The Dry Eye Zone

Dry Eye 101

A detailed introduction to create a context of understanding for dry eye disease.

Dry Eye 101


Brace yourself.

This is not the ubiquitous "Let's talk about the three layers of the tear film" explanation - dumbed down to the third grade level - that you can readily find on every other Google hit.

Nor is it laboratory science regurgitated in language designed to numb your brain into consumer compliance. 

Instead, it's me, Rebecca, a dry eye patient like you, inviting you to join me on a joyously jargonless journey into the science of what tears are, how they should work, why sometimes they don't, how you can find out what's wrong with yours, and how to explore your choices for what to do about it. 

So let's get started.


"Dry eye" doesn't mean dry eyes. It means sick tears.

"Dry eye" is not a disease, nor a specific problem, nor even simply a dearth of moisture. "Dry eye" is a bucket term for a lot of different types of problems. But the common element, no matter what kind of dry eye you have, is that the tears covering your eyes are sick, and to a certain extent perhaps even MIA.

What are tears?

Tears are a complicated concoction of wet stuff secreted by various glands and cells: 

  • watery stuff
  • oily stuff
  • sticky stuff
  • other stuff

The technical name for this concoction is the tear film.

What are tears for?

Well, obviously, to keep the eyes wet. But why would we want wet eyes?

First, for comfort. Your eyes are considerably more comfortable when wet, especially if you want to be able to blink, open your eyes, or see without pain. The cornea — that clear outer layer of the eye, through which you see — is very unhappy when not covered with its specially designed tear "film". If the cornea is unhappy, sooner or later you will be too, because the cornea is pretty much the most sensitive tissue in your entire body. Its ability to cause discomfort is profound. (If you're a skeptic, hold your eyelids open for just 60 seconds, and ask yourself how you would feel about it if that sensation continued for another 15 minutes.)

Second, for protection. Your corneas are designed to expect a wet surface. If they get too-too dry, they go from simple dryness to inflammation, dessication, erosions, abrasions, and ulcers and eventually scar over. Corneas simply can't live without being wet.

Third, for nourishment. Tears convey all kinds of proteins and nutrients and other goodies to your eye surfaces.

Fourth, for vision. Tears refract light. The tear film is called the "first refractive interface" of the eye. That’s a fancy way of saying that they do the same kind of thing that glasses and contacts do: they sit in front of the eye and help bend the light on its way to the back of your eye (the retina) so that you can see nice, sharp pictures. Not having a healthy tear film can be like looking through old, scratched, grubby glasses.

What are healthy tears like?

If you want to understand what's wrong with something, it's always helpful to first understand the way it works when nothing's wrong.

A brief digression on tears vs tears

Before any experts attack me with technicalities, I’d better explain that there are two kinds of tears: constant (or basal) tears, which are complicated, and reflex tears, which are simple, or at least simpler. 

To save you the trouble of looking them up, constant (basal) tears are the kind of tears your eyes are constantly secreting without your being aware of it, which keep your eyes — well, maybe not YOURS, but most people’s — reasonably well coated throughout the day and night so that said tears can meet all their job requirements — that is, the four tasks outlined above. Basal tears are complex in composition, with lots of fancy ingredients that we still don't fully understand, because they have so many responsibilities. Reflex tears, on the other hand, are simpler - more water, less fancy stuff. They are the type that your eyes kick out when the nerves of the corneas send an emergency signal to the brain saying "Ouch! HELP! Dirt / fumes / unknown foreign object / danger! It hurts, and who knows, it might make us go blind, and we don't have time to sit around and analyze it so for goodness' sakes please, QUICK, wash it out!" The brain passes an urgent signal to the lacrimal grands, which quickly twist the taps all the way to the left and try to flush everything out. Reflex tears are like a bucket of water to put out a small fire.

As luck would have it, we're mostly concerned with the complicated ones: the constant (basal) tears. There are people with certain types of dry eye who aren't even able to make reflex tears! But for most people with dry eye, it's the constant tears that are the biggest deal. (Oh, incidentally, in addition to constant and reflex tears, there are also emotional tears, which are slightly different yet, but they are similar enough to reflex tears that we will just lump them together for today's purposes.)

