much blood has escaped from the vessels, there will be red softening merging into yellow.

The characteristic morbid changes are the breaking up of the nerve tissue and fatty degeneration of the other structures. In the white substance drops of myeline, escaped from the medullary sheath, are found, and the axis cylinders are swollen. The fatty degeneration is manifested by the presence of abundant compound granular corpuscles, which, to some extent, are probably leucocytes or neuroglia cells, which have picked up the disintegrated myeline; but there may be abundant leucocytes apart from these cells. There is also fatty degeneration of the walls of the blood-vessels, the fat here having a similar source.

The myelitis rarely goes on to suppuration, but usually passes into a chronic stage. The fat, both of the medullary sheath and of the compound granular corpuscles, is absorbed, and a condition of grey softening remains. As the inflammation becomes more chronic there is new-formation of connective tissue of a cicatricial character. In the midst of the connective tissue there are often to be found large cells with radiating processes, sometimes called "spider cells " or " Deiter's cells." These are very obscurely visible in the normal cord, where they form part of the neuroglia, but here they are much enlarged. In this way the proper elements of the cord at the part affected may be replaced by a cicatrix or by a cyst, and its conduction interrupted. In some cases the interruption is not complete, and there may even be a partial regeneration of the conducting fibres, and a partial recovery from the paralysis.

As a consequence of the lesion there is extensive sclerosis in the immediate neighbourhood, as shown in Fig. 313. This localizes itself above and below the seat of lesion as an ascending and descending grey degeneration of the usual distribution. There may be also an extension of the inflammation upwards in the pyramidal tract for a short distance from the seat of the lesion (as in the specimen figured).

Gowers has described an appearance as if a new-formation of nerve fibres were taking place, and certainly the fact that there may be a partial return of power after motor paralysis which has lasted twelve months, would seem to indicate that there may be some such new-formation.