The method of measuring the amount of strabismus by prisms is applicable in all cases of manifest or latent squint in which the patient can recognize binocu lar diplopia. It consists in placing before the eyes such prisms as will cause the true and false images to coincide in spite of the strabismus. Such prisms will sub stitute binocular vision for diplopia; and will do away with all movements of deviation and recovery, or of apparent movement of the point of light looked at, when the cover is shifted from one eye to the other in rapid alternation.
Prognosis.—For apparent squint due to displacement of the cornea, we can do nothing except at the cost of binocular vision. Comitant strabismus is outgrown in a few cases in early childhood without treatment; and in a much larger per centage of cases may be permanently cured by the wearing of glasses, and proper orthoptic exercises. Comitant strabismus in adults, if intermittent, may be cured by correcting lenses; but if con stant will generally require an operation. All cases of comitant squint are capable of relative cure by operations judiciously chosen and skillfully performed, except such as suffer from diplopia when the image is thrown on the fovea of the deviating eye. The exactness and per manence of the cure depend on the pos sibility of establishing true binocular vision.
Paralytic strabismus may be cured by cure of the paralysis causing it. If the paralysis be very marked, it will prob ably not recover in less than six weeks, or after six months will not recover at all. After incomplete recovery from paralysis of one of the eye-muscles, operative treat ment may give practical relief. Strabis mus due to a permanent complete paral ysis cannot be cured. The diplopia of comitant strabismus usually ceases to be annoying or disappears entirely. Diplo pia from paralytic squint, except when it has occurred in childhood, will commonly last throughcut life.
Treatment.—In every case of strabis mus, any obstacle to easy binocular vision, in the form of an error of refraction, should be removed by the constant wear ing of correcting lenses. All eve-work or habits tending to cause or perpetuate the strabismus should be discontinued. If due to an ocular palsy this should be treated. If of recent origin, orthoptic exercises should be resorted to. If the strabismus be constant and of long stand ing, and not much influenced by the wearing of correcting lenses, and if throwing the image on the fovea of the deviating eye when the fixing eye is also in use does not cause diplopia, an opera tion should be done.
Correction of any error of refraction is the first step. It may be done at a very early age. Children two years old can have their correcting lenses deter mined by skiascopy; and will readily and gladly wear them, if they are much needed and accurately adjusted. The avoidance of injurious use of the eyes may require the use of a mydriatic to suspend all effort of accommodation. Or it may include, for monolateral strabis mus, the covering of the fixing eye, or the placing of it alone under the influ ence of a mydriatic. to compel the patient to use the eve he would otherwise allow to deviate.
Orthoplic exercises include: the view ing of special diagrams and pictures through the stereoscope; the exercise of muscles that are relatively inefficient by placing prisms so that they will bring the true and false images close enough to gether for the muscles to complete their fusion, in actual squint, or so that the prism will require special exertion to "overcome" it in latent squint. They also include the use of "fusion tubes," which are applied one to each eye and turned so that the eyes can just fuse the minute openings in the distal ends of the tubes. Also the employment of the "reading-bar," an opaque bar supported above the page in such a way that it cuts off a portion of each line from one eye, and another portion from the other eye, compelling fixation with both eyes for the reading of each line. Under this head also comes the practice of exercising con vergence, by fixing on a point that is gradually made to approach the eye until the requirement of convergence becomes too great to be sustained. Or the prac tice of viewing through strong prisms, turned with the base toward the nose, a point which starts near the eyes, but is slowly withdrawn until the limit of the power of abducting the eyes is reached.
Operations on the ocular muscles are of two kinds: tenotomy, designed to lessen the influence of an overacting mus cle; and advancement, designed to in crease the influence of a muscle relatively weak or inefficient. Tenotomy is the simpler and less formidable operation. T;ut it tends to lessen the total mobility of the cyc, and if injudiciously performed may cause the eye to deviate in the op posite direction. Advancement is more di fficult, but it does not lessen the mo bility of the eye, and is not likely to cause a strabismus of the opposite kind.