PACHYMETRY
Pachymetry:

Since the publication of the Ocular Hypertension Treatment Study (OHTS), central corneal thickness as a significant risk factor for glaucoma has been central to the discussion of its management. The study defines several risk factors that increase the probability of progression to glaucoma including older age, higher IOP, greater pattern standard deviation, larger vertical cup/disc ratio and thinner central corneas. What is remarkable is the strength of the correlation to the development of glaucoma over time and the relative lack of knowledge in this area prior to the publication of this seminal study.

Changing glaucoma diagnosis

Data from the study revealed that central corneal thickness (CCT) varied by race, with African Americans having corneas approximately 25 microns thinner than Caucasians in a cohort of 1,636 patients. The study also revealed that a difference of 30 microns (555 vs. 585) increased a patient's risk of developing glaucoma three fold. Over the five-year follow-up, results showed patients with thinner corneas (<555 microns) have between 13% and 30% greater risk compared with those who had a corneal thickness of 588 microns, dependent on IOP. Since the initial discovery of this concept, numerous articles have described the power of this tool in the identification and treatment of the entire spectrum of the disease from ocular hypertension to glaucoma. The literature shows some general trends and also defines the concept so we can better articulate and apply the information.

Broader implications

The OHTS study showed us the value of pachymetry only in ocular hypertensive patients and did not describe its use in glaucoma suspects or glaucoma patients. Additionally, the OHTS did not address the dynamics of CCT and its interpretation. Since the initial publication, several authors have analyzed the role of CCT in other areas and found it to be an equally valuable tool in the diagnosis and treatment of all forms of glaucoma.

Clinical evidence

In one study, pachymetry was performed on 188 patients with existing diagnoses of ocular hypertension, glaucoma suspect and existing glaucoma. The addition of the CCT measurement changed either the diagnosis or therapy in 70% of patients.

Another study demonstrated a direct relationship between thinner corneas and more advanced disease in patients undergoing glaucoma therapy. Results showed that the Advanced Glaucoma Intervention Score (AGIS) was higher in patients with thinner corneas. Therefore, it could be used as a predictor of the potential rate of progression and/or risk for loss of vision in patients currently undergoing glaucoma therapy. While OHTS defined risk ratios for ocular hypertensives, this data suggests that there is a direct relationship in glaucoma patients as well. In clinical practice this information can be used to determine the level of disease, set IOP goal levels and intensity of follow-up and determine the need for additional testing.

Another study reviewed the relationship between varying forms of glaucoma (COAG, normal tension, and pseudoexfoliative) and ocular hypertension. All types of glaucoma patients showed a decrease in corneal thickness compared with ocular hypertensive patients. Yet another study demonstrated that patients with decreased corneal thickness had an increased risk of early loss of visual field function with Frequency Doubling Technology (FDT) compared with patients who have thicker corneas.

You must also consider racial differences. Studies have identified population groups with statistically thinner corneas. One such study showed that Afro-Caribbeans' central thickness measurements were thinner than African Americans', and considerably thinner than Caucasians'. In correlation, the Barbados Eye Study demonstrated that Caribbean patients have rates of glaucoma that exceed any levels previously described in United States populations.

Accurate interpretation

The application of this technology in the clinical setting is relatively straightforward and can be accomplished with minimal investment on your part. There are, however, several caveats to consider when interpreting the measurements and their application to the diagnosis of OHTN, glaucoma suspect and glaucoma patients. One study shows that CCT stabilizes approximately one hour after awakening and remains constant throughout the day after that. Another demonstrated that CCT remains constant regardless of the patient's diurnal fluctuation of IOP. Finally, a recent study revealed that refractive error plays little role in the level of CCT, but that IOP increases with increasing myopia.

Clinically, there are several concerns relative to accurate interpretation of CCT that you'll need to consider when using this technology so as not to over- or under-interpret the values. These include "pseudo" CCT findings, as found in patients with keratoconus, forme fruste disease and corneal dystrophies such as Fuchs' and advanced anterior basement membrane disease. Additionally, patients with corneal scars or herpetic disciform disease can show significant variability from true readings. The best way to calculate is to evaluate the fellow eye and extrapolate the data based on the AAO 2002 presentation that shows CCT varies less than 10 microns in over 98% of eyes in this group of patients.

Additional considerations

Other cases that require interpretation are patients who present wearing extended-wear contact lenses or those in whom you're concerned about the potential for corneal edema. Less than 7% edema produces little to no visible evidence of corneal change. However, 7% of 550 is equal to 38.5 microns. A 30-micron change in the OHTS study imparted a three-fold change in risk factor for the disease. Given this, it's reasonable to switch patients to daily-wear before you make a final decision.

Another concern is the relationship of CCT and the post refractive surgical patient. With over 1.5 million procedures performed each year, there is an ever-increasing pool of patients in whom this measurement is eliminated as a tool to diagnose OHTN and glaucoma suspect status or the potential for risk in active glaucoma cases. Unfortunately the only alternative is to obtain the original CCT before LASIK or PRK, otherwise the post surgical measurements have little or no value. In our office, we give the patient a card with both their pre-operative pachymetry and pre-operative keratometry readings prior to refractive surgery. This assures that they will have this valuable information available in the future when concerns regarding glaucoma or cataract develop.

Pachymetry is a remarkable tool and has rapidly become a mainstay of clinical practice in the area of glaucoma. The question of its status as a standard-of-care has, to a large degree, been answered by the clinical community. The American Academy of Ophthalmology deemed CCT an integral part of the glaucoma evaluation and along with perimetry, IOP, gonioscopy and optic nerve assessment, is an essential element in the evaluation of the glaucoma patient. The American Optometric Association incorporated CCT into its recommendations for Preferred Practice Patterns. Medicare issued a unique reimbursement code more rapidly than for any other technology in the history of ophthalmic science.

While all of these acknowledgements indicate the importance of CCT in glaucoma, the real measure of the standard of care is adoption of a technology by the community of clinicians in the care of their patients. Pachymetry has become a mainstay of practice and is the most rapidly accepted technology in the history of eye care. Despite no formal designation as a "standard-of-care" having been issued, there is no doubt that among practitioners this technology is something that competent clinicians cannot do without when it comes to diagnosing and managing their patients.

Dr. Thimons is a nationally and internationally acclaimed speaker and serves as medical director at Ophthalmic Consultants.