CHAPTER 57 Strabismus surgery
Terminology, history, and epidemiologic considerations
The term strabismus is derived from a Greek word, strabismos, which means to squint. Strabismus has been recognized since antiquity. However, the first strabismus surgery was not reported until 1839 – a medial rectus myotomy by Johann Dieffenbach on a 7-year-old child with esotropia1. Within a few years, strabismus surgery was widely practiced throughout Europe. Strabismus is one of the most common ocular diseases in children. It is also present in many adults either because it was not corrected during childhood or secondary to a condition having an adult onset. The most common types of strabismus vary depending on the ethnicity of the population. In North America and Europe, esotropia is the most common type of strabismus. This probably reflects the high incidence of hyperopia in this population that predisposes patients to accommodative esotropia. However, in Asia, exotropia is the most common type of strabismus, perhaps because of the lower incidence of hyperopia in this population2.
Anatomical considerations
Gross anatomy of the extraocular muscles
There are six extraocular muscles – the medial, lateral, superior and inferior rectus muscles, and the superior and inferior oblique muscles. The rectus muscles all have insertions that are 10–11 mm in width. The four rectus muscles all arise from the apex of the orbit at the annulus of Zinn and then insert at varying distances from the limbus. The medial rectus muscle inserts closest to the limbus and the superior rectus muscle furthest from the limbus. The outward spiraling of the insertions of the rectus muscles, beginning with the medial rectus muscle and ending with the superior rectus muscle, is referred to as the spiral of Tillaux (Fig. 57.1). The inferior and superior rectus muscle insertions are slanted nasally whereas the medial and lateral rectus muscles are not usually slanted. The rectus muscles have varying arcs of contact with the globe – the longest being 10 mm for the lateral rectus muscle. Certain disorders can be associated with ectopia of the rectus muscles. For example, patients with craniosynostosis often have medial rectus muscles which are abnormally high and lateral rectus muscles which are abnormally low, which can result in a V-pattern mimicking overaction of the inferior oblique muscles.
Fig. 57.1 The spiral of Tillaux.
From Coats DK, Olitsky SE, editors. Strabismus Surgery and its Complications. Berlin: Springer, 2007.
There are two distinct layers to the rectus muscles – the global and orbital layers. The orbital layer inserts directly into the muscle pulleys while the global layer inserts into the globe3 (Fig. 57.2).
Physiology of the extraocular muscles
The extraocular muscles are some of the fastest and most fatigue resistant muscles. Unlike skeletal muscles, which are exclusively singly innervated, the extraocular muscles are both singly and multiply innervated4. Singly innervated muscle fibers are thicker and produce rapid all-or-none contractions, while multiply innervated fibers are thinner and produce slow, graded contractions. Thick fibers are primarily located in the global layer of rectus muscles while thin, tonic fibers are primarily located in the orbital layer of rectus muscles. In the oblique muscles, a core of thick fibers is surrounded by an outer layer of thin fibers. Thick fibers are believed to be primarily responsible for saccades while thin, tonic fibers are responsible for small, refixation movements of the eye.
The extraocular muscles have complicated actions based on the angle and location of their insertions. While the medial and lateral rectus muscles only have the primary actions of adducting and abducting the eyes respectively, the other four extraocular muscles also have secondary and tertiary actions in primary position (Table 57.1). All of these actions must be taken into account when planning strabismus surgery. The eyes are yoked together and the movement of one eye is closely linked to the movement of the fellow eye. Sherrington’s law of reciprocal innervations states that, when the agonist muscle receives a stimulatory impulse, the antagonist muscle in the same eye receives an inhibitory impulse or no impulse. Herring’s law of equal innervations states that the muscles responsible for rotating the eyes in a certain direction (yoke muscles) receive equal innervation thereby assuring that the eyes move in the same direction.
