Summary
Surgery on the inferior oblique muscle involves varying degrees of weakening and, in some cases, transposition of the muscle to alter its vector force and path which normally originates at the maxillary bone and inserts on the globe near the macula.
6 Inferior Oblique Muscle Surgery
6.1 Goals
Resolve vertical and torsional diplopia.
Improve an anomalous head position, which may be due to incomitant vertical strabismus caused by a unilateral trochlear nerve paresis or associated with a dissociated vertical deviation. 1
Improve inferior oblique overaction which may be an underlying cause of a manifest dissociated vertical deviation or associated with a large V-pattern in horizontal strabismus.
6.2 Advantages
Treatment with inferior oblique surgery is more effective than prism glasses if torsional diplopia and significant incomitance are present.
Patients undergoing strabismus surgery for a horizontal deviation with a large V-pattern may benefit from simultaneous inferior oblique surgery.
6.3 Expectations
Visualization of the inferior oblique muscle with adequate exposure prior to hooking the muscle.
Isolation of the entire inferior oblique muscle, including identification of multiple muscle bellies, 2 if present, without disrupting adjacent orbital fat or the inferotemporal vortex vein, or injury to the inferior and lateral rectus muscles.
6.4 Key Principles
The inferior oblique muscle can undergo weakening by various degrees, or transposition, depending on the procedure performed:
Recession with reinsertion of the inferior oblique measured from the temporal pole of the inferior rectus insertion.
Myectomy can be graded from moderate to large.
Anterior transposition which converts the inferior oblique to a depressor. 3
Denervation and extirpation, during which a large myectomy is performed and the neurofibrovascular bundle is transected, weakening the inferior oblique by the greatest amount and reserved for only severe, recurrent cases.
6.5 Indications
Unilateral trochlear nerve (4th cranial nerve) paresis causing diplopia or ocular torticollis.
Dissociated vertical deviation with associated inferior oblique overaction or tightness on forced duction testing.
Inferior oblique overaction, typically associated with early onset strabismus with poor fusion.
Large V-pattern horizontal strabismus.
6.6 Contraindications
If multiple inferior oblique weakening procedures have been performed for persistent or recurrent overaction with vertical strabismus, an alternative approach may need to be considered to attain the desired outcome. Denervation and extirpation of the inferior oblique muscle may be considered if other options are unlikely to be effective.
6.7 Preoperative Preparation
The superior and inferior oblique muscles are assessed for underaction or overaction on versions and ductions.
Evaluation of fundus torsion may be performed by assessing the position of the foveal reflex with respect to the optic nerve during indirect ophthalmoscopy of each eye (Fig. 6.1) while the patient fixates on a target, such as the tip of a pen, held between the condensing lens and the examiner in cooperative patients, or without fixation in younger patients.
Torsional diplopia, if present, may be assessed with double Maddox rod testing or with Lancaster red-green testing in older children and teenagers.
Confirmation of a suspected tight inferior oblique muscle by bilateral exaggerated traction testing, which is performed while the patient is under anesthesia before the start of surgery.
6.8 Operative Technique
6.8.1 Exaggerated Traction Testing
Evaluation of the tightness of the oblique muscles should be performed bilaterally for comparison, including in cases of unilateral oblique muscle surgery. 4
After placement of an eyelid speculum, the globe is grasped with toothed forceps at the nasal limbus:
To evaluate the inferior oblique muscle, the globe is retropulsed and adducted first. Then the globe is depressed and intorted, rocking the surface of the globe back and forth over the inferior oblique, which is felt as a “bump,” to determine the presence of laxity or restriction (Fig. 6.2). The tightness of the inferior oblique is compared to the tightness of the ipsilateral superior oblique tendon, as well as the tightness of the contralateral inferior oblique muscle and superior oblique tendon.
To evaluate the superior oblique tendon, the globe is retropulsed and adducted first. Then the globe is elevated and extorted, rocking the surface of the globe back and forth over the superior oblique tendon. The tightness of the superior oblique is compared to the tightness of the ipsilateral inferior oblique muscle, as well as the tightness of the contralateral superior oblique tendon and inferior oblique muscle.
Following creation of an inferotemporal fornix incision, as described under Chapter 3.8.1 Fornix Incision, the steps for inferior oblique muscle surgery are described below.
6.8.2 Isolation and Disinsertion of the Inferior Oblique Muscle
The eye continues to be grasped at the limbus, and the conjunctival incision is held open with forceps. A Stevens hook is used to isolate the lateral rectus muscle with the handle nearly perpendicular to the insertion, keeping in mind the approximate location of the insertion from the limbus. Once the muscle is securely hooked, the assistant can release the forceps from the limbus, while keeping the conjunctival incision open with forceps.
A Jameson hook then hooks the lateral rectus muscle by placing the hook just posterior to the small hook, which is then removed (Fig. 6.3). The Jameson hook is held by the assistant to provide traction and to slightly displace the lateral rectus away from the inferior oblique during surgery. Alternatively, a 4–0 silk suture can be placed under the lateral rectus using a Gass hook.
A small Desmarres or Conway retractor is placed in the inferotemporal conjunctival incision to retract the conjunctiva and Tenon’s capsule inferotemporally. Posteriorly, the pink inferior oblique muscle can be visualized and will appear adherent to the overlying retracted tissue and not in contact with the globe (Fig. 6.4). The inferotemporal vortex vein may be visible near the posterior border of the inferior oblique, just temporal to the inferior rectus and should be avoided during subsequent steps.
A Stevens hook is used to hook the inferior oblique from posterior to anterior, and the inferior oblique is pulled forward (Fig. 6.5a). The surrounding Tenon’s capsule is carefully unhooked with toothed forceps (Fig. 6.5b), so that only the inferior oblique muscle remains on the hook, and the tip of the small hook can be exposed between the muscle and the connective tissue without orbital fat exposure.
The inferior oblique is hooked with a Jameson hook adjacent to the Stevens hook, and the hooks are slightly separated to examine the space bordered by the temporal and nasal aspects of the inferior oblique muscle and the sclera in the inferotemporal quadrant (Fig. 6.6). This area should not appear pink as shown in Fig. 6.6aand should be white as shown in Fig. 6.6b to indicate that the entire inferior oblique has been hooked. If this area is pink, the Jameson hook is kept in place, and the Stevens hook is used to hook the remaining portion of the inferior oblique muscle, which is rejoined with the previously hooked portion of the muscle, and steps 4 and 5 are repeated.
Once the inferior oblique is confirmed to be entirely hooked, the Stevens hook is removed and the surrounding fascia is bluntly dissected from the muscle both nasally and temporally with blunt Westcott scissors (Fig. 6.7).
The inferior oblique is then clamped near its insertion with one click of a small straight clamp, allowing room to be able to visualize the muscle at its insertion and for blunt Westcott scissors to be used to disinsert the muscle. Both tips of the clamp must be visualized around the muscle before securing the clamp (Fig. 6.8).
The inferior oblique muscle can be first strummed at its insertion with closed blades of the blunt Westcott scissors for tactile confirmation of its location. The inferior oblique is then disinserted without excessive traction on the muscle, using small consecutive snips to be certain of the location of the scissors nearly flush to the sclera (Fig. 6.9). Once the inferior oblique is disinserted, additional fascial attachments can be bluntly dissected more nasally.
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