Editor's note: I wrote this more than 12 years ago. Some sections, especially about treatments, are out of date, but the core concepts about the tear film are still sound enough to be a good starting point for understanding dry eye.
What the tear film is and why it is so important
The tear film is a layer of substances secreted by glands around the eyes and intended to keep the eye surfaces constantly lubricated.
Here are some of the main reasons we have tears:
1) Tears protect the eye. Whilst we hate to state the obvious, they keep your eyes wet. But why would you want wet eyes? First, they’re more comfortable that way — especially if you expect to be able to blink. Second, your corneas are built to expect a wet surface and they’d get really unhappy without it. In fact, if your eyes get too dry you will develop erosions, abrasions and even ulcers and scarring on your eyes. Some of you have experienced that and you know exactly how unpleasant it is.
2) Tears provide all kinds of proteins and nutrients for the eye. You could say they nourish the eye. That is a Good Thing. You want as much of that nourishment as you can get.
3) Tears refract light. That’s a fancy way of saying that they do the same kind of thing that your glasses do — they help bend the light on its way to the back of your eye (the retina) so that you can see nice and sharp images. So, if you don’t have the tears you need, you might not be seeing nice, sharp images... and vice versa.
The three components of a healthy tear film
First of all, before the experts start attacking 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 simpler. As luck would have it we really are only concerned with the complicated ones today. After all if it was that simple, you’d be out enjoying the sunshine rather than reading about dry eye treatments.
(Okay, okay, to save you the trouble of looking them up: constant tears are the kind your eyes are constantly producing without your being aware of it and which keep your eye - well, maybe not YOURS but most people’s - reasonably well coated throughout the day and night. They are complex. Reflex tears, on the other hand, are simpler and more watery, the type that your eyes produce in large quantities if you get grit in your eyes. Naturally, it’s the complex ones that we have to deal with.)
Now, back to those constant tears. There are four basic tear components you should know about: oily stuff, watery stuff, sticky stuff, and other stuff.
Oily stuff (lipids)
Remember in your science lessons about 20 (30? 40? more?) years ago where you learned that oil floats in water? Well, guess what, that’s true here too! The lipid layer is oily stuff that sits on top of your tears. It is made by your meibomian glands, glands sitting in both your lower and upper eyelids. The purpose of the lipids is that they keep the watery part of your tears from evaporating. If you think about it, that’s a good thing. Suppose you’ve got lots of tears, but no lipids, well, your tears will evaporate and your eyes get dry. Fine, your eyes will (well, for most of you anyway) just keep making more tears, but what happens if your tears are evaporating faster than you can make new ones? Big problem. That is the problem, or rather one of the problems, that a lot of us have. In fact, if you don’t seem to have a problem with the watery stuff but still have classic "dry eye" symptoms, this is almost certainly a big part of your problem. Oh... and by the way, please don't be startled to find that your first five or ten eye doctors never mentioned this to you.
The main reason you might not have enough oily stuff (lipids) is that those meibomian glands, the things that make it, get clogged. The oil is supposed to be nice and warm and runny, but sometimes it can get thick and gets stuck. This can often be improved, but may well be chronic requiring regular treatment. Lid therapy - particularly heat treatment - helps deal with this by first kind of melting the stuff that’s thickened up, then gently squeezing it out. Some oral antibiotics such as doxycycline can sometimes help too. Some nutritional supplements such as flaxseed oil may be useful as well.
Watery stuff (aqueous)
This is the main part of your tears. It’s secreted by the lacrimal glands. You either make enough, or you don’t. If you don’t, this is a problem - the closest thing to true "dry eye" that there is. If you don’t have enough tears of your own, usually you’ll be told to add some fake tears (artificial tear supplements) or, if you’re so dry that you’re adding tears all day long, you’ll probably be given tiny little plugs in the little holes your tears drain out through, so that you can trap the tears you have in your eyes as long as possible. If those work really well, some people burn the holes shut so they don’t have to worry about the plugs falling out (cautery). And if that’s not enough, well, first you need to make sure you haven’t got any of the Other Problems or that if you have, you’re treating them. And if that’s not enough, and you're completely miserable, there are always more types of treatments to look into.
