The role of intraocular pressure measurements in glaucoma management

Intraocular pressure (IOP) has an important role to play in the diagnosis of glaucoma and its treatment. Out of the multiple diagnostic criteria, IOP is the most easily measurable and reproducible. Inability on the part of many ophthalmologists to understand the importance of IOP measurement values from the right perspective results in glaucoma being managed inappropriately. The purpose of this editorial is to clarify and reiterate the exact role of IOP in the entire spectrum of glaucoma management.

High IOP does not always mean glaucoma. Similarly, low IOP does not mean that the person has no glaucoma. Broadly speaking, four groups of population can be identified based on IOP levels:

1. Normal IOP and no glaucoma

2. Ocular hypertension, i.e., the IOP is high, but the person has no glaucoma

3. Hypertensive glaucoma, or usually referred to as glaucoma i.e. the person is having glaucoma with high IOP and

4. Low tension glaucoma i.e. the IOP is within the so-called normal range but still the person has glaucoma.

Although IOP alone is not enough for the diagnosis of glaucoma, it clearly indicates that certain other factors along with IOP should be considered and combined for it.

Glaucoma is a group of eye diseases characterised by chronic progressive optic neuropathy with typical optic disc changes with corresponding field defects, wherein the IOP becomes an important risk factor. Many randomised controlled trials have confirmed that reduction of IOP in the treatment of glaucoma has helped in controlling disease progression. IOP is the only known risk factor in the course of glaucoma treatment, which, if modified in a positive way by reducing the IOP, shows positive results. How do we apply these observations in glaucoma management?

To begin with, the limited, but exact role of IOP measurement during the treatment should be clear in the mind of the treating physician. IOP is only a surrogate measure. The main purpose of treatment is to reduce or stall the progression of nerve damage, which IOP measurements do not indicate. IOP measurements only indicate whether the given intervention (surgery/medical/laser) has reduced the IOP to the extent of the mean reported IOP lowering effect as per the literature reports of that particular intervention. If it has not produced a considerable effect, it is advisable not to continue with that medication, and one should switch to other medications and follow the same course of action.

If the initial intervention had produced the desired effect on IOP, we can conclude that it has been effective and we should continue it. Once the effectiveness is established, the next step would be to judge whether it is sufficient or not. This is judged by the stability of structure (by optical coherence tomography [OCT] and disc evaluation) and stability of function (by perimetry), making it imperative to measure IOP, perform perimetry and structural assessment on OCT as well as a detailed fundus evaluation at regular intervals. Hence, it can be said that IOP measurements help us know the “efficacy” of any intervention, but the “sufficiency” of the intervention will have to be judged by the serial analysis of perimetry, OCT, disc photos and clinical evaluation of disc, followed by gonioscopy.

The gold standard for IOP measurement is, without doubt, applanation tonometry in normal conditions. Having said that, we must acknowledge a few other tonometers with their limited role that are used in certain special circumstances. The guest editorial in Indian Journal of Ophthalmology also affirms that a complex matrix needs to be developed to assess the progression in glaucoma rather than relying on IOP alone.^[1] To conclude, we will all agree that we do not treat IOP but utilise and apply the value of IOP measurements in a proper way in the entire spectrum of glaucoma management. I hope this editorial will help in utilising the value of IOP recordings in the correct perspective in the management of glaucoma.