For centuries it has been humankind’s instinct to cover the mouth and
nose when coughing or sneezing. Common sense would dictate this
instinctively reduces the dispersion of aerosol and droplets and thus
the spread of contact and airborne infections.
Aerosol generating procedures (AGPs) have become a new byword for
procedures that put clinicians at increased risk of contracting
COVID-19. Whilst the title suggests the risk is simply in aerosols, the
science is much more interesting. Droplets and aerosols are different,
with the distinction between them based on size. Whilst experts disagree
on the absolute size when an aerosol becomes a droplet, the general
acceptance is that anything bigger than 50 microns (0.05mm) is a droplet
and those smaller are aerosols.
In most contagious respiratory infections, the principal transmission
agents are droplets (1). This is due to the relatively high viral load
in a droplet, purely due to its large size, and also the fact that large
droplets have weight, and so gravity pulls them down onto surfaces that
others can touch – so passing it on. This is why washing hands is so
effective against droplet spread.
Aerosol transmission is thought to be a much less frequent cause of
transmission, mainly due to the very small viral load (given the aerosol
itself is by definition very small). However, it is clearly more
concerning as these very small, and therefore very light particles, can
travel large distances on air currents and can be directly inhaled. That
said, it is thought to only play a minor role in transmission compared
to droplet spread.
During the COVID-19 pandemic PHE (Public Health England) updated
guidance on what it considers (AGPs) Aerosol Generating Procedures.
Included within this list were examinations of the upper aerodigestive
tract in ENT. Any procedure enacting air over a fluid mucosal surface
therefore poses a risk of viral dispersion within both droplets and
aerosol. Healthcare workers were recommended to reduce endoscopy of the
nose and throat. Any essential examination had to be performed using
high level PPE including an respirator (N95 or FFP3)(2).
Anfinrud et al (3) graphically represented a visual reduction in aerosol
production by creating light sheet from a 532-nm green LASER.
Comparisons were made between a person talking with and without a cover
for the mouth, in their instance, a slightly dampened wash cloth. Light
flashes were recorded to evaluate the number of droplets ranging between
5-200 microns. They showed that by covering the mouth, virtually no
light flashes were seen. This observation supports the well-known
concept that covering the mouth does indeed reduce droplet production.
On impact with smooth surfaces droplets disperse to smaller sizes and
can aerosolise. Similarly impact onto soft surfaces absorbs droplets
reducing their projection as well as the tendency to aerosolise (3).
As the pandemic plateaus in countries across the world various
strategies are to be considered to return to a new normal. This would
facilitate the resumption of diagnostic services whilst maintaining the
protection to healthcare workers. One suggestion is the use of facemasks
to help reduce the risk of inadvertent droplet dispersion (2). Despite
the ‘soft surface’ barrier masks create, in the ENT setting, facemasks
obscure access to the nasal cavity thus preventing nasoendoscopy.
The ‘SNAP’ (Safe Nasoendoscopic Airway Procedure) developed by
endoscope-i Ltd (West Midlands, UK) is a single-use, valved endoscopic
port, retrofitted to any surgical mask (Figure 1), permitting entry of a
4mm flexible and rigid endoscope to examine the naso and pharyngolarynx.
The valve, a 10.9 mm cylindrical tube truncated by two opposing 45
degree inclined membranes 700 microns thick, approaches a point but
terminates in a 700 micron thick and 500 micron wide plateau, creating a
‘duck bill’ formation The valves are formed using a FFF (fused filament
fabrication) 3D printing technique with a Flashforge Creator Pro 3D
printer. The plateau atop the valve serves to provide a reference for
introducing a slit using a hardened steel razor blade that is 400
microns thin. The blade is mounted in a jig to ensure angle, penetration
depth and position are controlled as it is driven through the membrane.
These measures ensure that the valve opening is observably consistent
and less than 50 microns.
Once the SNAP is fitted to a
surgical mask, any cough or sneeze generated during the procedure is
caught within the mask. The valve is configured such that pressure from
the patient side will serve to collapse the walls of the valve membrane
thereby further sealing the slit in the valve. This seal has been in
vitro tested with aerosolised fluorescein(figure 2). The 45-degree angle
of the valve walls from the non-patient side similarly allows the blunt
tip of the nasoendoscope to deform the valve walls with ease. The
cylindrical form of the walls encourages the valve membranes to return
to their original flat shape following withdrawal of the endoscope.
During the COVID-19 pandemic our tertiary head and neck cancer referral
centre managed 120 urgent 2ww cases. Using the Tikka et al calculator
(4) 40% of referrals were redirected back to the GP. The remaining 60%
either went direct for imaging or underwent endoscopy. In total 40 cases
were endoscoped, 9 of which using the SNAP. All 9 cases scoped with the
SNAP were completed without any adverse effect. No cough or sneeze was
elicited during any of the examinations and observations between the two
groups were identical. Subsequently one consultant lead FEES examination
was performed under controlled conditions. Again, the procedure was
completed without any complications. The patient self-remarked on the
comfort of the endoscopy as a result of the stability provided by the
SNAP device in the alar region the prevented inadvertent movement during
the chin tuck and head turn exercises.
Our observations demonstrate the SNAP device is a practical and safe
tool to aid reduction in droplet dispersion whilst performing
nasoendoscopy. We hope to see the inclusion of such a device in recovery
guidelines by national bodies in order to facilitate the return of safe
nasoendoscopy in the post COVID Pandemic era.