Weakening the inferior oblique muscle
Table 14-3 lists muscle weakening procedures, including those involving the inferior oblique muscle. These procedures are most commonly used for treatment of overelevation in adduction when it is believed to be due to inferior oblique muscle overaction. In all these procedures, the surgeon must be sure that the entire inferior oblique muscle is weakened, because the distal portion and the insertion can be anomalously duplicated (Videos 14-6, 14-7).
Strabismus surgery: inferior oblique—partial and complete hooking.
Courtesy of John D. Ferris, FRCOphth, and Peter E. J. Davies, FRANZCO, MPH.
Inferior oblique weakening procedures.
In cases that show marked asymmetry of the overactions of the inferior oblique muscles and no superior oblique muscle paralysis, unilateral surgery only on the muscle with the more prominent overaction is often followed by a significant degree of overaction in the fellow eye. Therefore, some surgeons recommend bilateral inferior oblique muscle weakening for asymmetric cases. A symmetric result is the rule and overcorrections are rare; however, inferior oblique muscles that are not overacting at all—even when there is overaction in the fellow eye—should not be weakened.
Secondary overaction of the inferior oblique muscle occurs in many patients who have superior oblique muscle paralysis. A weakening of that inferior oblique muscle typically corrects up to 15Δ of vertical deviation in primary position. The amount of vertical correction is roughly proportional to the degree of preoperative overaction (see Chapter 11). Frequently, a weakening procedure is performed on each inferior oblique muscle for V-pattern strabismus (see Chapter 10).
Moving the insertion of the inferior oblique muscle anteriorly to a point adjacent to the lateral border of the inferior rectus muscle (inferior oblique anterior transposition, inferior oblique anteriorization) weakens the normal actions of the inferior oblique. Because the neurofibrovascular bundle along the lateral border of the inferior rectus muscle can then serve as the effective origin for the distal portion of the muscle, anteriorization also allows the inferior oblique muscle to actively oppose elevation of the eye; that is, this muscle becomes an anti-elevator (see Chapter 3). This procedure is effective for treatment of dissociated vertical deviation (DVD) and is especially useful when DVD and inferior oblique overaction coexist.
Weakening the superior oblique muscle
Procedures to weaken the superior oblique muscle include tenotomy (Video 14-8); tenectomy; split-tendon lengthening; placement of a spacer of silicone, fascia lata, or nonabsorbable suture between the cut edges of the tendon to functionally lengthen it; and recession. The purpose of spacers is to prevent an excessive gap between the cut edges, but they have the disadvantage of possible adhesion formation, which can alter motility. Unilateral weakening of a superior oblique muscle is not commonly performed except as treatment for Brown syndrome (see Chapter 12) or for isolated inferior oblique muscle weakness, which is rare. Unilateral superior oblique muscle weakening can affect not only vertical alignment but also torsion, potentially creating undesired extorsion. Many ophthalmologists favor a tenotomy of only the posterior 75%–80% of the tendon to preserve the torsional action, which is controlled by the most anterior tendon fibers.
Superior oblique muscle tenotomy.
Bilateral weakening of the superior oblique muscle can be performed for A-pattern deviations and can be expected to cause an eso-shift in downgaze and almost no change in upgaze. If this procedure is performed on patients with normal binocularity, it may cause vertical or torsional strabismus with subsequent diplopia, which must be considered and discussed with the patient preoperatively.
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.