Relation Of Lesion To Function

The lesion in the cord and nerves affects the sensory tracts, and yet the most prominent symptom is inco-ordination of motion. Violent pains are indeed commonly present at an early stage, and there is loss of sensation, more or less pronounced, when the disease is fully established. The inco-ordination is not a motor paralysis, but is due to interference with the centripetal nerve fibres, especially those which connect the muscles with the cord. The ganglia in the anterior cornua of the cord are, as we have seen, the immediate centres for the muscles, being centres of a low order dominated by higher ones. The centres are in relation with centripetal fibres and give off centrifugal ones, thus forming a complete nervous system of a low order. The centripetal fibres come to a large extent from the muscles, and keep the centres informed, so to speak, of the position and state of contraction of the muscles. Any break in the arc will set the whole apparatus at a disadvantage and interfere with the completeness of the action. Here there is a break in the centripetal system, probably more particularly in the fibres coming from the muscles, as evidenced by the very early loss of muscle reflex (according to Erb, whenever the ataxia is developed). In consequence the contractions of the muscles will be to some extent at random and without that accurate adjustment which exists normally. Probably also the normal tonicity of the muscles is, as Lockhart Clarke suggested, due to continuous stimulation of the ganglia in the cord by stimuli conveyed from the periphery. The interruption of the centripetal fibres will interfere with this stimulation, and the contraction of the muscles will start at a disadvantage. The absence of the knowledge of the state of the muscles which is implied in a break in the reflex arc may be partly compensated by using the eyes to direct the movements, the motor centres in the cord being, as it were, informed from above of the state of the muscles. The occurrence of true paralysis indicates that the neighbouring motor tracts in the lateral columns have become involved.

Until recently such were the views generally current among neuro-pathologists, but our conceptions of the disease have undergone essential change, and we are no longer inclined to look upon tabes as a primary sclerosis of the posterior columns, but rather to think of this as consecutive to changes elsewhere, - in the posterior roots or rather in their ganglia. As has already been pointed out, the posterior columns must be considered as made up in great part of the axons or axis cylinder processes of the cells of the ganglia of the posterior roots, and that the sensory fibres in the peripheral nerves represent their protoplasmic processes or dendrons, and hence both own as their trophic or nutritive centres the cells of those ganglia. So long as these fibres are in organic association with their trophic centres, and they in turn are in a normal condition, then the nutrition of both sets of fibres is unaltered; but, should the nutrition of the ganglion cells suffer alteration, then there follow degenerative changes in those fibres which go to form the posterior nerve roots and the posterior columns of the cord; such degeneration being merely consecutive or sequential and not a primary degeneration, in like manner, the changes in the sensory fibres of the peripheral nerves must be regarded as a consequence of the alterations in their trophic centres in the ganglia of the posterior roots. In support of such a view there is, so far, not a great body of anatomical proof, but it is to be remembered that it is anything but the habit to examine these ganglia in cases coming to a post-mortem examination, and besides the detection of slight changes either in them or in the nerves is difficult and uncertain. Still, such changes have been observed and figured by Oppenheim and probably others; and Babinski, according to Marie, in the year 1891, expressed the opinion that the origin of the degeneration of the posterior root fibres, and their continuation in the posterior columns of the cord, was to be sought in a "functional derangement" of the trophic centres in these ganglia. Marie, while not accepting such an explanation as representing the whole of the mechanism of the degenerative process in tabes, is, however, satisfied that changes do occur in the spinal ganglia in all cases, and that these changes are of the greatest importance as regards the origin of such degenerations. He is also inclined to think that these same changes explain, in part at least, the occurrence of the degenerations in the peripheral nerves in this disease. Marie, further, is of opinion that there exist what he terms peripheral or terminal ganglion cells - cells which in the process of development have become separated from the neural plates and carried along with the centrifugal developing organs, skin, muscles, tendons, - from which certain of the sensory fibres are developed, and which find their way through the ganglia and go to form part of the posterior nerve roots and posterior columns of the cord. Thus, according to him, the nerve fibres of the posterior roots come from two distinct genetic and trophic centres, one in the cells of the corresponding spinal ganglia and the other in those peripheral ganglionic cells.

The changes in the posterior roots are, in any case, a prominent feature in cases of tabes, and have long been recognized; and, since the disease usually manifests itself first and chiefly in the legs, it is in the roots of the cauda equina and those of the lumbar portion of the cord that these changes are most manifest.

Trophic Lesions

In addition to the conditions already referred to, Charcot has called special attention to certain trophic lesions which occasionally occur in locomotor ataxia. These are of three kinds, namely, certain cutaneous eruptions, muscular atrophy, and affections of the joints.

The Cutaneous eruptions occur in the earlier periods, usually coinciding with the attacks of lightning pains, and they are in the form of local eruptions of herpes, lichen, pemphigus.

Muscular atrophy is not very common in locomotor ataxia, and is of late occurrence. The muscles may waste as a direct result of paralysis - a slow atrophy from disuse. In some cases, however, there are localized atrophies of special muscles similar to those in progressive muscular atrophy and in bulbar paralysis. In these cases the disease has extunded-to the anterior cornua in which are situated the cells of which the nerve fibres are prolongations, and there is destruction of these cells.

The Affections of the joints consist in comparatively acute swellings, with exudation, followed by atrophy of the ends of the bones. Dislocations of the joints may occur in consequence. These trophic disturbances of the joints seem, like those of the muscles, to occur in cases where the disease extends to the anterior cornua, the whole organs of locomotion being apparently affected by the degeneration of the motor nerve centres. The propagation of the disease to the anterior cornua, leading to muscular atrophy or trophic affections of the joints, seems to be not uncommon, and may occur at a comparatively early period. This can hardly be regarded as remarkable when we consider that there are, for purposes of reflex action, direct connections between the posterior root-zones and the anterior cornua.

General paralysis of the insane is often complicated with ataxia, and the spinal cord presents sclerosis of the posterior columns similar to that in ordinary ataxia.

Friedreich's Hereditary Ataxia

A considerable number of cases have been described in which motor inco-ordination has appeared at an early age in several members of the same family. There is, in these cases, probably a congenital faulty development of the cord, and the ataxia is the consequence rather of a degenerative than, of an inflammatory process.

The lesion is not confined to the posterior columns, but affects also the lateral columns, involving usually the pyramidal and the direct cerebellar tracts, and sometimes extending forward at the periphery of the cord. There seems little tendency to extension to the grey matter, and hence proper paralysis and muscular atrophy as well as anaesthesia are uncommon. There is seldom any evidence of irritation in the sensory fibres such as we have in ordinary ataxia, evidenced by the lightning pains.

The sclerosis of the posterior columns is similar to that in tabes, and as in that condition the posterior nerve roots are also implicated. According to Gowers the degeneration of the lateral columns always involves the pyramidal fibres; but Marie, although admitting that the lesion occupies the position of the lateral columns, disputes that it is the crossed pyramidal fibres which are actually implicated in the change. The direct cerebellar tract and Gower's tract are also involved.

The disease frequently extends upwards to the medulla oblongata, as evidenced by the occurrence of disturbance of speech and nystagmus. Atrophy of the cerebellum, pons, and olivary body has been described in one case.