Piece CCT Into the Glaucoma Puzzle

Pachymetry is now part of the standard of care for glaucoma patients. Here’s how it can affect your diagnosis and management decisions.

Gregory S. Black, O.D., and Diane Calderon, O.D.

A healthy 53-year-old black male presents for a routine exam. His intraocular pressure measures 23mm Hg O.U and his cup-to-disc ratios measure 0.70 x 0.70 O.U. His rim tissue is healthy, and no nerve fiber layer defects are detected. Automated visual field testing and nerve fiber analysis reveal no glaucomatous defects. Gonioscopy reveals open angles with trace trabecular meshwork pigmentation with no peripheral anterior synchiae or angle recession. He has no family history of glaucoma. How would you manage this patient if his CCT measured 500µm? Or 600µm?

The Ocular Hypertension Treatment Study (OHTS) has changed the way eye care practitioners diagnose and treat glaucoma. Eye care practitioners now consider central corneal thickness (CCT) measurements an important piece of the glaucoma puzzle, as these measurements may affect your diagnosis and management decisions.

 
CCT Findings

The objective of OHTS was to determine the safety and efficacy of topical hypotensive medications in delaying or preventing the onset of primary open-angle glaucoma (POAG) in patients who had ocular hypertension.1 Researchers found that:

• Patients with ocular hypertension who received treatment had a 4.4% chance of developing POAG compared with 9.5% in patients who were not treated.2

• Patients with ocular hypertension whose CCT measurements were 555µm or less had a three-fold greater risk of developing POAG than those with measurements greater than 588µm.2

• Participants who developed glaucoma had an average CCT of 553µm vs. 574µm in those who did not develop glaucoma.2

• Black patients had an average CCT measurement of 556µm and white patients had an average measurement of 579µm.1

• Overall, CCT was a strong predictor for progression from ocular hypertension to glaucoma.1

Another study found that a lower CCT is associated with a worse Advanced Glaucoma Intervention Study (AGIS) score (i.e., a measure of visual field defects), decreased mean deviation of visual field, and increased vertical and horizontal cup-to-disc ratios at the initial examination.3 A low CCT was the only significant predictor of the AGIS score, which improved 0.31 points when CCT increased by 10µm.3 Therefore, thinner CCT measurements may also predict an increased incidence of glaucoma.

Average CCT

Average CCT measurements vary according to the type of instrument used and the age and race of the patient. In one literature review, researchers found an average CCT of 534µm in healthy eyes in all pachymetry studies, 530µm in slit lamp-based pachymetry studies and 545µm in ultrasound-based studies.4 Researchers have also found that CCT averages approximately 580µm in full-term newborns, but stabilizes to adult values by age three.5 Other studies found an average CCT of 547µm in Hispanic patients and 530µm in black patients.6,7 Additionally, in a study of patients undergoing refractive surgery, researchers reported the following average CCTs: 535µm in black patients, 550µm in Asian patients, 551µm in Hispanic patients and 553µm in white patients.8

CCT and IOP

Goldmann applanation tonometry is typically the method of choice for measuring IOP, but CCT may affect the results. When first describing the tonometer, Goldmann and Schmidt assumed a CCT of 500µm.9 They believed that CCT measurements deviated little in the absence of corneal disease, but postulated that CCT might have some correlation to readings obtained via applanation tonometry. They reported first that CCT and scleral rigidity variations may affect IOP readings on applanation tonometry.

Additional studies have demonstrated that other possible effects of CCT on IOP measurements with the Goldmann tonometer include:

• IOP measurements with the Goldmann applanation tonometer are accurate only when CCT measures 520µm.10

• IOP measurements are over- or underestimated by approximately 5mm Hg for every 70µm of deviation from 520µm.10

• A systematic error occurs in Goldmann tonometry if CCT is greater or less than 520µm and due to corneal rigidity.11

Systematic errors of Goldmann tonometry tend to occur at very high or low CCT ranges. A thick cornea secondary to corneal edema may even demonstrate a negative IOP when correction factors are calculated. Therefore, rule out corneal disease if there is an asymmetry of 25µm or greater.6

Central corneal thickness can affect the results of Goldmann applanation tonometry.

Use of Pachymetry

Pachymetry has become an important test for diagnosing and managing glaucoma since the importance of CCT was discovered. One study found that CCT led to a change in medical therapy for 8.5% of patients, a change in laser therapy decisions for 2.1% of patients, and a change in glaucoma surgery decisions for 3.2% of patients.12 In the same study, IOP was adjusted 1.5mm Hg or more for 55.9% of patients due to CCT, 1.5 to 3mm Hg for 35.6% of patients and more than 3mm Hg for 20.2% of patients.

Pachymetry may be helpful to practitioners when:

• Patients have elevated IOP but normal optic nerve appearance and visual field or nerve fiber analysis.

• Patients have normal IOP but suspicious optic nerve appearance and borderline visual field or nerve fiber analysis.

• Patients have glaucoma.

You can use pachymetry readings as an additional piece of information to help you diagnose and manage glaucoma. For example, if the patient in the case presented above has a thick CCT (>600µm) and an IOP of 23mm Hg, his true IOP is likely lower than the Goldmann tonometer measurement so you can monitor him. If the patient has a CCT of 500µm, consideration of medical therapy would be warranted prophylactically.

