Estimate and measure of the region of view and myopia resulting from vitreous gas

Purpose: After vitreous gas injection, patients notice better acuity in dow nward gaze than in horizontal gaze. The authors evaluated the refractive er ror and the size of the region within which vision improves. Methods: For the vitreous fluid-gas interface, the authors calculated the a ngle of total internal reflection and the expected myopic shift and then me asured them in nine consecutive patients. The volume of gas, declination an gle at which perception of small targets occurred, and preoperative and pos toperative refractive error in downward gaze were measured. Results: Total internal reflection occurs at 41.5 degrees declination. Pati ents perceived a region of improved acuity below 41 degrees (+/-5 degrees) declination, The difference between the calculated and clinically measured gas-induced myopia was less than 25% for five of nine patients. The largest measured induced myopia was -23.9 diopters (60% gas volume). In eight eyes , patients read 5-point type or smaller. Conclusions: Patients accurately perceive that their acuity improves in dow nward gaze; the boundary of this region corresponds with the angle of total internal reflection. Calculations predict that vitreous gas produces a myo pic shift and aberration. These data support the notion that ocular positio ning by patients with vitreous gas can be enhanced by instructing them to r egard near targets in downward gaze.