Stimulus and response AC/A ratios in intermittent exotropia of the divergence-excess type.

Abstract

SUMMARY Objective infrared recording devices were used to measure simultaneously and continuously both accommodation and accommodative vergence to near stimuli in 4 subjects with intermittent exotropia of the divergence-excess type (2 simulated and 2 true). In addition standard clinically determined stimulus accommodative convergence to accommodation (AC/A) ratios were measured. Results showed the mean group response AC/A ratio to be 5-9/1 (range 4 5-8 0/1) with no differences between true and simulated divergence-excess. Similar AC/A ratios were found after 45 minutes of monocular occlusion. Our results clearly demonstrate relatively normal response AC/A ratios in these subjects. Thus, contrary to what is believed by many clinicians, the reduced ocular deviation at near compared with distance vision cannot be attributed primarily to an abnormally high AC/A ratio. We believe that fusional convergence after-effects and/or proximal convergence effects contribute to inflate the clinically determined stimulus AC/A ratios.The divergence excess type of intermittent exotrope, with the deviation being greater at distance than at near vision, ' is commonly believed to have a very high accommodative convergence to accommodation (AC/A) ratio.25 This notion has been based largely on distance/near measurements of the horizontal oculomotor exodeviation (i.e., tropia or phoria). The average deviation is 29 A at far and 9 A at near,6 resulting in an average stimulus AC/A ratio of approximately 14-0/1. However, reported values of the AC/A ratios in these patients show much variation. Ogle et al.7 found the mean stimulus AC/A ratio to be 3 0/1 using the fixation disparity method.8 Moore et al.,' grouping all exotropes (divergence excess, convergence insufficiency, and basic exotropes) together (45% were the divergence excess type), found the mean stimulus gradient AC/A ratio to be 4-7/1. Calculations from Brown' suggest presence of a stimulus AC/A ratio of 13-0/1 or greater based on the +3-00 dioptre gradient technique. Von Noorden'°found a wide range of stimulus AC/A ratios (3-3 to 9-0) using the +3-00 dioptre gradient technique in a large sample of patients. In 2 cases presented by Burian Correspondence to Dr Kenneth J. Ciuffreda.and Smith" gradient stimulus AC/A ratios were 6-0/1 and 8-0/1, while they were 14-0/1 by the distance/near phoria method.There are 2 basic problems with the above mentioned studies. First, only stimulus AC/A ratios were determined. One is never certain of the actual accommodative response but rather assumes it to be equal to the stimulus demand. This assumption results in an

The fixation target influences the near deviation and AC/A ratio in intermittent exotropia

“…Kushner, 3,4,8 Cooper and colleagues, 9 and Arnoldi and Reynolds 5 have suggested that adaptive fusional vergence (''tenacious proximal fusion'' or ''fusional vergence aftereffect'') plays a more significant role in reducing the size of the deviation at near than a patient's accommodative convergence. This is supported by the clinical finding that many patients with a high AC/A ratio do not develop a postoperative consecutive near esotropia.…”

“…[5][6][7][8] Studies have also indicated that monocular occlusion is required prior to measurement of the AC/A ratio 4,5,9 due to adaptive fusional vergence effects, commonly referred to as ''tenacious proximal fusion'' or ''fusional vergence aftereffect,'' contaminating the measurement. 4,9 However, the AC/A ratio is influenced not only by the presence of tenacious proximal fusion but also by a patient's accommodative response to lenses during testing. The purpose of using plus or minus lenses in measuring the AC/A ratio is to, relax or induce accommodation and the related accommodative convergence, respectively.…”

“…Studies that have objectively measured accommodation have found mismatches to commonly occur between a patient's accommodative response and with presented stimuli (plus or minus lenses). [9][10][11][12] This assumption, when violated, can significantly affect the outcome, grossly underestimating the AC/A ratio. When testing the stimulus AC/A, as is done clinically, one must therefore make every effort to ensure that the patient has relaxed or induced accommodation appropriately through the stimulus provided.…”

Dissociated vertical deviation in patients with intermittent exotropia

“…8,20 In general, patients with intermittent exotropia, especially when the eyes are aligned, may have relatively normal binocular function. [21][22][23] When the Titmus stereo test was used to measure stereoacuity in patients with intermittent exotropia, it averaged 41 arcsec, similar to the stereoacuity level in normal populations. 13,24 Compared with this level of stereopsis, our patients with intermittent exotropia and DVD had poorer stereopsis, with an average of 147 arcsec.…”

Survey of British and Irish orthoptists in the measurement of the AC/A ratio

“…Interestingly, there is little difference in the responses for H compared with NG, DG or SG, with participants stating 3:1 to be the normal ratio most often in all four methods, closely followed by 4:1 -yet anecdotal evidence and published data 5,7,8 have demonstrated that there are significant differences in measured AC/A ratios between these methods. Commonly clinicians will anecdotally attest that H often over-estimates the AC/A ratio compared with either gradient method, and in fact Garretty 8 demonstrated a 5-6:1 ratio using H compared with 2-3:1 with NG and DG, which was statistically significant ( p < 0.01).…”