Dr.Nikhil Balakrishnan

Dr. ROHIT SHETTY, Dr. LUCI KAWERI, Dr.Pooja Khamar

Semi Finals

Abstract

Purpose:To study the proclivity of aerosol generation during NCT&its impact during the COVID-19 pandemic

Methods:NCT was performed on human eyes under Normal settings, with a single& 2 drops of lubricant.Aerosols& droplet generation was imaged using high speed shadowgraphy& frontal lighting techniques

Results:In natural settings there was no splash or aerosol production. A single small droplet of around 470μ was observed being ejected from the tear film when 1 drop of lubricant was used prior to NCT.The trajectory of which was then computed.When 2 drops of lubricant was used we noted copious amount of tear ejection in the form a sheet which broke up into multiple small droplets which traversed back to the tonometer

Conclusions:There was no visible aerosol generation during NCT performed in natural settings&is thus safe to perform even in the COVID era.NCT is best avoided in conditions with high tear volume(natural or artificial)as it would lead to droplet spread& tactile contamination

Full Text

Deciphering the conundrum of Aerosolisation during Non-Contact Tonometry(NCT)

Purpose: To study the proclivity of aerosol generation during NCT & its impact during the COVID-19 pandemic

Methods: NCT was performed on human eyes under Normal settings, with a single drop & 2 drops of lubricant. Aerosols & droplet generation was imaged using high speed Shadowgraphy & frontal lighting techniques.

Results: In natural settings there was no splash or aerosol production. A single small droplet of around 470μ was observed being ejected from the tear film when 1 drop of lubricant was used prior to NCT, the trajectory of which was then computed. When 2 drops of lubricant were used, we noted copious amount of tear ejection in the form a sheet which broke up into multiple small droplets which traversed back to the tonometer

Conclusions: There was no visible aerosol generation during NCT performed in natural settings & is thus safe to perform even in the COVID era. NCT is best avoided in conditions with high tear volume (natural or artificial) as it would lead to droplet spread & tactile contamination.

Introduction

The World Health Organization (WHO) declared COVID-19 as a “Public Health Emergency of International Concern” in late January.[1] The pandemic is escalating at an alarming rate despite numerous outbreak control measures.[2] The transmission of the novel corona virus (2019-nCoV) occurs predominantly through direct contact[3] or via droplets when an infected individual coughs or sneezes.[4] Medical staff are at high risk as social distancing is difficult to maintain amidst their work environment and several patients may be asymptomatic.[5] While it is easier to safeguard against contact based infection, aerosols and droplets generated during diagnostic and surgical procedures pose a concern to healthcare workers since they may become airborne and not easy to track. Precise measurement of intraocular pressure (IOP) via tonometry is an essential practice in an ophthalmology clinic.[6]^,[7] The ideal device must be easy to use, fast, safe and precise, irrespective of patient posture, age, patient compliance and operator bias.[8] The Goldmann applanation tonometry (GAT) is a widely used method for measuring IOP.[9]^,[10] However, noncontact tonometer (NCT) (air-puff tonometry) is also used widely. It uses an air-puff to flatten the cornea. This method has advantages since no topical anaesthetic or risk of corneal abrasion is involved.[11]

There is some evidence of tear film dehiscence and aerosol formation during NCT.[12] The SARS-CoV-2 virus may reside in the tears and conjunctival secretions of symptomatic COVID-19 patients.[13] Hence, there is concern regarding the usage of NCT during the COVID-19 pandemic.[14] Based on their size, droplets can be sub grouped as large (>10-20 μm) and small (<10-20 μm) droplets. The larger droplets don’t remain suspended in air, settle quickly and hence don’t get deposited in the lower respiratory tract.[15]^,[16] By definition, aerosols are suspensions in air (or in a gas) of solid or liquid particles and small enough to remain suspended in air due to low settling velocity. Many studies have a size cut-off of <5 μm for aerosols. Particles with diameter lesser than 3 μm don’t usually settle.[17] Most particles of size > 6 μm can get trapped in the upper respiratory tract.¹⁶ To quantify aerosols and droplets, shadowgraphy is a visualisation technique in which the object to be imaged is inserted between the light source and the camera. Due to the density difference between the medium (air) and object (aerosol and droplet), light rays naturally bend and create a shadow which defines the boundary of the object. Shadowgraphy is a preferred technique for high speed imaging where the sensitivity is low at high frame rates.[18] We have previously used this technique to quantify the spread of aerosols and droplets during phacoemulsification and flap cut with a microkeratome. Both the studies revealed generation of droplets of large sizes which had minimal risk of aerosolization.[19]^,[20] Frontal lighting is another imaging technique where in light is directly shone upon the device to be imaged. Therefore, the aim of this study was to quantify the spread of aerosol and droplet generation during NCT using shadowgraphy and frontal imaging.

