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  • Updated 02.15.2024
  • Released 12.12.2002
  • Expires For CME 02.15.2027

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 two epidemiologic studies (50). These are distinguished from paralytic strabismus, which may result from extraocular muscle paresis or under action 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 a 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 (50).

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 an underlying neurologic disease or are part of a systemic syndrome and may require neurologic work-up. Also, mild paralytic strabismus may be subtle and masquerade as nonparalytic comitant strabismus on cursory examination. 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 establish or restore binocular vision, to correct abnormal head posture, and for psychosocial reasons.

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

• Mild paralytic strabismus may masquerade as a comitant nonparalytic strabismus on cursory examination.

• Nonparalytic strabismus can present as an association of a general condition of neurologic importance, such as cerebral palsy, craniosynostosis, and traumatic brain injury, with particular considerations in diagnosis and management.

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 single binocular vision and stereopsis (depth perception). 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.

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

• Hirschberg test or corneal light reflex test: The Hirschberg test gives an approximate estimate of the angle of ocular misalignment by observing the displacement of the corneal light reflex from the center of the cornea. Each 1 mm of displacement requires approximately 15 PD to correct. So, a displacement of 3 mm (midway between the center of the pupil and the limbus) indicates approximately 45 PD, and a displacement of 4 mm indicates approximately 60 PD.

• Krimsky light reflex: The Krimsky light reflex is useful in uncooperative patients (eg, in children) and in patients with a single seeing eye. It measures the misalignment by centering the corneal light reflex of both eyes using an appropriate prism in front of the one seeing eye, without the need for alteration of fixation between both eyes.

• Cover test: This test involves covering one eye at a time while the patient fixates and watching the other eye for refixation movement, which would indicate tropia. The alternate cover test is the most commonly used and most accurate method of measurement, where a patient fixes on a target, and the eyes are alternately covered. Any saccade of an eye after being uncovered suggests that the eye has deviated in the opposite direction when covered. The alternate cover test should ideally be done in the nine cardinal directions of gaze, with the patient fixing on a distant target at 20 feet. It should also be performed at near, with the patient fixing on a target at 33 cm. The deviation can be quantified in prism diopters.

• Maddox rod: Maddox rod is a technique for separating and quantifying horizontal and vertical strabismus when associated with double vision and requires specialized equipment and the patient’s cooperation.

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