Slit lamp examination during COVID-19: Where should the protective barrier be?

Purpose: The aim of this study was to describe a poly-vinyl chloride air condition (PVC AC) curtain shield placed at the intervening space between the chin rest and the illuminating and optical arm of the slit lamp microscope (distal barrier) instead of the present position at the oculars (proximal barrier) to guard against severe acute respiratory syndrome Coronavirus 2. This experimental study was done to validate and compare the protection offered by the conventional breath shields and the one described by us in a simulated environment.

Methods: In this experimental study, 12 puffs of fine mist were sprayed over a period of 1 minute using "magenta-colored dye" and "cyan-colored dye" for analyzing proximal barrier and distal barrier respectively. To access the amount of contamination of the slit lamp, caused by sprayed "magenta" and "cyan" colored dye, we covered the entire slit lamp with appropriately sized white-colored cotton sheet. The stained sheets were individually photographed and then the images were first cropped, then color threshold adjusted and then converted to binary and finally fraction of surface area stained was calculated using the ImageJ software. (P Surface Area (%) during analysis of "proximal barrier" and P Surface Area (%) during the analysis of "distal barrier", where P refers to various parts of slit lamp.

Results: The entire surface area of the cloth covering the slit lamp was 9912.45 cm. The surface area of the cloth which was stained when using the "proximal barrier" was 567.50 cm whereas when using the "distal barrier" was 222.93 cm. When using proximal barrier, 97.71% of the staining was present on the slit lamp base, viewing arm, illumination arm and the pivot of the slit lamp. While using the distal breath shield no parts of the viewing arm, illumination arm, the pivot or the mechanical base with joystick were stained.

Conclusion: The novel PVC AC curtain shield provides better barrier against the simulated contaminated stream of patient's breath directed towards the working parts of the slit lamp as compared to conventional slit lamp oculars mounted breath shields.