The three "layers" of the tear film

There are four basic elements of the tear film you should know about: oily stuffwatery stuffsticky stuff, and other stuff. But since the "other stuff" comes along for the ride in one or more of the first three, you will almost always hear the "tear film" talked about as having three parts — that is, three layers. (Though funnily enough, now that we've all been saturating in education about a three-layered tear film for I don't know how many years, scientists are starting to tell us it's really only two layers, or rather two "phases", because the bottom two layers are kind of more blended together than we used to think. But never mind that for now — sticking to a little conventional wisdom shall not hurt us.)

So, three "layers" it is: Watery stuff sandwiched between oily stuff and sticky stuff, with other stuff thrown in here and there for good measure.

Oily stuff on top, a/k/a the lipid layer

Remember your grade school science lessons 20 (30? 40? 50+?) years ago when you learned that oil floats on water? It's still true, even in your tear film!

The "lipid layer" of the tear film is oily stuff that sits on top of your tears. It is made by your meibomian glands, weird squiggly shaped glands that secrete oil through little openings in the lid margins, those skinny little strips of skin between your eyelashes and eyes. There are 20-odd such openings in each of your lower and upper eyelid margins. When you blink, teeny weeny amounts of this oil seep into your tear film. That is, assuming those oil glands are healthy.

The purpose of the oily stuff (lipids) is to keep the watery part of your tears from evaporating. And that evaporation-prevention function is critical to a healthy tear film. Suppose you’ve got lots of tears (see Watery Stuff, below), but not enough oil, or maybe icky oil that doesn't do its job well. The water will be exposed to the air and will evaporate. An oily covering is what keeps it from evaporating before more water comes in to replace it.

Watery stuff, a/k/a The aqueous layer in the middle

This is the main part of your tears — the water — and there are not a whole lot of exciting things to say about it. It’s secreted by your friendly little neighborhood lacrimal glands, which are usually described as almond-shaped and live up above each eye toward the outer side above the lid crease. Google it for a nice little diagram, because here at the DryEyeZone we're a lot bigger on language than pretty pictures. This water also contains other goodies to nourish the eyes.

Sticky stuff, a/k/a the mucin layer

This part still kind of falls in the Rocket Science category: really high-tech stuff that no one actually understands all that well, though they seem to understand it a lot better now than they did around 2004 when I wrote the very first version of this article. Whether it's understood or not, there are now an awful lot of companies making claims to be able to help this mucous layer when it's sick compared to ten years ago or so. Anyway, the mucin layer is mucous and electrolytes and stuff, secreted by what we call goblet cells living in the surfaces of the eye called the conjunctiva.

One of the main purposes of the mucin layer is to make your eyes wet. "Hmm," you will say. "But if my eyes are secreting tears at all, doesn’t that, well, by definition, sort of mean that my eyes ARE wet?"

Nope. Let's talk about what "wet" means. 

Look at your car when it’s raining lightly. Is your car wet? Yes, in the sense there are water droplets scattered across it. But is the surface of, say, the hood of your car covered by an unbroken layer of water, the way your cornea needs and wants to be? Nope, not unless you drive it into a pond. See, I’m no scientist, but it's been my observation that water does not do a good job of making smooth, solid surfaces "wet" across their entire surface unless they are submerged. On a horizontal surface, water beads up and sits there in fat little blobs. On a vertical surface, it runs down and falls off, leaving just a few beads behind. One of my earliest dry eye friends and educators was the brilliant Dr. Frank Holly, an expert in surface chemistry among other things, and he was really great at explaining how it takes a special kind of substance to stretch and spread around and stick to your eye in ways that don't also mess with your vision or make you uncomfortable. See, I was serious when I called it Rocket Science.

Anyway, so the watery stuff (aqueous) sits on top of the sticky stuff (mucous), and the mucous serves to glue the watery stuff down as long as possible (bearing in mind there's some gravity coming into play too, pulling water down the tear drains, a/k/a puncta) till the water gets replenished.

