General principles in the surgical treatment of paralytic strabismus
Paralytic strabismus is challenging to treat because the amount of ocular misalignment varies depending on the direction of gaze [1?3]. This incomitance makes it impossible to manage these situations successfully with prisms or standard strabismus techniques that work best when the size of the deviation is the same in the major gaze positions. The problem becomes more complicated if multiple muscles are affected. This is especially true with third cranial nerve palsies where both horizontal and vertical muscles are paretic. In such circumstances, both types of deviations need to be addressed to get a satisfactory result. In patients with fourth cranial nerve palsies there may also be a torsional issue that can be an extremely bothersome, and failure to eliminate the torsion can result in an unsatisfactory outcome. Finally, in patients who have had a long-standing paralysis, the ocular motility defect may have a restrictive component. The most common type of restriction is a contracture of the antagonist muscle. The more profound the muscle weakness, the more likely a restriction will result. Recognition of this phenomenon is paramount in designing a successful surgical procedure. The incomitance of paralytic strabismus must be treated with surgical procedures which produce an incomitant result [4?19]. What is necessary to restore useable binocular function is an operation which has a greater effect in one field of gaze than in another. Developing the appropriate "incomitant" strabismus surgery can be achieved by taking three fundamental principles into consideration: 1. Improve ocular movement of the involved eye. 2. Balance yoke muscles (create matching weaknesses in the other eye). 3. Minimize the creation of new incomitant deviations. While the first two principles may seem obvious, avoiding or anticipating the "creation" of a new incomitant deviation is not always appreciated until it unexpectedly occurs. The "unexpected" part is relative because almost always it was predictable. It is important to consider the gaze positions where this new deviation may be created by the proposed surgical intervention and to use another approach or use the creation of a new deviation to assist in the overall surgical plan. In most cases the one gaze position where the eyes are aligned prior to any surgical intervention is located in the opposite direction to the field of action of the paralytic muscle. (A patient with right sixth cranial nerve palsy often sees singly in left gaze.) Strengthening or tightening the paralytic muscle will limit ocular rotation in the opposite direction creating a deviation where none existed before the surgery. The choice of surgical options should take into account what new deviations may be created. A recession may result in a new deviation since the muscle is now weaker or a resection may restrict the ocular movement in the opposite direction. Planning for, and dealing with, these predictable outcomes will help in obtaining the widest field of useable vision. This is an extremely important point. Often patients are extremely distressed when the previous area of single binocular vision is eliminated in an attempt to improve the diplopia in other fields of gaze. This becomes especially disconcerting if the resultant field of single binocular vision is extremely narrow, or if the diplopia changes direction with small gaze changes (i.e., esotropia in one field and exotropia in another). © 2009 Springer Berlin Heidelberg.
- Pediatric Ophthalmology: Current Thought and A Practical Guide
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International Standard Book Number 13 (ISBN-13)
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