Risk calculators can help guide treatment decisions for ocular hypertension

Issue: May 2008

ByMurray Fingeret, OD

Mrs. Jones, a 73-year-old African American female, comes to your office for a comprehensive eye examination. During the examination you measure her IOP as 25 mm Hg in each eye. Her optic nerves appear healthy and, based upon her having elevated IOPs, perimetry is performed, with the visual fields being full. What management should be recommended for this individual with ocular hypertension (OHTN)?

Murray Fingeret

Before the results of the Ocular Hypertension Treatment Study (OHTS) were published, there was little evidence as to the best course of management. Who to treat for ocular hypertension was not well defined. Some clinicians would initiate therapy to reduce the IOP when it reached 22 mm Hg while others would wait until the IOP was at least 25 mm Hg. Still others would not treat until the IOP was in the 30s. There was little consistency among clinicians at what point therapy should be initiated based on the IOP.

OHTS illustrated that in addition to IOP, other risk factors were equally important and included age, central corneal thickness (CCT), visual field status using pattern standard deviation (PSD) measure and vertical cup-to-disc ratio. Conventional wisdom has been that about 1% of individuals with ocular hypertension develop glaucomatous damage (optic nerve or visual field loss) in a given year.

OHTS revealed that some individuals have a risk of converting to glaucoma as high as 7% to 10% in a year, depending on the number of risks they have. Thus OHTN in a select group is not a benign condition with low risk but may be associated with a significant chance for glaucoma developing in the near future.

The purpose of performing risk assessment for each patient with OHTN is to use a statistical model to recognize which individuals are at greatest risk of converting to glaucoma and consider therapy specifically for those patients.

Roots in cardiovascular medicine

The concept of risk assessment comes from the field of cardiovascular medicine and is used to estimate a person’s overall risk for onset of a disease outcome based on multiple risk factors. It is based on evidence from clinical trials and long-term studies and is used to guide treatment decisions.

The objective of the Framingham Heart Study, which began about 60 years ago, was to better understand which risk factors are associated with the development of cardiovascular heart disease (CHD). By following a group of individuals free of disease at study entry over many years, it was found that a series of modifiable risk factors were associated with the development of CHD.

Other clinical trials have validated this work and have led to a better understanding of CHD risks, which include high blood pressure, elevated cholesterol levels, smoking, diabetes, obesity and physical inactivity. Internists and cardiologists use this information to understand the risk of their patient developing an event such as myocardial infarction or cerebrovascular accident. Therapy is instituted to modify one or several of the risk factors when the risk exceeds a certain level.

Data on glaucoma risk factors

Data is now available to identify risk factors associated with the conversion of OHTN to glaucoma. OHTS, with its rich data set, allowed the concept of risk assessment to be applied to individuals with OHTN, allowing the recognition of which individuals are at greatest risk and may be treated prophylactically. To a large extent, this is a disease prevention strategy being applied to individuals at highest risk.

While not comparable diseases, CHD and glaucoma have several similarities. Both are chronic conditions and both are associated with at least one modifiable risk factor, although CHD can result in sudden death.

Integrating this information on a case-by-case basis is difficult, which is why risk calculators can be valuable. They allow the clinician to produce a quantitative risk of disease for a specific patient rather than a qualitative assessment, leading to improved consistency.

Many clinicians believe that they can predict the risk for their patients in their head.

Mansberger and colleagues evaluated this issue, asking five glaucoma specialists to review a series of cases. Each clinician on average considered only three of five risk factors identified in OHTS, with a great deal of variability found in regard to the predicted risk. This indicates another advantage of a risk calculator: a reduction in personal bias.

Available risk calculators

The first of the risk calculators was developed by Steve Mansberger and is available at the Discoveries in Sight Web site (www.discoveriesinsight.org). It uses information from OHTS, with a formula similar to the Framingham Heart Study to assess risk.

Medeiros and Weinreb were involved in the next version of the risk assessment tool, which became known as the STAR (Scoring Tool to Assess Risk) calculator. Based upon their work from the Diagnostic Innovations in Glaucoma Study, the OHTS data set was validated against this cohort of comparable individuals.

The most recent risk tool is available from the OHTS Web site (http://ohts.wustl.edu/risk/calculator.html) and is based upon the OHTS data using the placebo arm of the European Glaucoma Prevention Study (EGPS) as the validation set.

