Dry eye syndrome

What it actually is

Tears are not simply salt water. The film covering the eye has three layers, and each does a different job.

The innermost mucin layer coats the surface of the eye and lets the watery layer spread evenly over it rather than beading up. The middle aqueous layer is mostly water with a little salt, and carries oxygen and nutrients to the cornea, which has no blood supply of its own. The outer lipid layer is oil, produced by the meibomian glands along the rims of the eyelids, and its job is to seal the surface so the water beneath does not evaporate.

Dry eye is a failure somewhere in that system. The current definition, agreed internationally in 2017, describes it as a disease of the ocular surface marked by loss of a stable tear film, with inflammation and damage to the surface and changes in the nerves that supply it.

That phrasing matters, because it is a long way from “not enough tears”. Dry eye is a chronic condition that tends to drive itself: an unstable film leaves the surface exposed, the exposed surface becomes inflamed, inflammation disrupts the film further. Treatment aims at breaking that cycle rather than topping up a reservoir.

The name has shifted along with the understanding. Dry eye syndrome remains the common term, and the one most people search for, but clinical literature now generally says dry eye disease. The change is deliberate: it signals a chronic condition with an identifiable mechanism rather than a loose collection of symptoms.

Two kinds, and why the distinction decides everything

Dry eye divides into two broad types, which sit on a continuum rather than in separate boxes. Many people have some of each.

Aqueous-deficient dry eye is the one the name suggests: the lacrimal glands are not producing enough of the watery layer. Sjögren's syndrome, an autoimmune condition that attacks the glands producing tears and saliva, is the clearest example.

Evaporative dry eye is the other, and far more common, type. Enough tears are produced, but they evaporate too fast — usually because the meibomian glands in the eyelids are blocked, inflamed or producing oil of the wrong consistency. This is meibomian gland dysfunction, and across both clinic and population studies it is the single leading cause of dry eye.

The consequence is practical. If the problem is that the oil layer has failed, adding more water to the surface is a holding measure: it washes away at much the same rate as the tears already there. Unblocking the glands addresses the cause. This is why people who use artificial tears faithfully for months and find they help only for twenty minutes at a time are not doing it wrong — they are treating the wrong half of the problem.

Why dry eyes water

The commonest reason people dismiss the diagnosis is that their eyes stream.

It is not a contradiction. An irritated, exposed corneal surface triggers reflex tearing, the same mechanism that floods the eye when something blows into it. Reflex tears arrive in a sudden watery rush, carry little of the oil and mucin needed to make a stable film, and drain away almost immediately. The eye produces a great deal of fluid and remains badly lubricated throughout.

Watering that comes in episodes, leaves the eye no more comfortable, and is worse in wind or cold is a dry eye symptom rather than evidence against it.

What raises the risk

Age is the strongest factor, and the reason is mechanical: oil production declines, so the sealing layer thins. Dry eye becomes markedly more common past 50, and women are affected more than men, with hormonal changes around menopause a recognised contributor.

Beyond age:

• contact lens wear
• long periods looking at screens without a break
• air conditioned or heated rooms, and windy, dry or dusty conditions
• smoking, or time spent in smoky environments, and heavy alcohol use
• certain medicines, including some antidepressants, blood pressure medicines, diuretics and antihistamines
• blepharitis, an inflammation of the eyelid margins which commonly accompanies dry eye and shares its treatment
• autoimmune conditions, particularly Sjögren's syndrome and lupus
• conditions that reduce blink rate, including Parkinson's disease
• injury or surgery to the eye or eyelids, including laser vision correction

Two points about that list. Medicines should never be stopped on the strength of it — that is a conversation with whoever prescribed them. And persistent dry eye alongside a dry mouth is worth mentioning to a GP, because that combination is how Sjögren's syndrome commonly announces itself, and it is frequently missed for years.

Vitamin A deficiency is a genuine cause of a dry, damaged ocular surface, but in countries with an adequate food supply it is rare, and it belongs in a list of global causes rather than a list of likely explanations here.

Screens, and the blink

Screen work is the modern aggravating factor, and the mechanism is not the screen itself.

