Chapter 80 – Forms of Nonsurgical Strabismus Management
GARY R. DIAMOND
• Sector occlusion: occlusion of a portion of a spectacle lens with translucent adhesive paper to treat strabismus or amblyopia.
• Orthoptics: a wide range of techniques used to expand fusional vergence amplitudes and permit improved single binocular vision.
• Numerous techniques other than glasses are available to treat strabismus and should be considered before strabismus surgery is performed.
A popular form of strabismus management in Europe, not well known in the United States, involves the use of partial lens occlusion, termed “sector occlusion.” A sector is a piece of translucent adhesive paper applied to the posterior surface of a lens or lenses to obstruct vision of an eye in a particular direction ( Fig. 80-1 ). This method, unlike prisms or optical penalization, does not alter the shape, size, or localization of a viewed object.
Sector occluders are devised to obstruct the areas responsible for diplopia and visual confusion. A large frame is provided to enable exact placement of the sector occluders.
In one study, in 384 children who had constant horizontal strabismus, orthophoria was obtained in 169 (44%). Orthophoria was obtained in 85% of those who had a deviation less than 30?, in 25% of those who had a deviation in the range 30–50?, and in only 3% of those who had a deviation greater than 50?. An additional 57 (15%) achieved a final deviation less than 15?, and the remaining 157 children (41%) required surgery.
Sector occluders of different shapes are used for the treatment of amblyopia ( Fig. 80-2 ).
The treatment of strabismus by means of orthoptics (literally, “straight eyes”) has a long history, although perhaps this technique is used more extensively in Europe than in the United States. Its basic principle is the gradual expansion of fusional vergence amplitudes by exercises, either in open space or using targets viewed through devices intended to isolate the eyes—haploscopic devices, such as the major amblyoscope. Fusional vergence amplitudes are expanded through the use of gradually stronger prisms held in the appropriate direction before one or both eyes or through the use of spherical lenses to utilize the accommodation-convergence relationship to change ocular alignment.
Historically, the success of orthoptic training was greatest in cooperative patients who had phorias or intermittent strabismus of comitant nature. The patients who suffered alphabet pattern strabismus, torsional symptoms, or highly incomitant strabismus and who gave little cooperation fared less well. Patients affected by long-standing constant tropias were less likely to regain fusional vergence amplitudes despite orthoptic training.
Today, the most common indications for orthoptic expansion of fusional vergences include convergence insufficiency, intermittent exotropia of relatively small amplitude, and decompensating accommodative esotropia. Published series of successfully treated patients followed for long terms are difficult to find, and it is unclear whether orthoptic training must be continued throughout life to be effective. It also is unclear whether preoperative fusional expansion improves long-term surgical results.
Diplopia recognition training for patients who have anomalous retinal correspondence and suppression is performed rarely today because of the risk of intractable diplopia.
Prisms may be very useful in the treatment of certain patients who have a small degree of horizontal and vertical strabismus. 
Patients who have superior oblique palsy and a vertical deviation in primary position may benefit from a vertical prism before the paretic eye. The usefulness of this technique is confounded by the simultaneous excyclotorsion associated with this palsy; some patients, however, cyclofuse successfully if the vertical deviation is collapsed with a prism. The minimal amount of prismatic correction necessary to provide comfortable single binocular vision is prescribed after an office and home trial with Fresnel membrane prisms, as described subsequently. Because many patients who suffer this palsy have incomitant deviations when viewing from right to left, the field of single binocular vision is likely to be limited. In addition, vertical fusional vergence amplitudes are likely to wither under prism correction, and the patient becomes more strabismic when the prism is removed.
Some patients who have sixth nerve palsies and esotropia in primary position benefit from a base-out prism over the paretic eye; this may obviate the need for a face turn to view in the forward position. Some patients who have congenital nystagmus and a compensating face posture may benefit from prisms before one or both eyes to position the null point of least nystagmus and best acuity in the forward position, with the head straight. In the case of a horizontal face position, the bases of the prisms are placed in the direction of the face turn. Because the amount of prismatic correction is quite large, this approach is usually impractical.
The use of prisms to treat patients who have typical intermittent horizontal and vertical strabismus is often contraindicated because they place fusional vergence amplitudes at rest and consign the patient to permanent, often increasing, prismatic correction. However, certain elderly or debilitated patients may benefit from prismatic correction when surgery is not indicated or when the deviation is small and symptomatic.
Figure 80-1 Sector occlusion treatment. Various forms for strabismus, diplopia, and visual confusion.
Figure 80-2 Sector occluder shapes for amblyopia.
The prism adaptation test involves the preoperative use of prisms to neutralize a deviation with prisms for a given period of time, followed by surgery for the amount of strabismus fully neutralized by the prisms. Thus, the prism neutralization may be used to predict the outcome of surgery for a given deviation, to determine the maximum deviation, and to estimate fusion potential at that deviation (see Fig. 80-3 ). In addition, some patients exhibit a different deviation with the prism adaptation test than with cover testing. It remains to be shown whether the former deviation provides better surgical results than the latter. In a controlled, randomized study of patients who had acquired esotropia, 60% underwent prism adaptation and 40% did not; of those who responded to prisms with motor stability and sensory fusion, half underwent conventional surgery and half underwent augmented surgery based on the prism-adapted deviation. Success rates were highest (89%) in the prism adaptation responders who underwent augmented surgery, 79% in those who underwent traditional amounts of surgery, and lowest (72%) in those who did not undergo prism adaptation.
