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Eye Muscle Surgery and Infantile Nystagmus Syndrome

Description

Infantile Nystagmus Syndrome (formerly congenital nystagmus CN) is an ocular motor disorder of unknown etiology, which presents at birth or early infancy and is clinically characterized by involuntary oscillations of the eyes. These movements most commonly have a slow and fast phase although they may be purely pendular. They are usually horizontal with a small torsional component and may (rarely) have a vertical component. Other clinical characteristics, with variable association, include: increased intensity with fixation and decreased with sleep or inattention; variable intensity in different positions of gaze (usually about a null position); changing direction in different positions of gaze (about a neutral position); decreased intensity (damping) with convergence; anomalous head posturing; strabismus; and the increased incidence of significant refractive errors. INS can occur in association with congenital or acquired defects in the visual sensory system (e.g., optic nerve hypoplasia, albinism, achromatopsia, and congenital cataracts). Children with this condition frequently present with a head turn, which is used to maintain the eyes in an “eccentric” position of gaze (point of minimum nystagmus). This is particularly prominent when the child is concentrating on a distant object, since this form of nystagmus tends to worsen with attempted fixation. The head turn is an attempt to stabilize the image under these conditions. Head oscillations are common in INS, but are not used as the strategy to improve vision. Head oscillations in most patients with INS probably reflect underlying instability of neck muscle control.

The world moving (oscillopsia) is almost never constantly present in INS. In those rare patients with intermittent oscillopsia, the oscillopsia tends to occur at gaze angles in which the nystagmus is maximal or a new sensory system defect develops (e.g., retinal disease). Numerous studies of INS in infants and children confirm an age-dependent evolution of eye movement waveforms during infancy from pendular to jerk. This is consistent with the theory that jerk waveforms reflect modification of the INS oscillation by growth and development of the visual sensory system. These studies reemphasize that continued clinical classification of INS as either "sensory" or “motor" is confusing and often inaccurate. Accurate and repeatable classification and diagnosis of nystagmus in infancy as INS is best accomplished by a combination of clinical and eye movement recording findings; in some cases, the latter are indispensable for diagnosis.

Etiology

Eye movement calibration/development is an active process that may start in utero and continues at least through early infancy. The ability to see “sensory-system development” is a parallel visual process that has been more thoroughly studied and also continues to develop through the first decade of life. Previous studies have documented connections between these two visual processes (cross-talk) that modify, instruct, and coordinate these systems. INS probably results from abnormal cross-talk from a defective sensory system to the developing motor system at an early time during the motor system’s sensitive period. This theory of the genesis of INS incorporates a role of VISION in its genesis and modification. The primary ocular motor instability underlying INS is the same in all patients with this syndrome but it’s clinical and eye movement recording expressions are modified by both initial and final developmental integrity of all other visual processes.

Treatment

Numerous treatments have been described for INS. These include dietary manipulation, drugs, contact lenses, prisms, biofeedback, intermittent photic stimulation, acupuncture, transcutaneous vibratory or electronic stimulation of the face and neck, injection of botulinum toxin and a variety of surgical procedures. Excepting those treatments which directly improve visual acuity (spectacle and contact lens correction of refractive errors), all these treatments have the common goal of reducing the nystagmus intensity directly, and indirectly showing an increase in visual acuity.

Surgery

Extraocular muscle surgery has been performed routinely for about 150 years and is the third most common eye surgery performed in the United States (about 1.2 Million/Year). Although common indications for extraocular muscle surgery in patients with INS include associated strabismus and “convergence damping”, surgery is most commonly performed on those patients in whom there is a null position in eccentric gaze and the patients assumes an anomalous head posture enough to be “clinically significant” (a combination of a certain amount of head posture plus a certain amount of time posturing). Common technical names eye muscle surgical procedures include; recession, whereby the muscle tendon is cut (tenotomy, “otomy” comes from “to cut”) at its insertion on the globe and the entire muscle is moved backwards on the eye, resection, whereby the muscle tendon is cut (tenotomy) at its insertion on the globe and a piece of muscle and tendon is removed (tenectomy and myectomy, “ectomy” comes from “remove”) thus shortening the muscle, transposition, whereby the muscle tendon is cut (tenotomy) at its insertion on the globe and the entire muscle in displaced upward, downward or the right or left of its normal location (“transposed”). The purpose of recessions, resections and transpositions are to “move” the eye ball to correct an eye deviation or a head posture. All eye muscle procedures have in common that they begin with cutting the tendon off the eye or a “tenotomy.”

Research accomplished over the last 25 years has shown that INS patients who had eye muscle surgery performed for strabismus or a head posture had “unanticipated” beneficial effects due to reducing their nystagmus. There was no ready explanation for these observed effects, which could be related to simply mechanically shifting the position of the eyes until an animal model appeared in members of a family of Belgian sheep dogs. It was shown in this animal model that tenotomy with reinsertion of the muscle directly back at the original insertion (no eye positioning change) duplicated the damping effects first documented in human patients after undergoing the full recession, resection or transposition procedures.

About 40-60% of all INS patients will have a ”traditional” indication to perform eye muscle surgery, i.e., strabismus, head posture or convergence damping. In addition to improving those three primary goals of surgery they will also receive the beneficial “secondary” effects due to their nystagmus being reduced. Those 40-60% of patients in whom there was no “traditional” reason to perform extraocular muscle surgery could also benefit from “cutting” the eye muscles and reducing the nystagmus. This is the reason for the “tenotomy” surgery in this 60% group of patients. In patients who do not need “repositioning” of the eyes the tendon is simply cut and reattached at the original insertion, thus no “orthopedic” effect but only the “neurologic” effect. This is not new surgery, but, an old procedure for a new group of patients, i.e. a new “indication.”

Measures other than visual acuity are improved after surgery on the eye muscles, and probably contribute to the visual “well-being.” These include vision in side gaze, speed of visual recognition time, and improved binocular field (due to a more normal head posture). Recent data collected on INS patients having had eye muscle surgery of all types supports the hypothesis that surgical manipulation of the extraocular muscles in patients with oculographically diagnosed INS “improves” the oscillation and visual functions.

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