Introduction  

Intraocular pressure measurement or eye pressure exam is also referred to as a tonometry test. It is an essential examination that helps eye professionals to evaluate a patient's eye health. Intraocular pressure (IOP) is the internal pressure or the pressure in the eyeball.

Eyes are always producing aqueous humor, the fluid that occupies the central chamber of the eyeball. To keep the pressure in the eye stable, a tiny drainage opening at the corner of the eye allows the excess liquid to flow out. Therefore, as fresh aqueous humor enters the eye chamber, an equal amount drains out through the drainage canal. 

In cases where the drainage system is faulty or is not functioning correctly, the fluid accumulates in the chamber, causing a build-up in the intraocular pressure. An increase in the IOP can lead to eye trauma or injury. A persistently high IOP can damage the optic nerve and lead to the development of glaucoma. 
 

Also Known As

Tonometry
 

Types

  • Pneumotonometry
  • Air-puff tonometry
  • Rebound tonometry
  • Applanation tonometry
  • Other forms of tonometry
     

Purpose

Intraocular pressure measurement is a component of a comprehensive eye exam and can be done in the eye doctor's office or at the eye clinic by:

  • a technician
  • an optometrist
  • an ophthalmologist
     

The doctor checks the IOP and maintains a record of any changes over time. Often, glaucoma is an insidious disease and displays no symptoms, and so regular eye exams are vital. Glaucoma can result in a loss of vision. Identifying high intraocular pressure and taking appropriate measures to lower it may help to prevent blindness that is caused by glaucoma. Early and regular screenings enable the doctor to protect a patient's eyesight and slow down vision loss.  

People with the following characteristics should go for regular IOP tests because they have higher chances of getting glaucoma: 

  • Over 40
  • Diabetic
  • Get migraines 
  • Have had an eye trauma
  • African, Asian or Hispanic 
  • The near-sighted or far-sighted  
  • Have a family member with glaucoma
  • Have circulation (blood flow) problems
  • Have corneas that are thin in the middle
     

Preparation & Expectation

Intraocular pressure measurements are painless and straightforward and only take a few seconds. If the doctor uses the "puff of air" method, the patient may experience a small amount of pressure against their eye. Some of the tests may require eye drops to be applied in the patient's eyes to numb them. 

Procedure

During the eye pressure exam, it is helpful for the patient to relax and breathe normally. There are various methods to measure eye pressure. The common ones include: 

Applanation tonometry-
Goldmann applanation tonometry is considered the "gold standard" eye pressure test.
The test requires numbing drops to anesthetize the patient's eyes. A little amount of non-toxic dye is put in the eye. The patient sits and positions their head into an instrument called a slit lamp with an applanation tonometer. A small tip lightly touches the patient's eye surface, and the device measures the eye pressure. The tool measures the eye pressure depending on the force used or needed to flatten a fixed section of the cornea softly.

The results of the test may be affected by severe factors, including too much or little dye in the eye and the corneal thickness or thinness. The measurements might be artificially low if the patient's cornea is thin. The thin cornea may be natural or caused by procedures such as laser corrective eye surgery. 

It is important to note that naturally thin corneas can be an independent risk factor for glaucoma. In such cases, the patient should have a corneal thickness exam. It can be done during the patient's initial visit and subsequent visits as part of a comprehensive eye exam.

Pneumotonometry-
It is an especially useful method for patients with scarred corneas. The procedure also requires numbing drops to anesthetize the patient's eyes. However, it doesn't need any dye. The test uses a measuring instrument known as a pneumatonometer whose readings are less influenced by the thickness of the cornea. 
 
Rebound tonometry-
 The method uses a small plastic-tipped probe bounces softly on the surface of the cornea. There are also portable measuring instruments such as iCare, that one can use at home. It is lightweight, easy to use, and suitable for children and patients who are unable to comply with the traditional eye pressure exams. Tonopen is a type of electronic indentation tonometry. It is also a portable and easy to use IOP test device. However, unlike iCare, the gadget requires the patient to use numbing drops.
 
Air-Puff Tonometry-
It is non-contact tonometry that employs a rapid air pulse to flatten (applanate) the cornea. The modern forms of the instruments correlate with Goldmann tonometry.
 
Other Forms of Tonometry-
New IOP test techniques are being developed, and there are different forms of eye pressure measurements that one may encounter.
 

Outcome

Because tonometry is a portion of a comprehensive eye exam, its results, together with other vision exam results help the doctor to have a better understanding of the patient's eye health. The doctor will also discuss the patient's health history and any symptoms the patient may have.

Eye pressure varies from person to person. The normal range is between 12 and 22 mmHg. Most patients diagnosed with glaucoma have an intraocular pressure that is over 21 mmHg. However, some patients get glaucoma with the IOP within the normal range. The doctor will determine the intraocular pressure range that is healthy for the patient.

Ocular hypertension is a situation where the patient's IOP is high, but the optic nerve looks normal. The patient may not exhibit any symptoms, but over time, could lead to glaucoma.

If the IOP test shows a patient has high intraocular pressure, the doctor will require the patient to go for regular testing for monitoring purposes. He/she may also decide to prescribe eye drops to lower the pressure. The eye drops help to protect the patient's optic nerve from damage and may save their sight.
 

Risks & Complications

Intraocular pressure measurement is an easy and straightforward eye test without risks or complications.