Other stuff

There’s all kinds of other stuff in your natural tears, like Substance P and lyposomes and other goodies that nourish the surface of the eye (the epithelium) and kill bugs. It’s good stuff. The more of it, the better. In fact, unless you are truly and totally dry (meaning you haven’t got anywhere near enough Watery Stuff), it's really unfortunate that so often you end up having to use so many artificial tear supplements just to stay comfortable, because they might just be washing any remaining natural goodness out of your tears.

What are sick tears like?

In a nutshell

Sick tears mean:

  • There's not enough of them, and/or
  • They're not good enough quality, and/or
  • They're not behaving, i.e. staying put on the surface of the eyes.

There are ever so many reasons why one or more of those things might be happening. Most, but not all, of them have to do with something wrong with the tear film itself. You should find out as much as you can about what's happening to YOUR tears, and, if possible, why.

Why you need a detailed diagnosis: a philosophical digression on names, nails and hammers

One of the problems with the sick tear film is that everybody calls it "Dry Eye" in spite of the misleading, inaccurate, trivializing catch-all misnomer nature of this term (click here for a fuller discussion about the meaning, use and abuse of the term "dry eye".) The "Dry Eye" label gets attached to anything and everything that could possibly go wrong with your tears. There are actually quite a few very different things that could go wrong with your tears. And if you want to solve a problem, it helps to know which problem you have. Specifically, that is. (I can never write that word without hearing echoes of how my daughter used to pronounce it when she was about five: "pacifically". Then, because we're rather a wordy family, she would brace herself for the inevitable comeback, which usually included the word "polemically". No doubt I'll be footing the therapy bills 20 year hence. But I digress. Again.) I always tell people that an eye doctor saying "You have dry eye"... and stopping there... is like a cardiologist saying "You have heart trouble" and stopping there. Pleeeeeease may we have some specificity?

You need a detailed diagnosis, not just "dry eye". Without a detailed diagnosis, we all look like homogenous "dry eye patient" nails ready for homogenous pharmaceutical hammers wielded by homogenous well-qualified and well-meaning but undiscriminating eye doctors who just don't happen to have enough dry eye training for our needs. Restasis is a beautiful example of how this works. $1.5 billion sales in 2016 is strong evidence of the abundance of willing hammers with which equally willing nail-like objects get whacked in defiance of scientific evidence and patient feedback alike, which support considerably lower efficacy rates than the sales suggest.

Hammers are excellent inventions for actual nails. But what about the non-nails? Light bulbs? Coffee beans? What are you? What are your needs?

The best way to not get slapped with a generic Dry Clean Only label and processed with everyone else is by doing exactly what you're doing right now: read up, educate yourself, then go back to your doctor with questions. If they can't deliver answers, learn better questions to ask, or else focus on finding someone with better dry eye specialty training. 

Sick tear problem #1: sick oil (Evaporative Dry Eye)

Suppose you're making plenty of tears, but they’re evaporating faster than your tear-making apparatus can replace them. This is the most common kind of dry eye: evaporative dry eye (EDE).

First, how does the doctor know whether the problem really is oil-related?  Quite likely from a simple Tear Break-Up Time test. They put a little dye on your eye and look at you through the slit-lamp thingy and start counting. The dye is attracted to dry spots, and when dry spots start appearing, that’s called tears breaking up. As soon as that starts happening they stop counting. If they made it to 10-15 seconds or so, your break-up time is fairly normal. If it's less than 10, it’s not normal. Some people are way down at 2, 1 or even 0. Their tears are very unhappy, and so are they.

How does your doctor pin it down more specifically, and start figuring out what's wrong with the oil glands? A great step in the right direction is actually taking the time to examine the glands. (Duh. That sounds simple. But it's frequently not done.) A nice healthy little oil gland should let a doctor squeeze out nice clear runny oil when they put a little gentle pressure on it with their fingertips or a little tool. Sick glands, though, may just sit there and do nothing when pressed. Or they may be horribly clogged with a toothpaste-like substance that was oil in another lifetime. Such problems are called meibomian gland dysfunction (MGD). This might happen as a result of chronic blepharitis, that is, eyelash dandruff and inflammation. Or you might have rosacea (skin condition) or your oil glands might be affected by hormonal change or by an overpopulation of (ew, but we've all got them) demodex mites. If it's way out of control or has been left alone too long, the clogging in the glands might have turned to hard waxy caps in the openings, or some glands may be shriveling up and dying (atrophy).To complicate matters, in addition to diseases, there's also lots of environmental and other factors that contribute to EDE, from low humidity to computer use to long term contact lens wear to leaky CPAP masks and eyelids that stay partly open when you sleep. 

