What is meibomian gland dysfunction (MGD)?
MGD is the most common cause of dry eye, and it's almost certainly what nobody explained to you. Here is what is happening in your eyelids.
Read guideYou have tried the drops on the chemist shelf, one after another, and none of them fixed it. Part of the reason may be that dry eye isn’t one condition. There are two main types, they have different causes, and they need different treatment. If you have been treating the wrong one, no amount of the same drop was ever going to work.
Knowing which type you have is the difference between guessing and getting somewhere. The two mechanisms differ in cause and treatment, and many people sit somewhere between them.
What are the two main types of dry eye?
The internationally recognised framework for dry eye, set out in the TFOS DEWS II report, splits the disease into two broad mechanisms.
Evaporative dry eye means your tears evaporate off the surface of your eye too quickly. You may be making a perfectly normal amount of tears, but they don’t last, because the oily layer that seals them in is missing or poor quality.
Aqueous-deficient dry eye means the opposite: the watery part of your tears is in short supply. The lacrimal gland, which produces the watery layer, is underperforming, so there simply isn’t enough tear volume to keep the surface comfortable.
The reason this matters is that the treatment for each is different. Restoring oil glands does little for a tear-volume problem, and flooding the eye with watery drops does little when the real issue is evaporation.
What causes evaporative dry eye?
Evaporative dry eye is by far the more common of the two. In most cases the cause is meibomian gland dysfunction (MGD): the tiny oil glands in your eyelids become blocked or produce thickened, poor-quality oil, so the tear film loses its protective top layer and evaporates fast.
Other things speed up evaporation on top of MGD: long screen sessions that cut your blink rate, air-conditioning and heating, wind, ceiling fans overnight, incomplete blinking, and eyelids that don’t close fully during sleep.
The classic pattern is eyes that feel fine first thing but burn and tire as the day goes on, watering in wind or air-con, and drops that help for less than an hour before the discomfort returns.
What causes aqueous-deficient dry eye?
Aqueous-deficient dry eye is less common on its own, and it’s more likely to point to an underlying medical cause worth investigating.
The most significant is Sjögren’s syndrome, an autoimmune condition where the immune system attacks the glands that produce tears and saliva, so dry eyes come alongside a dry mouth. Aqueous deficiency can also be linked to other autoimmune conditions, some medications that reduce tear production, damage or scarring to the lacrimal gland, and age-related decline in tear output.
Because aqueous deficiency can be a sign of something systemic, persistent dryness combined with a dry mouth, joint pain or fatigue is worth raising with your GP, who may consider blood tests.
Can you have both at the same time?
Yes, and this is the crucial point most people miss. The two types aren’t a strict either-or. A large share of people have mixed dry eye, with both a failing oil layer and reduced tear volume feeding into each other.
This is exactly why one treatment so often only helps part of the problem. If you have mixed dry eye and only the evaporative side is being treated, the aqueous side keeps you symptomatic, and vice versa. It also explains the frustrating experience of a treatment that clearly did something but didn’t finish the job. The part it targeted improved; the part it ignored didn’t.
How does a clinic work out which type you have?
You can’t reliably tell the two apart from symptoms alone, because they feel very similar from the inside. A proper dry eye assessment measures the two mechanisms separately.
- For the evaporative side: meibography images the oil glands to show which are healthy, blocked or dropped out, and a tear break-up time measures how many seconds your tear film stays intact before it breaks apart. A fast break-up points to an oil problem.
- For the aqueous side: tests of tear volume estimate how much watery tear you’re producing. A low volume points to aqueous deficiency.
- For the damage: tear osmolarity (the saltiness of your tears) and surface staining show how stressed and inflamed the eye surface has become, regardless of type.
The pattern across these tests is what identifies your type, and it’s why a five-minute look with a torch can’t do the job.
Why does getting the type right change everything?
Because the treatments diverge.
Evaporative dry eye is treated by getting the oil glands working again: warm compresses done properly, gentle expression, lid hygiene, and, when home measures aren’t enough, in-clinic options like IPL, thermal pulsation and professional gland expression. Lipid-based artificial tears can help hold the tear film together, where watery drops alone often don’t.
Aqueous-deficient dry eye is treated by protecting and topping up the tears you do have: preservative-free lubricating drops used regularly, punctal plugs to slow tear drainage so tears stay on the eye longer, treating inflammation, and chasing down any underlying cause such as a medication or an autoimmune condition.
Mixed dry eye needs both approaches, layered together, which is why a single product rarely settles it.
Can you tell the type apart yourself at home?
Not reliably, and it’s worth being honest about that, because the two types feel almost identical from the inside. Both burn, both feel gritty, both get worse on screens and in air-conditioning, and both can water. Symptoms simply don’t sort cleanly into “evaporative” and “aqueous”.
There are a few soft clues. Evaporative dry eye linked to MGD often comes with red or crusty lid margins, symptoms that build through the day, and watering in wind. Aqueous deficiency is more likely to come alongside a dry mouth, and sometimes joint pain or fatigue if an autoimmune cause is involved. But these are hints, not a diagnosis, and plenty of people break the pattern. Treating yourself based on a guess is how so many people end up cycling through the wrong drops for years. The clues are useful for knowing what to raise with a clinician, not for self-diagnosing.
What should you do with this?
If you have been buying drops off the shelf and swapping brands hoping one lands, the missing step is a diagnosis. Ask for an assessment that images your glands and measures both tear quality and tear volume, so treatment can be matched to your actual type instead of the average. Treating the type you genuinely have is the shortest path out of the cycle you’re stuck in.