Incomitant Strabismus/Module 2: A/V Patterns

Strabismus in Art: Portrait of a Young Man, Agnolo Bronzino, circa 1535-1540

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