Strabismus also known as heterotropia (and including the three variants cross-eye, lazy-eye andwalleye), is a condition in which the eyes are not properly aligned with each other. It typically involves a lack of coordination between theextraocular muscles, which prevents bringing the gaze of each eye to the same point in space and thus hampers proper binocular vision, and which may adversely affect depth perception. Strabismus can present as manifest (heterotropia) or latent (heterophoria) varieties, and can be either a disorder of the brain in coordinating the eyes, or of the power or direction of motion of one or more of the relevant muscles moving the eye. Strabismus is primarily managed by ophthalmologists, optometrists and orthoptists. Strabismus is present in about 4% of children. Treatment should be started as soon as possible to ensure the best possible visual acuity and stereopsis.
One eye moves normally, while the other points in (esotropia), out (exotropia), up (hypertropia) or down (hypotropia).
Strabismus may be associated with “lazy eye” or “amblyopia” which is due to brain’s ignoring input from one eye. Amblyopia results in impaired vision in the affected eye. During the first 7–8 years of life the brain learns how to interpret the signals that come from an eye. This process which is called visual development may be interrupted by strabismus. This is because the deviated eye is not used to focus, and the brain does not receive signals from this eye which causes failure in the normal visual development of the affected eye in childhood.
“Cross-eyed” means that when a person with strabismus looks at an object, one eye fixes on the object and the other fixes with a convergence angle less than zero; the visual axes over-converge. “Wall-eyed” means that when a person with strabismus looks at an object, one eye fixes on the object and the other fixes with a convergence angle greater than zero; that is, the visual axes diverge from parallel.
Non-paretic squints are generally concomitant, which means that the amount of deviation remains the same in all directions of gaze. Nonparetic squints are not caused by a lesion reducing innervation.
The squint may be associated with refractive error in one or both eyes. This refractive error causes poor vision in one eye and so stops the brain from being able to use both eyes together.