The Goldmann applanation tonometer (GAT) is considered the gold standard instrument for the measurement of intraocular pressure (IOP). It is connected to the slit lamp (Figure 1) and features an adjustment knob, a control weight insert with housing, and a mobile rod or feeder connected to a bi-prism. The principle behind Goldmann tonometry is the Imbert-Fick law. Fluorescein dye is placed on the eye along with a topical anesthetic, and the split-image prism divides the image of the tear meniscus into 2 arcs. The tool measures the force necessary to flatten a 3.06-mm diameter area of the cornea, a diameter at which the cornea’s resistance to flattening is counterbalanced by the attraction of the tear film to the tonometer head. The force in grams is taken when the arcs’ inner margins touch, and this number is multiplied by 10 on the adjustment knob to reveal intraocular pressure in mmHg.^1,2

Despite its accuracy, there are many potential sources of IOP measurement errors associated with GAT:

1. Variable corneal thickness, with very thin corneas leading to underestimation of IOP and very thick corneas leading to overestimation of IOP;
2. Corneal scarring;
3. High astigmatism or irregular corneal curvatures;
4. Thick or thin mires due to an excess or deficit of fluorescein, respectively, leading to underestimation or overestimation of IOP;
5. Artificially induced external pressure on the globe: Pushing on the globe when elevating the lid can compress the globe and lead to higher IOP;
6. Valsalva-related events: This occurs when the patient holds their breath during measurements. This may be more prevalent in patients with larger body habitus who may strain to reach the slit lamp;¹ and
7. Head position on the slit lamp: Overextension of the neck can lead to falsely elevated IOP.

More recently, mask mandates implemented globally due to the novel coronavirus pandemic have led to another potential source of GAT IOP measurement errors. First, patients have been observed in our clinic to more often hold their breath during measurements when wearing a mask. Second, when positioning on the chin rest, poor fitting masks can drift up and place external pressure on the globe during measurement. Finally, and most importantly, this phenomenon can occur when a patient’s mask touches the feeder arm of the applanation tonometer and disrupts the balance of the bi-prism tip and the weighted insert leading to a falsely elevated IOP reading.³ This article is intended to further describe this phenomenon with several commonly used masks worn by patients in our clinic and to make suggestions on how to avoid mask-related measurement errors with GAT.

3M Health Care Particulate Respirator and Surgical Mask

This cup-style, FDA-approved N95 and surgical mask is used frequently in hospital settings to prevent particulate inhalation (Figure 2). The mask is composed of a polypropylene filter with polyurethane foam and polyester shell. The strap is braided polyisoprene and the nose clip is aluminum. It meets N95 requirements of a minimum 95% filtration against solid and liquid aerosols not containing oil.⁴ The polyurethane foam helps hold the shape of the mask and the Goldmann applanation tonometer’s feeder arm is easily displaced posteriorly if care is not taken to position the mask such that it clears this feeder arm. Displacement of the feeder arm can lead to falsely elevated IOP measurements.

Halyard Fluidshield Surgical N95 Respirator Mask

This 4-layered mask has a flat-fold style with elastic straps (Figure 3). It is also National Institute for Occupational Safety and Health N95 approved and has a soft inner lining with a large breathing chamber with adjustable nose wires.⁵ The long profile of this mask, along with multiple layers for particulate filtration, can lead to displacement of the feeder arm if care is not taken to adjust the mask away from the tonometer.

BYD Single-use Surgical Mask 2311X

These single-use surgical masks are widely available online and, therefore, accessible to the broad patient population (Figure 4). The masks consist of 3 layers. The outer layer is nonwoven polypropylene, the middle layer is melt-blown polypropylene, and the inner layer is spun-bound polypropylene. The ear loops are a polyester/nylon and spandex blend and the nose clip is unspecified metal wiring.⁶ This mask is not an N95 and is used only as a physical barrier without the ability to filter particulate matter. As there are fewer layers without rigid support, this mask is more likely to comply with pressure from the feeder arm. However, care should still be taken to position the mask appropriately to clear the Goldmann tonometer.

