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Excerpt from Esotropia and Exotropia, A-patternsSynonyms, Key Words, and Related Terms: A-patterns, A-pattern esotropia, A-pattern exotropia, A-pattern strabismus, alphabet pattern strabismus, vertically incomitant horizontal deviation Please click here to view the full topic text: Esotropia and Exotropia, A-patternsBackground: Horizontal deviations can be divided into 2 broad categories, as follows: esotropias and exotropias. Esotropia designates a convergent horizontal strabismus; exotropia designates a divergent horizontal strabismus. Horizontal deviations are subdivided further into comitant and incomitant deviations (also referred to as concomitant and noncomitant, respectively). Comitant refers to an ocular deviation that does not vary with the direction of gaze; incomitant describes an ocular deviation that varies with the direction of gaze.Vertically incomitant describes a horizontal misalignment of the eyes in which the magnitude of the horizontal deviation differs in upgaze when compared to downgaze. The following are common patterns seen in vertically incomitant horizontal deviations: A-patterns, V-patterns, and, less commonly, Y-patterns; lambda-patterns; and X-patterns. These patterns are named using letters of the alphabet whose shapes have visual similarities to the ocular motility patterns that they describe. Vertical incomitance may be seen with both esotropias and exotropias. The term A-pattern designates a vertically incomitant horizontal deviation in which there is more convergence in midline upgaze and less convergence (increased divergence) in midline downgaze. By convention, an A-pattern is not considered to be clinically significant unless the distance measurements of the ocular deviation in midline upgaze (25° above primary gaze) and midline downgaze (25° below primary gaze) differ by at least 10 prism diopters. The term A-pattern is used because the vertical lines that comprise the letter A converge near the apex of the letter and diverge at the bottom of the letter. The appearance of the letter A reflects the clinical situation. An A-pattern esotropia is an inward deviation of the visual axes in which there is more inward deviation of the eyes in midline upgaze than in midline downgaze. An A-pattern exotropia is an outward deviation of the visual axes in which there is more divergence of the eyes in midline downgaze than in midline upgaze. Lambda-pattern is used to describe a subtype of A-pattern strabismus. In this situation, little change occurs in the amount of ocular deviation from midline upgaze to primary position, but increased divergence occurs between primary position and downgaze. Pathophysiology: Various conditions may cause A-pattern incomitance of horizontal deviations in vertical gaze. Some individuals may have more than one factor underlying their A-pattern strabismus. Etiologies are outlined below.
Oblique muscle dysfunction
With significant A-patterns, version testing usually reveals superior oblique muscle overaction. The tertiary abduction effect of the superior oblique muscle is believed to produce the A-pattern. The abducting force is greatest in downgaze, the superior oblique's primary field of action, causing an increased relative divergence of the eyes in downgaze.
Generally, 2 types of oblique muscle dysfunction are associated with an A-pattern, primary superior oblique muscle overaction and secondary superior oblique muscle overaction. Primary superior oblique muscle overaction refers to overaction of the muscle with no identifiable etiology. The exact cause of the overaction remains unclear. Several hypotheses of the overaction exist. Why some individuals manifest oblique muscle dysfunction is unknown. Neurologic and mechanical hypotheses have been proposed. Inferior oblique muscle paresis is a rare entity that can cause secondary overaction of the ipsilateral superior oblique muscle.
Horizontal rectus muscle dysfunction
As a proposed cause or contributing factor in the development of an A-pattern strabismus, horizontal rectus muscle dysfunction could explain why A-pattern strabismus may occur without apparent superior oblique muscle overaction.
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