Techniques of Strabismus Surgery

Introduction

Strabismus can lead to frank diplopia, visual shadowing, visual confusion, asthenopia, headaches, dizziness, anomalous head position, decreased visual field, impaired stereopsis, and a large impact on an individual’s social and economic positions. Because of the myriad symptoms and functional limitations, treatments are commonly warranted. Strabismus surgery is an important part of the treatment armamentarium in addressing strabismus. In most patients, nonsurgical techniques have been employed with suboptimal results before surgical intervention. Many of these nonsurgical methods are discussed in Chapters 11.6 – 11.12 . It is important that before surgery, reproducible measurements are obtained, that the patient is systemically stable enough to be able to tolerate the procedure and the accompanying anesthetic/analgesia, and that the patient (or parent/s) understands the goals and limitations of the planned surgery. In addition, careful preoperative testing to evaluate for postoperative diplopia is imperative to ensure patient satisfaction.

Except in unusual cases, the following are goals of all strabismus surgery:

• Attaining peripheral fusion with fusional vergence amplitudes sufficient to maintain alignment of the eyes

• Achieving comfortable single binocular vision (horizontal, vertical, and torsional) to enable the patient to perform visual tasks without asthenopia

• Minimizing anomalous head positions to maintain single binocular vision

• Restoring appearance to as close to normal as possible

Historical Review

The first eye muscle procedure (horizontal rectus muscle tenotomy) was probably performed in the middle of the 18th century by Chevalier John Taylor. The first successful operation for strabismus with the use of horizontal rectus tenotomy was performed on a living patient (a 7-year-old boy with esotropia) in 1839 by Johann Dieffenbach. Von Graefe performed a partial tenotomy in 1851 and advancement of a rectus muscle in 1857. By the late 19th century, sutures were being used to shorten a muscle, and measured resections and recessions followed soon afterward. More recent contributions include the use of adjustable sutures and techniques for operation on the oblique and vertical rectus muscles.

In modern times, advances have been made in magnification techniques, improved overhead and loupe-mounted lighting, instrument technology, suture development, injectable strabismus treatments, and imaging. Contemporary sutures for strabismus surgery often are absorbable and swaged to needles. The needles are typically spatulated, with the cutting edges directed away from the center of the globe. The cutting edges are on the sides of a spatulated needle, making cutting through a tissue plane easier while not cutting above or below the needle, thus decreasing the risk of scleral perforation.

Preoperative Evaluation and Diagnostic Approach

The ideal preoperative evaluation of the strabismus surgical patient includes quantification of the misalignment in the primary position at distance and at near, in the nine diagnostic gaze positions, and in right and left head tilt ( Box 11.13.1 ). In most patients, the maximal deviation under conditions of complete dissociation of the visual axes is the deviation for which surgery is targeted. This is altered in cases of anomalous sensory adaptation (see Chapters 11.4 and 11.5 ). Patients with diplopia are assessed with prisms in free space to determine the amount of horizontal and/or vertical correction needed to fuse in both the primary position and at near. Patients with sensory adaptations, such as suppression, are evaluated to determine if they have the potential to fuse and are tested for postoperative diplopia. Finally, duction and version testing and, when appropriate, forced duction and force generation testing are performed. These topics are discussed in greater detail in Chapter 11.3 .

Anesthesia

The choice of anesthetic technique is individualized and depends on the circumstances of the procedure chosen, patient age, patient comorbidities, the ability of the patient to tolerate discomfort, and the patient’s choice. Strabismus surgery on children younger than age 16 years usually requires general anesthesia. Many adult patients are uncomfortable with the operating room setting and/or the concept of eye muscle surgery and request general anesthesia. The benefit of general anesthesia is an immobile, insensate patient. The disadvantages include the attendant risks of general anesthesia and increased recovery time. In cases where local anesthesia is used, conversion to general anesthesia occasionally may be necessary.