Cloth Masks with Silicone Mask Bracket

The CDC recommends the public choose masks composed of 2 or more layers of washable, breathable fabric that completely cover the nose and mouth. Depending on the number of layers, the type of fabric, and the shape of the mask, these may also lead to displacement of the feeder arm of the tonometer.⁷ A second element, a silicone mask bracket, may be hiding under the cloth mask for comfort and may not be readily apparent to the measurer⁸ (Figure 5). These brackets are much less pliable than multilayered cloth and may displace the feeder arm without the ability to adjust the mask to clear the tonometer. In these cases, the patient may need to remove the silicone bracket prior to tonometry.

Recommendations

To minimize these mask-induced artifacts in GAT measurements, a number of steps can be taken by technicians and physicians during the pandemic. First, ensure patients are comfortably seated and the slit lamp table is adjusted to avoid the patient straining while positioned in the chin rest. Prior to tonometry, note the mask type worn by patients and identify those with broad profiles that are more likely to come in contact with the GAT feeder arm. During measurement, observe mask position in relation to the GAT tonometer and feeder arm. If any contact is observed, consider one or more of the following:

1. Adjust patient mask to the side while still covering the nose and mouth, if possible.
2. Tilt the patient’s head slightly away from the tonometer arm to clear the feeder arm, while asking the patient to keep the gaze forward.
3. Compress the patient’s mask with your hand or a cotton tip applicator to prevent any contact during measurement.
4. If none of the above adjustments are successful, consider an alternative tonometry method using a hand-held tonometer, such as the iCare or Tonopen tonometers.

Conclusion

Goldmann applanation tonometry remains the gold standard for measuring IOP. We rely on accurate IOP measurements to gauge treatment effects and to guide need for therapy alteration or advancement. Despite the reliability of GAT, it has long been known that great care must be taken during measurements to avoid various factors that induce falsely low or high recordings. We can now add a new potential cause for error related to mask wearing when masks come in contact with the applanator. While all masks may lead to displacement of the feeder arm of the tonometer, less pliable masks, including N95s or masks with silicone brackets, may pose the biggest risk to accurate measurements. It is important to observe masks for bulk and shape prior to measuring IOP and to remain vigilant for mask-related GAT errors that could lead to escalation of treatment based on faulty data. GP

References

1. American Academy of Ophthalmology. Basic and Clinical Science Course Section 10: Glaucoma. Singapore: American Academy of Ophthalmology, 2008.
2. Barnett T. Goldmann applanation tonometry. PowerPoint presentation. September 2000. Accessed December 22, 2020. http://webeye.ophth.uiowa.edu/dept/resfelo/goals/videos/Tests&devices/Goldmanntonometry.ppt
3. Davanian AM, Donahue SP, Mogil RS, Groth SL. Mask-induced artifact impacts intraocular pressure measurement using Goldmann applanation tonometry. J Glaucoma. 2020 Dec 2. doi:10.1097/IJG.0000000000001746
4. 3M Health Care. 3M Health Care particulate respirator and surgical mask, 1860sS, N95, small technical specification sheet. 2017. Accessed December 22, 2020. https://multimedia.3m.com/mws/media/1425066O/3m-health-care-particulate-respirator-and-surgical-mask-1860s-n95-small-technical-specifications.pdf
5. Halyard Worldwide, Inc. Fluidshield surgical N95 respirator mask. 2018. Accessed December 22, 2020. https://products.halyardhealth.com/infection-prevention/facial-respiratory-protection/masks-respirators/fluidshield-n95-particulate-filter-respirator-and-surgical-mask.html#frp_safety_seal_978
6. BYD care. Single-use surgical mask-2311X. Accessed December 22, 2020. https://www.byd.care/pages/single-use-surgical-mask-2311x
7. US Centers for Disease Control and Prevention. How to select, wear, and clean your mask. Updated December 21, 2020. Accessed December 22, 2020. https://www.cdc.gov/coronavirus/2019-ncov/prevent-getting-sick/about-face-coverings.html
8. Cleveland Clinic. Do face mask brackets work—and are they safe? October 6, 2020. Accessed December 22, 2020. https://health.clevelandclinic.org/do-face-mask-brackets-work-and-are-they-safe