Tenon’s fascia and muscle pulleys
The paths of the rectus muscles have been shown to be highly uniform in normal patients in primary position using high resolution magnetic resonance imaging5. In secondary positions of gaze, the paths of the rectus muscles exhibit small but consistent shifts opposite to the direction of gaze. Patients with strabismus generally have little if any change in their rectus muscle paths either in primary position or in other positions of gaze. Rectus muscles pass through connective tissue sleeves in Tenon’s fascia near the equator of the globe. Demer et al6. have labeled these fascial connections which suspend the rectus muscles in the orbit as ‘muscle pulleys’. Ectopic pulleys are a rare cause of strabismus and may be identified by high resolution magnetic resonance imaging7.
Indications for strabismus surgery
The primary indications for strabismus surgery are: (i) diplopia, (ii) reduced or threatened binocularity, (iii) an abnormal head position secondary to ocular misalignment or nystagmus, (iv) a constricted visual field secondary to strabismus, and (v) poor cosmesis secondary to ocular misalignment. The indications for strabismus surgery vary depending on the age and the particular needs of the patient. In children, the primary indication for strabismus surgery is often to align the eyes to permit the development of binocular vision. Other common indications in children include anomalous head postures that can be associated with nystagmus or ocular misalignment. In adults, perhaps the most common indication for extraocular muscle surgery is the elimination or reduction of diplopia or asthenopia. Finally, an important indication for surgery for patients of all ages is the restoration of normal ocular alignment since discrimination in the workplace, and psychological and physical dysfunction are all well-documented consequences of uncorrected ocular misalignment8–15.
Preoperative assessment
Examination
The majority of the preoperative examination should focus on the sensory–motor evaluation. This evaluation should be tailored to the patient and his or her individual problem. It is important to identify whether an abnormal angle kappa or eccentric fixation is present, which can be done by having the patient fix on a light with each eye separately and noting the position of the corneal light reflex relative to the center of the pupil. A normal angle kappa is slightly positive (displaced slightly toward the nose). Ductions and versions are tested, and recorded. Overacting or underacting muscles should be identified and documented. In very young children, or in patients with very poor vision, the Hirschberg light reflex test or Krimsky test is used to estimate the ocular deviation, but when possible alternate prism and cover testing are preferred to document the magnitude of the deviation in the nine diagnostic positions of gaze and at near. In patients with vertical deviations, the alignment should also be tested with right and left head tilt. The AC/A ratio should be assessed in patients with a distance–near disparity with either the lens gradient or the heterophoria method. For patients with intermittent exotropia, the alignment should also be assessed at near with +3.00 D lenses and at distance with −2.00 D lenses to determine if a high AC/A ratio is present. Testing in the far distance and after monocular occlusion is also helpful since it may bring out a larger deviation16. Common patterns that can be elucidated with motility testing include ‘A’ patterns, which are often associated with superior oblique muscle overaction, and ‘V’ patterns, which are often associated with inferior oblique muscle overaction. Other common patterns include ‘T’ or ‘Y’ patterns, ‘X’ patterns, and lambda patterns. Orbital pulley and innervational anomalies can give rise to atypical strabismus patterns17.
Sensory testing can be helpful in predicting postoperative outcomes and in directing surgical management. For example, adult patients with anomalous retinal correspondence should be warned of an increased possibility of postoperative diplopia. Decreasing distance stereoacuity can be an indication that control of an intermittent exotropia is worsening and that surgical intervention is warranted18. Surgery on the superior oblique muscles should be approached cautiously in adult patients with high grade stereopsis because of the risk of inducing vertical or torsional diplopia. The sensory evaluation should be tailored to the individual patient.
Orbital imaging is not routinely performed in all strabismus cases, but can be helpful in identifying anomalous anatomy, identifying slipped or lost muscles, or assisting with surgical planning in complicated cases7.
Surgical decision making
Table 57.2 summarizes some of the most common forms of strabismus and outlines accepted treatment practices. Table 57.3 gives guidelines for surgical dosages based on Parks’ recommendations. Adjustable sutures can be used in cooperative patients when surgical outcomes are likely to be unpredictable. The use of adjustable sutures is discussed in the section on p. 28. Each surgeon should modify these tables based on their own outcomes.
