Intraocular Pressure Measurement

Intraocular pressure is the tissue pressure within the eye, which is determined by the balance between the production and drainage of aqueous humor, the clear fluid inside the eye.

In healthy persons, intraocular pressure generally ranges between 10 and 20 mm Hg. Slight changes in eye pressure from one season to another — or even during the course of a day — are normal.

Changes in intraocular pressure can be caused by anatomical problems, inflammation of the eye after trauma or infection, genetic factors, and medication use.

Intraocular pressure varies with changes in heart rate or respiration, and may also be affected by exercise and fluid intake.

Alcohol consumption and caffeine use also cause temporary changes in intraocular pressure, as can coughing, vomiting, or straining to lift heavy objects.

A significant change in intraocular pressure that persists and remains untreated can eventually cause vision problems and lead to eye disease.

Abnormal intraocular pressures usually don’t cause symptoms. For this reason, going to the eye doctor regularly is important, as eye pressure changes can be detected during a routine office visit.

Ocular Hypertension

An increase in blood pressure is called hypertension, and an increase in intraocular pressure is called ocular hypertension.

An eye is considered to have ocular hypertension if intraocular pressure is consistently 21 mm Hg or higher. Although ocular hypertension can affect anyone, it may be more common in persons with certain risk factors.

Ocular hypertension is a risk factor for glaucoma, a disease that damages the optic nerve. In a study published in the European Journal of Ophthalmology, researchers found that patients with intraocular hypertension had a higher risk of developing open-angle glaucoma if:

  • They had a family history of glaucoma
  • They were age 60 or older
  • They suffered from axial myopia
  • They suffered from arterial hypertension

Although hypertension is a risk factor for glaucoma, you can have higher than average eye pressure and not have glaucoma. Glaucoma typically causes no pain and produces no symptoms, but if left untreated it can eventually affect peripheral vision. If the condition continues to progress, permanent eye damage and blindness can result.

Hypotony: When intraocular pressure is less than 8 mm Hg, a person is considered to have hypotony. When eye pressure is too low it can cause problems with vision.

The risk of vision problems increases when eye pressure falls below 6 mm Hg, although this number can vary. While some persons may have visual symptoms at 10 mm Hg, others may have no symptoms at 6 mm Hg.

Hypotony may be treated with a variety of techniques, depending on the cause.

All adults attending an eye unit should have their intraocular pressure (IOP) measured, unless there is a contraindication (e.g. trauma or corneal ulcer).

Many people with glaucoma have no symptoms and do not know they have the condition. All children who have had cataract surgery should also have their IOP measured at every follow-up visit, if possible.

Finding glaucoma early allows treatment to be given which will preserve sight. Although elevated IOP is not the only sign of glaucoma, measuring it is simple and quick to do. It should therefore be done routinely on all adults attending eye care facilities.

Applanation tonometry, described in this article, is the preferred method (the ‘gold standard’). Schiötz tonometry, which will be described in a future issue, can also be used to measure intraocular pressure and is a useful screening test.

Equipment

  • Tonometer, either Goldmann (used on slit lamps) or Perkins (hand-held)
  • Applanation prism
  • Local anaesthetic drops
  • Fluorescein strips
  • Clean cotton wool or gauze swabs.

Preparation

Ensure the prism has been disinfected with isopopyl alcohol 70% (methylated spirit) or sodium hypochlorite 1%. The prism must be rinsed in sterile water and wiped dry with a clean swab (residue of the disinfectant may cause a caustic burn on the cornea).

Check that the graduation marked ‘0’ on the measuring prism is aligned with the white marker point on the tonometer head

Check the calibrated dial of the tonometer is set at 10 mmHg

Ensure that the patient is sitting comfortably at the slit lamp: at the right height, with their chin on the rest and their forehead against the headband (or in a chair with their head supported, if using the Perkins tonometer)

Set the magnifcation of the slit lamp at ×10.

