CHAPTER 10

Pattern Strabismus

Pattern strabismus is present when a horizontal deviation changes in magnitude between upgaze and downgaze. The most common type is a V pattern, in which the horizontal deviation is more divergent (less convergent) in upgaze than in downgaze. An A pattern is present when the horizontal deviation is more divergent (less convergent) in downgaze compared with 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.

Etiology

Ophthalmologists have considered each of the following conditions to be a cause of A and V patterns, but this issue remains unsettled:

Dysfunction of oblique muscles. Apparent inferior oblique muscle overaction is associated with V patterns (Figs 10-1, 10-2), and superior oblique muscle overaction with A patterns (Figs 10-3, 10-4), reflecting the ancillary abducting action in upgaze and downgaze, respectively, attributed to these muscles. Whether a true primary overaction of the oblique muscles exists, especially with respect to their vertical actions, is controversial (see Chapter 11). Because the terms overelevation in adduction (OEAd) and overdepression in adduction (ODAd) accurately describe the abnormality without implying an etiology, they have been suggested as alternatives to the traditional terminology.

Abnormalities of the orbital pulley system. 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, which 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 10-5) may show A and V patterns because of an underlying variation in orbital configuration 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 exaggerated altered muscle pathways.

Dysfunction of horizontal rectus muscles. One early school of thought attributed A and V patterns to varying effectiveness of the lateral rectus muscles in the upper half of the vertical gaze field, and of the medial rectus muscles in the lower half of this field. According to this concept, increases in lateral rectus or medial rectus muscle action produce a V pattern; decreased action of these muscles produces an A pattern. Although, empirically, horizontal rectus muscle surgery that includes displacement of the insertions may effectively treat the underlying deviation, there is no convincing evidence that innervation of these muscles varies in vertical gazes.

Selective innervation of horizontal rectus muscles. This is a possible contributing factor and is under investigation (see Chapter 3).

Dysfunction of vertical rectus muscles. Increases or decreases in the tertiary adducting action of these muscles may result in a less convergent or more divergent alignment in upgaze or downgaze and a corresponding pattern.

Figure 10-1 V-pattern esotropia. Note overelevation and limitation of depression in adduction.