Calculating Glaucoma Risk
BY LEO SEMES, OD, FAAO
An evolving strategy for glaucoma management involves the use of risk factors in clinical decision making. A risk calculator can estimate the risk of converting from ocular hypertension (OHT) to glaucoma.
The concept of risk evaluation grew from the Ocular Hypertension Treatment Study (OHTS), which found that risk of conversion decreased about 50 percent with treatment. The researchers suggested that patients at highest risk were those who had IOP >25.75, CCT <555μm and vertical C/D >0.5. This comprised the "Rule of Fives," which represented the first semi-quantitative approach to assessing the risk of developing glaucoma for patients who have elevated IOP but normal optic discs and visual fields (ocular hypertensives).
Refining the Calculation
Since this initial attempt, Mansberger offered a more detailed scheme for conversion from OHT to glaucoma (discoveriesinsight.org; click on risk calculator). This paradigm accounts for structural and functional characteristics of the patient. The structural items include C/D, CCT and IOP, while the global index of pattern standard deviation (PSD) represents functional deficit. So, this calculator represents a more sophisticated version of the initial risk profiling suggested from the OHTS results.
One item that deserves mentioning before we look at the profile
itself is diabetic status. The OHTS determined diabetes to be protective (reducing
the risk) against conversion. This is an artifact of the methodology; researchers
excluded patients from the study if they had diabetic retinopathy and determined
no blood sugar nor A1c levels. And patients could participate if they denied being
diabetic on questioning. Diabetes probably represents an increased risk of glauco-
The functional representation is the PSD value from the visual field results. With each of these items plugged into the calculator, you can hit the "calculate risk" button and return a number that estimates the patient's risk of conversion from OHT to glaucoma over the next five years.
The interpretation guidelines are as follows:
Risk: <5 percent little evidence for a treatment recommendation.
5 percent to 15 percent consider a treatment recommendation, weighing other factors heavily (family member who has glaucoma, migraine, myopia, etc.).
>15 percent strongly consider treatment recommendation.
Medeiros et al (2005) have validated these data.
Using the Risk Calculator
The introduction and validation of a risk calculator for conversion from OHT to glaucoma can assist us in making clinical decisions. It's important to recognize the caveats and limitations of the calculator. It won't provide an answer, only additional clinical decision-making power. You must ensure that the results you enter are from accurate data. You may have to run more than one visual field to obtain reliable and consistent results, for example.
Another potential application of risk calculators would be to change the established baseline values. If your patient's C/D is 0.2 and you want to see the risk when the C/D is 0.4, simply hold all other values constant and recalculate. This may reassure the patient and give you additional comfort in your clinical decision.
A few additional caveats apply:
The risk calculators are not to be applied to glaucoma suspects or normal tension patients.
Remember to use average results to generate risk.
This is the first model for risk assessment in ocular hypertension, and refinements are sure to improve our recommendations for our patients.
The validated glaucoma risk calculator is the Scoring Tool for Assessment of Risk (STAR), available from Pfizer Ophthalmics.
For references, visit www.clspectrum.com/references.asp and click on document #126.
Dr. Semes is an associate professor at the University of Alabama at Birmingham School of Optometry.