Intraocular pressure (IOP) — the fluid pressure inside your eye — is invisible, painless, and the most important modifiable risk factor for glaucoma, Korea’s leading cause of irreversible blindness. A single 30-second measurement at your eye exam can change everything.
What Is Intraocular Pressure (IOP)?
Your eye is not a hollow ball. It contains a clear fluid called aqueous humour that continuously flows in and out, nourishing the lens and cornea. When this drainage system is too slow or blocked, pressure builds — just like a blocked drain causes water to back up.
IOP is measured in millimetres of mercury (mmHg). The measurement takes seconds and is completely painless with modern non-contact tonometers (the classic “puff of air” test) or the gold-standard Goldmann applanation tonometer, which we use at Nunehim Eye Center.
IOP Reference Chart
(Check for
other causes)
Range
(mmHg)
(Monitoring
needed)
(Treatment
often needed)
The normal range is 10–21 mmHg. However, “normal” is population-based — some patients develop glaucoma at 16 mmHg, while others tolerate 24 mmHg without damage. This is why IOP is one piece of a complete glaucoma assessment, not the whole picture.
Why High Eye Pressure Damages Vision
The optic nerve, which carries visual signals from your eye to your brain, is particularly vulnerable to pressure. Sustained elevated IOP compresses and gradually kills optic nerve fibres. Once lost, these fibres do not regenerate.
The damage begins at the edges of your visual field — the peripheral (side) vision — and slowly moves inward. Because central vision is preserved until late in the disease, most patients have no symptoms until 40–50% of optic nerve fibres are already dead.
This is why regular IOP screening is so important even when your vision feels perfect.
What Causes High IOP?
Poor Aqueous Drainage
The trabecular meshwork — the eye’s drainage filter — becomes less efficient with age or in primary open-angle glaucoma.
Steroid Use
Eye drops, nasal sprays, skin creams, and oral steroids can all raise IOP. Even OTC decongestant eye drops may cause short-term elevation.
Genetics
Having a first-degree relative with glaucoma raises your own risk by 4–9×. IOP tendency also runs in families.
Age
Drainage efficiency naturally declines after 40. Korean adults over 40 should have IOP measured at every eye exam.
Body Position & Sleep
IOP is highest in the morning and when lying down. Tight neckties and face-down sleeping positions also temporarily raise pressure.
Secondary Causes
Uveitis (eye inflammation), previous eye trauma, and pseudoexfoliation syndrome all impair drainage and raise IOP.
🚨 Acute Angle-Closure Glaucoma: A Medical Emergency
- Sudden, severe eye pain (often described as the worst headache of your life)
- Nausea and vomiting accompanying the eye pain
- Blurry or hazy vision with haloes around lights
- Eye that feels rock-hard to the touch
- IOP can spike to 50–70 mmHg within hours — go to an emergency eye clinic immediately
Ocular Hypertension vs. Glaucoma: What’s the Difference?
Ocular hypertension (OHT) means your IOP is above 21 mmHg but your optic nerve and visual field are still normal. About 5% of adults over 40 have OHT. Of these, roughly 1 in 10 will develop glaucoma within 5 years without treatment.
Normal-tension glaucoma (NTG) is the flip side: optic nerve damage occurs even with IOP in the normal range (≤21 mmHg). NTG is unusually common in Korean and Japanese populations — making comprehensive optic nerve evaluation essential beyond just checking pressure.
At Nunehim Eye Center, every IOP measurement is combined with optic disc photography and OCT (optical coherence tomography) of the nerve fibre layer for a complete picture.
Common Myths About Eye Pressure
| ❌ Myth | ✅ Reality |
|---|---|
| “I’d feel it if my pressure was high.” | Chronic high IOP is completely painless in the vast majority of cases. Only acute angle closure causes pain. |
| “Normal IOP means no glaucoma risk.” | Normal-tension glaucoma is responsible for ~30–40% of glaucoma in Korea — optic nerve evaluation is still essential. |
| “Glaucoma only affects the elderly.” | While risk increases with age, glaucoma can develop in your 30s and 40s, especially with a family history. |
| “If IOP is treated, glaucoma is cured.” | Lowering IOP slows progression but does not reverse existing optic nerve damage. Early detection is critical. |
| “Glasses and IOP are related.” | Refractive errors (myopia) do not cause high IOP, though high myopia is an independent glaucoma risk factor. |
How IOP Is Measured at Nunehim Eye Center
Non-Contact Tonometry (Air Puff)
A quick puff of air flattens the cornea momentarily. No drops, no contact, takes under 5 seconds. Used for initial screening in all patients.
Goldmann Applanation Tonometry
The gold-standard measurement. Anaesthetic drops are applied, and a small probe gently touches the cornea. More accurate, especially for corneas of non-average thickness.
Corneal Thickness (Pachymetry)
A thin cornea makes IOP appear falsely low; a thick cornea makes it appear falsely high. We measure corneal thickness to interpret IOP accurately — especially important for myopic patients.
Optic Nerve OCT
Even with normal IOP, subtle thinning of the retinal nerve fibre layer can be detected by OCT years before any visual field loss appears.
✅ Who Should Get IOP Checked?
- Everyone over 40 — annual eye exam including IOP measurement
- Family history of glaucoma — start at age 35
- High myopia (−6D or more) — more frequent monitoring
- Long-term steroid users — any route of administration
- Diabetes or hypertension — increased vascular risk for optic nerve
- Previous eye injury — trauma can impair drainage years later
Treatment Options When IOP Is Too High
Prescription Eye Drops
The first-line treatment for elevated IOP or early glaucoma is usually once or twice-daily pressure-lowering drops. Prostaglandin analogues (such as latanoprost) reduce IOP by 25–35% and are typically taken at bedtime. They must be used every day indefinitely.
Laser Treatment (SLT)
Selective Laser Trabeculoplasty is a 5-minute in-office procedure that improves drainage through the trabecular meshwork. It works in about 80% of patients, reducing or eliminating the need for drops. Effects typically last 3–5 years and the treatment can be repeated.
Surgery (Trabeculectomy / MIGS)
For advanced cases or when drops and laser are insufficient, surgical options create a new drainage pathway. Modern minimally invasive glaucoma surgery (MIGS) combines effective IOP reduction with faster recovery compared to traditional trabeculectomy.
Lifestyle Tips to Support Healthy Eye Pressure
Aerobic Exercise
Regular moderate aerobic activity (brisk walking, cycling) can lower IOP by 2–3 mmHg over time.
Limit Caffeine
High caffeine intake temporarily raises IOP. Moderate coffee consumption is fine for most people.
Hydration Pace
Drinking large amounts of water very quickly raises IOP briefly. Sip water throughout the day rather than drinking large volumes at once.
Avoid Inverted Poses
Yoga headstands and inverted poses significantly raise IOP. Speak with your ophthalmologist if you practise yoga regularly.
Check Your Eye Pressure at Nunehim Eye Center
A comprehensive glaucoma screening — including IOP, optic nerve OCT, and visual field — takes under 30 minutes. Protecting your vision starts with knowing your numbers.
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