Now, if you don’t have enough of this watery stuff, AND you don’t have enough lipids to help you keep the small amount of watery stuff your lacrimal glands can be persuaded to actually produce from evaporating as fast as it’s made, then you’ve got an Even Bigger Problem, and you need to address both problems separately.
Sticky stuff (mucins)
This is Rocket Science, we are told. Really high-tech spooky stuff that no one actually understands. — Just my luck, you say. The very thing that’s probably wrong with me, they don’t even know how it works or what to do about it when it doesn’t work. Yes, our feelings exactly — sigh.
Anyway, the point of the mucin layer is to make your eyes wet. "Hmm," you will say. "But if I have tears in my eyes, doesn’t that, well, by definition, sort of mean that they are wet?" Sorry, no. This just isn’t your day, is it? Look at your car when it’s drizzling outside. Could you swear that the entire surface is coated with water? Look carefully. No? See, I’m no scientist, but I kind of figure that unless you’ve got it totally submerged in water (and please don’t try this experiment with your car) a lot of things don’t actually stay 100% wet across their entire surface for very long at all. — Porous things that soak up water, fine, but smooth things, well, that’s a little harder. Funnily enough, I consulted a PhD surface chemist about this and he agreed. It really DOES take a special kind of substance to stretch and spread around and stick to your eye. Hence Rocket Science. So the watery stuff (aqueous) sits on top of the sticky stuff (mucin), which serves to glue down the watery stuff as long as possible. Where mucin goes aqueous will follow. Where mucin doesn’t go, well, that’s going to be dry. And unprotected, because that’s the other job of the mucins, standing between your corneas and the big bad world of bacteria.
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), we think maybe the fewer of those fake tears you use, the better, because they might just wash all this good stuff out of your tears.
What can go wrong with the tear film to cause "dry eye"
See, one of the problems with Dry Eye is that everybody calls it Dry Eye when dryness may or may not actually be the problem. Non Wetting Eye is closer to the mark, but that's too technical for most eye doctors, let alone patients. So we all use the term Dry Eye as a kind of catch-all to describe anything and everything that could possibly go wrong with your tears. There are actually quite a few things that could go wrong with your tears.
Usually, if you want to solve a problem, it helps to know which problem you have. Of course, you could walk around with a hammer and wherever you see a problem, hit it with the hammer. That's kind of the way Restasis is being used right now. And so long as there are plenty of willing nails around, well and good. Nevertheless, hammers aren't very good at changing light bulbs or making coffee. What are you? nail? bulb? unground coffee beans?
So, let’s try and get a rudimentary understanding of the possible problems.
Problem #1 - aqueous deficiency (not making enough of the watery part of your tears)
Truly dry: Now we’re talking dryness. This really is "dry eye", if you agree with our logic that absence of moisture = dryness and vice versa. Your lacrimal glands (the things in your upper eyelids) are seriously misbehaving themselves. They just aren’t pulling their weight at all — they are underproducing aqueous tears. This is a problem.
How do I know I have it? Good question. There is a test called Schirmers which you may have had, which is supposed to measure aqueous tear production. (If you aren't sure, this is where they stick a pre-measured strip of paper in your lower eye and wait to see how much tears it soaks up in 5 minutes.) You may or may not know by now that it is controversial. To anaesthetise or not to anaesthetise? How should the test be performed? Is it reliable? It figures, you moan. The one standardised test that could actually tell me whether I’ve got the all-important problem, and doctors are too busy sitting around debating the technicalities of whether and how it works to bother finding a way to ensure it WILL work. — We of course are of the opinion that you should be anaesthetised and left for about 5 minutes for the anaesthetic to take effect. Then we think they should blot the excess anaesthetic drop otherwise the first 10mm of the strip are going to soak up the drops, not your tears. But anyway, however the test is done, if you score less than 10 you’re almost certainly not producing enough aqueous tears. If you’re a lot less than 10, you’re in real trouble. If you're 0, you probably couldn't read this far down on the page - or if so you have a high pain threshold. If you’re 30, they probably did the test wrong. If you’re anywhere between 10 and 30 they might have done it wrong and you might actually be less than 10 but you won’t know. Bottom line, the test might be wrong when it tells you you don't have dry eye, but it's not going to be wrong when it tells you you do have dry eye.
