Correlation of central corneal thickness and Goldmann applanation tonometry among Filipinos

ACCURATE measurement of intraocular pressure (IOP) is essential not only in the diagnosis and management of glaucoma, but also in the postoperative monitoring of those who have undergone intraocular surgery. Currently, Goldmann applanation tonometry is the gold standard for clinical measurement of IOP. It works on the principle that the pressure inside an ideal sphere is equal to the force necessary to flatten its surface, divided by the area of flattening.1 In calibrating this tonometer, Goldmann assumed a standard central corneal thickness (CCT) of 0.55 µm.1 He predicted that variations from this standard corneal thickness, which he assumed to be rare, would be a source of error in measuring IOP. Currently, there is accumulating
evidence that Goldmann’s theoretical prediction is correct. Several studies involving simultaneous IOP measurements by applanation tonometry and by manometry have demonstrated that thicker corneas yield an overestimation of IOP by applanation.2-4 Population-based studies have also demonstrated a positive correlation between CCT and IOP measured by Goldmann applanation tonometry5, 6 or Tono-Pen.7 A
recent metaanalysis of IOP measured by applanation and CCT, which pooled 133 data sets, revealed a statistically significant correlation between the two.8 Studies involving patients who have undergone Photorefractive Keratectomy (PRK)9-13 and Laser in Situ Keratomeleusis (LASIK)14 have demonstrated a corresponding drop in postoperative IOP with the cornea ablated. Numerous studies have shown that patients diagnosed with ocular hypertension have significantly thicker corneas compared with normal subjects, suggesting that IOP in this group of patients is being overestimated. 15-18 Similarly, patients diagnosed with normal-tension glaucoma have thinner corneas than normal subjects, suggesting that IOP in this group of patients is being underestimated.16, 18, 19, 20 Given these, several authors have recommended that CCT be considered in interpreting IOP or adjustments be made in IOP based on CCT.2-5, 7, 11, 13-15, 17, 18, 21, 22 To date, there are no published studies of CCT among Filipinos. It may be inappropriate to apply studies on IOP and CCT done in other countries to the Philippine setting because several studies have demonstrated interracial differences in the range of CCT and factors affecting these values.7, 8 This study determined the following among Filipinos: the distribution of CCT, the correlation between CCT and IOP, and the association between CCT and other factors such as age, gender, diabetes, thyroid disease, hypertension, trauma, eye surgery, and family history of glaucoma.

METHODOLOGY
This is a prospective cross-sectional study involving Filipino patients seen consecutively at the General
Ophthalmology Clinic of the Philippine General Hospital Outpatient Department from October 17 to 19, 2000. Patients over 10 years old with normal IOP, optic-nerve head, and cornea were included in the study. Patients with the following findings were excluded: glaucoma, use of glaucoma medication for the past month, intraocular surgery for the past 3 months, intraocular or retrobulbar tumor, contact lens use for the past 1 month, uveitis, retinal detachment, conjunctivitis, corneal ulcer, corneal dystrophy, and corneal refractive surgery. Patient consent was obtained. Patients included in the study were seen by a single examiner (PG). A case-report form was completed indicating demographic and other factors that could affect IOP and CCT. Proparacaine (Alcaine, Alcon, Fort Worth, TX, USA) was given as a topical anesthetic before IOP and CCT measurements were taken. IOP was measured twice using a Goldmann applanation tonometer (HaagStreit, Bern, Switzerland) with both measurements within 1 mm Hg. The mean of the two measurements was then
recorded. CCT was measured 10 times until all 10 measurements had a standard deviation (SD) of less than 5 using an ultrasound pachymeter (Bio & Pachy Meter AL-2000, Nishi-ku, Nagoya, Japan). Both instruments were disinfected with 70% isopropyl alcohol between each use. The calibration of both instruments was checked daily prior to the start of each study session. Statistical analyses were performed using Epi Info version 6.02c (Centers for Disease Control and Prevention, Atlanta, GA, USA). P values less than 0.05 were considered
statistically significant. For all statistical tests, the mean of the ten CCT measurements was used. The correlation
between average CCT and IOP, and between CCT and age were determined by linear regression. The effect of gender; family history of glaucoma; diseases such as diabetes mellitus, systemic hypertension, and thyroid
disease; trauma; and eye surgrery on CCT was determined by analysis of variance (ANOVA). The right and left eyes were analyzed separately since IOP and CCT measurements between eyes are interdependent.