Causes of Changes in Eye Pressure

Changes in eye pressure may be caused by:

  • Excessive or decreased aqueous production
  • Inadequate or increased aqueous drainage
  • Long-term use of certain medications, in particular corticosteroids
  • Eye trauma
  • Underlying eye condition, such as pseudoexfoliation syndrome, pigment dispersion syndrome, or corneal arcus, or a thin cornea
  • Eye surgery, especially cataract surgery

Tests to Detect Changes in Eye Pressure

The test most commonly used to measure intraocular pressure is tonometry, which is often used to screen for glaucoma. A diagnosis of glaucoma will require more than just eye pressure measurement because it is possible to have glaucoma and have normal eye pressure.

If glaucoma is a concern, your eye doctor will also carefully examine your optic nerve for signs of damage.

Your eye doctor may use one of the following tonometry methods to measure intraocular pressure:

Applanation tonometry

This test measures the amount of force needed to temporarily flatten a part of your cornea. Your eye doctor may put fluorescein, the same dye used during a slit lamp examination, in your eye to make it easier to see.

The tonometer is briefly placed on the cornea to determine eye pressure. Before the test, you’ll receive anesthetic drops to numb the eye, so you won’t feel anything.

Noncontact tonometry

This test estimates the pressure in your eye using a puff of air. Your doctor will warn you to expect a quick pulse of air, which may be surprising. No instruments will touch your eye, so you won’t need an anesthetic.

If abnormal intraocular pressures are detected, then you may need more specialized tests to determine your risk of developing eye disease.

These tests may include ophthalmoscopy, perimetry or visual field testing, pachymetry, and optical coherence tomography.

Method

Instil the local anaesthetic drops and then the fuorescein. Only a very small amount of fuorescein is needed

For measuring the IOP in the right eye, make sure the slit beam is shining onto the tonometer head from the patient’s right side; for the left eye, the beam should come from the patient’s left side

Move the filters so that the blue filter is used to produce a blue beam

Make sure the beam of light is as wide as possible, and that the light is as bright as possible. This makes visualising the fluorescein rings easier (with the slit diaphragm fully open)

Ask the patient to look straight ahead, open both eyes wide, fix his or her gaze and keep perfectly still

With the thumb, gently hold up the patient’s top eyelid, taking care not to put any pressure on the eye

Direct the blue light from the slit lamp or the Perkins tonometer onto the prism head

Make sure that the tonometer head is perpendicular to the eye

Move the tonometer forward slowly until the prism rests gently on the centre of the patient’s cornea

With the other hand, turn the calibrated dial on the tonometer clockwise until the two fluorescein semi-circles in the prism head are seen to meet and form a horizontal ‘S’ shape. (Note: the correct end point is when the inner edges of the two fluorescein semi-circle images just touch)

Note the reading on the dial and record it in the notes

Withdraw the prism from the corneal surface and wipe its tip

Repeat the procedure for the other eye

Wipe the prism with a clean, dry swab and replace it in the receptacle containing the disinfectant.

Treating Changes in Eye Pressure

Changes in eye pressure that do not affect vision may not need to be treated, unless the patient is at risk for glaucoma. A variety of medications are used to treat ocular hypertension.

Topical therapy in the form of eye drops is often the first line of treatment. Patients with severe intraocular hypertension may require glaucoma surgery.

Adherence to eye drop therapy can be a problem for people with ocular hypertension because they are often asymptomatic. Also, the eye drops used to control eye pressure can cause side effects.

But non-compliance with a prescribed treatment regimen is a major cause of blindness resulting from glaucoma.

Prevention of Changes in Eye Pressure

There is no way to tell whether your eye pressure has changed. Maintaining good eye health is the only way to prevent vision problems associated with eye pressure changes.

Getting regular eye examinations from an ophthalmologist is important to maintaining your eye heath.

If you think your vision may have changed, make an appointment with your eye doctor as soon as possible to avoid potential vision loss that may result from an untreated condition.

Source  & More Info: ncb.nlm.nih.gov and eyehealthweb

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