The Basics
Lucas Bonafede, MD
All the extraocular muscles except the inferior oblique muscle originate from the orbital apex. The inferior oblique muscle originates from the anterior orbital floor lateral to the lacrimal sac.
The rectus muscles form the muscle cone in the orbit with the apex at their origin and base at their penetration of Tenon capsule.
The muscles are surrounded by fibrous capsules that are attached by a thin continuous membrane called the intermuscular septum.
The superior rectus and inferior rectus muscles have attachments to the upper and lower eyelids, respectively.
 FIGURE 42.1. Anatomic representation of extraocular muscle insertions, innervations, and function. |
Because of attachments to the upper and lower eyelids, vertical rectus muscle strengthening or weakening procedures can change the eyelid position.
The arterial blood supply to the rectus muscles is from the medial and lateral muscular branches of the ophthalmic artery, which give rise to the anterior ciliary arteries. Variable number of anterior ciliary arteries emerge from the rectus muscle tendons and enter the episclera by the limbus to perfuse the anterior segment.
Anterior segment ischemia is a risk of strabismus surgery, especially when operating on multiple muscles and in patients with risk factors (see Chapter 54).
ANESTHESIA (SEE CHAPTER 1)
The choice of anesthesia depends on patient factors (age, level of anxiety, ability to cooperate with surgery, comorbidities), surgical plan (ie, adjustable sutures, reoperation, bilateral), and surgeon preference.
General anesthesia:
Best option for children and uncooperative adults.
Risks of general anesthesia should be reviewed with the patient.
May limit ability to accurately test ocular alignment until full recovery.
Most suitable for pediatric patients, complex cases, reoperations, and bilateral surgery.
Topical anesthesia:
Good option for cooperative patients, patients in whom general anesthesia is contraindicated, and adjustable suture cases.
Usually administered in conjunction with light sedation.
Typically performed with tetracaine and lidocaine gel; reapplications may be necessary.
May require conversion to general or retrobulbar anesthesia if unable to complete surgery safely with topical anesthesia.
Most suitable for routine cases; use cautiously for reoperations or complicated strabismus.
Retrobulbar anesthesia:
May limit ability to accurately test ocular alignment until anesthetic has reversed.
Risks scleral perforation and/or retrobulbar hemorrhage (especially if the patient is on blood thinners).
Relative contraindications—increased axial length, bleeding diathesis, thyroid eye disease, space-occupying lesion within the orbit, and previous scleral buckling.
Because CN IV travels outside the muscle cone, the superior oblique may not be affected by a retrobulbar block until there is adequate diffusion of the anesthetic.
Take care to limit the volume injected of retrobulbar anesthesia to avoid excess sub-Tenon and subconjunctival fluid, which can make surgery more difficult. A modified Van Lint block can be used in conjunction with a retrobulbar block in order to provide akinesia to the orbicularis oculi.
Local anesthetic dosing:3
It is important to consider maximum doses for safe administration of anesthetics. This is particularly relevant in infant surgery, especially oculoplastics procedures such as frontalis sling and blepharophimosis repair.
Maximum dose recommendations are based on systemic toxicity, not local toxicity.
Toxicity: CNS dysfunction and depression, including respiratory and cardiovascular dysfunction.
Maximum single dose should be determined by the clinical judgment of the physician administering the medication. General guidelines based on the cited reference are as follows:
Lidocaine with epinephrine: 6-7 mg/kg (max 500 mg/dose).
Bupivacaine with epinephrine: 2.5-3 mg/kg (max 225 mg/dose).
POSITIONING—SURGEON AND PATIENT
Position the patient in the supine position with his or her neck extended (slight “sniffing position”) so that the operative area is flat.
Maintain close communication with the anesthesia team to ensure any head positioning is performed in a safe manner.
Tilt the bed and/or head to improve exposure if necessary.
Turning the head toward the surgeon can be especially helpful for scleral passes when recessing the horizontal rectus muscles.
Consider using a bed with a headrest in order to aid in patient positioning and comfort. Additionally, wrist rests or chairs with armrests may help provide support for the surgeon’s arms.
