2.4 Applications of Bilateral Feedback Control to Clinical Practice and to Future Research

Vertical Difference R-L

time [s]

Fig. 2.8 Vertical recordings, with head straight and tilted 45° to either side, after 45 min of adaptation, with head straight, to a left-over-right vertical disparity of 50+° fields of concentric circles. The relative positions of the two eyes are shown in the fusion-free, dissociated state. The negative values correspond to the induced right hypodeviation

vertical disparity. This arrangement simulated a relative right hyperdeviation, because the right eye had to move downward to fuse, and the left eye had to move upward. After adaptation, the relative positions of the eyes were measured in the fusion-free, dissociated state. The eyes had partially adapted to the simulated relative right hyperdeviation by developing a measured right hypodeviation. The relative shift of the right eye downward of 3° with head straight increased to 5° with right head tilt (RHT) and decreased to 0° with left head tilt (LHT) (see Fig. 2.8). These changes with forced head tilt are in the directions that are expected from increased tonus to the normal right superior oblique muscle and to the normal left inferior oblique muscle. This increased tonus was produced by vergence adaptation to the relative right hyperdeviation. The deviations recorded simply represent a basic cyclovertical deviation induced in a normal subject by vergence adaptation to a vertical disparity.

The demonstration of such head-tilt changes accompanying the induced cyclovertical deviation is in favor of the belief that many deviations currently called congenital superior oblique paresis are nothing more than basic cyclovertical deviations of the eyes. To explore this thesis, these adaptation techniques will be used to study not only normal subjects but also patients with congenital and acquired forms of apparent superior oblique paresis.

2.4Applications of Bilateral Feedback Control to Clinical Practice

and to Future Research

Practically speaking, the consequences of muscle length adaptation are often best appreciated under deep general anesthesia, when the anatomic positions of the eyes can be seen and careful forced ductions can be performed. The decision about which eye or eyes to operate on, and which muscles, may best be postponed until obtaining these intraoperative findings. This has been advocated by many, including Roth in Switzerland [61], Jampolsky in the United States [22], and the author [62].

Because version stimulation is only minimally e ective in changing extraocular muscle length, surgery designed to eliminate or minimize extraocular muscle contracture by creating chronic version stimulation, for example, by recessing the contralateral medial rectus muscle, on the sound eye, in cases of sixth nerve palsy [63], may not work as well as expected.

It has long been the teaching in the field of strabismus to wait for stabilization of the angle of deviation before intervening surgically. However, the consequences of unguided vergence adaptation and muscle length adaptation suggest a revision of this teaching. If there is potential for fusion, it now appears that every e ort should be made to realign the eyes without delay, using glasses, prisms, and surgery when necessary, and not wait for stabilization.Waiting for stabilization may actually be harmful if there is fusion potential, for it is now known [64] that the chances for successful restoration of binocular vision decrease with each month that misalignment persists. On the other hand, if fusion potential is truly not present, early surgery may best be postponed. The biases that exist in the unguided vergence and muscle length adaptation mechanisms may themselves change over time, altering the angle of strabismus naturally. Waiting for stabilization of these biases, as reflected by stability of the deviation, may indeed be warranted in such cases. The challenge, therefore, lies in the accurate determination of fusion potential.

Whenever strabismus is corrected, by whatever means, any fusion that develops will need to compete with any biases in the vergence and muscle length adaptation mechanisms in order for the eyes to remain straight. It is very possible that we shall learn in the future how to measure such destabilizing biases and learn how to minimize or counteract them by pharmacologic, surgical, or other interventional means, in order to help maintain good binocular alignment after we have achieved it.

For example, selective activation of vergence should be able to change not only vergence adaptation, but also

muscle lengths over time. This of course is currently the goal of fusional vergence exercises as part of orthoptic training. However, eventually we may be able to supply vergence stimulation from external sources, such as is

2currently done with the transcutaneous electrical stimulation used in orthopedic applications to correct or prevent scoliosis as well as contractures in cases of hemiplegia or cerebral palsy [65]. To do this, we shall need to discover the di erences between version and vergence stimulation of the extraocular muscles so as to be able to supply vergence stimulation selectively. To be sure, correction of strabismus in the future may possibly be by selective electrical stimulation rather than by surgery.

Summary for the Clinician

■At least a three-level feedback control system exists for the maintenance of binocular alignment. Of particular interest is the unique regulation of extraocular muscle lengths by vergence stimulation as opposed to version stimulation.

■Even though we may treat these mechanisms in a black-box fashion in the beginning, we can use this understanding to explain currently observed phenomena such as the development of socalled oblique muscle dysfunction with the development of A and V patterns. We also can use this understanding to appreciate previously unrecognized patterns of misalignment such as the basic cyclovertical deviation that mimics superior oblique muscle paresis.

■Not all answers are yet known, and some of the mechanisms proposed in this chapter are still quite speculative. However, from such speculation, models such as those formulated here can help in the understanding of not only how strabismus changes over time, but also the causes of the many forms of strabismus, facilitating the development of preventive measures as well as better and longer-lasting treatment methods for the future.

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