Strabismus from lack of development of the co-ordinating mechanism, hyper opia, or difference of refraction between the two eyes develops in early childhood, when it is also most likely to arise from practical blindness of one eye. From myopia it occurs a few years later, as the myopia usually develops during the period of school-life. From paralysis of the muscles it may develop at any time of life. Syphilis and rheumatism are the most common causes of these palsies. But acute infectious diseases, especially diphtheria, injuries, and chronic diseases —as diabetes and Bright's disease—also cause them. Spasm of the muscles is apt to be hysterical.
The onset of convergent strabismus during the wearing of a bandage for in jury or disease in childhood, or during any affection which temporarily pre vents binocular vision, is not uncom monly observed. F. W. Marlow (Oph thalmic Record, p. 117, '97).
Varieties.—When strabismus is due to paralysis of certain muscles, causing in ability to move the eye in certain direc tions, it is called paralytic. Where the squint is due, not to inability to move the eye, but to a false co-ordination of the movements, so that while the two eyes move freely in all directions they still keep their false relation to each other (as always too convergent or too divergent, or one turned too high for the other) the condition is called con comitant or comitant strabismus.
When the eyes converge too much it is internal or convergent strabismus. When they diverge, or do not converge enough for near seeing it is external or divergent strabismus. When one eye turns higher than the other it is vertical strabismus. When it is always the same eye that deviates it is monocular or mono lateral strabismus. When it is sometimes one eye, sometimes the other, that devi ates, it is alternating strabismus. When a comitant deviation is always pres ent, it is called constant, although it may vary much in degree; if sometimes absent it is called intermittent or peri odic.
Paralytic strabismus only appears when the affected muscles are called on to perform their function. It is divided into varieties corresponding to the mus cles affected, and usually spoken of as paralyses of those muscles; as paralysis of the internal rectus, paralysis of the inferior oblique. Paralysis of all the muscles supplied by a certain nerve trunk may also be designated, according to the nerve affected. as abducens paral
ysis, oeulomotor paralysis, paralysis. Paralysis of all the extra ocular muscles, oplithalrnoplegia externa, causes some kind of squint whenever an attempt is made to look out of the direc tion in which the affected eye is turned. If both eves are affected the strabismus is usually constant.
Latent strabismus, also called heter ophoria, muscular insufficiency, or im balance, or dynamic squint, is that condi tion in which a tendency to strabismus exists but is overcome by a special effort of the appropriate muscles, in order to avoid diplopia and preserve binocular vision. The insufficiency may be of any one or more of the muscles, shown only or chiefly when the particular muscle is called into action: a sort of lateral paralytic strabismus. Or it may be found to be about the same, whatever the direction in which the eyes are turned, a latent comitant strabismus. To the latter variety the term heter ophoria (from the Greek ZTEpOC,'. dif ferent, and (pop6;;, tending) may be ap plied. The varieties of heterophoria are esophoria, tending inward, latent convergent strabismus; exophoria, tend ing outward, latent divergent strabis mus; and hyperphoria, tending upward, or latent vertical strabismus. The latter may be right or left according to the eye which tends to turn above its fellow. Ortkophoria, right tending, or muscular balance, is the normal condition, the ab sence of heterophoria.
Diagnosis. — In a case of apparent strabismus we must first determine whether the apparent deviation is real. This is done by having the patient fix his gaze steadily upon some distant ob ject; and then, while watching his eyes, covering first one and then the other, so that he is compelled to fix with them alternately. He will fix with the uncov ered eye. Then on shifting the cover, if the other eye was also properly directed while it was covered, no movement will occur. But if the covered eye was deviat ing, it will have to move in order to fix the point looked at, and the eye which previously fixed will deviate; and these movements will be repeated every time the cover is shifted. The extent of such movements indicates the amount of the deviation, and the direction shows the variety of strabismus.