Informed consent of the strabismus patient
Informed consent is a process of communication between a patient and physician whereby the patient is informed of their diagnosis, the nature and purpose of the proposed procedure, the risks, benefits, and alternatives to the procedure, and the risks and benefits of not undergoing the treatment. This communication process is both ethically and legally required before a surgical procedure can take place. Although the physician is not required to disclose every conceivable risk, he or she should disclose any and all risks that a reasonable person would consider important in deciding whether or not to undergo a procedure. Once this communication has taken place, then the patient or guardian must sign a written consent as evidence that proper informed consent has taken place. This written consent becomes a permanent part of the medical record. More detailed information about the legal aspects of informed consent can be found in legal textbooks19.
Anesthesia
Postoperative pain and nausea are common following strabismus surgery particularly when general anesthesia is used. Box 57.1 lists precautions that can be taken to minimize postoperative pain and nausea in children.
Operative techniques
Forced ductions and traction testing
Traction testing is helpful for diagnosing restricted rectus muscles. It can also be helpful in diagnosing Brown syndrome or a lax superior oblique muscle. When performed under general anesthesia, depolarizing muscle relaxants should be avoided since they can make it more difficult to interpret forced ductions. When testing the horizontal rectus muscles, the conjunctiva is grasped with fine forceps at 6 and 12 o’clock 1–2 mm posterior to the limbus. The globe is then abducted and adducted without retropulsion. It is often helpful to compare the forced ductions in both eyes since a comparison between the two eyes will often allow a subtle difference to be detected. When testing the vertical rectus muscles, the conjunctiva is grasped at 3 and 9 o’clock and the globe is elevated and depressed. The restriction can be confirmed when the muscle is secured on a muscle hook. When testing the superior oblique muscle, the globe is grasped at 4 and 10 o’clock for the right eye and 2 and 8 o’clock for the left eye (Fig. 57.3). The globe is then retropulsed and elevated in a superonasal direction. Once in this position, it is moved back and forth temporally and nasally to determine how taut the tendon is. In an eye with Brown syndrome, it is generally difficult to fully elevate the eye. In an eye with a lax superior oblique tendon the eye can not only be fully elevated, but there is little resistance to rocking the globe while it is retropulsed and elevated20,21. The degree of laxity can be graded and used to determine whether to tuck the superior oblique tendon. Traction testing can result in conjunctival hemorrhages and tears and corneal abrasions, and thus should be performed carefully.
Conjunctival incisions and methods of muscle reattachment
Choice of type and location of incision
Fornix conjunctival incisions are made through the bulbar conjunctiva between adjacent rectus muscles at a point 8–9 mm posterior to the limbus to avoid the extraconal fat pad that begins 10 mm posterior to the limbus22. Fornix incisions are almost always used for surgery on the oblique muscles, and can also be used for surgery on the rectus muscles. Advantages of the fornix incision include access to a greater number of extraocular muscles through a single incision, ease of construction and closure of the incision, less scarring, and greater patient comfort. Disadvantages of the fornix incision include poorer exposure, especially when exposure to the posterior orbit is needed, an increased risk of conjunctival tearing, inability to resect conjunctiva in cases of large rectus muscle resections, and greater reliance on a skilled assistant.
Fornix incision technique
A fornix incision can be made in any of the quadrants between adjacent rectus muscles (Video: Medial rectus recession using fornix incision.). The following describes the procedure used to create an inferonasal conjunctival incision, which is the most common site used for accessing the medial rectus or inferior rectus muscles. The description is for a right-handed surgeon operating on a right eye. The fornix incision is initiated by having the assistant grasp the perilimbal conjunctiva at the 4:30 o’clock position; the assistant elevates and abducts the eye. The surgeon angles the blunt tipped Westcott scissors perpendicular to the globe and initiates the incision at a point 8 mm posterior to the limbus, extending the incision approximately 8 mm (Fig. 57.4A). Next the surgeon makes a second 8 mm incision through Tenon’s capsule and intermuscular septum, which is oriented perpendicular to the conjunctival incision (Fig. 57.4B).