Concentration suppresses blinking. Blink rate falls sharply during sustained close work, and many of the blinks that do happen are incomplete — the lids do not fully meet, so the oil is never properly pressed out and spread. The surface at the bottom of the eye, the part a half-blink misses, is where the dryness is usually worst.

What helps is dull and effective: breaks away from the screen, the monitor positioned at or below eye level so the lids cover more of the eye, and conscious, complete blinking. Looking down at a screen rather than up at one measurably reduces the exposed surface area.

The step most people skip

For evaporative dry eye — again, the common kind — the mainstay of treatment is not a bottle. It is heat and cleaning, applied to the eyelids, repeatedly, over weeks.

The standard NHS method for lid care is the same one used for blepharitis:

1. Soak a clean flannel or cotton wool in warm water and hold it against the closed eyelid for five to ten minutes.
2. Massage the eyelids gently for around thirty seconds.
3. Wipe along the edge of the eyelids with cotton wool or a cotton bud to remove any flakes or crusts. A small amount of baby shampoo in water can help.

Twice a day, and continued even after symptoms settle, because stopping lets the glands block again.

The heat matters more than it looks. Meibomian oil thickens when the glands are inflamed, and a compress that has gone lukewarm after ninety seconds does not warm it enough to flow. The massage is what actually expresses it.

This is unglamorous, takes a fortnight or more to show results, and is the single most effective thing available to most people with dry eye. It is also the part of the advice most often abandoned.

Drops, and the preservative problem

Artificial tears relieve symptoms. They are worth using, and for aqueous-deficient dry eye they do more than that. But they substitute for the tear film rather than repairing it, and expecting more from them is the commonest disappointment.

Several families exist — hypromellose, carmellose, sodium hyaluronate, hydroxypropyl guar, carbomer gels, and paraffin-based ointments. Thicker preparations last longer and blur vision more, which is why ointments are generally used at night. A pharmacist can match a product to the symptoms, and the useful advice is to choose by ingredient rather than brand and to give one a fair trial of six to eight weeks before deciding it has failed.

The detail worth knowing is preservatives. Most multi-dose bottles contain one, commonly benzalkonium chloride, and with frequent long-term use it can damage the ocular surface — the problem the drops are meant to be solving. Anyone using drops more than about four times a day should be using preservative-free, which comes in single-use vials or specially valved bottles. Preserved drops used occasionally are not a concern.

What the evidence does not support

Omega-3 supplements are marketed heavily for dry eye. The largest trial to test them randomised 535 people with symptomatic dry eye to a year of high-dose marine omega-3 or a placebo, and found no clinically meaningful difference between the groups on symptoms, signs or the need for other treatment. That is a single trial and not the last word, but it is a large one, and it does not support buying supplements for this.

Drinking more water appears in a good deal of advice, including NHS leaflets. Staying hydrated is sensible generally, and severe dehydration does affect tear production. As a treatment for established dry eye in someone drinking normally, the evidence is thin. It is not a reason not to, but it is not the answer either.

When self-care is not enough

The threshold for getting it looked at is a few weeks of consistent self-care without improvement. An optometrist can examine the tear film, assess how fast it breaks up, and look at the meibomian glands directly — and a sight test is the normal route in, free on the NHS for a long list of people.

Beyond self-care, the options include punctal plugs — small inserts that block the tear drainage ducts so what tears there are stay on the eye longer — and prescription anti-inflammatory drops for more severe disease, started by an ophthalmologist. Persistent dry eye with no clear cause is grounds for referral.

When it is not dry eye

Dry eye is uncomfortable rather than dangerous, which makes it a convenient explanation for symptoms that are neither.

Seek urgent advice for an eye that is painful as well as red, and for any red eye in someone who wears contact lenses, where infection has to be excluded quickly.

Go to an emergency department for any change in sight, including wavy lines, flashing or loss of vision; pain on looking at light; a red eye with a severe headache and nausea; an eye that is very dark red; or any injury to the eye or anything stuck in it.

None of those are dry eye, and the mistake that matters is waiting them out because dryness seemed the obvious explanation.