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Vertical Fusional Vergence the Key to Dissociated Vertical Deviation

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Vertical Fusional Vergence the Key to Dissociated Vertical Deviation CLINICAL SCIENCES Vertical Fusional Vergence The Key to Dissociated Vertical Deviation Edward W. Cheeseman, Jr, MD; David L. Guyton, MD Objectives: To test the previous findings of Enright that value of torsional shift was 1.15° ± 0.76° in the expected disparity-induced vertical vergence is mediated primar- direction. ily by the oblique muscles, and to relate this normal eye movement pattern to the eye movement pattern seen in Conclusions: Vertical fusional vergences in this study subjects with dissociated vertical deviation. were produced primarily by the oblique extraocular muscles. The eye movement patterns of these vertical Methods: Sixteen normal volunteers underwent 55 mea- vergences in normal subjects are qualitatively similar to surements of the cycloversion associated with prism- those seen in recordings of patients with dissociated induced vertical vergence using an afterimage appara- vertical deviation. Dissociated vertical deviation thus tus. A Vernier scale measured the direction and magnitude seems to be an exaggeration of a normally occurring eye of the torsional shift that occurred with recovery of fu- movement pattern. The cyclovertical component of dis- sion on removal of a 3– or 4–prism diopter prism. sociated vertical deviation may help stabilize the fixing eye by damping vertical nystagmus, while the accompa- Results: Of the 55 trials, the directions of torsional shift nying hypertropia is an incidental and undesirable side were consistent with the oblique muscles being the pri- effect. mary mediators of vertical fusional vergence in 51 (93%) (P = .03 using a binomial distribution). The mean ± SD Arch Ophthalmol. 1999;117:1188-1191 ISSOCIATED vertical devia- noted vertically divergent eye move- tion (DVD) is an enig- ments that were accompanied by a con- matic motility disorder jugate cycloversion. The direction of the that has defied explana- cycloversion implied predominant ac- tion since its original de- tion by the oblique muscles and not the Dscription more than a century ago.1 It may vertical rectus muscles. He concluded that occur spontaneously or as a result of cover testing. It may occasionally be found as an See also page 1216 isolated phenomenon, but it generally oc- curs in association with strabismus, with the bulk of the reflex vertical fusional ver- the most common association being con- gence was due to the superior oblique genital esotropia.2,3 muscle. The role of the inferior oblique The dissociated eye movement of muscle was considered passive and mi- DVD is normally elevation and extorsion nor. van Rijn and Collewijn7 attempted to of the covered eye, accompanied by intor- reproduce Enright’s findings using scleral sion of the fixing eye. It has been docu- search coil recordings. They confirmed his mented, however, that dissociated devia- cycloversion findings in normal subjects tions can be primarily horizontal or accompanying disparity-induced vertical torsional.4 vergences, but refuted his hypothesis that The eye movements of DVD have pre- only the superior oblique muscle was the viously been described as a form of verti- primary mediator of vertical vergence. In cal vergence.2,5 Enright6 reported the in- their study, part of the cycloversion ac- From the Wilmer teresting, and unexpected, finding that companying the disparity-induced verti- Ophthalmological Institute, vertical fusional vergence induced by ver- cal vergence was in the form of torsional The Johns Hopkins University tical disparity in normal subjects is asso- nystagmus. School of Medicine, ciated with binocular torsion. He placed The primary purpose of this study Baltimore, Md. a weak vertical prism in front of 1 eye and was to confirm Enright’s finding that ver- ARCH OPHTHALMOL / VOL 117, SEP 1999 1188 ©1999 American Medical Association. All rights reserved. Downloaded From: https://jamanetwork.com/ on 09/29/2021 SUBJECTS AND METHODS the order of possibilities randomly selected. Initially, a 2–prism diopter (PD) prism was introduced, and this was slowly increased to 4 PD, if possible. Two subjects were Sixteen volunteers, 8 men and 8 women, participated af- only able to fuse a maximum of 3 PD vertically on one of ter giving informed consent. Their ages ranged from 17 to the trials (Table). 