Some doctors may give you lots of detail on what they're seeing in the glands; others, none whatsoever. Don't let your eye doctor leave the room without an explanation of what condition your oil glands are in. These glands are important for all of us for the long term health of our tear film. On the other hand, you should not be horrified if you find out that some aren't working, or that they're all misbehaving somewhere. These oil glands tend to degrade a bit in general over your lifespan - you lose a little every decade. But sick oil glands often respond quite well to treatment.

Digression: Please don't be startled if your first three eye doctors never explained this to you. Education about evaporative dry eye has improved dramatically since I wrote the first version of this article, thanks in considerable measure to the highly motivated dry eye drug and device manufacturers that we love to hate, but there are still a LOT of dry eye patients just being told they have "dry eye" (not which kind) and given generic "dry eye treatments", not treatments specific to problems with the oil glands.

Sick tear problem #2: Sick water (aqueous deficient dry eye)

Now we go back to our friendly neighborhood water-producing lacrimal glands, which in *some* people may seriously misbehave themselves. They get inflamed, they get diseased, or they get damaged by medical treatments. They stop pulling their weight. They underproduce. This is the version of dry eye that actually lives up to its name! Medically, we call it Aqueous Deficient Dry Eye (ADDE). Or if you prefer Latin or Greek, try "keratoconjunctivitis sicca" or "xerophthalmia". Interestingly, this is not the most common form of dry eye.

How do I know I have it? Good question. The best way is to ask your doctor. You can ask them if you're making enough tears, or ask them to talk to you about your tear volume or your tear secretion rate. In my personal opinion, you should not try to micro-manage them when it comes to which specific diagnostic methods they are going to use. (If you don't have any confidence in their dry eye competence, why are you there at all? Go find a specialist.) "Back in the day", the most common testing standard was the Schirmer test, where they put a little paper strip in each eye and you close your eyes and they see how many tears you produce by how far down on the strip the moisture travels. There are some issues with this test in terms of inconsistencies in how it's given and how reliable the results are under various circumstances, so not everyone does it anymore. I don't want to bore you with details, but if you do undergo the penury of this test, pay attention to whether you've been given anaesthetic drops first, because if you haven't, this test will be measuring your reflex tears, not your constant tears, and those are very different things.

So why aren’t your lacrimal glands behaving? You could be taking medications that affect your tear production. If you have ever looked at the side effects lists on medications... well, an awful lot of them mention dry eye as a side effect. Or, you could have a disease that messes with the lacrimal glands. Sjogrens Syndrome, an auto-immune disease that attacks moisture glands in general, is a hands-down winner for causing ADDE. There are all kinds of other potential causes, though, from medical treatments and elective surgeries to disease, age and sex.

Sick tear Problem #3: sick sticky stuff (Mucin deficiency)

Remember when we were talking about the sticky part of your tears — the mucin layer, which is between the watery part of your tears and your eye surfaces themselves — the part that actually makes your eyes wet? Where mucous goes, aqueous flows. (Ha! Thanks to my assistant Aidan for that uncharacteristically poetic edit.) Where mucous doesn’t go, well, that’s going to be dry. And unprotected, because that’s the other job of the mucin layer of the tear film: standing between your corneas and the big bad world of bacteria. Goblet cells make mucous, so if you don't have enough mucous, it's because you don't have as many goblet cells as you should have. 