The most commonly used methods for obtaining CCT measurements include ultrasonic pachymetry, optical ray path analysis (i.e., Orbscan, Bausch & Lomb), and optical coherence tomography (OCT). Although researchers have found a high correlation among the three methods, ultrasonic pachymetry is the most commonly used method.13

Although ultrasonic pachymetry can efficiently measure corneal thickness, it does have disadvantages. These include required physical contact, use of anesthesia and technician error. The accuracy and reproducibility of ultrasonic pachymetry depends on precise probe placement.14 You must locate the center of the cornea and apply the probe perpendicularly. Locating the same central point in serial examinations may be difficult, even for experienced technicians.

A recent study reported that one CCT measurement may no longer be satisfactory for long-term patient follow-up.14 These researchers recommended you perform at least one repeat measurement at a follow-up appointment to ascertain whether the initial CCT value was accurate.

Meanwhile, another study showed that both intra- and inter-observer reproducibility of CCT measurements with ultrasonic pachymetry was high.15 This study suggests that well-trained technicians can accurately perform ultrasonic pachymetry and obtain repeatable and reliable objective CCT measurements.

Interpretation of CCT measurements varies among practitioners. Some practitioners alter measurements based on a pachymeter adjustment factor, whereas others classify measurements as thin, average or thick. Practitioners usually decide how to interpret CCT measurements based on whether they believe that deviations from normal CCT mask true IOPs measured by Goldmann applanation tonometry or that deviations in CCT are an independent risk factor for glaucoma.

Since the results of OHTS were published, physicians have debated whether IOP should be adjusted or corrected based on CCT measurements. Currently, no single algorithm for adjusting IOP based on pachymetry measurements is accepted because the relationship between IOP and CCT is believed to be nonlinear.16 Several researchers have studied this relationship and derived “correction factors.” For example, in one study, researchers calculated a correction of 2.5mm Hg for each 50µm change in CCT.4 Other researchers found a correction factor of approximately 2.0mm Hg for each 100µm difference in CCT.17 A recent study found that both the 5mm Hg correction model and a mathematical model overestimate the effect of CCT on IOP measurements.18

The variation in these correction factors illustrates the complexity of the relationship between CCT and IOP. The current conversion nomograms provide practitioners with only an estimate of the range of effects that CCT has on IOP.19 Until further research can account for the discrepancies, practitioners should be cautious and not adjust IOP, but use the conversion nomograms as guidelines. These guidelines can help you classify CCT measurements as thin, average or thick. Such a classification can provide additional information on IOP that might help you determine a patient’s risk of progression to glaucoma or need for more aggressive therapy to prevent glaucoma progression.

 OHTS taught us that patients who have a thin CCT have a greater chance of developing glaucoma. Therefore, pachymetry measurements provide us with another piece of the puzzle to help us develop better management strategies for glaucoma patients and suspects.

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2. Gordon MO, Beiser JA, Brandt JD, et al. The Ocular Hypertension Treatment Study: baseline factors that predict the onset of primary open-angle glaucoma. Arch Ophthalmol 2002 Jun;120(6):714-20.

3. Herndon LW, Weizer JS, Stinnett SS. Central corneal thickness as a risk factor for advanced glaucoma damage. Arch Ophthalmol 2004 Jan;122(1):17-21.

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5. Doughty MJ, Laiquzzaman M, Muller A, et al. Central corneal thickness in European (white) individuals, especially children and the elderly, and assessment of its possible importance in clinical measures of intraocular pressure. Ophthal Physiol Opt 2002;22:491-504.

6. Hahn S, Azen S, Ying-Lai M, Varma R, Los Angeles Latino Eye Study Group. Invest Ophthalmol Vis Sci 2003; 44:1508-12.

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10. Ehlers N, Bramsen T, Sperling S. Applanation tonometry and central corneal thickness. Acta Ophthalmol (Copenh) 1975 Mar;53(1):34-43.

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13. Fishman GR, Pons ME, Seedor JA, et al. Assessment of central corneal thickness using optical coherence tomography. J Cataract Refract Surg 2005 Apr;31(4):707-11.

14. Shildkrot Y, Liebmann JM, Fabijanczyk B, et al. Central corneal thickness measurement in clinical practice. J Glaucoma 2005 Oct;14(5):331.

15. Miglior S, Albe E, Guareschi M, et al. Intraobserver and interobserver reproducibility in the evaluation of ultrasonic pachymetry measurements of central corneal thickness. Br J Ophthalmol 2004 Feb;88(2):174-7.

16. Brandt JD, Beiser JA, Kass MA, et al. Central corneal thickness in the Ocular Hypertension Treatment Study. Ophthalmology 2001 Oct;108(10):1779-88.

17. Whitacre MM, Stein RA, Hassanein K. The effect of corneal thickness on applanation tonometry. Am J Ophthalmol 1993 May;115(5):592-6.

18. Gunvant P, O’Leary DJ, Baskaran M, et al. Evaluation of tonometric correction factors. J Glaucoma 2005;14:337-43.

19. Wunder H. Embrace measurement of corneal thickness. www.revophth.com/index.asp?page=1_533.htm. (Accessed October 14, 2005).

20. Shah S, Spedding C, Bhojwani R, et al. Assessment of the diurnal variation in central corneal thickness and intraocular pressure for patients with suspected glaucoma. Ophthalmology 2000 Jun;107(6):1191-3.

21. Pourjavan S, Deghislage C, Van Malderen L, Zeyen T. Pachymetry before or after applanation tonometry: does it matter? Bull Soc Belge Ophtalmol 2005;296:51-5.

22. Wickham L, Edmunds B, Murdoch IE. Central corneal thickness: will one reading suffice? Ophthalmology 2005 Feb;112(2):225-8.