Vertically incomitant pattern strabismus comprises 50% of infantile horizontal strabismus. The oblique muscle dysfunction has been associated with pattern strabismus. High-resolution orbit imaging and contemporary neurophysiology studies in non-human primate models of strabismus have shed light into the mechanisms of pattern strabismus. In this review, we will examine our current understanding of etiologies of pattern strabismus. Speculated pathophysiology includes oblique muscle dysfunction, loss of fusion with altered recti muscle pull, displacements and instability in connective tissue pulleys of the recti muscles, vestibular hypofunction, and abnormal neural connections. Orbital mechanical factors, such as abnormal pulleys, were reported as a cause of pattern strabismus in patients with craniofacial anomalies, connective tissue disorders, and late-onset strabismus. In contrast, abnormal neural connections could be responsible for the development of a pattern in infantile-onset strabismus. Pattern strabismus is likely multifactorial. Understanding the mechanisms of pattern strabismus is pivotal to determine an appropriate surgical treatment strategy for these patients. 1. Background “A” and “V” patterns describe horizontal strabismus that is vertically incomitant and is characterized by a substantial change in the horizontal deviation from the midline position in upgaze as compared to downgaze. Approximately 12–50% of strabismus patients have “A” or “V” patterns [1–4]. “A” and “V” patterns are relatively frequent in patients with infantile-onset strabismus. “A” pattern is present when a horizontal deviation shows a more convergent (less divergent) alignment in upgaze compared with downgaze. The “V” pattern describes a horizontal deviation that is more convergent (less divergent) in downgaze compared with upgaze. The “A” pattern is considered clinically significant when the difference in the measurement between upgaze and downgaze, each approximately 25° from the primary position, is at least 10 prism diopters ( ). The “V” pattern is considered clinically significant when the difference is at least 15 . Other patterns include “Y” where exodeviation is present only in upgaze, lambda (“λ”) where exodeviation is present only in downgaze, or “X” where exodeviation is present in downgaze and upgaze compared to the primary position. This review will focus on “A” and “V” pattern strabismus. Several theories have been proposed to describe the pathophysiology of “A” and “V” pattern strabismus. Initially it was believed that pattern strabismus is the
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