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Why is there pressure in the eye?

The eye is filled with a fluid and a jelly. These provide the eye with all the nutrients it requires and helps the eye maintain it's firm structure. The fluid in the eye needs to be constantly replenished, and to do this there are areas within the eye that produce fluids and an area where the fluid drains out. The constant changes in fluid production and drainage cause there to be pressure changes within the eye which changes throughout the day and over time. This pressure is called the intra-ocular pressure (IOP).

Why measure eye pressure?

Think blood pressure! We all have it and if its too high or low your GP will probably monitor or treat it with medication. Well, it's the same for opticians and IOP. We need to monitor your IOP regularly because if it goes high it can cause glaucoma. Glaucoma is a serious eye disease that effects the Optic Nerve and if left untreated can lead to blindness. A normal IOP reading can measure anything between 10-19 mmHg and varies between individuals. As we get older, the IOP increases naturally, putting us at more risk of glaucoma therefore this test is vital for the over 40's.

How is eye pressure measured?

There are three different ways of measuring the IOP and they all vary in levels of accuracy.

  • Non-Contact Tonometry
  • Goldmann Applanation Tonometry
  • Goldmann Applanation Tonometry with Pachymetry is the most accurate technique.

Non-contact tonometry

This is the technique used by most high street opticians to keep the costs down. This uses a machine which automatically blows a “puff of air” into the eye. The puff of air test is carried out three times because it gives an approximate reading ,which is then averaged out. This is the least accurate technique.

With this technique measurements can vary by 3-5mmHg between each puff.

Goldmann Applanation Tonometry 

This is considered to be a more accurate method of IOP measurement and is used in Eye Hospitals. Because Mr Shoker is a senior optometrist at the Eye Department in Maidstone Hospital we use this technique as standard.

It involves the instillation of a small drop of dye into the eye, this is painless and, in most cases, more comfortable than the ‘puff of air’ test. Goldmann Applanation Tonometry gives a more accurate reading of the IOP but, to obtain the TRUE (adjusted) IOP, PACHYMETRY MUST BE CARRIED OUT.

Your True Eye Pressure

The TRUE (adjusted) measurement of the intra-ocular pressure can only be taken with Goldmann applanation tonometry and pachymetry .

A pachymeter gives us a more personalised information about the thickness of your cornea (the front of your eye) and its effect on your IOP. As individuals we all come in different shapes and sizes and in the same way everyone's cornea are different thickness's. When we use Goldmann applanation tonometry alone, the IOP reading is made using a theoretical average corneal thickness.

With pachymetry we measure your actual corneal thickness, the pachymeter then substitutes this for the theoretical average thickness which gives your TRUE (adjusted) intra ocular pressure. It is almost imperative that all patients who have undergone Laser Eye Surgery have Pachymetry, since their cornea's are considerably thinner than average.