Limbal incision technique
Limbal incisions are commonly used when performing surgery on the rectus muscles (Video: Medial rectus muscle using a limbal incision.). The conjunctiva is grasped with forceps along one border of the muscle undergoing surgery, and blunt Westcott scissors are used to make a radial incision that begins at the limbus and extends about 3–4 mm posteriorly (Fig. 57.5A). It is important to not extend the conjunctival incision to the plica when operating on the medial rectus muscle and to the lateral canthus when operating on the lateral rectus muscle to avoid the formation of symblepharon. The scissors are then used to extend the incision along the limbus through an angle of approximately 3 clock-hours. Tenon’s capsule should be bluntly dissected from the conjunctiva prior to performing the conjunctival peritomy. A second radial incision is then created at the opposite end of the limbal incision (Fig. 57.5B).
Conjunctival closure
Limbal conjunctival incisions require suture closure. It is important not to suture Tenon’s capsule to the conjunctiva since this can result in unsightly hyperemia. Tenon’s capsule can be distinguished from conjunctiva by irrigating saline solution on its surface. Tenon’s capsule will absorb the solution and become thickened whereas the appearance of conjunctiva remains unchanged. The corners of the conjunctival flap should be identified with forceps. A suture is then passed through each corner of the conjunctival flap and sutured back to the conjunctiva adjacent to the limbus with interrupted absorbable sutures (Fig. 57.6). If the conjunctival flaps are large, more than one interrupted suture may be needed to close each flap.
Surgery on the rectus muscles
Hooking and dissecting rectus muscles using a limbal approach
Prior to hooking the rectus muscles, the blunt Westcott scissors should be used to separate the intermuscular membrane and Tenon’s capsule from the rectus muscles. A muscle hook with a blunt end (Graefe or Stevens hook) should then be advanced parallel to the margin of the rectus muscle and then rotated behind the muscle. It is important that the toe of the muscle hook be firmly pressed against the globe during this maneuver since the rectus muscle lies flat against the globe and the hook will otherwise only capture Tenon’s capsule. It is also important to consider the varying distances of the rectus muscles from the limbus. In addition, since the nasal edges of the superior and inferior rectus muscle insertions are generally closer to the limbus than the temporal edges, it is generally preferable to hook these muscles nasally rather than temporally. Once it has been determined that the muscle is securely hooked, the intermuscular membrane should be dissected off the insertion of the muscle. This can be facilitated by lifting the conjunctiva overlying the muscle with a Manson double ended strabismus hook (Fig. 57.7). The poles of the muscle should then be inspected to ensure that the entire muscle has been captured by the muscle hook. The Graefe or Stevens muscle hook should then be replaced with a Green or Jameson muscle hook, which have a toe at one end of the hook that will prevent the muscle from slipping off of the hook. When performing a recession, just enough of the intermuscular membrane should be removed to place the polyglactin suture in the muscle insertion. However, a more extensive dissection of the intermuscular membrane is required when performing a resection. In addition when performing a resection, a second Green or Jameson hook is placed to spread out the muscle.
Hooking and dissection of rectus muscles using a fornix approach
Hooking and dissecting the rectus muscles using a fornix incision is performed similarly to that described for a limbal incision. Blunt Westcott scissors are used to separate the intermuscular membrane and Tenon’s capsule in the quadrants between adjacent rectus muscles. A Stevens hook is used to identify the insertion of the rectus muscle, the muscle is placed on a Green or Jameson hook (Fig. 57.8A), and the conjunctiva is reflected over the hook (Fig. 57.8B). Blunt Westcott scissors are used to dissect Tenon’s fascia from the tip of the hook, and a ‘pole test’ is performed by sweeping a small hook around the pole of the muscle to assure that all rectus muscle fibers are contained on the Green hook (Fig. 57.8C). The small hook can then be used to pull Tenon’s fascia away from the rectus muscle so that is can be dissected off of the muscle with blunt Westcott scissors (Fig. 57.8D). For recessions of the horizontal muscles, dissection of the Tenon’s fascia is limited to the anterior portion of the muscles. For resections or for vertical muscles, the muscle is dissected posteriorly to allow for the desired amount of resection, and to separate the vertical muscles from the eyelid retractors.