When planning a retrobulbar injection, preinjection of intravenous dissociative agents or narcotics often is helpful, and a lid block is usually not required. Retrobulbar injection may provide sufficient akinesia and anesthesia for cooperative older teenagers and adults to enable them to tolerate strabismus surgery. The benefits of retrobulbar injection include obviation of general anesthesia risks, briefer recovery room time, and a more rapid return to normal routine. The disadvantages include risks of globe or nerve perforation, rare cases of presumed brainstem anesthesia with respiratory suppression, inability to immobilize the superior oblique muscle, inability to operate on both eyes in one session unless another technique is used for the second eye, and injection into a muscle belly with resultant damage. Particular caution must be exercised in those who have large globes or encumbered orbits (patients who have dysthyroid orbitopathy and enlarged extraocular muscles) because of the increased risk of globe perforation, intramuscular injection, or vascular compromise.

Many cooperative patients can tolerate epibulbar injection of anesthetic agent around the appropriate muscle(s). Generally, the patient who tolerates forced duction testing in the office is a candidate for epibulbar injection or even topical anesthesia, especially when augmented with intravenous sedation. The advantages of epibulbar injection are similar to those of retrobulbar injection; the disadvantages include somewhat greater discomfort (especially when the muscle is hooked) and rare cases of globe perforation. Reoperations dissecting through scar tissue may not be tolerated with local anesthesia modalities. The advent of the delayed adjustable suture technique, allowing adjustment much later in the postoperative period, eliminates the need to choose the type of anesthesia according to the desired time to perform an adjustment.

Newer anesthetic agents may decrease postoperative nausea and vomiting. Anesthesiologists need to be prepared to medically support bradycardia secondary to the oculocardiac reflex.

Visualization

Visualization is enhanced with surgical loupes or an operating microscope. Each method has its benefits and drawbacks. Newer features in some surgical loupes allow adjustable magnification during surgery, enabling the mobility of a face-mounted system with the dynamic magnification of a mounted microscope.

General Techniques

Strabismus surgery demands meticulous planning. Surgical findings of muscle configuration and forced ductions often alter the decision-making process. It is helpful to keep a description of the prior eye/strabismus surgeries, sensorimotor examination, and proposed surgical plan for ready reference before and during surgery.

The main types of strabismus surgeries are outlined in Box 11.13.2 . The exact location and number of conjunctival incisions depend on the muscles to be operated, preexisting scarring, and the patient’s previous surgical history. Limbal incisions more easily permit accurate surgery without the presence of a trained assistant, may expedite surgery in patients who have experienced multiple procedures on a given muscle, may minimize conjunctival shredding in older patients who have limited Tenon’s capsule and inelastic conjunctiva, and permit conjunctival recessions if the tissue is contracted. Patients, however, experience increased discomfort because of corneal proximity; the incisions may compromise the conjunctival contribution to the anterior segment circulation and affect corneal stem cell health, and the incisions heal more slowly than when other options are used.

Forniceal incisions permit rapid access to the muscles, stimulate less discomfort than limbal incisions, preserve conjunctival vascular supply, and may not require suture or diathermic closure. However, they do require the presence of a more skilled assistant, a better knowledge of muscle anatomy to avoid surgery on the wrong muscle, and relatively elastic conjunctiva. It is important not to incise conjunctiva more than 8 mm posterior to the limbus to avoid the risk of invading orbital fat. In cases of conjunctival shortening, conjunctivoplasty or recession may be required.

Incisions at the anterior border of the muscle tendon (Swan’s approach) have fallen from favor because of the tendency for significant postoperative subconjunctival scarring between the limbus and the muscle insertion.

Absorbable sutures generally are utilized when vascular healing of tissues occurs, as in typical recession and resection techniques. Small-caliber 8-0 conjunctival sutures generally absorb within 7–10 days if covered with conjunctiva, slightly longer if exposed, as opposed to the 6-0 Polyglactin 910 (Vicryl, Ethicon) muscle sutures that typically absorb within 56–70 days. Some advocate permanent sutures if avascular tissue, such as the superior oblique tendon, is harnessed, as in the superior oblique muscle tuck or silicone (Silastic) band-lengthening procedure. Many also use permanent sutures on the inferior rectus muscle to avoid late overcorrections and elongated scarring resulting from stretch scar.

Specific Techniques