Methodology

This experimental study was approved by the institutional research and ethics committee of Narayana Nethralaya Multispecialty Hospital, Bangalore, India and conducted in accordance with the tenets of the Declaration of Helsinki. The study was performed in collaboration with the Department of Mechanical Engineering of the Indian Institute of Science, Bangalore, India. First, NCT was performed on one eye of each subject under normal settings (no eye drop instilled before NCT). Then, NCT was repeated immediately after instillation of a single drop of a lubricant (Systane Ultra lubricant eye drop, Alcon Laboratories, Fort Worth, TX). NCT was again repeated immediately after the instillation of two drops of lubricant. A ten-minute interval was maintained between each NCT measurement. The above process was repeated in 6 subjects who volunteered for the study. The Shin-Nippon NCT-200 (Rexxam Co. Ltd., Osaka, Japan) was used for the experiments.[21]

The shadowgraphy technique involved the use of a high-speed camera, the Mini- UX100 (Photron USA Inc., San Diego, USA) coupled with a macro lens (ATX 100, 100 mm, f2.8D; Kenko Tokina Co., Ltd., Tokyo, Japan) for imaging. The resolution of the camera was 1280 × 1024 pixels. The aperture was set to f /32 for a maximum depth of field. The continuous illumination used a high-power LED source (Constellation 120, Veritas) which was positioned opposite to the camera. The NCT device was placed between the light and the camera for high speed shadowgraphy (side lighting setup in Figures 1A and B). The camera was manually triggered to acquire images before the NCT was triggered. For the frontal lighting setup of shadowgraphy, the illumination was placed in front of the NCT device so that the light fell directly on it (Figure 1C). The light emitting diode (LED) light source was placed behind the camera to image the tear droplets distinctly (Figure 1D) since the normal morphology of the human face, location of the eyes and tilt of the human head sometimes impeded the shadowgraphy technique. By placing a white tape on the subject’s nose, enough backlighting was possible to make the cornea appear like a shadow which allowed sharper imaging of the indentation of the cornea during applanation.

For the fluorescein dye analyses, we stained the conjunctiva of the volunteers with a sterile ophthalmic fluorescein sodium strip (Fluro Strips, Contacare Ophthalmics & Diagnostics, Vadodara, India) after moistening the tip with a lubricant drop and 1 mg of sodium fluorescein. The volunteers were requested to blink repeatedly following a period of eye closure after the application of the strips. Then, tonometry was performed. The fluorescein absorbed wavelengths between 460 and 480 nm and emitted fluorescence in the range of 530 to 560 nm. To capture these emissions from the illuminated eye, we used a 30W blue LED as an excitation source (450 nm) and imaged with a bandpass filter (527 nm) placed on the camera. We used 3 different cameras for the fluorescein dye analyses. Firstly, we used a 16 megapixel smart phone camera (Realme 3 Pro – Realme Mobile Telecommunications Private Limited, Haryana, India) for external video filming in a darkly lit room. Videos were captured in 4K resolution and at 30 frames per second (fps). Secondly, a Nikon D7200 digital single-lens reflex (DSLR) camera was used to capture high resolution images (24 megapixels) at a shutter speed of 2.5 seconds. Videos on the same camera were acquired at 25 fps. Thirdly, the Mini UX100 was used at low acquisition rate (50 fps) to check for aerosol and droplet generation. The lens used for all the experiments was the ATX Pro 100. Figures 1E and F show the fluorescein setup.

