What Causes High Intraocular Pressure (IOP)?

At a Glance:

  • Aqueous humor is a clear, watery liquid produced inside the eye that helps to support the eye, delivers nutrients to the cornea and provides other biomechanical benefits
  • This liquid naturally drains over time to maintain stable eye pressure
  • If the aqueous fluid is overproduced, or drainage becomes hampered this may lead to a buildup of pressure in the eye
  • Pressure inside the eye is referred to as intraocular pressure (IOP). High eye pressure is sometimes referred to as ocular hypertension 
  • High IOP is the primary cause of glaucoma

What is High IOP?

High intraocular pressure (IOP), also sometimes referred to as ocular hypertension is the leading risk factor for glaucoma. Intraocular pressure is measured in millimeters of mercury (mm Hg). The range of pressure we describe as “normal” is anywhere between 10 and 21 mmHg. It is important to remember that glaucoma can exist at any range of eye pressure. Some individuals with “high” pressures do not develop nerve damage, while others with “normal” or “low” pressures develop progressive nerve damage. 

IOP is measured with a tool called a tonometer. The most accurate instrument for measuring IOP is the contact tonometer which involves a probe usually attached to a slit lamp that gently makes contact with the front of the eye to take a measurement after the eye is numbed with a local anaesthetic drop. Eye pressure can also be measured with non contact instruments such as the air puff tonometer which does not require anaesthetic eye drops. 

High IOP is often asymptomatic therefore regular testing is required. There may be no warning signs that you may have ocular hypertension until damage to the optic nerve from the high eye pressure is quite progressed and unfortunately permanent. Therefore, it is important to have regular eye tests and diagnose high IOP before this permanent damage occurs. 

If high IOP is detected, an Ophthalmologist or Optometrist will perform other tests to determine the cause of the increased pressure. A gonioscopy is a technique that uses a specialised contact lens to examine the drainage angles (channels) in the eye. The channels may be open, closed or narrowed. Closed or narrowed drainage channels may cause elevated IOP due to the fact that the aqueous humour cannot naturally drain. 

In the case of closed or narrow drainage channels, the IOP may rise abruptly causing an ACUTE angle-closure glaucoma attack. Acute angle-closure is a medical emergency requiring urgent treatment. This sudden rise in pressure produces dramatic symptoms such as severe eye pain, nausea and vomiting , blurry vision, and seeing halos or rings of light. A person experiencing any of these symptoms must present to their local hospital emergency department without delay for treatment. The emergency department will also contact Ophthalmology to continue treatment and arrange ongoing follow up. 

It’s recommended that Australians have a comprehensive eye exam annually. This exam includes an eye pressure check, visual field test and OCT (scan of your optic nerve and macula). If you haven’t had an eye exam in the last 12 months, now is the time to schedule an exam with an optometrist . The easiest way to find and book an appointment for an Optometrist in your area is through MyHealth1st. 

Causes of High IOP

All hypertension in the eye is caused by an imbalance between the production of aqueous humor and the drainage of aqueous humor. There are a number of causes for this. Primary glaucoma refers to those cases in which the cause is not evident. Secondary glaucoma can be caused by trauma/injury, medications, or other underlying eye conditions.

  1. Excessive Aqueous Humor Production

    Located behind the iris, or the coloured part of the eye, is a structure known as the ciliary body. This structure is responsible for the production of aqueous humor. This clear fluid is similar to blood plasma in composition and contains water and a small amount of sugar, vitamins, proteins and other nutrients that are vital to eye health. The fluid flows from the ciliary body to the front of the eye to fill the space between the iris and the cornea. It also helps to give structure to the eye as well as providing nutrients to the cornea. If the production of aqueous humor falls out of sync with drainage, then pressure in the eye may increase. Think of it as filling a water balloon with a tiny hole in it. Although water may leak out of the balloon, the volume of water entering it will still lead to the balloon expanding as the hole is too small to keep up with the inflow. 
  1. Existing Eye Conditions
    Increased pressure in the eye has been linked to a number of eye conditions, including secondary causes of glaucoma such as pseudoexfoliation syndrome or pigment dispersion syndrome.
  2. Genetic Predisposition

    Race, age and family history all contribute to a person’s risk of developing ocular hypertension/glaucoma. Indigenous Australians, people of Asian, African and Hispanic descent are at higher risk than any other ethnic group.  People with genetic conditions such as diabetes or sickle cell anemia have an increased risk for high IOP, as are people with extreme myopia (short sightedness) or hyperopia (long sightedness). 
  3. Medications

    There are many medications that can affect eye pressure therefore providing your medication history to your Ophthalmologist or Optometrist is important. These can include corticosteroid medications that can also include corticosteroid eye drops (used in treating inflammatory eye conditions) and others such as certain asthma puffers. 
  4. Eye Trauma

    Trauma to the eye may change the balance of aqueous humor production and drainage. This change may not be immediate and may occur months or even years after the initial trauma due to scarring and anatomical damage to certain eye structures. When having an eye test it is important to tell your optometrist/ophthalmologist of any eye trauma you may have suffered in the past so that this may be factored into your testing. 
  5. Caffeine Intake

    A 2020 study titled “Effects of caffeine consumption on intraocular pressure during low‐intensity endurance exercise” has linked caffeine intake and low intensity exercise to an increase in IOP. While both exercise and caffeine intake were already known to affect IOP, this new study shows that caffeine intake, before low intensity exercise (such as cycling, jogging or swimming) counteracts the IOP lowering benefits of exercise. Before you panic, this does not mean that you should give up drinking coffee or tea.  Rather, the results of the study indicate that if a person has been prescribed low intensity exercise as part of their IOP treatment, they should avoid caffeine before exercising so they don’t negate the benefits of exercise. Further, glaucoma patients should moderate their caffeine intake as it has shown to increase IOP if consumed in excess.


Looking for an optometrist near you?