Inferior oblique weakening procedures presented in this chapter include chemodenervation, disinsertion, various myotomies, recessions, myectomy, nasal myectomy, anterior transposition, anterior nasal transposition, and denervations.
25 The Inferior Oblique: Surgical Techniques
25.1 Surgical Technique
Most procedures to weaken the inferior oblique (IO) require surgical exposure of its insertional end. This is best accomplished through an inferior, temporal, bulbar conjunctival incision made 8 to 10 mm from the limbus and 1 to 2 mm above the thickened portion of Tenon’s tissue, which is easily seen in the fornix. The incision should be made above the fornix to avoid excessive bleeding. To expose this area, the eye should be elevated and adducted to a maximum degree by grasping the inferior temporal conjunctiva with forceps at the limbus. The incision should be 8 to 10 mm long; using blunt-tipped Westcott scissors placed perpendicularly against the globe with the blades fully open, cut with mild pressure.
Following the conjunctival incision, Tenon’s capsule should be incised vertically, perpendicular to the conjunctival incision to access the sub-Tenon’s space. The incision should be large enough to avoid undue traction and movement restriction. The incision will often stretch and enlarge during surgery. Through this incision, double marginal myotomy, disinsertion, inferotemporal myotomy and myectomy, recession, anterior transposition, denervation and extirpation, and nasal myotomies can all be performed.
25.1.2 Isolation of the Inferior Oblique
Once the sub-Tenon’s space is entered, the lateral rectus (LR) is used for traction to maintain elevation and adduction of the eye. A 4–0 silk suture or muscle hook can be used with the LR to maintain the eye in the adducted and elevated position, allowing for the best exposure of the IO insertion.
A convenient variation of this technique, allowing omission of the silk traction suture, may be performed with a muscle hook alone. This is illustrated in Video 25.1.
A von Graefe hook, Desmarres retractor, or Manson hook is then placed in the sub-Tenon’s space to retract the intermuscular septum and conjunctiva inferotemporally. A Stevens hook is used to retract the muscle. A ribbon retractor, lens loop, or muscle hook can then be placed on the sclera, and light pressure against the globe will retract it nasally and allow direct visualization of the IO. A Stevens or von Graefe muscle hook is placed between the IO and the sclera with tip directed inferiorly. To capture the entire IO, the tip is rotated away from the sclera. The IO can easily split during this maneuver, so care must be taken to hook the entire muscle.
Constant awareness of the location of the adjacent rectus muscles and inferior temporal vortex vein is essential during IO surgery. The muscle hook should be placed so as to avoid direct contact with or indirect traction on the vortex vein. The IO should then be gently lifted away from the sclera. Penetration of Tenon’s capsule adherent to the muscle capsule should be avoided, as extraconal fat lies immediately externally. To accomplish this, lift all tissue except the IO off the muscle hook.
Once Tenon’s capsule is removed from the muscle hook, a sharp incision with Westcott scissors can be made along this posterior muscle border to penetrate any residual fascial attachments and expose the tip of the Stevens hook beneath the IO. A large muscle hook can then be placed from a posterior direction through this opening. Tenon’s capsule, the fat external to it, and the conjunctiva are now retracted temporally with this hook. The original hook retracting intermuscular septum and conjunctiva can now be removed. A second large hook now replaces the Stevens hook, which holds the IO. Lyse the remaining fibrous attachments surrounding the IO from this hook distal to the scleral insertion. The IO insertion should now be completely visible, and the selected weakening technique may be performed.
The proximity of the macula is important to consider if the surgeon attempts to preplace a suture in the IO near its insertion before disinserting it from the sclera. An Apt clamp or noncrushing vascular clamp is used to engage the IO before disinsertion and suture placement. This minimizes the risk of inadvertent scleral penetration. The importance of good assistance, good illumination, good exposure, and complete identification of the IO insertion cannot be overemphasized. It is also important to not distort the anatomy of the IO as it is disinserted. If the surgeon chooses to place a hemostat or clamp on the IO near the sclera before disinsertion, he or she should be certain that the muscle is retracted by the underlying hook in such a way as to prevent muscle rotation or twisting around its long axis. Suture preplacement in the IO insertion increases the chance of this error.
After disinsertion, most weakening procedures are best done by engaging the inferior rectus (IR) on a Green hook for exposure purposes. The surgical assistant must use gentle traction on the IR to prevent muscle rupture. If there is concern over IR traction, it is better to substitute locking forceps engaged on the sclera at the insertion of the IR rather than a hook to retract the globe superiorly.
Inadvertent incision of the inferior or lateral rectus may occur during IO surgery. This risk is increased when scarring from previous strabismus or retinal detachment procedures is present. To prevent disorientation, the IR and LR are first isolated as landmarks before further dissection is attempted. Reoperation on the IO with adjacent tissue scarring requires an experienced surgeon and assistant.
25.2 Inferior Oblique Weakening Surgeries
Although chemodenervation is a theoretical possibility for mild weakening of the IO, it should be approached with caution. Transconjunctival injection in an awake patient is possible, but the botulinum toxin fluid frequently spreads to other adjacent muscles. Injection under direct observation of the IO during surgery on other muscle is also possible. Since the amount of IO weakening achieved by this procedure is small, and most mildly overacting IOs can be left alone, it is rarely used. Botulinum toxin chemodenervation may be more useful in cases of secondary IO overaction when the primary position deviation is small but symptoms of diplopia exist in contralateral side gaze. 1
25.2.2 Double Marginal Myotomy
An option for mild IO overaction is double margin (Z) myotomy. Some surgeons utilize it frequently for mild overaction without abduction in elevation. This should be considered for the least weakening effect possible. Restrictive strabismus due to excessive scarring is a risk with this procedure.