The amount of prism to give a patient for comfortable single binocular vision may be assumed arbitrarily to be one third to one half of the maximal phoria obtained on cover testing, or it may be titrated to the subjective response of the patient. Fresnel membrane prisms are obtained easily, are relatively inexpensive, and are easy to adjust in strength ( Fig. 80-4 ). They are somewhat dysesthetic, yellow, peel after about 3 months in place, and do degrade acuity (about one line per 5?). Available in the range 1–30?, they may be confined to one or both lenses (plano carriers in the case of patients who are not wearing a correction); trimmed to fit a bifocal segment, distance correction, or part of the field of a lens; or prescribed to an oblique axis orientation for those who have both horizontal and vertical deviations. Patients are given a trial of Fresnel prism wear. If it is agreeable and the deviation is relatively small (under about 10?), the prism may be ground permanently into spectacles. Some prefer
Figure 80-3 Prism adaptation test. A child with esotropia is wearing a Fresnel membrane prism over the left eye, of sufficient strength to neutralize the esotropia.
Figure 80-4 A Fresnel membrane prism. The prisms are prepunched in circular format and the base is clearly marked.
to continue to wear the Fresnel membrane prism and simply change it after 3 months or so.
The use of type A Clostridium botulinum toxin (oculinum) to paralyze temporarily human extraocular muscles by chemodenervation and thus permit the antagonist to contract and effect a permanent alignment change is credited to Alan B. Scott and colleagues at the Smith-Kettlewell Eye Research Foundation. The toxin interferes with acetylcholine release from nerve endings by antagonization of serotonin-mediated calcium ion release. The toxin is usually injected in the conscious patient, with the syringe needle connected to an auditory electromyogram device that amplifies the muscle action potentials; upon successful injection, the muscle immediately becomes silent as the action potential disappears. Within 3 days, the injected muscle becomes paralyzed and an overcorrection is noted. Alternatively, the muscle may be injected under direct visualization and a general anesthetic.
Oculinum is provided as a freeze-dried lyophil in vials that contain 50?ng of toxin prepared under supervision of the U.S. Food and Drug Administration. The toxin is stable for up to 4 years at freezer temperature but degrades within 1 hour if thawed to room temperature. Immediately before use it is reconstituted with normal saline. Each vial contains less than 1/40 of the lethal dose for 50% of humans; the nanogram dose injected is too small to stimulate antibody formation.
Oculinum is predicted to be most useful in cooperative adults but has been injected successfully in children as young as 9 years. Younger children may be sedated in the operating room using ketamine and then injected with oculinum. The protocol suggests titrated dosages proportional to the deviation, with children’s dosages also proportional to body weight. Injections are preceded by administration of topical anesthetic drops.
Many series of patients have been reported. In a series reported by Biglan et al., best results were found in patients who had surgical overcorrections (87.5% controlled with oculinum) and mild sixth nerve palsy (43.7% controlled) and worst results in patients who had comitant exotropia (13.3% controlled) and infantile
esotropia (33.3% controlled). Significant complications included blepharoptosis, hypertropia, globe perforation, and subconjunctival hemorrhage. Complications reported by others include transient pupillary dilatation, retrobulbar hemorrhage, spread of paralysis to noninjected muscles, missed injection site, patient disorientation, diplopia, and corneal irritation.
Oculinum appears to be most useful in patients who are not medically fit for surgery (because of illness or a history of malignant hyperthermia), who refuse surgery, who have had multiple strabismus operations, or who have acute sixth nerve palsies and mild esotropia in primary position. Oculinum is contraindicated in patients who have a history of myasthenia gravis and is unlikely to be effective in patients who have mechanical restriction of eye movement caused by scar tissue or entrapment of tissue.
1. Sarniguet-Badoche J. Early medical treatment of strabismus. In: Reinecke R, ed. Strabismus II. Orlando: Grune & Stratton; 1984:83–9.
2. Kutschke PJ. Use of prisms: are they really helpful? Am Orthopt J. 1996;46:61–4.
3. Sinelli JM, Repka MX. Prism treatment of incomitant horizontal deviations. Am Orthopt J. 1996;41:123–6.
4. Prism Adaptation Study Research Group. Efficacy of prism adaptation in the surgical management of acquired esotropia. Arch Ophthalmol. 1990;108:1248–56.
5. Berard P. Prisms: their therapeutic use in strabismus. In: Knapp P, ed. International Strabismus Symposium: an evaluation of present status of orthoptics, pleoptics, and related diagnosis and treatment regimes. New York: Karger; 1968:339–44.
6. Scott A. Botulinum toxin injection of eye muscles to correct strabismus. Trans Am Ophthalmol Soc. 1981;79:734–70.
7. Biglan A, Burnstine R, Rogers G, Saunders R. Management of strabismus with botulinum A toxin. Ophthalmology. 1989;96:935–43.
8. Lingua R. Sequelae of botulinum toxin injection. Am J Ophthalmol. 1985; 100:305–7.