Effect of prolonged dissociation on the subjective measurement of near heterophoria

Abstract

Previous studies have demonstrated that vergence adaptation resulting from the prolonged decay of slow fusional vergence may prevent the accurate assessment of oculomotor deviations. Continuing changes in heterophoria have been reported after 27 days of monocular occlusion. However, since most slow fusional vergence will decay within the first few minutes of dissociation, a clinical assessment of both vergence adaptation and heterophoria could be made more rapidly. This report documents two investigations which examined changes in near horizontal heterophoria during 30 min and 180 min of continuous dissociation, respectively. Seven out ot the 16 subjects exhibited significant changes in heterophoria during the 30 min dissociation period, with a mean increase in exophoria of 3.4 1. The decay of slow fusional vergence took approximately 25 min to reach completion. Thus in the clinical setting, both the degree of vergence adaptation and a more accurate assessment of heterophoria may be obtained by maintaining dissociation for 25 min. Furthermore, the presence of vergence adaptation can be assessed after just 5 min of dissociation.

Vergence adaptation and the clinical AC/A ratio

“…However, the mean change in dissociated heterophoria was only 0.17 meter angles (MA), or approximately 1D. Since previous estimates of the repeatability of heterophoria measurements have suggested that the 95% limits of repeatability of this parameter range between 21.9 and 4.0D (Schroeder et al, 1996;Rosen®eld et al, 1997), it seems likely that this dierence observed by Jiang fell within the limits of repeatability of the measurement procedure. Furthermore, of the 12 (out of a total of 31) subjects who demonstrated positive accommodative adaptation greater than +0.25 D, only two (6%) exhibited an exophoric shift in dissociated heterophoria greater than 0.3 MA (approximately 2D).…”

“…Subjects were subsequently divided into two subgroups on the basis of their response to prolonged dissociation. Rosen®eld et al (1997) reported that the 95% limits of repeatability of near heterophoria were 21.9D. Accordingly, subjects were divided into adapters and non-adapters depending upon whether their near heterophoria changed by more than 22D.…”

“…Accordingly, if the oculomotor deviation is measured after only a few seconds of dissociation, then a substantial proportion of the slow fusional vergence response may remain, and an inaccurate heterophoria measurement will be obtained. Indeed, Rosen®eld et al (1997) observed sig-ni®cant shifts in near horizontal heterophoria during a 30-min period of continuous dissociation. They indicated that these changes in the ocular deviation following dissociation resulted from the extended time course of decay of slow fusional vergence.…”

The forced vergence cover test and phoria adaptation

“…, 1998). Additionally 2 Δ has previously been used to indicate a significant change in phoria (Rosenfield et al. , 1997), and is typically the smallest interval between prisms used clinically to measure phorias with the CT.…”

“…The phoria (dissociated phoria, heterophoria) can be described as the difference between the ocular vergence angle and the vergence demand of a target in the absence of a disparity vergence response (Rosenfield et al. , 1997).…”