Squint is also called strabismus, or heterotropia. In other words, in orthophoria there is binocular fixation all the time and under all conditions; in heterophoria it is present only when the two eyes are uncovered, so that both see the object looked at; while in squint it is not present at all.
Or, in still plainer terms, in orthophoria both eyes are straight all the time; in heterophoria both are straight, but only so long as both are uncovered; and in squint only one eye is straight, no matter whether both eyes are uncovered or not.
In squint, while binocular fixation is altogether absent, the ability to perform monocular fixation is almost always preserved; i.e., the squinting eye will “fix” at once if the other eye is covered. It is only when there is marked amblyopia, particularly as the result of a central scotoma (or spot on the cornea in the line of vision) that the squinting eye loses its power to fix at all, and wanders uncertainly about, receiving impressions now on one, now on another portion of the retina.
The term imbalance is often used to denote the two conditions opposed to orthophoria; i.e., to denote collectively heterophoria and squint.
Varieties of Heterophoria and Squint
1. Classification According to Direction of Deviating Eye: Heterophoria and squint may be classified according to the direction assumed by the deviating eye. Thus we have the following varieties of heterophoria:
| Heterophoria | |
| Lateral Deviations | |
| Either eye deviates | |
| In, or toward the nose | Esophoria |
| Out, or toward the temple | Exophoria |
| Vertical Deviations | |
| The right eye goes up and the left down | Right Hyperphoria |
| The left eye goes up and the right down | Left Hyperphoria |
In rare cases of vertical heterophoria, each eye has either an upward tendency (anophoria) or a downward tendency (cataphoria). These cases must not be confused with anatropia and catatropia. In anaphoria and cataphoria, there is binocular fixation when both eyes are uncovered, while in anatropia and catatropia one of the eyes squints. This shows the following squint condition:
| Squint |
| Lateral Squint |
| The deviating eye turns in, or toward the nose: |
| Esotropia (Strabismus convergens—Convergent Squint) |
| The deviating eye turns out or toward the temple: |
| Exotropia (Strabismus divergens—Divergent Squint) |
| Vertical Squint |
| The deviating eye turns up: |
| Hypertropia (Strabismus sursumvergens) (Right or left) |
| The deviating eye turns down: |
| Hypotropia (Strabismus deorsumvergens) (Right or left) |
In addition to these lateral and vertical deviations, conditions exist in which the vertical meridian of one eye, instead of maintaining its parallelism with the vertical meridian of the other, either forms (or tends to form) an angle with it (cyclotropia), but is kept in position through muscular effort (cyclophoria.)