Owing to the advance of clinical and anatomical knowledge made within the past fifteen years many forms of spinal disease classed with the inflammations have been recognized as distinct pathological entities, no longer to be confounded with simple acute myelitis, ordinarily so called. Special forms of acute spinal paralysis, notably acute poliomyelitis anterior of children and the corresponding chronic affection among adults, have become separated in this way, and are accordingly treated of in separate parts of this volume.86
86 For other and practical reasons the traumatic and compression forms of myelitis are also assigned a separate place.
Some dispute exists as to the propriety of making a distinction between acute and chronic myelitis, since an acute myelitis, if the initial attack be recovered from with life, presents a similar condition clinically as chronic myelitis; and this quiescent or slowly-progressing condition may extend over many years. The term acute with reference to inflammation of the spinal cord refers only to the active period of the disease. Just as an embolic softening of the brain is an acute affection, but may be followed by a chronic paralysis or aphasia, so the acute myelitic process may be followed by a chronic paraplegia. It is improper to call the latter a chronic myelitis. It is merely a protracted symptomatic sequel of the acute process. The latter is distinguished from chronic myelitis both clinically (by the rapidity of its onset) and anatomically (by the early dissolution of nerve-elements in the focus of disease). Limited in this sense, acute myelitis, excluding the special clinical forms already adverted to, is rather a rare disease.
MORBID ANATOMY.—The most recognizable change noted in an acute myelitic focus is one of consistency: the spinal substance is softened. In some cases the softening is so slight that the observer may doubt whether he has a pathological or cadaveric softening to deal with, the dorsal cord, which is most apt to be the site of an acute transverse myelitis, being precisely the part which is most apt to show the latter change even in fairly well-preserved bodies. In extreme cases the softening may be so intense that the cord-substance, completely fluidified, runs out of the meningeal sac, leaving the latter a collapsed membranous cylinder to mark the place where the cord once was. Where the cord-substance is sufficiently firm to permit of sections being made through it, the normal outline of the gray and white substance is found obliterated, either presenting the appearance as if the gray and white matter had been stirred up together or of a more uniform color-change. The color may be either white, reddish, yellowish, or chocolate-like. It depends upon the participation of the blood-vessels in the change. If there be much hyperæmia, there will be developed what is known as red softening; if there be much extravasation and commingling of blood with the diffluent cord-tissue, a chocolate color will mark the diseased area; and similarly one and the same focus may present different tints in different parts according to the age and intensity of the process and the more or less advanced retrogressive metamorphosis of the extravasated fluid. As already stated, the purulent form of softening or abscess does not occur in ordinary myelitis.
There is considerable variation in the extent of the affected areas of acute myelitis. In the typical and severe transverse form the whole thickness of the cord may be disorganized, and the disorganization may extend in the length of the cord, so as to involve the level of exit of from two to five pairs of nerves. In less furibund cases the area of absolute softening is confined to the gray substance and its immediate neighborhood, the submeningeal white substance being but slightly affected or escaping. Sometimes several foci of intense softening are scattered through a short length of the cord and connected by less severely involved areas of softening or œdema. Leyden distinguishes three types of distribution—the transverse, the longitudinal, and the disseminated insular or multiple form. He includes under the longitudinal type the so-called central softening of Albert, but undoubtedly many cases of syringo-myelia have passed under this designation. The submeningeal form of softening which, with Ollivier, he states to occur in association with spinal meningitis, must be a very rare affection, as it is difficult to find a well-established case recorded. The longitudinal form shows the same predilection for the gray substance which the acute myelitic process generally does, but I have seen a finely demarcated fascicular myelitis limited to the lateral column in a paretic negro. In this case the pyramid tract and the contiguous area in front of it were so intensely softened that for a length of twelve centimeters a hollow canal ran through the cord in the place previously occupied by the diseased substance. In recent cases of myelitis the diseased area is usually found surrounded by a transition zone in which, the morbid change gradually becoming less intense, the consistency is firmer, and which merges into that of the normal cord. In cases where death occurs after a few weeks a more abrupt demarcation is usually found; this is due to the reactive changes occurring in the neighborhood. The connective tissue becomes firmer, and thus the softening centre becomes surrounded by a sclerosing capsule. Ultimately, the centre undergoes complete disintegration and absorption, and a cavity is left behind filled with a clear fluid; in short, a cyst surrounded by a firm capsule represents the residua of disease. In cases where the softening at the centre of the focus does not proceed so rapidly nor reach so high a degree as to result in liquefaction, the less vulnerable elements, the blood-vessels and supporting tissues, survive the death of the ganglionic and conducting substance; the connective elements hypertrophy, and thus a firm sclerotic patch is formed, indicating the location of the previously softened field.
