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. 
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.
According to this theory, an A-pattern esotropia would be due to underaction of the lateral rectus muscles; an A-pattern exotropia would be due to underaction of the medial rectus muscles. Electromyographic studies of patients with A-pattern strabismus support this theory. 
Vertical rectus muscle dysfunction
Abnormally functioning vertical rectus muscles have been proposed as a cause or contributing factor in the development of A-patterns. This theory is based on the tertiary adduction action produced by the vertical rectus muscles in their field of action to produce the A-pattern.
According to this theory, overaction of the superior rectus muscles would result in increased adducting effect (increased convergence) in upgaze. Underaction of the inferior rectus muscles would result in decreased adduction (decreased convergence) in downgaze, thereby producing an A-pattern.
Abnormalities of extraocular muscles or globe torsion
A-pattern strabismus may be related to the complex interplay of the ocular muscles and orbital soft tissues. Abnormalities in the location of the orbital connective tissue sleeves (which act as mechanical muscle pulleys) can cause incomitant deviations, simulating overaction of the superior oblique muscle. The heterotopic muscle pulleys, probably a superiorly displaced lateral rectus muscle pulley, may be the etiology of A-patterns, and the oblique muscles may be normal. 
Torsion of the globe may be the etiology of horizontally incomitant strabismus. Globe torsion may be due to abnormal oblique muscle function or loss of fusion.  Globe rotation alters the relationship of the extraocular muscles and/or extraocular muscle pulleys. By altering the vertical alignment of the horizontal rectus muscle insertions, these muscles can act as partial elevators or depressors. Conversely, by altering the horizontal alignment of the vertical rectus muscle insertions, these muscles can act as partial abductors or adductors. The vector forces would be changed in both magnitude and direction. According to this theory, intorsion of the globe alters the vector forces of the rectus muscles, causing an A-pattern.
Anatomic abnormalities of facial structure
Although their exact significance remains unclear, certain facial characteristics have been noted in some individuals with vertically incomitant strabismus.
A-pattern esotropia has been associated with both flat lid margins and eyes in which lateral canthi are higher than medial canthi, producing a mongoloid appearance.
A-pattern exotropia has not been associated with the appearance of lid fissures.
The frequency of A-pattern deviations among the general population is unknown. Among individuals with strabismus, the prevalence of an A-pattern varies among different studies, ranging from as low as 4.5% to as high as 36%. The ratio of A-pattern esotropia to A-pattern exotropia is approximately 2.2:1.
In some individuals, the presence of an A-pattern strabismus may be insignificant and only cosmetically bothersome to the patient and the patient's family. Conversely, the misalignment of the visual axes of the 2 eyes may interfere with the patient's ability to fuse and develop normal binocular vision. It also may cause diplopia in children and adults. Abnormal vertical head postures may develop to place the eyes in a position of minimal deviation to restore single binocular vision.
A child with an A-pattern esotropia may be orthotropic in primary gaze, and even able to fuse in downgaze, but manifest a significant esotropia in upgaze. Although this situation may be functionally satisfactory, the cosmetic appearance of the inward deviation in upgaze may be quite disconcerting to the parents.
Conversely, a child with an A-pattern exotropia may appear aligned in upgaze but not be able to fuse in downgaze. Parents may be unaware of the eye misalignment because the eyes may appear straight when they look at their child. Although this may be cosmetically acceptable, this is functionally undesirable.
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