Quantifying the vertical fusion range at four distances of fixation in a normal population

Vertical vergence in nonhuman primates depends on horizontal gaze position

The goal of this study was to compare vertical fusion capability at different orbital eye positions in normal nonhuman primates and attempt to use this information to isolate the extraocular muscles (EOMs) that mediate vertical vergence. Scleral search coils were used to record movements of both eyes as two normal nonhuman primates (M1, M2) performed a vertical vergence task at different horizontal eye positions. In a control experiment, M1 was also tested at different angles of horizontal vergence. To elicit vertical vergence, a 50° x 50° stimulus comprising a central fixation cross and random dots elsewhere was presented separately to each eye under dichoptic viewing conditions. Vertical disparity was introduced by slowly displacing the stimulus for one eye vertically. Vertical fusion amplitude (maximum disparity that the monkey was able to fuse) and vertical vergence (maximum difference in vertical position of the two eyes) were measured. Vertical fusion capability differed at different orbital eye positions. Monkey M1 had significantly smaller vertical fusion capabilities when the right eye (RE) was abducted than left eye (LE) while M2 had significantly smaller vertical fusion capabilities when the RE was adducted and LE abducted. M1 also showed greater vertical fusion capability for near gaze. M1 data suggested that the vertical recti mediated vertical vergence in the RE and the oblique muscles in the LE while M2 data suggested that the oblique muscles mediated vertical vergence in the RE and the vertical recti in the LE. The variable results within the same animal and across animals suggest that EOM involvement during vertical fusional vergence is idiosyncratic and likely a weighted combination of multiple cyclovertical muscles.

Impact of intraocular lens displacement on the fixation axis

To investigate the impact of intraocular lens (IOL) decentration ≤±1  mm and IOL tilt ≤±10° on the fixation axis and spherical equivalent refraction (SE), 50 pseudo-phakic eyes were simulated using numerical ray-tracing. We computed the position of the object point whose image ends up at the virtual fovea for each scenario and estimated the corresponding change of fixation axis and SE. The eye turned opposite to the direction of IOL decentration or tilt to compensate for the associated prismatic effect (angle <1.2°). Decentration of the aspheric IOL resulted in a hyperopic shift (<0.57 D), and tilt in a myopic shift (<0.77 D).

Target Properties Effects on Central versus Peripheral Vertical Fusion Interaction Tested on a 3D Platform

PURPOSE

We investigated the impact of target properties on vertical fusion amplitude (VFA) using a 3D display platform; the performance of the subjects allowed us to assess how central and peripheral retina regions interact during the fusion process.

MATERIAL AND METHODS

Fourteen subjects were involved in the test. VFA was recorded by varying the viewing distance, target complexity, disparity velocity, lighting condition and background luminance. Base-up prisms were introduced to create vertical disparity in the peripheral retinal area, whereas an offset compensation was added in the central area. Data were analyzed in JMP software using T-test and repeated-measures ANOVA tests.

RESULTS

VFA is significantly affected by target properties including viewing distance, target complexity and disparity velocity; the impact from lighting condition and background luminance is not significant. Although central retina plays a crucial role in the fusion process, peripheral regions also affect the fusion performance when stimulus size on retina and contents disparity values are modified between central and peripheral vision.

CONCLUSION

Vertical fusion is affected by various target properties. For the first time, peripheral vertical disparity direction effects on central fusion and eye motion response have been explored. Besides, a quantitative interaction of central and peripheral fusion is observed, which could be applied in clinical measurement on binocular disease concerning central and peripheral vision conflict.

Adjustable small-incision selective tenotomy and plication for correction of incomitant vertical strabismus and torsion

PURPOSE

To evaluate the effectiveness of adjustable small-incision selective tenotomy and plication of vertical rectus muscles in correcting vertical strabismus incomitant in horizontal gaze positions and cyclotorsion.

METHODS

The medical records of all patients who underwent adjustable small-incision selective tenotomy or plication of a vertical rectus muscle for correction of horizontally incomitant vertical strabismus or cyclotorsion by a single surgeon at a single eye institute from July 2013 to September 2014 were retrospectively reviewed. Selective tenotomy and plication were performed on either the nasal or temporal side of vertical rectus muscles, based on the direction of cyclotorsion and incomitance of vertical strabismus.

RESULTS

Of 9 patients identified, 8 (89%) had successful correction of horizontally incomitant vertical strabismus, with postoperative vertical alignment within 4(Δ) of orthotropia in primary position, lateral gazes, and downgaze. Of the 8 patients with preoperative cyclotorsion, 4 (50%) were successfully corrected, with <5° of cyclotorsion postoperatively. Of the 4 patients in whom cyclotorsion did not improve, 3 had undergone prior strabismus surgery, and 2 had restrictive strabismus. Eight of the 9 patients (89%) reported postoperative resolution of diplopia.

CONCLUSIONS

Adjustable small-incision selective tenotomy and plication effectively treat horizontally incomitant vertical strabismus. These surgeries may be less effective for correcting cyclotorsion in patients with restriction or prior strabismus surgery. Advantages are that they may be performed in an adjustable manner and, in some cases, under topical anesthesia.