Hi EH Ohi,
The current definition of glaucoma does not include IOP as an criteria .It is retinal ganglion cell loss,with optic disc and field changes . In Normal tension glaucoma disc changes can be present at a normal IOP. Usually IOP ranged from 10 to 21 mmHg is considered normal ,but disc changes ,RNFL changes and risk factors like family history, thin CCT, diurnal variation etc have to be evaluated to decide the management
Hi Divya,
Thank you for your answer. Based on what you say, would it be fair to say that by the time glaucoma is diagnosed, there would already have been some high risk to vision loss?
I understand that in normal tension glaucoma, there is no reported elevation in IOP. In the case of primary open angle glaucoma, is IOP one of the criteria used for diagnosis?
I am not very familiar with some of the abbreviations you used. Could you let me know what RNFL and CCT means?
Thank you.
Ean Hin
RNFL stands for retinal nerve fiber layer thickness while CCT stands for central corneal thickness.
IOP is an important criteria in diagnosis of POAG in collaboration with disc and visual field changes .Some patients diagnosed as Normal tension glaucoma may be rediagnosed as open angle if IOP spike is noted in diurnal variation of IOP.IOP is important as it is the only modifyable risk factor in glaucoma. Pretreatment IOP is important for calculation of target IOP in collaboration with disc and visual field ichanges. RNFL is retinal nerve thickness measured by GDxr nerve fibre analyser ,it can pick up pre-perimeteric glaucoma .CCT is central corneal thickness measured by pachymects .CCT affects the IOP measured by goldmann applanation tonometer as goldmann caliberated for CCT of 540 microns.Thin CCT further increases the actual IOP measured after correction whereas thick CCT is protective .CCT is an important entity contributed by OHTS(Ocular hypertension treatment study)
Screenning for Primary open-angle glaucoma ( POAG) consists of Intraocular Pressure (IOP) measurement, visual field testing (Perimetry) and optic nerve evaluation by ophthalmoscopy
Glaucoma can occur at any eye pressure. However, the higher the eye pressure, the higher the risk for developing glaucoma.
It has been properly pointed out that eye pressure is not a diagnostic criterion for glaucoma. Glaucoma is diagnosed by identifying optic nerve damage (i.e. cupping of the optic nerve head) and typical patterns of visual field loss.
Ocular hypertension is elevated eye pressure with no signs of glaucoma (no optic nerve damage and no visual field loss). The Ocular Hypertension Treatment Study (OHTS) investigated the efficacy of treating ocular hypertension to prevent the development of glaucoma (to prevent optic nerve damage and visual field loss). This study showed that lowering eye pressure does lower risk for developing glaucoma.
However, a large proportion of those with ocular hypertension will never develop glaucoma. One can use data from the OHTS studies to help guide decisions about when to treat ocular hypertension. There is a risk calculator that estimates the risk for developing glaucoma in 5 years given a set of clinical parameters (i.e. age, eye pressure, corneal thickness, cup-to-disc ratio, visual field test results).
http://ohts.wustl.edu/risk/calculator.html
Bottom line, clinical judgement is needed to evaluate eye pressure data and to decide when to treat ocular hypertension with pressure lowering medicines.
If glaucoma has been diagnosed, practitioners typically set a target pressure at which they believe the glaucoma is likely to be stable and try to bring the patient's eye pressure to the target using medicines, lasers, and/or surgeries. Still lower targets may be set if a patient has progressive disease while at a given target pressure.
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