RESULTS
The study included 222 eyes of 112 patients of whom 36 were male and 76 were female. Two patients had 1 eye where IOP and CCT could not be measured because of corneal irregularity secondary to trauma. The patients’ ages ranged from 16 to 86 years with a mean of 52.3 ±17.5 years. Most of the patients were in the 51- to 70-year age group (Figure 1). CCT ranged from 451.0 µm to 653.6 µm with a mean of 531.5 ±33.8 µm (Figure 2). The IOP ranged from 11 mm Hg to 21 mm Hg with a mean of 16.2 ±2.3 mm Hg. A direct linear correlation was found between CCT and IOP (r = 0.63). The IOP was noted to rise by 4.3 mm Hg/ 100 µm CCT (Figure 3). No significant correlation was found between CCT and age, gender, diabetes, thyroid disease, hypertension, trauma, eye surgery, or family history of glaucoma (p > 0.05). Neither was there a significant relationship between IOP and these factors (p > 0.05).

DISCUSSION
There is no general consensus on what is the average normal CCT. It has been estimated to range from 518 µm24 to 580 µm.21 A recent metaanalysis of measured CCT in normal eyes pooled from 300 data sets worldwide reported a mean of 534 ±31 µm. 8 Our study population had a comparable mean CCT of 531.5 ±33.8 µm. The CCT values were distributed normally (Figure 2). Our study population also showed
a direct correlation between CCT and IOP measured by Goldmann applanation tonometry (r = 0.63). An increase of 4.3 mm Hg per 100 µm of CCT increase was noted. This correlation is lower compared with results reported in manometry studies. Ehlers et al.2 reported a 5 mm Hg rise for every 70 µm CCT (7.14 mm Hg/100 µm CCT) while Whitacre et al.4 predicted an error of 3.5 mm Hg for every 70 µm CCT (5 mm Hg/100 µm CCT). Compared with population-based studies by Dohadwala et al.,7 Foster, et al.,5 and Wolfs et al.,6 our computed
slope for IOP rise for every µm is steeper. Dohadwala et al.7 reported only a 2 mm Hg increase per 100 µm CCT. Their study, however, used a Tono-Pen to measure IOP. In contrast, Foster et al.5 used an optical pachymeter and reported a 1.8 mm Hg/100 µm slope for right eyes and 2.4 mm Hg/100 µm for left eyes. Wolfs et al.6 reported a rise of 1.9 mm Hg/100 µm. This study, however, only included patients aged 55 or older (Table 1). Variations in the results obtained may also be partly explained by the differences in the number of eyes examined. Dohadwala et al.7 reported a significant difference in CCT among Asians, Blacks, and Caucasians. It is likely that interracial variation could partly explain the difference in the magnitude of the relationship observed between other studies and our study. Differences in instrumentation and characteristics of the study population are also possible causes. Other studies have reported that factors such as age,25-27 gender,7, 28, 29 hypertension,7, 30-32 diabetes,7, 20, 28, 33, 34 and family history of glaucoma 28,35
have relationships to IOP and/or CCT. Our study, however, did not show any statistically significant relationship. This study showed that CCT among Filipino patients consulting at the General Ophthalmology Clinic of the Philippine General Hospital is normally distributed and is comparable to the distribution obtained by metaanalysis of worldwide data. The study also found a direct correlation between CCT and IOP. Variation in CCT is a source of systematic error caused by the cornea’s resistance to flattening. Further population-based and manometry studies with larger sample sizes are recommended to evaluate the relationship between CCT and IOP as measured by Goldmann applanation tonometry. Such studies could help generate a formula for adjusting IOP measured by applanation to compensate for variations in CCT.