22.214.171.124 Surgical Technique
As with any double margin (Z) myotomy, the IO is clamped along the anterior and posterior borders in an overlapping fashion but without the muscle being completely incorporated into either clamp. A distance of several millimeters should be left between the two clamps. Once the clamps are removed, the muscle is cut along the crushed tracts, resulting in muscle lengthening.
Disinsertion has been a periodically popular procedure that is still preferred by some surgeons in select clinical situations. Its greatest downfall is inconsistent results and lack of predictability. Scleral reattachment following disinsertion is extremely variable, often occurring at the original insertion. Some surgeons imbricate Tenon’s capsule over the cut end of the IO in an attempt to prevent its reattachment to the sclera.
126.96.36.199 Surgical Technique
Disinsertion can be accomplished by cutting the IO free from it scleral insertion. The IO is allowed to retract and no attempt is made to reattach it to the globe. Because the muscle can reattach at variable, unpredictable scleral locations, this procedure produces inconsistent results and a high percentage of residual IO overaction.
25.2.4 Myotomy at the Origin
Myotomy at the origin of the IO is of historic interest only. We are unaware of any clinical situation in which this procedure would not be replaced by another weakening technique, as the surgical approach to the IO through the orbital fat external to Tenon’s capsule is undesirable.
25.2.5 Myotomy Inferotemporally
Myotomy inferotemporally has the advantage of being a technically simple procedure. The muscle is often seen immediately after incision of the conjunctiva and underlying Tenon’s capsule in the inferotemporal quadrant. Risk of macular damage can be almost eliminated by performing the procedure proximal to the scleral insertion. If accurate anatomical isolation is not maintained, myotomy of the IR or LR can be inadvertently performed. Inconsistency and reduced predictability of the procedure stem from readherence of the muscle ends either directly or by intervening scar tissue. This frequently results in residual or recurrent IO overaction.
188.8.131.52 Surgical Technique
The IO can be lysed 5 to 8 mm proximal to its insertion. To control bleeding, two hemostats are placed across the IO on either side of the proposed myotomy. Once cut, the ends can be cauterized. The muscle ends can occasionally reattach directly or by way of a band of scar tissue between them. One can attempt to prevent the proximal end from reattaching to the sclera in the inferotemporal quadrant by imbricating Tenon’s capsule around the proximal end of the IO.
The IO can be recessed 2 , 3 along its anatomical path as much as 14 mm. Reattaching the muscle to the globe in a location of one’s choice increases the predictability of the procedure. Recessions are the most commonly performed type of controlled reattachment. The 10-mm recession is a time-honored and frequently used procedure. The procedure provides a high percentage of satisfactory results in moderate
IO overaction with or without abduction. It is also a procedure to correct IO overaction secondary to superior oblique (SO) underaction (Video 25.1).
It is not uncommon to see subtle but definite midline supraduction deficiencies following unilateral 10-mm recessions. This could account for the tendency of contralateral IO overaction to develop after a unilateral 10-mm recession as the supraduction defect produces pseudo-overaction in the other eye. Reinserting the IO adjacent, 2 mm temporal, or 4 mm temporal to the IR represents a 14-, 12-, or 10-mm recession, when accounting for Apt and Call’s 3 anatomical landmarks. To avoid an anterior transposition, the anterior suture should be placed 4 to 6 mm posterior to the IR insertion and the posterior suture placed 3 mm further, directly posterior. This will maintain a pure recession procedure along the long axis of the IO.
A true 14-mm recession would reattach at the lateral border of the IR, preferably 5 to 8 mm posterior to the IR insertion, to prevent any anterior transpositioning of the posterior arm. The recession techniques are pure weakening procedures of the IO muscle. Combining recession and anterior transposition weakens the overacting IO and converts half of the IO to a tonic depressor and antielevator.
184.108.40.206 Surgical Technique
Any IO weakening that requires reconnecting the muscle to the sclera in a chosen location can be referred to as a controlled weakening or reattachment procedure. Recessions require that a suture be placed in the IO near its insertion while the muscle is still attached to the globe or after the muscle has been clamped. Since the posterior border of the IO is very close to the macula, it is important for the surgeon to be very comfortable with preplacing sutures before disinsertion. Complicated suture placement can be avoided by relaxing the tension on the muscle before the suture placement to allow the muscle to reorient to its natural position. There is concern about possible macular damage from sclera penetration with suture placement this close to the sclera. Suture placement after disinsertion eliminates this risk.
Once the lock bites of the suture are properly placed in the anterior and posterior edge of the muscle, this orientation must be maintained. This can be done by leaving the suture extending from the anterior muscle border shorter than that from the posterior edge. Since the reattachment position of the posterior edge of the muscle affects the results of an anterior transposition, the location of those fibers must be maintained throughout the procedure.
Once the muscle has been disinserted and the suture properly placed, the IO can be reattached to the globe in a variety of locations. The two most popular sites have been designated 10-mm and 14-mm recessions. The true insertions of the temporal margin of the IR should be used as an initial reference for all controlled reattachment procedures. The lateral border if the IR is 14 mm from the insertion of the IO. Four mm temporal to the IR is therefore 10 mm from the IO insertion and in line with the vortex vein. To keep the recessed IO in line with the normal pathway of the muscle, the muscle should be sutured to the sclera 5 to 8 mm posterior to the lateral border of the IR insertion. The posterior suture should be placed 2 to 3 mm directly posterior to the anterior suture (Fig. 25‑1).