It seems to be generally admitted, with Hayem, that the blood found exuded in the hemorrhagic form of myelitis does not necessarily indicate an active determination, but is rather, like some forms of so-called red softening of the brain, the result of capillary rupture or necrosis in the midst of the disintegrated tissue, now rendered incapable of supporting the vessels. The existence of a purely white form of myelitic softening shows that a textural change is the primary occurrence, and that the participation of hyperæmia or congestion is not an essential feature of myelitis. The assumption of an initial inflammatory congestion is made rather on theoretical grounds than on the basis of observation. It is simply incredible that, as Ross87 claims, white softening should be a third stage, preceded by red and yellow softening as a first and a second stage! How the extravasated blood, which pathologists generally allow to leave long-lasting traces, manages to disappear, and how blood-vessels in the midst of necrotic or œdematous surroundings suddenly acquire such contractile energy as to produce a total emptying of their contents while the perishable nerve-elements remain behind, are problems which should be solved before attempting to assign to a condition which is often found to be a primary phase of myelitis the position of a late and regressive stage. Erb admits that red softening, to which he also assigns the position of a first stage, is very rarely seen, only traumatic and rapidly fatal cases of central myelitis offering opportunities of examining it. None of the various forms of exudation claimed to occur at this period under the names of vitreous, colloid, or hyaline deposit have been confirmed in any recently well-studied case.88 The great mass of authorities, however, still agree in regarding the minute changes of the initial stage of myelitis to correspond to those of ordinary inflammation. The vessels are described as injected, the adventitial spaces as crowded with the formed elements of the blood, and the vascular walls and the neuroglia infiltrated with granule-cells and fatty granular matter. By some, inflammatory changes of the neuroglia are described, but I am unable to find a single case in which these were determined in early fatal cases. As far as our observation goes, the hypertrophy of the neuroglia is a later occurrence.
87 A Treatise on Diseases of the Nervous System, 1882, vol. ii. p. 280. The author states no authority, nor does he advance his own observations in support of this statement.
88 Baumgarten's case of hyaline exudation, Archiv der Heilkunde, vol. xvii. 276, was an infectious myelitis and associated with anthrax.
As to the nervous elements themselves, they are always found affected. The nerve-cells appear inflated, their processes fragile, sometimes suddenly swollen in their course, at others very thin and brittle. Multiplication of the nuclei of the large multipolar cells has been described. It must be an unusual occurrence, as it has been confirmed by but a few of the numerous observers who have examined into this question. The protoplasm of the nervous elements loses its normal striation and fine molecular granulation; it becomes either coarsely granular or hyaline. The axis-cylinders, both in their intracinereal and their intramyelinic course, show changes similar to those of the cell-processes in the gray matter. Particularly frequent are swellings in their course, the diameter of the cylinder being so much increased as to almost equal that of the myelin tube. This increase in diameter is regarded as an inflammatory swelling by some, as secondary to disturbed nutrition by others; it precedes disintegration: the substance becomes granular, fragile, and in the end dissolves. In the mean time the myelin loses its continuity, irregular segments of it fusing into round and oval masses.89