A simple one-dimensional analysis of the aerosol and droplet spread was performed similar to our earlier work.^19,20 Assume that a droplet of diameter (D µm) was ejected with a velocity u_d (horizontal component) during the course of an air puff tonometry. Our out-patient departments (OPD’s) do not have controlled air-conditioning and a natural air velocity (u_air) ~ 0.1 to 0.2 m/sec in the room was considered assuming that the room was closed from inside. However, the presence of an air-conditioning unit or a table fan can enhance u_air to as much as 1 m/s.^19,20 The appropriate governing drag equation for the droplet can be written as:

[1]   where , r=D/2, u_d was the settling rate of the droplet, μ_f was the viscosity of air, ρ_f was the density of air, ρ_d was the density of droplet and g was the acceleration due to gravity. The droplet evaporates as well as settles due to gravity simultaneously. The evaporation timescale can be estimated from the D² law ^[22] while the appropriate settling rate (v_d in m/sec) was estimated from the Stokes equation:

[2]The calculation assumed that the human eye was positioned approximately 50 cm from the table on which the NCT was placed. Thus, the timescale of droplet settling can be obtained from equation 2 as follows:

[3]    The final horizontal distance (x) travelled by the droplet was determined from the smaller of the two quantities, the evaporation time scale and settling time scale. Additional details about the properties of air and droplet used for these calculations were provided in our earlier study. The droplet sizes were measured using custom algorithms as described in our previous studies.^19,20

Results

Unlike our previous studies. shadowgraphy was difficult to perform as the anatomy of the human face and the setting of the human eye obstructed distinct imaging of the cornea and conjunctival surfaces. The cornea appeared as a thin white film and deformed upon impact of the air puff. Figures 2A to E show a sequence of frames captured with shadowgraphy. Some deformation was evident in Figure 2C. After applying one drop of lubricant, a large droplet of diameter ~470 μm was observed originating from the eye (Figure 2E). On superimposition of the sequential images, we could chart the trajectory of the same (Figure 2F). To overcome the shortfalls of shadowgraphy imaging, we repeated the measurements using frontal lighting. The images were acquired at a rate of 2000 fps and shutter speed of 1/20000s. We first imaged without the instillation of any eye drop. We observed the presence of a tear film with minimal pooling of tears in the lower meniscus prior to NCT. No formation of any droplet or aerosol was observed during the procedure (Figure 3). On instillation of a drop of lubricant, we witnessed pooling of fluid in the lower meniscus, along with tear drops on the lower eyelashes prior to tonometry (Figure 4). Impingement of the air puff caused deformation of the cornea which caused the fluid film on the ocular surface to be displaced. In one instance, ejection of a tear drop was noted which traversed towards the lashes of the upper lid. The drop was seen as suspended from the upper lashes following return of the cornea to its normal state (Figures 4C and D). In another instance following corneal deformation, the fluid from the lower meniscus moved along the meniscus laterally up to the lateral canthus of the eye and was evidently imaged (indicate by white circle in Figure 5). On repeating the test for a third time, we observed break-up of the fluid film and ejection of two droplets (Figure 6).

