Ogle & Dyer (38) used fixation disparity methods to

calculate stimulus AC/As. They reported an average

AC/A of 3-4/1 in the DEX(T) (see Figure 2, right).

Interestingly, fixation disparity measured AC/As do not change significantly after surgery while distance near stimulus AC/AS do. Cooper, Ciuffreda & Kruger (40) simultaneously recorded accommodation and vergence responses using an infrared optometer and an infrared eye movement monitor. From their recordings they calculated response AC/As in both true and simulated DEX(T) patients. They found the response AC/A ratios to range from normal to slightly high. The mean response AC/A was 5.9/1 (with a range 4.5-8/1) (see Figure 3, top, next page >>>). Furthermore, they found no difference between response AC/As for simulated and true DEX(l’). They postulated that proximal convergence in true DEX(T) and vergence aftereffects (slow vergence) in simulated DEX(T) were responsible for the discrepancy between distance/near AC/A ratios and gradient AC/A ratios in these patients.

Any discussion of the effects of occlusion on X(T)s must address the concept of vergence aftereffect. Vergence aftereffects or slow vergence are a temporary change in oculomotor position which result from any sustained fusional induced vergence (42). For example, Ogle & Pragen (43) have shown that if 6 diopters of vertical prism is slowly added with a Risley prism, e.g., 1 pd per minute, in front of some subject’s eyes and those subjects completely adapt, both cover testing and forced fixation disparity curve before and while wearing the prism will be identical, e.g., orthophoria with and without the prism. Significant adaptation to the prism occurs within a few minutes. Abrupt removal of the prism usually results in diplopia with a slow recovery of fusion. The rate of recovery depends upon the duration of the vergence stimulus (44). Recovery occurs when the original phoria is attained (see Figure 4, right below).

This phenomenon, described as vergence or prism adaptation, may occur if an orthophoric patient wears horizontal prism, such as 10 pd base Out, for an hour. Complete vergence adaptation occurs when cover testing with and without prism does not change, i.e. orthophoria. Upon immediate removal of the prism, cover testing will reveal a moderate esophoria.

These findings demonstrate that elimination of fusional impulses by occlusion, such as cover testing does not result in an immediate loss of vergence related impulses. The slow change in oculomotor position over time secondary to disruption of fusional impulses has been called prism adaptation, vergence aftereffect, or slow vergence (45). These vergence aftereffects are time dependent and receive their input from the fast, fusional, disparity vergence system (46). This is supported by the observation that the amount of diplopia, which is induced by vergence aftereffects, is not affected by either darkness or sleep (47). The position that the eyes are in just prior to going to sleep is the same position that one awakes with. Thus, the oculomotor position of the eyes is not altered by darkness or sleep.

The total disparity induced vergence response is made

up of the sum of the fast fusional and slow adaptive

190

Major Review: Intermittent Exotropia; Basic and Divergence Excess Type

Figure 2 (Cooper & Medow): Typical fixation disparity curve at near and far, and derived AC/A ratios before and after surgery for DEX(T). The presurgical measurements were 30t pd X(T) and 14A X’ (AC/A = 12/1). The fixation

disparity curve before and after surgery was not changed. The derived fixation disparity AC/A was 5/1 both before and after surgery. (‘Reprinted with permission from Ogle K Dyer TA: Observations on Intermittent Exotropia Arch Ophthalmol 1965; 73:58-73. Copyright A,n Med Assn 1965).

Figure 4 (Cooper & Medow): Eye movement recordings for three different subjects demonstrating the slow decay of signals after occlusion. At point A a step vergence stimulus is introduced, i,e,. wedge prism. All three subjects demonstrate a rapid fast fusional response to eliminate disparity (closed loop). At B 15 seconds and B 160 seconds the loop is reopened by occlusion. The rate of relaxation of fusional vergence varies from individual to individual (RC greatest; RK least) and with the time of the adapting prism, 4e,. 15 seconds versus 160 seconds. (Reprinted from Vision Research Vol. 19, Schor CM The relationship between fusional vergence eye movement and fixation disparity 1359-67, copyright 1979 with permission from Pergamnon Press Ltd., Headingham Hill Hall, Oxford (X30BW, UK).