94 According to the first observer, this is probably due to structural differences. The extramedullary fibres have a sheath and annular constrictions which are absent in the intramedullary.
In a large number of cases myelitis is a limited affection; that is, its ravages remain confined to the area originally involved. But occasionally the morbid process involves the next segments above or below, extending with specially great rapidity through the anterior gray horns. Exceptionally, the entire cord may thus become the site of a generalized myelitis. There is one segment of the cord which may be regarded as possessing an acquired vulnerability when a myelitic focus is in its neighborhood, and that is the lower end. It seems that while the results of a transverse myelitis in the middle dorsal cord may remain stationary for ten or more years, those of a transverse myelitis at the upper lumbar level do not; on the contrary, the entire cord below the lesion appears to be doomed to undergo the same degeneration by contiguity. This is the only occurrence which seems to deserve the name of a descending myelitis: an ascending extension is more frequently noted in other parts of the cord, but the frequency of both the so-called ascending and descending types has been unduly magnified by the inclusion of the secondary degenerations, which are constant sequelæ of all complete destructive transverse lesions of the cord, but which are rather passive phenomena, and probably influence the clinical progress of the case but little, except under such conditions as are potent in that chronic form of myelitis which underlies tabes dorsalis.
CLINICAL HISTORY.—The symptoms of acute myelitis usually correspond to those of any more or less completely transverse lesion of the cord, and accordingly vary with the altitude of the upper level of the lesion. In a general way, they may be stated as consisting of—
First, paralysis of movement in the parts supplied from the nerves given off below the level of the lesion. The reason for this can be easily recognized in those cases where the pyramid tract, which conveys voluntary impulses centrifugally, is interrupted by the softening.
Second, paralysis of sensation in the parts supplied by the same nerves. This is equally explained by the pathological interruption of the centripetal impressions normally conveyed brainward.
Third, alterations in the nutrition of the parts supplied by the nerves arising in the affected level.
Fourth, abolition of those reflexes which are translated in the level of the lesion.
Speaking crudely, then, the symptoms of a transverse myelitis fall into two natural groups. The one which includes the first and second categories enumerated are symptoms due to interruption of cerebral functions; the other, which comprises the last two categories, being due to abolition or perversion of spinal functions. There is a third group comprising certain constitutional symptoms.
Aside from those variations due to the distribution, extent, and intensity of the lesion there are others which depend on the rapidity of its invasion. There are three types in this respect—the apoplectiform, the ordinary acute, and the subacute. The term apoplectiform has been used in two different senses, one being clinical, and referring to the rapidity of onset of the symptoms; the other anatomical, and referring to the nature of the lesion. It is, however, doubtful if a distinction in the latter sense is practically valuable. The presence of a blood-clot in a myelitic focus is itself secondary to the softening, and the intensity and rapidity of the process must have shown itself in the development of the latter.95 The term ought, therefore, to be used in a clinical sense only.