CHAPTER 14
Surgery of the Extraocular Muscles
While orthoptic exercises or prism spectacles are sufficient for some patients with strabismus, many require surgery in order to correct their alignment. Most often, this is achieved with incisional surgery. Historically, a variety of incisional procedures have been used, but chemodenervation, covered at the end of this chapter, has emerged as an alternative for some patients.
Evaluation
The history and a detailed motility evaluation, as part of a complete ocular examination, provide the information necessary for the surgeon to plan optimal strabismus surgery. Evaluation, tailored to the type of case, may include sensory binocularity testing, forced-duction testing, active force generation, saccadic velocity measurement, and simulation of the target postoperative alignment with prisms or an amblyoscope to assess the risk of diplopia and the opportunity for single binocular vision. (See Chapter 7 for discussion of these tests.) Preoperative discussions should address the expectations of the patient and family, as well as the risks and potential complications, especially if surgery on the only eye with good vision is considered.
Indications for Surgery
Surgery of the extraocular muscles (EOMs) is performed to improve visual function, appearance, or patient well-being or any combination of these. It may relieve asthenopia (a sense of ocular fatigue) in patients with heterophorias or intermittent heterotropias, or it may relieve the double vision that is often present in patients with adult-onset strabismus. Alignment of the visual axes can establish or restore fusion and stereopsis, especially if the preoperative deviation is intermittent or of recent onset. Correction of esotropia expands the binocular visual field. Some patients require an abnormal head position to relieve diplopia or to improve vision (eg, with nystagmus and an eccentric null point; see Chapter 13). For these patients, surgical treatment may not only increase the field of binocular vision but also shift it to a more useful, centered location. Correction of strabismus should be considered reconstructive rather than merely cosmetic, as it has many functional benefits.
Kraft SP. The functional benefits of adult strabismus treatment. Am Orthoptic J. 2008;58(1):2–9.
Planning Considerations
Visual Acuity
When a child has amblyopia, some surgeons prefer to treat the amblyopia before surgery, while others believe that the prognosis for binocular vision is better if surgery is not postponed. If a patient
has dense amblyopia or permanent vision loss due to other causes, surgery is usually performed only on the eye with poor vision.
General Considerations
Symmetric surgery
The amount of surgery is based on the preoperative deviation. One commonly used set of guidelines for medial rectus muscle recession or lateral rectus muscle resection for esodeviations is given in Table 14-1 (also see the section Rectus Muscle Strengthening Procedures). Surgical options for infants with large-angle esotropia (>60Δ) include combined recession-resection of 3 or 4 rectus muscles or bilateral medial rectus muscle recessions of 7.0 mm. Augmentation of the latter with botulinum toxin has been advocated.
Table 14-1
Surgical guidelines for exodeviation are provided in Table 14-2. Some surgeons employ bilateral lateral rectus muscle recessions of 9.0 mm or greater for deviations larger than 40Δ. Others prefer to limit lateral rectus recession to no more than 8.0 mm and add resection of 1 or both medial rectus muscles for larger-angle exotropias.
Table 14-2
Lueder GT, Galli M, Tychsen L, Yildirim C, Pegado V. Long-term results of botulinum toxin–augmented medial rectus recessions for large-angle infantile esotropia. Am J Ophthalmol. 2012;153(3):560–563.
Monocular recession-resection procedures
The figures given in Tables 14-1 and 14-2 may also be used in monocular recession-resection procedures, with the surgeon selecting the appropriate number of millimeters for each muscle. For example, for an esotropia of 30Δ, the surgeon would recess the medial rectus muscle 4.5 mm and resect the lateral rectus muscle 7.0 mm. For an exodeviation of 15Δ, the surgeon would recess the lateral rectus muscle 4.0 mm and resect the antagonist medial rectus muscle 3.0 mm. Monocular surgery for exotropia beyond these guidelines (ie, >50Δ) is likely to result in a limited rotation; thus, a 3-or 4-muscle procedure is preferable if there is at least moderately good vision in each eye.
Incomitance
When the size of the deviation varies in different gaze positions, the surgical plan should be designed with a goal of making the postoperative alignment more comitant.
Vertical incomitance of horizontal deviations
The treatment of horizontal deviations that change in magnitude in upgaze and downgaze—such as A
or V patterns—is discussed in Chapter 10.
Horizontal incomitance
When the esodeviation or exodeviation changes significantly between right and left gaze, paralysis or restriction is suggested. In general, restrictions must be relieved for surgery to be effective, and the surgical amounts usually used to correct a misalignment of a given size may not be applicable.
When there is no restriction to account for an incomitant deviation, the deviation is treated as if it were caused by a weak muscle, whether from neurologic, traumatic, or other causes. If the weak muscle exhibits little or no force generation, transposition procedures are usually indicated. Otherwise, treatment consists of some combination of resection of the weak muscle (or advancement if it has been previously recessed) and weakening of its direct antagonist or yoke muscle.
In some cases, both restriction and paralysis are present, particularly in long-standing paretic strabismus, and a combination of treatment strategies is necessary. Forced-duction and active force generation testing are helpful in these cases.
Distance–near incomitance
Treatment of horizontal distance–near incomitance has classically consisted of medial rectus muscle surgery for deviations greater at near and lateral rectus muscle surgery for deviations greater at distance. Recent evidence suggests that, regardless of which muscles are operated on, the improvement in distance–near incomitance is similar.
Archer SM. The effect of medial versus lateral rectus muscle surgery on distance-near incomitance. J AAPOS. 2009;13(1):20–26.
Cyclovertical Strabismus
In many patients with cyclovertical strabismus, the deviation differs between right and left gaze and, on the side of the greater deviation, often between upgaze and downgaze as well. In general, surgery should be performed on those muscles whose field of action corresponds to the greatest vertical deviation unless results of forced-duction testing reveal contracture that requires a weakening procedure for a restricted muscle. For example, for a patient with a right hypertropia that is greatest down and to the patient’s left, the surgeon should consider either strengthening the right superior oblique muscle or weakening the left inferior rectus muscle. (Strengthening and weakening of the oblique muscles are discussed later in this chapter.) If the right hypertropia is the same in left upgaze, straight left, and left downgaze, then any of the 4 muscles whose greatest vertical action is in left gaze may be chosen for surgery. In this example, the left superior rectus muscle or right superior oblique muscle could be strengthened, or the left inferior rectus muscle or right inferior oblique muscle could be weakened. Larger deviations may require surgery on more than 1 muscle.
Prior Surgery
In the surgical treatment of residual or recurrent strabismus after previous EOM surgery, EOMs that have not undergone prior surgery are technically easier and somewhat more predictable to operate on than those that have. Unfortunately, when previous surgery has resulted in muscle restriction or weakness with limited duction (due to excessive recession, slipped or lost muscle), reoperation on the involved muscle is usually necessary. If the restriction is a result of retinal detachment surgery, correction can usually be accomplished without removal of scleral explants. Consultation with the patient’s retina surgeon is advisable in case such removal becomes necessary. For an eye that has previously undergone glaucoma surgery such as trabeculectomy or implantation of a glaucoma drainage device, strabismus surgery should be planned to minimize the risk of disrupting the filtering