216 Thèse de Paris, 1878.
NYSTAGMUS.—This is a term applied to a periodic type of involuntary oscillatory or rotatory movements of the eyeballs. The oscillatory are due to rapid alternate contraction of the straight muscles, while the rotatory indicate either similar actions of the oblique muscles alone or in conjunction with the straight. The oscillatory motions are usually horizontal, but instances of vertical nystagmus occur, as in the case recorded by Soelberg Wells.217 Nystagmus may be either congenital or acquired; the latter variety being much the more frequent form of the affection. Congenital nystagmus is usually associated either with cataract or imperfect development of the optic nerve and retina. It is a very frequent accompaniment of albinism and pigmentary retinitis. We often see the acquired form arise during the first few months of life, when the child in its effort to see is hindered by corneal or lenticular opacities resulting from ophthalmia neonatorum. One of the most interesting of the acquired forms is that which occurs amongst coal-miners, rendering a considerable number of those thus affected unfit for work. At first the symptoms are that the lights in the mines and the objects on which the patients endeavor to fix their attention begin to dance, this being accompanied by a sensation of dizziness and discomfort. In the first part of the attack they disappear when work is stopped, and the miners come up into the daylight; but if work be persisted in they become permanent and exaggerated. When the nystagmic motions have ceased, they may often be called into activity by placing the patient in a dark room and getting him to direct his eyes to a candle held above the horizontal line of the field of vision. The motions are usually lateral, or in some cases the centre of the cornea describes an ellipse or circle which causes the patient to see a ring of light. It has been observed to occur much more frequently in those working in shafts where there is a good deal of fire-damp; which has caused some writers to assert that the nystagmus has been dependent upon the action of the gas. This view would seem to receive some support from an instance reported by Bright of nystagmus, in a case of suffocation from the fumes of burning coals, which he attributed to cerebral pressure. In these cases it is more probably due to fatigue of the eye and its nerve-centres in the endeavor to see in the dim light and strained position which the miner is often obliged to maintain, which is intensified by the enfeeblement of the nerve-centres due to the action of the gas: these, associated with the diminution of the light caused by the wire gauze of the safety-lamp, would further increase the strain in those obliged to work in the shafts pervaded with fire-damp. The statements of Dransart,218 founded on the examination of a large number of miners, probably give a correct idea as to the frequency of the affection. He states that among 12,000 workmen employed by one company, there were 30 under treatment for nystagmus, which would give about two and a half patients per thousand. In any form of nystagmus the motions of the eyes usually become more rapid when they are used for near work. According to Nagel,219 excessive convergence will at times cause a temporary cessation of all nystagmic motion; and he further proved this by putting extra strain on the interni by means of prisms with their base out. The true pathology of the various forms of nystagmus is still imperfectly known. Arlt220 supposes that there is a rapid repetition of reflex movements in the endeavor to attain distinct vision in those forms which develop on account of corneal and lenticular opacities. He explains this by the supposition that the retinal impression is strengthened by the same retinal areas being rapidly and repeatedly subjected to the action of the rays of light from the same object, while a longer period of fixation would cause retinal fatigue and blur; showing the same principle by reminding us that our perceptive powers for a test object, upon first being brought into view at the periphery of the field of vision, are much stronger when the object is shaken than when it is brought quietly toward the fixation-point. Some forms of the affection, however, are manifestly due to fatigue of the nerve-centres, and have been by some authors placed in the same category as writers' cramp. For its causation we would naturally look for the anatomical changes either in the cortical centres for the eye-muscles or in the nuclei of the third and sixth pairs. Vulpian221 states that wounds of the medulla in dogs cause nystagmus, and Schiff asserts that wounds of the white substance of the cerebellum near the peduncles give rise to the same phenomenon; while Ferrier has produced it by the influence of electricity on the cerebellum of apes. Cohn222 records a case of gunshot wound of the right parietal bone (near the angular gyrus) which produced nystagmus. Merkel's case, occurring in a patient with embolism of the artery of the fissure of Sylvius, would also point to lesion near the angular gyrus. Stintzing223 gives a case where there was thrombosis of the basilar and Sylvian arteries. Oglesby224 relates two cases where nystagmus came on suddenly with dilatation of the pupils, the autopsies showing a clot which pressed on the medulla. Fienzal225 also gives a case where there was a tumor in the left peduncle of the brain. It is often seen during epileptic convulsions. According to Raehlmann,226 the motions of both eyes are under the control of psychic centres which regulate them according to the necessities of vision: for Willbrand227 it is a sign of weakness of the voluntary cortical centres which fail to regulate the reflex activity of the middle brain and cerebellum. The latter author shows that the extent of the field of vision is increased in the direction of the oscillations in those cases where direct vision is not much impaired, while there is marked contraction of the field in cases where the direct visual acuity is much diminished. He also states that there is contraction of the field in the nystagmus of miners, which is greater during the intervals of the paroxysm than during their occurrence, and, further, that the contraction is greater where the case is one of long standing.
217 Lancet, 1871, p. 662.
218 Annales d'Oculistique, 7, 82, p. 177.
219 Graefe u. Saemisch, vol. vi. p. 226.
220 Krankheiten des Auges, Bd. iii. p. 335.
221 Comptes Rendus de la Société de Biologie, 1861 (quoted by Robin, p. 157).
222 Schussvorletzungen des Auges, p. 19.
223 Jahresbericht f. Ophth., vol. xiv. p. 306.
224 Brain, vol. iii., 1880.