What do dry eye tests mean? (Part 3)

In most offices (including the offices of dry eye specialists), it is common to place a drop of orange (fluorescein) dye into each eye, in order to “see” the tears (now stained by the dye) and to see any “dry spots” (where dryness-damaged cells will take up the dye and show the degree of damage being done to the surface of the eye from a lack of good tears). Watching the dye become scattered by drying (evaporation) has been a standard method of judging the quality of a tear for generations (using the doctor’s slit lamp - or examining microscope) and is called a Tear Break Up Time (TBUT). The act of placing a drop of dye in the eye - with whatever “carrier” (some use anesthetic drops while other prefer saline) will “change” the results, because the drop alters the characteristics of the patient’s tear.

Many modern keratographers (instruments used in dry eye centers to investigate the causes of dry eyes) allow a so-called non-invasive tear break up time (NITBUT) measurement. This records the time it takes to see dry spots on the surface of the eye caused by evaporation and is not altered by drops, as it looks with projected light rings on the patient’s eye - to see “breaks” in the rings caused by evaporation (and runs a timer to see how long that takes from the time of a blink). This is considered a more “objective” test (not influenced by the attention span of the observing doctor) and is extremely useful in determining degrees of Meibomian Gland Dysfunction, because MGD means having less healthy oil floating on the surface of the tear. This oil will normally stabilize the tear by preventing evaporation (water has a hard time evaporating through an adequate amount of oil, akin to Saran Wrap sealing moisture from leaving a sealed container in the fridge). Without oil providing the “liquid Saran Wrap” function, comes rapid evaporation and dry spots. These dry spots function like little open sores on the surface of the eye, and are largely responsible for many of the classic dry eye symptoms - as salty tears will irritate these open sores, causing stinging, burning and reflex tearing.

Healthy tears commonly take 20 seconds or longer to begin evaporating (under standard humidity conditions). The “average” inter-blink-interval (the time between blinks) varies with the task and environment. The key driver of the blink reflex is when the cornea has a dry spot. When we are visually engaged (as in staring at computer screens), the brain can “override” the sense of a dry spot and allow considerably longer intervals between blinks (which can result in many dry spots that can interfere with vision and ultimately cause inflammation - as a response to irritation). Under in-office conditions, a “normal” blink interval can be closer to 10 seconds - so in my office we look for a NITBUT of 10 seconds or greater (meaning it should take 10 seconds or longer to see a single dry spot on a staring cornea) to indicate a good tear.

Courtesy of Topcon’s Brochure for their Topcon CA 800 Keratographer (used by the Excellent Vision Dry Eye and Rejuvenation Centers in Portsmouth, NH and Stoneham, MA). The red zones are the driest and fastest to dry - in as little as 3.4 seconds in the Left eye and 5.5 seconds in the Right eye for this “test patient.” Their “Interblink Interval” is 5.2 seconds - so potentially “keeping up” with the Right eye but not the Left’s rate of evaporation. These measurements indicate this patient does not have good tears - and is likely to have significant MGD as a cause of their dry eyes.

As I’ve stated before, a blink is the “heartbeat” of the eye’s surface. https://www.eyethera.com/blog/avoiding-a-heart-attack-on-the-surface-of-your-eye The interval between blinks is critical to maintaining a healthy surface, as frequent blinking can refresh that surface and prevent dry spots - as long as the interval between blinks keeps up with the evaporation and as long as the blinks are sufficiently strong and complete - to engage the oil glands and to fully turn over a tear. Some keratographers can assess that interval, as well as the time to acquire the dry spots - and can provide a graphic analysis that is easy to understand and to track over time.

Unfortunately, many factors can confound adequate measurement of the tear break up time, such as the humidity in the room (less humidity means tears will evaporate more easily into that dry air, vs more humid air), the force of the blink and ability of the blink to express oil from the glands, eye rubbing, recent artificial tears, air currents (fans or air conditioning) that provide fresh dry air to the eyes, and even the amount of germ activity along the lids with enzymes that turn oils into soaps. Even a lot of oil - if contaminated with these soapy residues - can quickly break down and allow rapid evaporation even when the oil otherwise seems fairly abundant. Lastly, some equipment is not up to the task of identifying very rapid blinks, and may continue to run the clock as the computer continues looking for evaporation. A rapid blink can keep the surface from otherwise evaporating, and therefore show a longer Tear Break Up Time (TBUT), than would be clinically true. Fortunately, the older technique of using the orange dye called fluorescein to stain the tears, and then to observe with a microscope during an eye exam can still be useful in identifying the more accurate tear break up time when the computer’s information appears suspect. Additionally, the foam is clearly evident during that exam.

A dry eye specialist will commonly look at computer driven data in the context of their own exam and the history from the patient, to best assess the condition of the Meibomian Glands and the amount and quality of oil they are producing.

To schedule an appointment with Dr. Jaccoma, call Excellent Vision at either of these two dry eye offices:

(1) 155 Griffin Rd, Portsmouth, NH 03801 (603) 574-2020

(2) 3 Woodland Rd, STE 112 Stoneham, MA 02180 (near Boston) (781) 321-6463