Management of strabismus

Pharmacologic injection treatments can be given to cooperative adults under local anesthesia in an outpatient setting, and for some agents, under light general anesthesia.[3][4] In the former case, it is possible to bring the injection needle to an optimal location in the desired muscle using EMG guidance[5] as the alert patient looks in diagnostic directions, the needle is advanced until the electromyogram (the electrical signal from an activated skeletal muscle) indicates it is optimally positioned, whereupon the injection is completed. Some agents (e.g., botulinum toxin) can be injected at the insertional end of a muscle under visual guidance, using special forceps[6] and allowed to diffuse posteriorly, whereas others (e.g., bupivacaine) must be distributed throughout the body of the muscle,[7] which requires non-visual guidance. EMG guidance generally provides more effective injections, but is only suitable for alert, cooperative adults. Because injection treatment does not result in the scarring that is often a troublesome consequence of conventional strabismus surgery, if therapeutic goals are not achieved with one injection, additional injections or surgical treatments can readily be given.[8][9]

Replacement of strabismus surgery with less invasive procedures began in Alan B Scott’s San Francisco lab with his development of botulinum toxin injection treatment.[10]

Some forms of strabismus can be corrected by weakening an extraocular muscle. Botulinum toxin blocks the neuromuscular transmission and thus paralyzes injected muscles.[11][12][13] Paralysis is temporary, and it might seem that injections would always need to be repeated, except that muscles adapt to the lengths at which they are chronically held, so that a paralyzed muscle tends to get stretched-out by its antagonist and grows longer by addition of serial sarcomeres (the contractile units of skeletal muscles), while the antagonist tends to grow shorter by deletion of sarcomeres,[14] thereby maintaining re-alignment when the toxin-caused paralysis has resolved. If there is good binocular vision, once muscular imbalance is sufficiently reduced, the brain mechanism of motor fusion (which points the eyes to a target visible to both) can stabilize eye alignment.[15]

Botulinum A toxin (introduced as Oculinum), now called Botox, is the principal drug used to temporarily paralyze extraocular muscles, and is widely accepted as an alternative to surgery for many types of strabismus.[16][1] Crotoxin, a snake neurotoxin, is being developed in Belo Horizonte, Brazil as a potential alternative.[17]

Treatment is usually surgical, performed at the insertional ends of extraocular muscles (where they attach to the globe). Resection surgery removes tissue in order to stretch a muscle, increasing its elastic force; recession moves an insertion so as to reduce stretch, and so reduce elastic force; transposition moves an insertion “sideways”, sacrificing one direction of muscle action for another; posterior fixation relocates a muscle’s effective insertion to a mechanically disadvantageous position. All are kinds of compensatory impairment. Pharmacologic injection treatments, in contrast, offer the possibility of directly increasing or decreasing contractile muscle strength and elastic stiffness, as well as changing muscle length, without removing tissue or otherwise compromising orbital mechanics.[28][21][22] The idea of treating strabismus by cutting some of the extraocular muscle fibers was published in American newspapers by New York oculist John Scudder in 1837[29]

Spherical lenses and miotic eye drops can provide relief in some types of horizontal strabismus by biasing the neural link between convergence (orienting the lines of sight for near objects) and accommodation (focusing), and prism lenses can relieve diplopia (double vision) by refracting the visual axis,[30][31] but these treatments don’t address the underlying muscular imbalance, and are not further considered here.

With surgery, results are seen in a few days. After bupivacaine injection the muscle is inactivated by the drug’s anesthetic effect for a day, and weakened by myofiber destruction for a week or so, after which regeneration and hypertrophy over 2–3 weeks gradually achieves the corrected alignment. If bupivacaine injection is combined with a small dose of botulinum toxin in the antagonist muscle, eye deviation during regeneration is minimized. Strabismus surgery generally sacrifices one mechanical effect to gain another, and always causes scarring, both of which may make any subsequent procedures more difficult. Bupivacaine injection treatment, in contrast, directly increases muscle strength and reduces length. Strabismus surgery requires an operating room, anesthetist, and other personnel, whereas bupivacaine injection in cooperative adults is an office procedure taking only a few minutes. Bupivacaine injection is not effective in paralyzed or atrophic muscles, or where there are restrictions to movement elsewhere in the orbit (e.g., fibrotic muscles). Very small misalignments might be better treated surgically because of the risk of “overcorrection”, which tends to cause diplopia (double vision).[21][22][9][23]

A complex approach to non-surgical management of strabismus (wandering eye), amblyopia (lazy eye) and eye movement disorders may include a variety of vision therapy methods, primarily directed at the abnormal retinal correspondence management such as eye occlusion with an eye patch, binocular vision training using a haploscope and many others. The orthoptic therapy can be used either before or after the surgical treatment, as it is prescribed by an eye care specialist.