2020–2021 BCSC Basic and Clinical Science Course™
6 Pediatric Ophthalmology and Strabismus
Part I: Strabismus
Chapter 10: Pattern Strabismus
Clinically significant patterns (see the section Clinical Features and Identification of Pattern Strabismus) are typically treated surgically, in combination with correction of the underlying horizontal deviation.
Surgical Correction of Pattern Deviations: General Principles
The following are strategies for surgical correction of pattern deviations. See Chapter 14 for further discussion of some of the procedures and concepts mentioned here.
For pattern strabismus associated with apparent overaction of the oblique muscles (OEAd, ODAd), weakening of the oblique muscles is performed.
For patients with no apparent overaction of the oblique muscles or a pattern inconsistent with oblique dysfunction, vertical transposition of the horizontal muscles is performed. The muscles are transposed from one-half to a full tendon width. The medial rectus muscles are always moved toward the “apex” of the pattern (ie, upward in A patterns and downward in V patterns). The lateral rectus muscles are moved toward the open end (ie, upward in V patterns and downward in A patterns). A useful mnemonic is MALE: medial rectus muscle to the apex, lateral rectus muscle to the empty space. These rules apply whether the horizontal rectus muscles are weakened or tightened (Fig 10-4).
If transposition of horizontal rectus muscles is used to treat pattern strabismus when there is associated ocular torsion, it will exacerbate the torsion (extorsion with V pattern and intorsion with A pattern), which itself can contribute to the pattern. Conversely, when rectus muscle transposition is used to treat torsion, it will make any associated pattern strabismus worse.
When horizontal rectus muscle recession-resection surgery is the preferred choice because of other pertinent factors (eg, prior surgery, unimprovable vision in 1 eye), displacement of the rectus muscle insertions should be in mutually opposite directions, according to the rules stated previously. Unlike what occurs when both horizontal rectus muscles of an eye are moved in the same direction, this displacement has little, if any, vertical effect in the primary position. This procedure should be used with caution in patients with binocular fusion as it can produce symptomatic torsion.
Some surgeons adjust the amount of horizontal surgery because of the potential effect of oblique muscle weakening on the horizontal deviation, particularly for superior oblique muscle surgery, but this is controversial. Some believe that bilateral superior oblique weakening causes a change of 10Δ–15Δ toward convergence in primary position and suggest modifying the amount of horizontal surgery to compensate for this expected change. For inferior oblique muscle weakening procedures, the amount of horizontal rectus muscle surgery does not need to be altered, because the inferior oblique muscle weakening does not substantially change primary position alignment.
Surgery on the vertical rectus muscles (eg, temporal displacement of the superior rectus muscles for A-pattern esotropia or temporal displacement of the inferior rectus muscles for V-pattern esotropia) is rarely used because transposition of the horizontal rectus muscles that are being operated on for the underlying esotropia or exotropia is usually sufficient.
Figure 10-4 Direction of displacement of medial rectus (MR) and lateral rectus (LR) muscles in procedures to treat A-pattern (left) and V-pattern (right) deviations.
(Reprinted with permission from von Noorden GK, Campos EC. Binocular Vision and Ocular Motility: Theory and Management of Strabismus. 6th ed. St Louis: Mosby; 2002:388.)
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.