For most rectus muscles, sit across from the operative muscle to provide optimal exposure. Sit superiorly or on the opposite side if operating on the inferior oblique muscle. For the superior rectus or superior oblique muscles, determine the position that optimizes exposure and facilitates use of the dominant hand (eg, a right-handed surgeon can access superior muscles most easily when seated on the patient’s right).
PREPPING AND DRAPING
Some surgeons prefer exposing only the operative eye, others prefer having both eye exposed. This may be dependent on the choice of anesthesia.
Prep the operative eye, both eyes, or the upper face with 5% povidone-iodine, per surgeon preference.
Place a towel or gauze over the laryngeal mask airway (LMA) or endotracheal tube to prevent inadvertent extubation of the patient during drape removal.
Some surgeons use a bladed speculum, others prefer a wire speculum. If a wire speculum is planned, use Tegaderm or a plastic drape to isolate the lashes from the operative field.
Surgeries performed under topical anesthesia may benefit from prepping and draping both eyes in order to test alignment intraoperatively without needing to re-prep for adjustments.
Covering the eyebrows with Tegaderm can reduce risk of accidental removal of brow hairs when taking off an adhesive drape.
FORCED DUCTION AND EXAGGERATED TRACTION TESTING
The forced duction and exaggerated traction tests can be helpful to detect restrictions or laxity (especially superior oblique laxity). These tests are performed either with topical anesthesia (tetracaine drops or lidocaine gel) or after the induction of general anesthesia.
The benefit of using topical anesthesia is the ability to also test for force generation, which is helpful to identify paralytic strabismus.
Description of procedure:
Use toothed forceps to grasp close to limbus where the conjunctiva and Tenon capsule fuse 90 degrees away from the muscle being tested.
Put the muscle being tested on stretch to isolate its field of action when testing for restriction (ie, tightness appreciated in lateral gaze indicates possible medial rectus muscle restriction).
For horizontal and vertical recti, proptose the globe.
For oblique muscles, retropulse the globe.
Observe how the muscle behaves and what position it returns to after its release for information regarding restriction and the elastic properties of the muscle.
Complications include subconjunctival hemorrhage, tearing or stretching of conjunctiva, and corneal abrasions/lacerations from a slipped instrument.
When testing the superior oblique muscle, a “popping” sensation may be felt when rotating the eye in an elevated and adducted position as the globe slips over the superior oblique tendon.6 The degree of tightness during this test correlates with clinical overaction.
SUTURES/NEEDLES/INSTRUMENTS
Suture/needles:
Traction suture: 4-0 to 6-0 silk.
Muscle: 6-0 absorbable suture (ie, Vicryl or Polysorb, single or double armed depending on surgical technique). A spatulated needle (S14, S24) is commonly used.
Load the needle with a second instrument as opposed to using hands or fingers to decrease the risk of postoperative infection and needle sticks.
Avoid holding the needle with the needle holder during tying or manipulating the suture, as it may dull the needle tip and/or lead to accidental needle sticks.
Single or double lock the knots at each pole to ensure the muscle is secured.
Secure sutures with 2-1-1 knot (Figure 42.2).
Conjunctival closure: 8-0 Vicryl/Chromic gut/Plain gut.
In cases where the conjunctiva is thin and friable (ie, older patients), use a BV-1 taper needle, which is very small and delicate and less likely to damage fragile tissues.
When tying the 2-1-1 knot, leaving one side of the suture longer than the other facilitates tying a square knot; always start a throw by grabbing the longer side of suture (Fig. 42.2).
 FIGURE 42.2. A-D. Square knot 2-1-1 technique. |
Instruments (Fig. 42.3):
Locking or nonlocking forceps—eg, Bishop Harmon, Castroviejo 0.5 mm (locking and nonlocking), Curved Moody.
Needle holder—locking or nonlocking, curved or straight.
Scissors—eg, blunt-tip Westcott scissors.
Muscle hooks—eg, Stevens “small,” Jameson, Green, Wright, Von Graefe, Culler.
FIGURE 42.3. Strabismus surgery set.
Measuring device—calipers, ruler, scleral ruler (Scott curved).
Muscle clamp or hemostat.
Eyelid speculum.
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