How will you know, and why does it happen? Sorry, out of luck. This one is way beyond the scope of a one-page dry eye crash course. Even in the entirety of the TFOS DEWS II report on diagnostic methodology for dry eye, mucin and mucous only get about a dozen mentions. It's things like limbal stem cell deficiency and Stevens Johnson Syndrome and mucous membrane pemphigoid. If that's territory you are interested in, have fun researching, and tell me what you learn!

sick tear problems #4+ - The lids and drains

Getting back to more normal problems, not all dry eye problems are about the tear film layers (oil, water, mucous). There are other factors that come into play, including:

Eyelid problems: Eyelids are central players in healthy tears, because they secrete part of the tears, they spread tears around, and they keep a lot of the tears covered up a lot of time time — all this, through the amazing and seriously undervalued function of blinking. But what happens when you don't blink all the way? Or when you don't blink frequently enough because you're staring at a computer (like right now!)? Or when you can't blink because of disease or injury to eye muscles? And what happens when your lids won't stay closed, so your tear-covered surface never gets the covering and protection it needs? Or when your lids stay open while you sleep?  Or when your eyelids twitch frequently (blepharospasm), again, leaving the eyes exposed too much?

For many people with dry eye, some version of lid malfunction, whether very mild or very severe, is part of the picture. Find out how your lids are doing. Get the earliest appointment of the day if possible, if you want them to be able to detect signs of your lids having been opening while you slept. Your doctor can tell you whether you have a complete blink. Some of them have fancy gizmos with cameras to analyze this in great detail, but they can also figure out a fair amount just by looking. 

Tear drainage problems: The tear "drains" in the corners of your eyes let excess tears (and eyedrops) drain into your nose instead of spilling over and running down your cheeks. When there's a flood (you're crying, or having a lot of the "emergency" tears we talked about), the drainage system gets overwhelmed and tears spill down your cheeks. But... sometimes a drain gets blocked, and they spill down your cheeks because they have nowhere else to go. It's also possible for the drains to be too big, resulting in losing too many tears, too fast for one of the least obvious reasons. It's also possible for problems to develop in these drains because of a history of having plugs (more about those shortly) put in them - just ask any oculoplastic surgeon, that is, the specialists that get tasked with surgically hunting down and cleaning up after plugs that have gone MIA.

Red herring tear problems

There are a lot of things out there with symptoms that can mimic dry eye in one way or another. These can get overlooked amidst all the symptom noise, so sometimes they accidentally end up misdiagnosed as "just" (don't you love that!) dry eye.

Here are some examples:

  • Allergic conjunctivitis
  • Blocked tear duct
  • RCE (Recurrent corneal erosions), especially when mild
  • SLK (Superior Limbic Keratoconjunctivitis). 
  • GPC (Giant Papillary Conjunctivitis)
  • Neuropathic corneal pain (pain that feels like severe dry eye pain, plus or minus actual dry eye)

What makes these babies particularly troublesome is when you have one or more of them in addition to dry eye, which brings me to my final point about finding out what's wrong with your tears:

Combo meal deals

Miserable tear glands seem to love company. Suppose you don’t have enough of the watery stuff, AND you don’t have enough oily stuff to help you keep the pitiful amount of watery stuff your lacrimal glands can be persuaded to produce from evaporating as fast as it’s made. Throw in a little eyelid closure problem increasing your evaporative loss, or a little extra inflammation from mild allergy, or a leaky CPAP mask, or a little neurotrophic something-or-other from LASIK a few years ago, or a daily dose of a glaucoma drop with a toxic preservative, and suddenly you're adding layers of complexity.

None of the tear problems we've discussed, nor yet the other problems masquerading as tear problems, are mutually exclusive. These are "check as many as apply" types of questions, not "select the correct answer" questions. 

This is why you need to be seeing a doctor who has a really solid, broad experience of all ocular surface diseases. Don't just doctor-hop. Dry eye patients with severe symptoms often get on a relief-chasing treadmill that leads to nowhere but expense and frustration really fast. You need to be intentional and thoughtful about finding the right kind of specialist, whether it's a corneal specialist ophthalmologist who has a proven interest in dry eye, or an exceptionally well-equipped optometrist who has ditto. If you want to take good care of your eyes, gear up to advocate for the kind of care you'll need.

Dealing with sick tears

Let's talk solutions! What do you do about sick tears? There are 3 types of things you can do:

  1. Supplement what's missing by adding fake tears.
  2. Trap moisture on the eye surface longer. 
  3. Make more and/or better tears (or, rather, tear parts).

Nearly all dry eye treatments and remedies fall into one (or more) of those categories. Which ones make most sense for your sick tears depends, at least in part, on how sick they are and what's wrong with them.