On instillation of two drops of lubricant, there was excessive fluid in the lower meniscus of the eye along with presence or droplets in the lower lashes (Figure 7). On impingement of the air puff and after corneal deformation, the tear film was pushed away from the ocular surface. A sheet of fluid was initially noted which disintegrated into droplets (Figure 7). The diameters of these droplets were found to be in the range of 100 to 500 μm among all subjects (Figure 7). In all the subjects, we noted that the droplets originated from the fluid lake along the lower lid margin. On air impingement, a redirection of air impulses from the centre of the cornea towards the conjunctival fornices occurred that led to separation of eyelid margin from the sclera. If pooling in the lower fornices was present, NCT caused an excursion of fluid. Since the smallest detectable droplet diameter was ~100 µm, the process was controlled primarily by the settling timescale. To compute the distance traversed by these droplets, we used the drag equation 1 taking into consideration two settings of room air velocity, ~0.1 m/s and 1 m/s. Figure 8 shows the estimated distance traversed by the droplets as a function of diameter. This distance would be traversed by the droplet if left unobstructed. The initial value of droplet’s horizontal velocity (u_d) was calculated as 1 m/sec from the sequential images shown in Figure 7. The distance between the point of contact of the human eye and the pneumatic port of the tonometer was approximately 11 mm.²¹ Hence, droplets smaller than 300 μm can settle on the device even in case of low velocity of 0.1 m/sec (red circles in Figure 8). The spread distance increased with greater air velocity of 1 m/sec (blue circles in Figure 8). The simple calculation presented here does not account for the clustering effect observed in sprays.^[23]

The fluorescein experiments confirmed the observations our frontal imaging experiments. No aerosol production in a natural eye setting (Figure 9A) (without the instillation of eye drop) was seen on either smartphone imaging (Figure 9B), DSLR camera (Figure 9C), or high-speed imaging (Figure 9D). When one drop of lubricant was instilled, we noted supplementary volume of fluid in the lower fornix (Figure 10A) and minimal aerosol production on tonometry. The smartphone (Figure 10B) and DSLR camera (Figure 10C) picked up droplets being emitted from the ocular surface on air impingement, which appeared to settle on the nose bridge of the subject. High speed photography revealed emission of droplets from the lower tear meniscus which were emitted initially as a sheet and subsequently disintegrated (Figure 10D). When two drops of lubricant eye drop were instilled, we observed pooling of fluorescein stained tears in the lower fornix with some spillage towards the medial and lateral canthi (Figure 11A). Via the nightscape smartphone videography, there was generation of droplets (Figure 11B and C), which traversed back up to the NCT contaminating it (Figure 11D and E). The DSLR (Figure 11F and G) and high-speed imaging (Figure 11H and I) too demonstrated ample dispersion of droplets. Droplet size and trajectory in fluorescein analysis could not be computed because of the halo around the droplets obtained and the low speed (50 fps) of acquisition respectively.

Discussion

The fact that aerosols and large droplets can transmit viruses is well known.¹⁷ Hence, the transmission of COVID-19 could occur through respiratory droplets.[24]^{,[25],[26],[27]} Thus, the mucosa (mouth or nose) or conjunctiva are at risk of being exposed to the infected respiratory droplets. Another mode of transmission of the COVID-19 virus was through fomites in the environment around the infected person.[28] Airborne transmission differed from droplet transmission as it denoted the presence of aerosols that were smaller than 5μm in diameter. These have the potential of surviving in air for longer periods of time and also travel greater distances.[29] Thus. the air or the objects around a potentially infected person may be a potential source of infection. Hence, the procedures or treatment modalities that generate aerosols could result in airborne transmission of COVID-19.

Recommendations from physician bodies indicated that NCT should be avoided.^14,[30] Some bodies suggested the use of single use disposable tonometer tips as cleaning of the tips with 70% alcohol weren’t effective in disinfecting tips infested with the SARS-CoV-2 virus.[31] Due to these guidelines, many eye clinics may have discontinued the use of NCT and moved on to contact tonometry. The goal of this study was to precisely determine if there was risk of aerosolization from NCT procedure. We observed no aerosolization from the eye of the subjects without the instillation of eye drop. This was a critical finding. On instillation of a single lubricant drop, we observed accumulation of fluid in the lower fornix. On air impingement, dispersion of this fluid was observed. Most of the fluid was either drained along the lower meniscus into the lake near the lateral canthus or a small amount of fluid was ejected in the form of droplets. However, these droplets only travelled up to the lashes and weren’t emitted outside the eye. Only in one of the subjects did we observe droplets being ejected which traversed back up to the pneumatic port of the tonometer. This could possibly be a source of infection if the infected patients had ocular symptoms of COVID-19. The pneumatic port once infected may act as a reservoir for the virus and could transmit the same to the successive patients undergoing tonometry. In our final set of experiments, instilling two drops of lubricant resulted in significant splatter of fluid from the ocular surface. These drops may not only stay suspended in the air for long periods of time because of their size but also may contaminate the surfaces on which they land

