What do dry eye tests mean? (Part 5) Salt & why you don’t drink seawater.

As I noted in the prior testing segments, the Tear Break Up TIme (TBUT) is an indication of evaporation - and gets shorter (less time until evaporation) when the oil doesn’t “seal in” the water. The flip side of evaporation, is that only the water leaves - which means the stuff that is otherwise dissolved in that water remains behind. Since tears are salty by nature (and the fire hose is mostly salt water), the salt levels can climb quickly under conditions favorable to evaporation of tears. The ultimate conditions favoring evaporation are dry, windy air. Add to that, conditions that favor having eyes more open and you have a recipe for toxic levels of salt in the tears.

Salt levels are measured in terms of. “osmolarity” - a chemical term that quantifies the amount of salt in a solution. The higher the number, the more salt in that solution. Every cell in your body is subject to salt levels and blood carries a “normal” salt level of 275-295 “milliosmoles” of osmolarity. This is a reflection of the desired salt levels inside every cell in your body - and the surface cells on eyes are no different. Once salt levels rise above the level of salt inside the cells, water will seep across the cell’s border membranes in the direction of the salt, to “normalize” the salt on both sides of that membrane. As a result, cells “dehydrate” - becoming crinkled, shriveled up ghosts of themselves - before giving up the ghost completely. (Sorry, it is getting close to Halloween and ghosts must be on my mind).

Most know that if you get stranded at sea, the last thing you should do is drink the seawater, as it will only dry you out further and make you progressively more thirsty (until you get sick and die from thirst). Salt levels in tears can concentrate to levels higher than seawater and risk drying out the surface to toxic levels. Our “firehose” tears will become necessary to wash all that salt away - but those tears are not the “complete” tears from our “sprinklers” (which make water, salt, protein and oil) - so our surface gets progressively weaker, more damaged and drier.

Many dry eye specialists will measure salt levels with a tiny “bird-beak-like” device that dips into the tears lining your lower eyelid and “sips” a tiny amount of tears into the tip. A commercial unit found in many offices is the “TearLab” and their website offers additional information for those who wish to dive deeper (see the link below). Their device has a “test card” calibrated with a range of electrical elements that measure the flow of electricity across that salty water (higher salt levels conduct faster and better than lower) and gives a precise level of salt in the tears measured in those milliosmoles (mOsm/kg). Normal levels in tears range from 280-295, but once it climbs to 308 or more, it starts to become “toxic.” 316 -324 is “bad” but we commonly see levels higher than the salt-testing machine can register (it goes up to almost 400 and then becomes too high to measure). I find those with the highest levels to generally have the worst oil and faster evaporation (a TBUT generally less than 3 seconds and often less than 2 seconds).

Since TBUT is a good proxy for evaporation-related dryness (which is a proxy for poor oil), you might wonder why we would bother to also check tear salt-mOsm. Some patients have very active “fire hoses” which can largely keep up with salty elevations. Since each eye is separated on sides of the face and can have differing degrees of evaporation related to “which way the wind blows,” as well as having variable degrees of oils expressed with degrees of blinking, it is common to see wide swings in salt levels. When both eyes are very high, this can mean trouble with the fire hose keeping up and spell worse troubles for the surface trying to stay healthy. Even a difference of 8 mOm or more is considered significant, since salt is normally so carefully regulated by our eyes (and the consequences can be so great once salt levels get out of whack). High salt levels wreak havoc with the surface, causing burning, stinging, (reflex) tearing, blurring and scratchy sensations - basically every dry eye symptom since high salt leads to drier eyes. Some symptoms from other diseases can overlap with dry eye, so having more lab and clinical findings to help sort out the true causes can be a help.

Many patients ask if they can reduce their tear salt levels by avoiding salt in their diet. While this makes intuitive sense, it isn’t clinically true - again because of the innate, careful regulation of salt levels as part of our normal tear “recipe.” Just like blood is highly regulated, so too are our tears - and for the same reason - cells are delicate and salt levels are important. So the answer isn’t to reduce salt intake (at least for this purpose - check with your PCP if your blood pressure or other health issues might require you to alter the salt in your diet since a certain amount of salt is essential to your health). The better approach is usually to pay more attention to your oil intake and the function of the Meibomian Glands, that make the precious oils for your tears.

For a link to TearLab, see: https://www.tearlab.com/

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