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 damages the lacrimal glands — auto-immune diseases are good at that. Or, you could have a hormone deficiency. We women are so lucky. You could be getting old. Or, you could have lost some of the sensitivity in your corneas by not blinking enough, wearing contact lenses too much, staring at the computer too much, or having LASIK. For further discussion of the causes, click here.
So what do you DO about aqueous tear deficiency? Traditionally, the standard methods have been to supplement your tears by using fake ones from the pharmacy, and trap your existing tears by plugging the drains that they escape through (using punctal plugs or cautery). These days assessing and treating "the inflammatory component" is increasingly vogue. Restasis is commonly prescribed, and there are more dry eye drugs in development. For severe cases, an excellent treatment is scleral lenses, which act as a constant liquid bandage over the cornea.
Problem #2: Lipid deficiency: They’re evaporating too fast
Okay, so maybe I’m making plenty of tears, but they’re disappearing as fast as I can make them. I can’t keep up. On the other hand maybe I’m not making enough tears, and even the few I am making are disappearing as I speak. Help!
How do I know this is happening? Probably from a Tear Break-Up Time test. It’s a stupid name for it (since what they really mean is evaporating) but basically, they put a little dye on your eye and look at you through the slit-lamp and start counting. The dye is attracted to dry spots, and when dry spots start appearing, that’s called tears breaking up (evaporating). As soon as that starts happening they stop counting. If they made it to 10-15 seconds or so, it’s working something like normal. If they didn’t, it’s not normal. If like most of us you’re down around 1 second or less, well, you’re in real trouble, as if you didn’t already know. The other way you will figure it out is just empirically: If you’ve done everything you can to make sure you have enough tears, and if the doctors swear you aren't dry, but you’re still dry, chances are it’s because the tears are evaporating faster than you can make or supplement them and trapping them with plugs doesn’t help because it only prevents them from draining out through the puncti, not evaporating.
So why are they evaporating too fast? First, remember when we were talking about the lipid layer of your tears? That’s what keeps your tears from "breaking up", i.e. evaporating, i.e. floating off into the air. So, if your tears are evaporating too fast you don’t have enough oily stuff. This is made by your meibomian glands. They may be making it but it’s stiffening up and getting stuck and not being secreted properly onto your eyes. That’s called meibomian gland dysfunction (MGD) and is particularly common if you’ve got blepharitis. Second, if you’ve got any tendency towards too much evaporation, or you haven’t got very many tears in the first place, environmental facts will contribute to the evaporation. Heat and air conditioning dry the air and suck moisture from your eyes. Dry climates and wind make it worse. Third, if your eyes are just getting over-exposed to the air, this will also result in too much evaporation. Strangely enough, it seems many of us sleep with our eyes slightly open without realizing it so surface evaporation may be even more of a problem.
So how do I keep them from evaporating? First and foremost is making more of the stuff that keeps them from evaporating — the oily stuff (lipids) that the meibomian glands produce and keeping your lids nice and clean so the glands don’t keep getting clogged up. Lid therapy (hot compresses, lid scrubs and lid massage — don’t do these without reading the instructions) does all of those things. Some medications like doxycycline are really helpful for this, and so can some nutritional supplements. Second, you can make a nice friendly environment that at least doesn’t increase evaporation: Keep your house humid, which means not overdoing it with the heat. Same with your car — don’t overdo air conditioning in summer, do wear wraparound eyewear. Third, since nights can be the worst, check out our tips on protecting your eyes at night.
Problem #3: Mucin deficiency: They’re not sloshing around and sticking on your eyes the way they should.
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? If you are mucin deficient, for some reason you either don’t have enough or what you have isn’t doing its job properly. There are tests of some kind for this but they don’t seem to be very common and your eye doctor may cross his eyes when you start talking about the assays you looked up on Medline. So most people figure this out empirically: if they are maximally treating aqueous and lipid deficiency and they still have the same symptoms, one explanation is mucin deficiency.
There is no "fix" for mucin problems but this is an area of a lot of current research. Keep an eye out for references in the Dry Eye Digest to mucin secretagogues. And keep an eye out for anything that talks about goblet cell density, since it is directly related.