Incomitant Strabismus/Module 2: A/V Patterns

Characteristics of Alphabet Patterns

An A or V (or alphabet) pattern occurs when there is horizontal incomitance from the primary position to up gaze and/or down gaze.

V esotropia - an increase in the convergent angle in down gaze
V exotropia - an increase in the divergent angle in up gaze
A esotropia - an increase in the convergent angle in up gaze
A exotropia - an increase in the divergent angle in down gaze

There are modifications of these patterns:

X pattern - an increase in the divergent angle in up gaze and down gaze
◊ pattern - an increase in the convergent angle in up gaze and down gaze
Y pattern - an exotropia is present in up gaze only
λ pattern - an exotropia is present in down gaze only

Causes

There are several theories as to the cause of A/V patterns. Some more common causes are as follows:

overaction or underaction of the horizontal recti
paresis of the cyclovertical muscles
anatomical anomalies, including orbital configuration and muscle insertion

Clinical Diagnosis

The diagnosis of an A/V pattern can only be made following a measurement (or at least estimation) of the angle in primary position, up gaze and down gaze. As a slight V is a physiological phenomenon, there must be a difference of approximately 15∆ between the angles in up gaze and down gaze before a V pattern is considered significant. A difference of approximately 10∆ is considered significant for an A pattern.

Management of A/V Patterns

The surgical options are based on whether the A/V pattern is associated with an eso- or exo-deviation, and the likely aetiology or the coexistence of superior oblique overaction or IV CN palsy as two examples. In most cases of A/V patterns, this is obvious. The three choices in the surgical treatment of A/V patterns are operating on the:

the horizontal recti
the vertical recti
the obliques