ACTA OPHTHALMOLOGICA SCANDINAVICA 2000 -

morphometric parameters was evaluated in EOAG as compared to OHT eyes. The cOnCIUsions by multiple regression analysis. areas under the ROC curve were: PERG, Functional and morphometric measures

0.77; CDA, 0.69; RA, 0.71; CSM, 0.71. of early optic nerve damage display com- Results Diagnostic accuracy did not differ sig- parably good accuracy in discriminating Mean PERG amplitude was smaller nificantly across measures. In the whole between clinically defined EOAG and (p<O.Ol) in EOAG as compared to OHT study population, PERG amplitudes were OHT eyes. eyes. Mean CDA and CSM were greater negatively correlated with CSM (r= (p<O.Ol), and RA was smaller (p<O.Ol) -0.60, pCO.001).

Corneal thickness in glaucoma: an important parameter? €! Brusini, E Miani, C. Tosoni Eye Unit, S: Maria della Misericordia, Udine

Summary Central corneal thickness was measured by pachometry in 49 patients with pri- mary open-angle glaucoma (POAG), 41 with ocular hypertension (OHT), 14 with normal tension glaucoma (NTG) and 48 normal subjects. The mean corneal thick- ness of the OHT patients was significant- ly greater than that of the normal control group. The NTG patients, on the other hand, had a cornea on average thinner than the normals. These differences may cause misclassification of normals with a thick cornea as ocular hypertensive eyes or, contrarywise, cause those normals whose IOP is underestimated because of a thin cornea to be classed as NTG pa- tients.

Key words: primary open-angle glaucoma - ocular hypertension - cornea1 thickness - pa- chometry.

Introduction Notwithstanding its scarce sensitivity and specificity, intraocular pressure is still an important parameter in the diagnosis and follow-up of glaucomatous patients. Goldmann’s applanation tonometry is still today the most widespread intraocul- ar pressure measuring method, even though certain factors, corneal curvature and thickness to name but two, may in- fluence its reliability. In regard to this, various authors (Kruse & Ehlers 1971; Ehlers et al. 1975; Johnson et al. 1978; Whitacre et al. 1993; Argus 1995; Copt et al. 1999; Bron et a!. 1999) have demon- strated a correlation between the central corneal thickness and the ocular tension

value. Intraocular pressure can, therefore, be overestimated or underestimated in corneas thicker or thinner than normal, respectively.

Aims This work was undertaken to evaluate the differences in corneal thickness in 3 groups of patients affected, respectively, by primary open-angle glaucoma (POAG), ocular hypertension (OHT) and low (or normal) tension glaucoma (NTG), and to compare them against a sample of normal subjects.

Materials and Methods The study populations were:

49 POAG patients, aged between 41 and 85 years with a mean of 72.959.7 years, in medical treatment,

41 OHT subjects with IOP>22 mmHg on more than two occasions, without peri- metric defects, aged between 26 and 80 years with a mean of 64.3210.2 years, and

14 NTG patients, with typical alterations of the optic nerve head and perimetric defects, and with IOPc21 mmHG al- ways, aged between 50 and 87 years with a mean of 70.32 11.3 years,

while the control group contained 48 nor- mal subjects aged between 21 and 90 years with a mean of 61.22 19.3 years,

Corneal thickness was measured in both eyes of each study member, but only one eye per patient chosen at random was used in the analysis of the results.

All the patients had previously been examined by computed automatic per-

imetry (programme 30-2, Humphrey or G1 Octopus). Diabetic patients and eyes which had undergone surgery were ex- cluded from the study.

The central corneal thickness was measured with the ultrasonic pachometer Altair Optikon 2000. Each eye was meas- ured 5 times and the lowest value of the five was the one chosen for the analysis. Student’s t-test for independent samples was used for the statistic analysis of the results.

Results The mean corneal thicknesses in the vari- ous groups were:

a. POAG: 555.5238.7 p (range 465-651

b. OHT 566.2228.5 p (range 493412

c. NTG: 512.9544.6 p (range 43C580

while in the controls (normal subjects) the mean thickness was 542.6236.0 p (with a range of 467-614 p).

Statistically significant differences were found between normal and OHT eyes (p<O.OI), between normal and NTG eyes (p<0.05), between WAG and NTG eyes (pc0.01) and between OHT and NTG eyes (pCO.001). Not statistically signifi- cant differences were recorded between normal and POAG eyes and between OHT and POAG eyes.

Discussion Goldmann’s applanation tonometer at over 40 years from its introduction (Goldmann & Schmidt 1957) is still con- sidered the gold standard for ocular pressure measurement. It is an instru- ment designed for use in standardized conditions and, therefore, it may err in eyes which do not meet those standard- ized conditions, for example, in eyes whose corneal thickness is not “stan- dard“ (Whitacre & Stein 1993). The find- ing of a corneal thickness quite different

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to the normal in the OHT subjects (higher values) and in NTG eyes (lower values) is recently acquired knowledge, notwithstanding the fact that some authors had already reported it a good number of years ago (Kruse & Ehlers 1971; Ehlers et al. 1975; Johnson et al. 1978). It has been hypothesized on the basis of these data that the intraocular pressure in some patients with POAG and thin cornea might be underestimated, or that there could be an incorrect diagnosis of NTG (false negatives). On the other hand, some patients with OHT but no identifiable glaucomatous damage could well have an increased corneal thickness with a consequent overestimate of tono- metric values (false positives). Ehlers and his associates (1975) have calculated that at an intraocular pressure of 20 mmHg, applanation tonometry could make an underestimate of 5.2 mmHg in eyes with a corneal thickness of 450 p and an over- estimate of 4.7 mmHg in eyes with a cor- neal thickness of 590 p.

