glaucoma
OHTS One Year Later
The Ocular Hypertension Treatment Study (OHTS) was hailed as a landmark upon publication -- how has it changed our treatment of glaucoma?
BY J. JAMES THIMONS, O.D., F.A.A.O., Fairfield, Conn.

The Ocular Hypertension Treatment Study (OHTS) is now more than one year out from publication and has had more impact on the thinking behind the primary care management of glaucoma than any study in recent history. Now that we've had the opportunity to absorb the information it brought us, the real challenge is to translate it into improved care for our patients.

What's it worth?

As with all studies, the highlights are what are remembered -- the real value, however, is in understanding the nuances and how we can use them to improve our decision making.

For example, while one of the most notable features in OHTS was the discovery of the relationship between central corneal thickness and the progression to glaucoma, other factors were equally important and deserve our attention when we consider therapy. Additionally, understanding the general construction of the study and it's goals and objectives is critical to maximizing it's use on our glaucoma patients.

Examining OHTS

OHTS was developed to assess the role of intraocular pressure (IOP) in glaucoma as it pertained to patients have show no evidence of optic nerve damage or visual field loss.

One of the interesting aspects of the study's construction that's frequently overlooked is the level of IOP permitted at entry: IOP of 24 mmHg to 32 mmHg was required in one eye and the other eye was permitted to be between 21 mmHg and 32 mmHg.

The study lasted five years during which time researchers randomized the patients into two groups -- those who they managed medically with all available topical agents and those who they simply monitored.

Think twice about IOP

While the data from the OHTS study showed a distinct advantage to therapy, it also showed that elevated IOP is not as dangerous in the short term as many of us once thought. When we analyzed the trend curves over the first year, there was little difference in the rate of glaucoma development in the treated vs. the untreated group.

This is valuable information because it confirms that we can take the time necessary to fully evaluate the patient's status (serial IOPs, optic nerve assessment, visual fields, central corneal thickness [CCT], etc.) before rushing into treatment and committing these patients to a lifetime of therapy that may not have been necessary.

Race introduces a twist

Another intriguing lesson from the study is the difference in the rate of development of glaucoma in African Americans vs. other racial groups. The overall rate in the study was 4.4% in the treated group vs. 9.5% in the observation group. The rate in the subsection of the study that observed African Americans vs. all others was 6.9 % for African Americans vs. 3.6% for others in the treated group, and 12.7% vs. 10.2% in the observation group.

The differentiation in rates is significant information in the doctor's consideration to institute therapy; given equality in other factors, the data supports treatment in this sub-group that may not have been considered in the overall population.

Searching for risk

As most clinicians are aware, the study cohort was composed of 1,636 individuals aged 40 to 80 years who represented a normal population distribution, including 25% African American. One of the goals of the OHTS was to identify variables that represented risk for patients who had ocular hypertension and that we could use in the consideration for therapy.

The multivariate analysis demonstrated these factors:

• higher age
• larger vertical or horizontal cup-to-disc ratio
• higher IOP
• greater pattern standard deviation (PSD)
• thinner CCT.

While several of these factors aren't surprising, the role of the visual fields, specifically PSD, had not been previously noted in the development of glaucoma. The discovery of the relationship to CCT is monumental on a relative scale.

PSD is a basic measure of performance of a patient on visual field assessment after it's been adjusted for variables such as lens changes, refractive error, etc.

Typically, we've used this information to assess the validity of patient's outcomes; however, it hasn't been applied to the initial consideration for treatment. OHTS used Humphrey Visual Field analysis with full 30-2 algorithms and showed that patients with PSD levels that exceed the normal range are more likely to progress to primary open-angle glaucoma.

For clinicians, the implication of this data is that we can use this aspect of the patients' visual field information as a tool in the decision-making process -- in those cases were other factors don't clearly define the need for therapy, this component may be the deciding factor.

OHTS's shining moment

As previously mentioned, every study has a great moment: for the OHTS, it was the discovery of the role of CCT in glaucoma. Most of the original work in this area was done in the refractive surgery field because of concern over the alteration of CCT and how it would influence glaucoma diagnosis in the future. The OHTS was the first to proactively assess the role of CCT in the diagnosis and decision for therapy.

The data are remarkable in that they demonstrate a clear and almost linear relationship between CCT and IOP. The study found that the mean CCT was 578 +/- 39.0 µm and that 24% of the patients had CCTs of greater than 600 µm. Analysis of the subgroup of African Americans showed that the mean CCT was 555.7 +/- 40.0 µm (23 µm thinner).

Other factors associated with greater CCT were lower age, female gender and diabetes. It's interesting to note that the average corneal thickness for all patients in the general population is approximately 545 µm.

One of the points this data demonstrates is that the patient base in OHTS was a thick cornea group and that we should look at the patients we treated for elevated IOP before this information was available to verify that they in fact warrant continued therapy.

The other crucial lesson from this data is that CCTs must become part of our baseline glaucoma assessment in all ocular hypertensive patients and in the majority of our glaucoma suspects. For example, a patient with a pachymetry of 555 µm is three times more likely to develop glaucoma than one who has a CCT of 588 µm.

Another measure of the importance of this data is a recent post-OHTS study of pachymetry showing that in a population of ocular hypertensives and glaucoma suspects, approximately 40% of the diagnoses were influenced by the pachymetry measurements independently of other variables.

O.D.s, stay on your toes

OHTS has shown itself to be one of the most important publications in the field of glaucoma in recent years. The data it has generated are simply the beginning of the knowledge that we will derive. Clinicians who manage patients who have glaucoma need to stay abreast of the many areas in which the OHTS study will affect the care of our patients in the future.