Etiology. — Ninety per cent. of the acute cases occur within the first decade of life and more than half of all cases within the first three years of life. Among children the two sexes seem about equally susceptible. Among adults it is comparatively rare in the female. The disease is no respecter of cast or class, nor does it manifest any special racial proclivities, though the negro is comparatively exempt and the disease is more common in centres of dense lation than in rural districts. Polio myelitis is often a sequel to the febrile infections of childhood, especially let fever, measles, and diphtheria. In this respect, as well as others, its etiology is quite similar to that of epidemic and sporadic cerebro-spinal meningitis. Pol iomyelitis may also occur as an demic, many such having been reported, though no specific micro-organism has as yet been demonstrated.
Poliomyelitis anterior acuta is believed to be of an infectious or an infectio toxic nature. This view is based on the numerous experiments in which the dis ease has been produced in animals by the injection of different bacteria and their toxins. There is, perhaps, a secondary inflammation due to a primary focus situated in the intestine or elsewhere. Billow-liansen and F. Harhitz (Norsk Mag.. for Laeg.. Nov., '98).
Study of 20S cases of poliomyelitis observed over a space of twelve years in the polyclinic of Vienna further sus thins the theory of the infectious origin of poliomyelitis, first pointed out by Striimpell and Marie. 'lids is now gen erally admitted, and one of the strong arguments in favor of this view is the srrhtlini, or epidemic, appearance of the disease. These 20S eases show that from 1SSG to 1897 the number of cases ranged between 3 and 18; in 1S9S this average arose to 42, to fall to 6 in 1899. Store over, of the 129 eases observed between 1886 and 1897, there were 35 which be gan between the months of January and July, and from July to January there were 75, while in 19 cases the date of onset was not ascertainable. Even if all the uncertain cases were to be as cribed to the earlier half of the year the number occurring from July to January would still be notably the larger. The preponderance of cases in the summer months thus seems quite evident. In the epidemic of IDS four cases developed from January to July; in July, 5 cases; in August, 11 cases; in September, 12 cases; in October, 4 cases; in November, 3 cases; and in December, 1 case. Among these cases and also among those of preceding years there was not one instance of family in fection, nor were the cases confined to one street, or house, or to a certain section of the city. It therefore seems certain that, if infantile palsy is an in fectious disease, its power of contagion is very limited, in which respect it re sembles cerebrospinal meningitis. Zap
pert (Ja lirb. f. Kinderh., B. 3, S. 125, 1901).
In not a few instances trauma appears as the exciting cause; exposure to ex treme cold or to excessive or violent exer cise may induce the disease. The season has its influence, many more cases occur ring in summer than in winter. This is especially noticeable in seasons of pro longed excessive heat. Among adults violent exercise, exposure, trauma, de bilitating excesses, and syphilis are recog nized as potent factors. Heredity is ex tremely rare.
Pathology. — The essential lesion in acute anterior poliomyelitis is an atrophic destruction, more or less complete, of the larger ganglion-cells (giant cells) of the anterior horns. This destruction of the cells of the anterior horns occurs as the result of an inflammatory myelitis proc ess limited to the anterior gray matter, established through some, as yet unde termined, micro-organism in many in stances, but probably not in all, the me dium of invasion being the branches of the anterior spinal artery. The cells of the lower dorsal and midcervical seg ments are most frequently affected. The anterior nerve-roots are also affected secondarily with degenerative changes, and this is true of the muscles to which the affected nerves are distributed. The atrophied muscles show a distinct dim inution in the size and number of fibres, the normal tissue being replaced by fat and connective tissue.
Prognosis.—It is of some interest to know or to approximate during the acute febrile stage the extent or degree and the distribution of the final more or less per manent paralysis. There is no positive guide, but the severity of the constitu tional disturbance, including tempera ture, is sometimes an index. Occa sionally after the constitutional disturb ance subsides, the loss of power may remain rather widely distributed. In such instances the electrical response affords information to the experienced examiner. If the quantitative response grows less or the qualitative change greater from day to day in certain mus cles or a limb, just in proportion is there likely to be a permanent residual paral ysis in such muscles or limb or vice versa. In all cases some permanent paralysis will remain, but it may be six months from the onset before the limits of this paralysis can be determined. The pa tient is handicapped physically in after life to a greater or less extent, but never mentally. The prognosis depends largely upon the ability of the parent to carry out instructions in faithful, patient, per sistent treatment.