First: Coping and keeping comfortable

There are a lot of purely practical measures that anyone whose dry eyes are really, really bothering them tend to take, regardless of the causes - whether they have bad oil, not enough water, lids that don't want to close or whatever. At a certain point the "why" just doesn't matter nearly as much as the "how the heck do I get through my day". These practical measures don't interfere with any medical treatments, but they do make us feel better, hence their popularity:

  • humidifiers
  • redirecting vents that are blowing at your eyes in your home, car and workplace
  • dry eye glasses, especially for computer use or in dry homes/offices
  • wraparound sunglasses (preferably foam-lined)
  • goggles, masks, shields, patches or tape to keep eyes moist while sleeping
  • smart strategies for computer use, like positioning the screen low, adjusting the lighting and color, having visual reminders to blink more frequently, and maybe a desktop humidifier unless you're using dry eye glasses

Treating evaporative dry eye (sick oil)

How do I keep my tears from evaporating when I don't have enough oil? There's loads of practical things to reduce the effects of evaporative tear loss - they are listed above. On the medical side, treatments for problems with the "oily stuff" should be specific to a specific diagnosis. You may find yourself really frustrated if you use cookie cutter treatments on the one hand, or on the other, if you start throwing everything and the kitchen sink at it, without an understanding of the details. Get thee to a specialist, insist on a detailed diagnosis and full discussion of the details, and discuss your treatment options. There's a long list of options, depending on the specifics, and plenty of those options are not drugs. There are umpteen lid hygiene products (please don't use the conventional baby shampoo, it's been proven to be harmful to the tear film even while it helps the lids) and warm compresses which are great for maintaining healthy lids. There are antibiotics (in pill, eyedrop and ointment forms) and fancy schmancy expensive in-office treatments from Blephex to Lipiflow to IPL to probing. There are also fish, flaxseed and any number of other oils that are newly vogue in dryeyeland and may find their way into your diet or your daily supplements. Some people self-treat with additional dietary changes, such as eliminating sugar, or going to an anti inflammatory diet altogether.

But it all starts with good diagnosis. Don't play the part of passive nail to the most convenient hammer.

Treating aqueous deficient dry eye (not enough water)

If you don’t have enough watery stuff (aqueous tears) of your own, usually you’ll be told to add some fake tears (lubricant eye drops, hopefully preservative free) or, if it's severe, gels or even greasy ointments, or you may have some really sophisticated artificial tears made from your own blood (autologous serum or platelet-rich plasma) If you’re so dry that you’re adding tear supplements frequently, you may be given tiny little plugs in the little holes your tears drain out through (punctal plugs), so that you can trap the tears you have in your eyes as long as possible. If those work really well, you might have those holes sealed shut (punctal cautery) so you don’t have to worry about the plugs falling out or, which is much worse, falling in and getting lodged somewhere where they can do some harm. In addition to those things, prescription drugs (Restasis, Xiidra) may be recommended to increase tear secretion and/or reduce inflammation. If you're so dry that the cornea is getting damaged, which can be extremely painful and can start to become a threat to vision, there are many other treatments to protect the surface, including putting amniotic membrane on the eye (e.g. ProKera), or using soft "bandage" contact lenses, or for longer term protection, PROSE or scleral lenses. One of the latest things on the market is a device that makes you tear by stimulating nerves in your nose (TrueTear). There are many pipeline drugs that might eventually get FDA approved, and many "off-label" treatments going on. And there's more, and things are changing all the time. But the important thing for you to know is that plugs, drops and a drug or two are only the beginning, not the end, of available dry eye treatments.

Wrapping it up

I could go on, and on, and on. There is so much more information to be had. There are so many details and variations on themes. But if you made it to the end of this page, you should have a great foundation to move forward with more self-education about dry eye and to have many productive conversations with your eye doctor. So go to it!


Editor's note: The first version of this article was called Dry Eye for Dummies. It was written around 2004 and consisted mostly of "Everything I learned from my doctor-friend Sandra in a single pithy little email that led to a lot of great conversations". It's been expanded and polished many times, and the March 2018 update was a major re-write and expansion. But the most important concepts really never changed.  - Rebecca