As long as the eye was in its natural condition, performing NCT would be safe. However, the dispersion of droplets from the ocular surface could be a potential source of infection in cases of epiphora. Hence, NCT should be avoided in such patients, e.g., allergies, meibomian gland disorders, dry eyes[32]. Since epiphora is common after phacoemulsification[33], pterygium surgery[34], refractive surgery[35], lacrimal apparatus surgery[36], squint correction[37] and trabeculectomy[38], NCT should not be performed immediately after the surgery. The use of a protective shield on ophthalmic equipment such as slit lamps, optical coherence tomography(OCT) and fundus cameras was recommended.[39] However, the use of the same on a tonometer would not be effective as placing of a shield between the pneumatic port and the eye would prevent the air puff from reaching the eye. To our best knowledge, only one previous study evaluated the dispersion of aerosols during NCT. Britt et al. used fluorescence photographic technique to study the presence of aerosols.¹² A major drawback of this method was the inability to gauge the size of the aerosols, chart their trajectory and spread. Contrary to our findings, they reported the dispersion of aerosols and droplets on NCT in most eyes.¹² This could be explained by the fact that they used a drop of fluorescein to stain the ocular surface in every subject. Hence, we used a moistened fluorescein strip instead of fluorescein drops in all subjects to replicate the state of the normal human eye. The observation of excessive splatter on addition of a drop of methylcellulose was akin to our findings.¹²

They also used two different NCT’s, the American Optical (AO) NCT II (Cambridge Instruments Inc, Cambridge, Mass) and Keeler Pulsair (Keeler Instruments Inc, Broomall, Pa) tonometer. The AO NCT used a piston generated air impulse that linearly increased over the first 8 milliseconds after which it progressively decayed. The Keeler Pulsair used an electrical pump to create a pressure gradient and a ramped air impulse for applanation. It consisted of a sub-30 mm and a supra-30 mm mode based on the IOP of the patient. The Supra-30mm mode created a more forceful impulse which in turn would lead to more splatter. In comparison to this, we used the Shin Nippon-200 NCT which used a newly developed Smart Puffing Controlled system. This system had an integrated algorithm so as to adjust air-puff pressure instantly based on the patients IOP.²¹ Although we did not use an adjustable triggering device for the synchronization of the camera (for shadowgraphy and frontal imaging) with the tonometer, we tweaked the frame per seconds rate of the cameras so as to ensure that we capture the entire video of the air puff indenting the cornea up until the droplets being emitted.

By using three different imaging set-ups (shadowgraphy, frontal lighting and fluorescein), we were able to image the fluid dynamics of splatter from the eye during NCT.

It is important to note the stochastic nature of droplet creation from the fluid splatter. Atomization of the fluid depends on several factors, e.g., air-puff pressure, duration of air-puff, thermo-physical properties of the fluid, angle of inclination of the eye to the puff. Nonetheless, our experiments showed that the droplet diameters were bounded up to 500 µm (Figure 8) in the all the captured frames of the side lighting videos. For each eye, as many as 1000 frames were captured over a 2 second period. However, the distribution of the droplet sizes between the frames differed sharply due to the stochastic (random) nature of droplet creation in such experiments. This is a well-known phenomenon in the field of droplet fluid mechanics.[40] Thus, assessing repeatability between frames or between the eyes was physically unrealistic. We evaluated nearly 8000 frames (8 eyes) and focused on the frames which yielded the smallest droplet diameter. Based on our detailed experiments, we concluded that NCT did not lead to droplet or aerosol generation, when the eye was in its natural state. However, any condition which could lead to watery eyes (natural or artificial) should be an exclusion criterion for performing NCT.

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