Pathophysiology: Multiple factors may be responsible for V-pattern strabismus, and, in a given individual, one or more factors may contribute toward its cause.
Inferior oblique muscle overaction
The most widely accepted mechanism contributing to the causation of V-pattern strabismus is associated with inferior oblique muscle overaction and/or relative underaction of the superior oblique muscles. Inferior oblique muscle overaction produces significant horizontal deviation in upgaze. In upgaze, the horizontal action of the inferior oblique muscles is abduction; therefore, inferior oblique muscle overaction produces relative divergence in elevation. A V-pattern results in greater exotropia (or less esotropia) in upgaze and greater esotropia (or less exotropia) in downgaze. Not all V-pattern strabismus is associated with oblique muscle dysfunction. See Images 7-8.
Facial and orbital configuration
An association exists between individuals with hypoplasia of the malar bones, anti-Mongoloid lid slants, and S-shaped contours of the lower lid, and overaction of the inferior oblique muscles and V-pattern strabismus. Patients with craniosynostosis have shallow orbits, and the angle between the visual axis and the insertion of the inferior oblique muscle is increased. This increased angle increases the abduction ability of the inferior oblique muscles in upgaze, resulting in a V-pattern. Contributing to this condition is the complex interplay of these factors with excyclotorsion of the globe.
Effect of the pulley systems
Upward displacement of the pulley systems around the lateral rectus muscle may cause V-pattern strabismus.
Horizontal muscle overaction
Initially, this theory was believed to be responsible for V-pattern strabismus, but no convincing evidence supports it. In V-pattern esotropia, the medial rectus muscle was believed to overact, thereby increasing esotropia in downgaze, and, in V-pattern exotropia, the lateral rectus muscle overaction was responsible for increased exotropia in upgaze.
Vertical rectus muscle imbalance
One theory states that V-pattern strabismus occurs because of underacting superior recti muscles and corresponding overacting inferior oblique muscles. This hypothesis was applied when nasal and temporal displacements were used to treat A- and V-patterns. This theory has been abandone .....
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