Chapter 10: Pattern Strabismus
Pattern strabismus is a horizontal deviation in which there is a difference in the magnitude of deviation between upgaze and downgaze. The term V pattern describes a horizontal deviation that is more divergent (less convergent) in upgaze than in downgaze, while the term Apattern describes a horizontal deviation that is more divergent (less convergent) in downgaze than in upgaze. An A or V pattern is found in 15%–25% of horizontal strabismus cases. Less common variations of pattern strabismus include Y, X, and λ (lambda) patterns.
The following conditions are associated with various types of pattern strabismus or considered causes of these patterns.
Oblique muscle dysfunction. Apparent inferior oblique muscle overaction (overelevation in adduction [OEAd]; see Chapter 11) is associated with V patterns (Fig 10-1), and apparent superior oblique muscle overaction (overdepression in adduction [ODAd]) with A patterns (Fig 10-2). These associations may be due to the tertiary abducting action of these muscles in upgaze and downgaze, respectively; however, oblique dysfunction is frequently associated with ocular torsion that can also contribute to A or V patterns (see below).
Orbital pulley system abnormalities. Abnormalities (heterotopia) of the orbital pulley system (see Chapter 3) have been described as a cause of simulated oblique muscle overactions and of altered rectus muscle pathways and functions that can result in A or V patterns. These pulley effects may help explain the observation that patients with upward- or downward-slanting palpebral fissures (Fig 10-3) may show A or V patterns because of an underlying variation in orbital configuration, which is reflected in the orientation of the fissures. Similarly, patients with craniofacial anomalies (see Chapter 18) may have a V-pattern strabismus with marked elevation of the adducting eye as a manifestation of rotation of the orbits, pulley system, and muscle pathways.
Ocular torsion. While not as consequential as pulley dystopia, ocular torsion displaces the anterior path of the vertical rectus muscles. Extorsion displaces the superior rectus muscle temporally and the inferior rectus muscle nasally, which tends to produce a V pattern. Intorsion displaces the superior rectus nasally and the inferior rectus temporally, which tends to produce an A pattern.
Restricted horizontal rectus muscles. Contracture of the lateral rectus muscles in large-angle exotropia may result in an X pattern, with globe slippage in adduction.
Anomalous innervation. Sometimes seen in isolation and sometimes associated with other congenital cranial dysinnervation disorders (see Chapter 12), this most commonly produces a Y pattern.
Selective innervation of superior or inferior compartments of the horizontal rectus muscles. This is a possible contributing factor to A and V patterns and is under investigation (see Chapter 3).
Figure 10-1 V pattern with exotropia in upgaze and esotropia in downgaze. Note overelevation in adduction and limitation of depression in adduction.
Figure 10-2 A-pattern exotropia with overdepression and underelevation in adduction.
(Modified with permission from Levin A, Wilson T, eds. The Hospital for Sick Children’s Atlas of Pediatric Ophthalmology and Strabismus. Philadelphia: Lippincott Williams & Wilkins; 2007:11.)
Figure 10-3 Palpebral fissures that slant downward temporally, sometimes associated with a V-pattern horizontal deviation.
(Courtesy of Edward L. Raab, MD.)
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.