How is Glaucoma Diagnosed??

[13/03/2012]

How is Glaucoma Diagnosed??

Intraocular pressure (IOP)

As mentioned previously, glaucoma is associated with raised intraocular pressure. However, diagnosing someone with glaucoma is not as simple as measuring the pressure as it is now understood that there isn't a standard "normal" pressure. Conventionally, a normal level of 21mmHg was the upper limit beyond which someone would have glaucoma. But as our understanding of this diseases increases, we have found that damage to the optic nerve can also occur at pressures less than 21mmHg. As such, to diagnose glaucoma, the IOP is only one aspect which is looked at.

The standard method of IOP measurement is with an applanation tonometer - this is the most accurate measure. There are of course many other methods (some shown below) which all have their advantages and disadvantages:

Goldmann Tonometry Tonopen Air puff tonometer

 

Gonioscopy

This is necessary to assess the drainage angle of the eye to differentiate between open and closed angle glaucoma as the treatment can be quite different.



This involves gently placing a special lens on the surface of the eye. It is not painful as the eye is first anesthetized with eyedrops first.

Central Corneal Thickness (CCT)

This parameter is important as it affects how we measure the IOP. Studies have shown that the thicker the cornea, the higher the measurement of IOP and the thinner the cornea, the lower the IOP measured. As such, it is important to know the thickness of the cornea to decide if the pressure is indeed really abnormal or is it because the cornea thickness has artificially skewed the IOP measurements.

Visual field assessment

This is an automated test which assesses the extent of functional loss of vision (if any) as a consequence of damage to the optic nerve. In early stages, there may be minimal loss which affects the peripheral vision but in advanced stages, only tunnel vision may remain.



(Humphrey visual field assessment on the left and the printout of visual field on the right)

Optic disc photos

Photographs of the optic discs are taken to serve as a baseline for future comparison. Damaged optic discs become pale and the 'cups' (hollow in the middle of disc) become large.



Optical Coherence Tomography

This is a sophisticated piece of equipment used to analyze the nerve fibre layer. It uses a specialized laser to obtain very precise and high definition images which helps to quantify the amount of damage (if any) to the optic nerve.



All these tests are necessary to thoroughly evaluate a patient for glaucoma. There should be no shortcuts as a diagnosis of glaucoma carries with it the implications of life long treatment with medication. As ophthalmologists, we have to be certain and with the advanced technology we have now, it is imperative that we backup our clinical skills with evidence of disease before condemning a patient to a lifetime of medication.