CHAPTER 2

Strabismus Terminology

The term strabismus is derived from the Greek word strabismos—“to squint, to look obliquely or askance”—and means ocular misalignment. This misalignment may be caused by abnormalities in binocular vision or by anomalies of neuromuscular control of ocular motility. Many terms are employed in discussions of strabismus, and unless these terms are used correctly, confusion and misunderstanding can occur. Unfortunately, some terms still in use are not correct physiologically.

Orthophoria is the ideal condition of perfect ocular alignment. In reality, orthophoria is seldom encountered, as a small heterophoria can be found in most people. Some ophthalmologists therefore prefer orthotropia to mean correct direction or position of the eyes under binocular conditions. Both terms are commonly used to describe eyes without manifest strabismus. Heterophoria is an ocular deviation kept latent by the fusional mechanism (latent strabismus). Heterotropia is a deviation that is manifest and not kept under control by the fusional mechanism (manifest strabismus). It is sometimes helpful to identify the deviating eye. This practice is particularly helpful when the clinician is measuring vertical deviations or restrictive or paretic strabismus, or when amblyopia is present in a preverbal child.

Prefixes and Suffixes

A detailed nomenclature has evolved to describe the various types of ocular deviations. This vocabulary uses many prefixes and suffixes based on the relative positions of the visual axes of both eyes to account for the multiple strabismic patterns encountered.

Prefixes

Eso- The eye is rotated so that the cornea is deviated nasally. Because the visual axes align at a point closer than the fixation target, this state is also known as convergent strabismus, one type of horizontal strabismus.

Exo- The eye is rotated so that the cornea is deviated temporally. Because the visual axes are diverging from the fixation target, this state is also known as divergent strabismus, another form of horizontal strabismus.

Hyper- The eye is rotated so that the cornea is deviated superiorly. This describes one type of vertical strabismus.

Hypo- The eye is rotated so that the cornea is deviated inferiorly. This describes another type of vertical strabismus.