Case study

Let us go back to the case described in the introduction. I would want more information than simply Mrs. Jones’ age and IOP. For a clinician to make the best decision, her CCT, vertical cup-to-disc ratio and visual field status are also needed. It turns out that her CCT is 515 µm OD and 520 µm OS, vertical cup-to-disc ratio is 0.6 OD and OS and the Humphrey Field Analyzer (Carl Zeiss Meditec, Dublin, Calif.) PSD measure is 1.8 dB OD and 1.9 dB OS.

I then used this information to calculate her risk of developing glaucomatous damage, which turned out to be 41% in 5 years. Using this risk calculation allows a discussion of the treatment options with the patient, with an understanding of what risk this individual has to develop glaucoma in the near future.

It may appear that the calculation of risk is a best guess for a patient; however, the calculators have been validated against a similar cohort, so in a large patient pool these assessments are accurate. Still, for a given patient, there is a potential error involved with any assessment.

Several measurements needed for risk calculation

A risk calculation is determined for a patient, not per eye, with several measurements needed for most parameters. IOP is based upon the use of the Goldmann applanation tonometer, averaging three IOP readings performed at different visits. CCT measurements are also based upon an average of three measurements for each eye, while visual field PSD measures should come from two reliable, full visual fields. With the OHTS-EGPS calculator, either a Humphrey Field Analyzer or Octopus perimeter (Haag-Streit, Mason, Ohio) may be used.

Fields with artifacts from a trial lens or eyelid or with diffuse loss associated with learning should not be used. Also, risk assessment is indicated for individuals with ocular hypertension and not for those with suspicious optic nerves with the IOP in the normal range.

When OHTS was released several years ago, diabetes was seen as a positive risk factor and used in the risk assessment model. Since then, new information has found that diabetes is neither protective nor harmful for the development of glaucoma and is no longer part of the risk calculation.

When should therapy be initiated?

A group of experts convened as part of a think tank on risk assessment was asked the question: “At what risk should therapy be initiated for OHTN?” The consensus was that a 5-year risk of 5% was low and should carry the recommendation of monitoring for this individual. When a risk of 5% to 15% was present, this was seen as moderate and would have the recommendation of considering treatment. A risk of 15% or higher was seen as high and would suggest a recommendation of treatment for these individuals.

Still, this is a guideline used along with the other information when deciding whether therapy is indicated. Patient input, ocular and family ocular history, medical history and age are other important variables.

Evaluating patients’ life expectancy

The American Geriatrics Society recommends that a patient’s life expectancy be incorporated in medical decision making. Thus, the decision to treat a chronic disease must consider the possibility that a person may die before developing any symptoms from the condition.

The Charleson Index is one method of evaluating life expectancy. In this case, weights are assigned for certain diseases. Myocardial infarction or diabetes is assigned a score of 1, severe liver disease a score of 3, and metastatic solid tumor or AIDS a score of 6. The score is computed for a person and then compared against a table, using a person’s age as the other variable.

A person with a Charleson index of 6 and an age of 65 has a 5-year predicted risk of dying of 86.5%, while a person age 75 with a Charleson Index of 1 has a 5-year predicted risk of dying of 28.1%. A person with a high 5-year risk of developing glaucoma, indicating the need for therapy, may have a much lower risk when life expectancy is taken in account. Individuals with short life expectancy may not require therapy at all.

On the other hand, risks increase over time, as in the case of a 40-year-old in excellent health with ocular hypertension and an expected life expectancy of 40+ years. If this hypothetical case has a 5-year risk of converting to glaucoma of 12%, the risk is far, far greater when taking into account the person’s long expected lifetime.

The decision to treat is exceedingly complex, based upon more than simply IOP. In addition, information used with the risk calculators is evolving over time. Just as cholesterol levels are changing in regards to the CHD risk model, so is IOP and other factors in regard to the OHTN risk calculator.

One way a clinician may make an informed decision is to use one of the risk assessment tools to recognize which individuals are at greatest risk and deserve treatment. Still, the risk calculation should not be used as a cookbook but, rather, as another piece of information, along with patient input, family history and other factors when deciding if therapy for ocular hypertension is indicated.

For more information:

Murray Fingeret, OD, is chief of the optometry section at the Department of Veterans’ Affairs Medical Center in Brooklyn and Saint Albans, N.Y., and a professor at SUNY College of Optometry. He is also a member of the Primary Care Optometry News Editorial Board. He may be contacted at St. Albans VA Hospital, Linden Blvd. and 179th St., St. Albans, NY 11425; (718) 298-8498; fax: (516) 569-3566; e-mail: murrayf@optonline.net.

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