53 years. All had a best-corrected visual acuity of at least Once either 3 or 4 PD of vertical disparity were fused, 20/20 OU. None of the volunteers had manifest strabis- the subject was asked to look at a small white dot that was mus or any evidence of amblyopia. painted centrally between the 2 slits on the flash unit. The An apparatus was constructed using an ordinary pho- flash was then fired, and the prism was rapidly removed tographic flash unit that was attached to a standard rotat- from the eye. Gaze was maintained on the white dot until able optical mount (Figure 1). The mount could be ro- the induced vertical diplopia resolved (normally between tated manually for 360°, and the outer edge had an engraved 5 and 10 seconds). The room lights were then turned off, 360° Vernier scale that allowed measurement with a pre- and the subject rotated the entire flash unit until the phos- cision of 0.1°. The flash aperture was masked using black phorescent reference line was parallel to the 2 parallel af- tape such that 2 horizontal slits were created parallel to one terimages that had been created (Figure 2). The magni- another. The 2 slits were separated by 4 mm. Below and tude and direction of any torsional movement occurring, parallel to the 2 slits, a thin line of phosphorescent paint associated with the vertical fusional vergence to recover was applied to the black tape. single binocular vision, were then read from the scale to A near reading card (Bausch & Lomb, Rochester, NY) the nearest 0.1°. Each subject performed 2 to 4 trials in this was then taped to the front surface of the flash unit, with a manner (Table). slot cut in the card to expose the underlying parallel slits To ensure reliability in determining the position of the and phosphorescent line. Lettering (size 20/50) was im- afterimage relative to the reference line, each subject ini- mediately adjacent to the slits. tially performed 2 trials without any prism in place. Sub- The entire apparatus was placed on an optical rail at jects were instructed to look at the central white dot, and a distance of 40 cm from a headrest–chin rest assembly. the flash was fired. The assembly was then rotated 30° from Two cam-type rings were mounted on either side of the the horizontal position, and the lights were turned off. The headrest that, when rotated into place, served to stabilize subject was instructed to rotate the assembly back such that the head and ensure that there were no significant head the phosphorescent reference line was again parallel to the movements. afterimages. If a subject could not perform this task within Each subject was instructed to fixate on the 20/50 size 0.5° of the original horizontal position on each trial, the lettering while progressively stronger vertical prisms were volunteer was not considered reliable. The 16 volunteers placed either base up or base down in front of 1 eye, with selected fulfilled these criteria; 1 volunteer was rejected. tical fusional vergence is mediated primarily by the ob- lique muscles. We used a small prism to induce a verti- cal fusional movement in 16 normal volunteers. We then used a basic afterimage apparatus to measure by subjec- tive means whether a clockwise or counterclockwise tor- sional movement occurred during recovery of fusion af- ter the prism was removed. A second purpose of the study was to relate the apparently normal eye movement pat- tern of vertical vergence to the eye movements seen in subjects with DVD. RESULTS Each subject participated in 2, 3, or 4 trials using the af- Figure 1. Afterimage apparatus. A flash is triggered when the subject verifies terimage device, for a total of 55 trials (Table). In 51 (93%) fusion with the vertical prism, creating an afterimage. of the 55 trials, the direction of the cycloversion that ac- companied the vertical vergence required to recover fu- sion was consistent with the oblique muscles being the tical fusional vergence was classified as “inconsistent.” primary mediators of vertical fusional vergence. That is, Any individual with all observations consistent with ob- in most cases, removal of a base-down prism from the lique muscle mediation was classified as “consistent.” The right eye or a base-up prism from the left eye resulted in criteria for consistent were thus quite strict. The hypoth- a counterclockwise torsional movement (levocyclover- esis of equally likely mediation of the vertical fusional sion) as fusion was regained. Similarly, removal of a vergence response by the oblique muscles or the verti- base-up prism from the right eye or a base-down prism cal rectus muscles was tested by considering each sub- from the left eye resulted in a clockwise torsional move- ject’s consistency or inconsistency as an independent ob- ment (dextrocycloversion) as fusion was regained. servation. A binomial distribution was used with P = .50 Any individual having at least one observation that to describe the chance of observing certain numbers of was not consistent with oblique muscle mediation of ver- subjects with consistent and inconsistent results. ARCH OPHTHALMOL / VOL 117, SEP 1999 1189 ©1999 American Medical Association. All rights reserved. Downloaded From: https://jamanetwork.com/ on 09/29/2021 Torsional Shift of the Afterimage on Removal of the Vertical Prism* Prism, Subject No. PD Orientation Eye Degrees Direction 1 4 Base down R 1.8 CCW 4 Base up R 1.5 CW 4 Base down L 1.9 CW 4 Base up L 1.8 CCW 2 4 Base down R 1.2 CCW 4 Base up R 1.0 CW 4 Base down L 2.2 CW 4 Base up L 1.0 CCW 3 4 Base down R 0.8 CCW 4 Base up R 1.8 CW Figure 2.
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