Assessment of Simulated Respiratory Droplet Spread During an Ophthalmologic Slitlamp Examination

Abstract

This study simulated respiratory droplet spread during an ophthalmologic slitlamp examination to help establish risk of infectious disease contagion in this setting.

The coronavirus disease 2019 (COVID-19) pandemic has brought challenges to the medical community. One of the concerns is regarding the detection of high titers of virus in the oropharynx early in the disease course.¹ Furthermore, the relatively long incubation period and environmental contamination of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) place others at high risk of exposure to the virus. Personal protective equipment (PPE), including gloves, face masks, goggles, face shields, and gowns, is warranted when appropriate to reduce the risk of spread of infection to relevant health care professionals from patients who might harbor SARS-CoV-2 in the asymptomatic or presymptomatic period. The face-to-face proximity during the slitlamp biomicroscope examination presumably places ophthalmologists at high risk of contracting various respiratory pathogens. As such, the use of commercially available slitlamp barriers or breath shields as an added measure of protection has been recommended by the American Academy of Ophthalmology.

Methods

In a simulation conducted from March 2020 to April 2020, an ophthalmologist who had donned standard PPE, including a face mask (ASTM level 2 [3M]) and eye protection (safety glasses [Ultra-Spec 2000]), was positioned looking through the oculars of the slitlamp (BM 900 [Haag-Streit]). The slitlamp had a commercially available breath shield hung on oculars, measuring 9.75 inches in width and 10.5 inches in height (Carl Zeiss Meditec AG). A manikin (Vera cardiopulmonary resuscitation model [Canadian Red Cross]) was placed at the chin rest of the slitlamp to simulate a patient under examination. To standardize the target distance, the slitlamp was focused on the manikin’s right eye. A patient cough was simulated using a small latex balloon, which was compressed with oxygen and 1.25 mL of washable fluorescent dye that was run through tubing inside the manikin and placed inside the oral cavity. The balloon was inflated until it burst at 5 PSI, which has been previously reported as the force for a voluntary cough and laryngeal cough reflex.² The simulation was performed under ultraviolet light conditions (a light-emitting diode 395NM ultraviolet flashlight [WJZXTEK]) to visualize emission of fluorescent small particles, which included a mixture of dry and wet particles measuring 30 to 100 μm for varied particle size distribution (ultraviolet neon fluorescent blacklight paint kit [Paint Glow]).³ The simulation was repeated for 10 rounds to confirm the repeatability of the results. These methods have been previously validated for visualization of cough droplets.^4,5 Conduction of this simulation was permitted by the Institutional Quality Improvement Review Board at the Department of Ophthalmology and Vision Sciences, University of Toronto, Toronto, Ontario, Canada. The findings of the simulation are reported in a descriptive manner, without statistical analysis.

Results

The greatest droplet spread was found on the slitlamp and the breath shield. Smaller droplet particles were noted in up to a distance of 5 m from the manikin. This simulation suggests that the use of slitlamp breath shields and standard PPE does not eliminate the projection of droplets onto the examiner’s field and his or her chest, shoulders, and arms (Figure). The spread of smaller droplets was also identified on the hair, hands, and shoes of the examiner. Further contamination of the floor, walls, and window covers was identified within the room. A video of the simulation that was slowed down by a factor of 8 to 240 frames per second is available online (Video).

Figure. Image Sequences Captured Under Ultraviolet Light Conditions of a Simulated Patient Cough With Ejected Fluorescent Dye Droplets at the Slitlamp With a Breath Shield.

A, The variation in brightness of emitted fluorescent dye in image sequences captured represents differences in the size of the particles, and the streaked dye suggests that particles are moving faster than can be captured on freeze-motion images. Spread of droplets noted onto the examiner’s field at the slitlamp (B) and chest, shoulders, and arms (C).

Video. Simulated Respiratory Droplet Spread During a Slitlamp Examination.

Download video file ^{(83.5MB, mp4)}

Simulation of a patient cough to demonstrate potential spread of respiratory droplets during a slitlamp examination.

Discussion

These findings are consistent with previous reports. These have suggested ejection of up to 2 m and 8 m away from the patient for large and small evaporating droplets, respectively.³

We acknowledge some limitations of this simulation. The presumptions behind this simulation have not been validated independently. An overproduction of the volume of the cough was intentionally implemented to try to account for various scenarios of droplet spread. This may or may not account, though, for the potential scenario in which a patient is able to sense the onset of a cough and tends to back away from the slitlamp prior to coughing. Furthermore, the spread of aerosolized particles smaller than 30 to 100 μm cannot be accounted for in this simulation.³

Conclusions

In summary, these findings support the use of adjuncts to the current standard PPE and protective barriers, such as breath shields,⁶ to try to minimize cross contamination during slitlamp examinations. These may include disposable gowns that provide coverage of the shoulders and arms, gloves, and surgical caps for the examiner. Most importantly, the use of masks for the patient at the slitlamp should be further explored in future studies, because it may offer an easy and inexpensive means of providing protection for the examiner.