A Guide for Patients

Understanding Intraocular Pressure (IOP)

Intraocular pressure (IOP) refers to the pressure inside your eye, which is essential for maintaining its shape and function. Just like the air inside a balloon keeps it inflated, the fluid inside your eye keeps it firm and healthy. However, when the pressure becomes too high, it can lead to conditions like glaucoma, which can damage your vision if left untreated. This section will explain what IOP is, how it works, and why it’s important to manage it properly.

If you’ve been diagnosed with raised intraocular pressure or glaucoma, your eye specialist will work with you to manage and monitor it effectively. Treatments might include eye drops, laser therapy, or surgery to lower your pressure and prevent further damage to your vision. Regular check-ups and measurements of your eye pressure are important to ensure your treatment is working as intended.

What is Intraocular Pressure (IOP) ?

OP is the pressure created by the fluid in your eye. The eye is filled with two types of fluids: aqueous humour, which is found in the front part of the eye between the iris and the cornea, and a jelly-like substance called vitreous humour, located at the back between the lens and the retina. The aqueous humour is produced by the ciliary body, just behind the iris, and flows through the pupil into the front of the eye. From there, it drains out through the trabecular meshwork, located where the iris meets the cornea, in an area known as the drainage angle.

The balance between the production and drainage of this fluid is what creates intraocular pressure. If the drainage is blocked or slowed, or too much fluid is produced, the pressure inside the eye increases. Imagine your eye as a sink: the tap (ciliary body) is constantly producing fluid, which fills the basin (anterior chamber) and drains out through the plughole (trabecular meshwork). If the drain is blocked, the water (or fluid) builds up, causing the pressure to rise.

Glaucoma and Intraocular Pressure

In open-angle glaucoma, the drainage system doesn’t work as efficiently as it should, causing the fluid to build up and increase the pressure inside the eye. Treatments for this include eye drops to reduce fluid production or help improve drainage, laser therapy, or surgery to create new drainage pathways.

In angle-closure glaucoma, the drainage angle is blocked by the iris, which acts like a rubber stopper over the plughole. This blockage causes a sudden spike in pressure, which can be very painful and requires emergency treatment. This might involve medication to lower the pressure or a laser treatment (called a laser iridotomy) to create a small hole in the iris, allowing fluid to flow freely.

How Do We Measure Intraocular Pressure?

Eye pressure is measured using a special instrument that gently presses against the cornea to gauge how firm or soft the eye is, similar to testing the firmness of a balloon by pressing on it. The measurement is given in millimetres of mercury (mmHg), and this helps eye specialists assess whether your eye pressure is within a healthy range.

What is a Normal Eye Pressure?

Most people have an eye pressure that falls between 10 and 24 mmHg. However, what’s considered a “normal” pressure can vary from person to person. Some people with eye pressure below 21 mmHg may still develop glaucoma (normal-tension glaucoma), while others with pressures above 24 mmHg may not show any signs of the disease (ocular hypertension).

According to the National Institute for Health and Care Excellence (NICE), if your eye pressure repeatedly measures 24 mmHg or higher, you should be referred to a hospital eye service for further investigation. However, some individuals with glaucoma can experience damage even with lower pressures, especially those with advanced disease.

Factors That Influence Intraocular Pressure

Intraocular pressure can fluctuate throughout the day due to various factors. These include:

Body Position: Lying flat can increase your eye pressure compared to sitting upright, which is why night-time spikes in pressure may occur.
Exercise and Activity: Physical activity, breathing, fluid intake, and even wearing a tight shirt collar can affect your eye pressure.
Medications: Both systemic (whole body) and topical (eye drops) medications can influence your pressure.
Caffeine: Drinking caffeinated beverages may cause temporary increases in eye pressure.

For people with glaucoma, these fluctuations are particularly important to monitor, as spikes in pressure can contribute to further damage to the optic nerve.