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  • Updated 07.25.2020
  • Released 12.12.2002
  • Expires For CME 07.25.2023

Nonparalytic horizontal strabismus



The author reviews the different forms of nonparalytic horizontal strabismus (eye misalignment), including eyes in-turning (esotropia) and out-turning (exotropia) strabismus. This is the most common kind of strabismus with comitant (similar magnitude of ocular misalignment in all directions of gaze) heterotropia representing 67% to 74% or significant/decompensated heterophorias 8% to 17% according to 2 epidemiologic studies (Fetcher et al 1966; 56). These are distinguished from paralytic strabismus, which may result from extraocular muscle paresis underaction, resulting from neurologic deficit at any point along the extraocular motor pathway from the cerebral cortex, through brain stem, along the oculomotor nerves (III, IV, and VI), and the neuromuscular junction to the extraocular muscles. Paralytic strabismus is usually incomitant with higher magnitude of deviation (more pronounced misalignment) in the field of action of the paretic muscle and represents 7% to 10% of all strabismus cases (Fetcher et al 1966; 56).

Treatment of nonparalytic strabismus conditions includes nonsurgical measures like patching one eye or glasses. In some cases, strabismus surgery is helpful to restore binocular vision, to correct abnormal head posture, and for psychosocial reasons. Nonparalytic strabismus conditions may have different pathophysiologic mechanisms, some of which are yet to be discovered.

Although nonparalytic eye deviations are usually isolated ocular conditions, they rarely may have underlying neurologic disease and may require neurologic work-up. Also, mild paralytic strabismus may be subtle and masquerade as nonparalytic comitant strabismus on cursory exam. The aim of this update is to review different types of nonparalytic strabismus and how to differentiate them from subtle paralytic mimickers.

Key points

• Horizontal strabismus (eyes misalignment) can be classified by exotropia (turned out) and esotropia (turned in).

• Nonparalytic strabismus refers to ocular misalignment and is not due to cranial nerve or neuromuscular junction causes.

• Treatment of nonparalytic strabismus conditions includes nonsurgical measures like patching one eye or glasses, as well as surgery to binocular vision, to correct abnormal head posture, and for psychosocial reasons.

• Nonparalytic strabismus may rarely be an underlying neurologic cause and may require neurologic work-up.

Historical note and terminology

In primitive folklore and mythology, strabismus, which is misalignment of eyes relative to each other, was considered an affliction sent by a malignant spirit or evil god. The bad luck brought from the look of cross-eyed person forms the basis for the legends based on “the evil eye." Maya, the Egyptian goddess, squinted; and the statue of King Djoser (2600 BC), for whom the first pyramid was erected, depicts him as having a large internal ocular deviation. Hippocrates first differentiated between paralytic and nonparalytic type of squint and correctly stressed the hereditary aspect of the latter.

Ocular deviations are described as a phoria or a tropia (used as a suffix). A phoria is a misalignment of the visual axis kept in check by fusion, meaning that when both eyes are being used the eyes are aligned, but the misalignment occurs when vision in one is interrupted. Fusion is an innate drive to keep each fovea directed to the object of regard, thus, allowing stereopsis (depth perception) and single binocular vision. A tropia is a manifest misalignment of the visual axis not controlled by fusion, meaning that when both eyes are being used the misalignment is evident.

Terminology of ocular misalignment:

• Eso: a prefix that means inward “toward the nose” deviation

• Exo: a prefix that means outward “away from the nose or toward the ear” deviation

• Hyper: a prefix that means upward deviation

• Hypo: a prefix that means downward deviation

• Hetero: a prefix that means deviation

• Ortho: a prefix that means straight

• Apparent or pseudo-strabismus (apparent esotropia or exotropia): straight eyes that give false impression of apparent inward or outward strabismus or pseudo-esotropia or pseudo-exotropia, respectively.

• Prism diopter: Prism diopter is the measurement unit of ocular misalignment with larger numbers meaning larger deviation. One prism diopter deviates the light reflex 1 cm at 1 meter. This is sometimes called the angle of deviation.

Measurement methods of strabismus convenient for bedside examination by neurologists include:

• Hirschberg test or Corneal light reflex test: An estimate of the angle of ocular misalignment may be made based on observation of the corneal light reflex, which is called the Hirschberg test. The observer then estimates each millimeter displacement of the light reflex from the center of the cornea of the deviated eye. Thus, with the penlight held at one third of a meter from the patient, each 1 mm displacement of the reflex equals 70 of arc deviation. Because an average cornea is about 10 to 11 mm, a reflex falling at the limbus indicates a 5 to 6 mm displacement or a 35 to 40 degree deviation.

• Krimsky light reflex: The Krimsky light reflex measures the misalignment by centering corneal light reflex of both eyes using an appropriate prism in front of the seeing eye.

• Cover test: This test involves covering 1 eye at a time while the patient fixates and watching the other eye for refixation movement, which would indicate tropia. Alternate cover test is the most commonly used and most accurate method of measurement, where a patient fixes on a target preferably at 20 feet, and the eyes are alternately covered (12; 14). Any saccade of an eye after being uncovered suggests that the eye has deviated in the opposite direction when covered. Prisms can be placed before the fixing eye until no further movement of the deviated eye is observed when the cover is alternately placed from one eye to the other.

• Maddox rod: Maddox rod is a technique for separating and quantifying horizontal and vertical strabismus and requires specialized equipment.

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