Microsurgeons of all emphases, to thine own binocular refractive correction be true, a new study says. Performing surgery using a microscope requires a combination of highly-practiced skills, both fine motor technique and precise depth perception. For perceiving depth under a microscope’s objective lens, previous research has found that binocular vision outperforms monocular sight.

Also, monovision is suspected of diminishing stereoscopic vision. The impact of monovision on depth perception can be assessed with induced anisometropia; adding plus or minus diopter (D) values to the better optically-corrected lens on a lens frame. Investigators used this method, along with a newly developed microscopic stereotest (M-stereotest) to evaluate the question, according to a report in Clinical Ophthalmology. M-stereotest employs three test bars, similar to the distance three-rods test, but small wire bars are horizontally aligned to better replicate the typical procedure direction during vitrectomy.

Thirty healthy participants, mean age 22.5 years of age, aligned 3 wire bars under the microscope lens in 7 conditions: best optically-corrected binocular, monocular dominant and nondominant eye, binocular with added -3.0 D or +3.0 D to the dominant eye’s correction, and binocular with -3.0 D or +3.0 D added for the nondominant eye. Trial frames were used for the anisometropic states. Results were significant. Participants displayed significantly less exact bar alignment, and thus reduced depth perception in the binocular dominant eye -3.0 D or +3.0 D condition (P <.001), and the binocular nondominant eye -3.0 D (P =.003), or +3.0 D (P <.001) state, compared with the binocular best optically-corrected condition.


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“Therefore, it is desirable to correct monovision and anisometropia during microscopic surgery, including eye surgery,” the investigation says. The research speculates that findings in anisometric conditions resulted from the “defocusing of one eye.”

Binocular best optical correction was also significantly better than the monocular dominant eye condition (P <.001), and the monocular nondominant eye state (P <.001). Thus, the current analysis affirms findings of prior studies showing better depth perception with best-corrected binocular correction than with monocular vision. 

Further, stereopsis may be harder to achieve under a microscope than in typical viewing conditions. It is notable that the present data showed a strong correlation between results of M-stereotest and the standard distance three-rods perception test, both dynamic stereo assessments.

Twelve female and 18 male participants completed tests for this analysis, conducted in Japan. The cohort’s mean refraction was -3.96±2.50 D in the dominant eye, and -3.92±2.57 D in the nondominant eye. Investigators used a stereomicroscope with “Galileo-type parallel optical system” (Olympus Corporation). In addition to the M-stereotest; accommodation convergence/accommodation ratio, amplitude of accommodation, and fusional amplitude assessments were completed. Also, alternating prism cover tests were performed at 5 meters and 30 centimeters to determine eye position.

The cohort was relatively small, representing a study limitation. However, the significant correlation between M-stereotest and three-rods test was a strength. “Our results showed that in subjects with normal stereopsis, binocular vision and proper refractive correction are necessary for good depth perception under a microscope,” according to the research.

Reference

Shinomiya K, Fujikado T, Kitora A, et al. Depth perception with a newly developed microscope stereotest. Clin Ophthalmol. 2021;15(9):3901-3908. doi:10.2147/OPTH.S326695