Our study has produced similar results in that hypertensive eye corneas are on average thicker than normal and NTG eye corneas are the opposite. A fairly clear corneal thickness difference, even if not statistically significant, was found be- tween POAG and OHT. The difference,

which moreover is not significant, be- tween normal eyes and POAG eyes is less explainable.

These results confirm the hypothesis that the measurement of ocular tension can be influenced by the thickness of the cornea and can lead in some cases to a false diagnosis of OHT on the one hand and of NTG on the other hand. Given the important clinical effect of such an event, it would seem advisable to make a corneal pachometry a standard examina- tion in eyes with OHT at low risk and in patients who, notwithstanding typically glaucomatous anatomical and functional damage, consistently present an ocular pressure within normal limits. In this way, it would be possible to bring back into the ambit of complete normality, with obvious psychological benefits, a part of the subjects with suspect glau- coma, avoiding thus the introduction of useless and not always anodyne therapy. On the other hand, a part of the patients diagnosed NTG could be more correctly profiled and treated. In this group, a cor- neal pachometry could help to differen- tiate those cases in which vascular factors are pathogenetically preponderant from the cases in which the ocular tension, falsely underestimated, might be, instead, a factor of primary importance.

References Kruse HF & Ehlers N (1971): Elevated tonom-

eter readings caused by a thick cornea. Acta Ophthalmol49 775-778.

Ehlers N, Bramsen T & Sperling S (1975): Ap- planation tonometry and central corneal thickness. Acta Ophthalmol53: 34-43.

Johnson M, Kass MA, Moses R & Grodzki W (1978): Increased corneal thickness simulat- ing elevated intraocular pressure. Arch Ophthalmol 96: 664665.

Whitacre MM, Stein RA & Hassanein K (1993): The effect of corneal thickness on applanation tonometry. Am J Ophthalmol

Argus WA (1995): Ocular hypertension and central corneal thickness. Ophthalmol 102: 1810-1812.

Copt R-P, Thomas R & Mermoud A (1999): Corneal thickness in ocular hypertension, primary open-angle glaucoma and normal tension glaucoma. Arch Ophthalmol 117: 14-16.

Bron AM, Creuzot-Garcher C, Goudeau-Bou- tillon S & d’Athis P (1999): Falsely elevated intraocular pressure due to increased central corneal thickness. Graefe’s Arch Clin Exp Ophthalmol 3: 22&224.

Goldmann H & Schmidt T (1957): uber Applanationstonometrie. Ophthalmologica

Whitacre MM & Stein R (1993): Source of error with use of Goldmann-type tonom- eters. Surv Ophthalmol 38: 1-30.

1 15: 592-596.

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Quantitative mapping of the retinal thickness at the posterior pole in chronic open angle glaucoma I? Brusini, C. Tosoni, E Miani Dept. of Ophthalmology, General Hospital of Udine

Abstract The sensitivity of a new device, which is able to measure the thickness of the ret- ina at the posterior pole (Retinal Thick- ness Analyzer), was assessed in a group of 21 glaucomatous patients, and in 24 patients with ocular hypertension. The specificity of RTA was evaluated in a con- trol group of normal eyes. Of the glau- comatous eyes, 57.6% had a significant reduction in retinal thickness, while an- other 30.3% were borderline. In the ocu- lar hypertension group, 37.5% of eyes were abnormal, and 45.8% were border-

line. In the control group, no eye was ab- normal, though 42.9% had borderline alterations. In many glaucomatous eyes (81 3%) there was a relationship between retinal thickness loss and visual field de- fects.

Introduction The diagnosis of chronic simple glau- coma is still based on the presence of either typical visual field defects or struc- tural damage at the optic nerve head, usually associated with an IOP increase. As the glaucomatous damage is generally

irreversible, an early diagnosis is ex- tremely important. Computerized auto- mated perimetry is not sensitive enough for this purpose, as a consistent amount of retinal nerve fibre is usually lost before a visual field defect can be found (Quigley et al. 1989). On the other hand, non-con- ventional visual field testing techniques do not seem to have satisfactorily solved the problem (Brusini et al. 199811999).

More interesting seem to be the methods which either assess the optic nerve head or measure nerve fibre layer thickness around it. However, recent ex- perimental studies demonstrate that earliest glaucoma damage often affects ganglion cells in the perifoveal area (Glo- vinsket et al. 1993, Frishman et al. 1996).

Purpose To study the reliability, sensitivity and the specificity of a new computerized device, the Retinal Thickness Analyzer (RTA), in assessing and quantifying the structural damage in chronic open-angle glaucoma.

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