# BIOMETRIC CORRELATIONS OF CORNEAL THICKNESS

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A C T A O P H T H A L M O L O G I C A V O L . 5 3 1 9 7 5 Department of Ophthalmology, (Heads: N . Ehlers & V. A . Jensen), University of Aarhus, Denmark, and Debartment of Ophthalmology (Head: T . 1. Bertelsen), University of Bergen, Norway BlOMETRlC CORRELATIONS OF CORNEAL THICKNESS BY NlELS EHLERS, FINN KRUSE HANSEN and HENRY AASVED Central cornei thickness, depth of anterior chamber, thic-.ness of lens, length of vitreous, axial length, applanation (Goldmann) and indentation (Schietz) tension and rigidity (Friedenwald) were determined in three groups of patients; (1) 16 patients with glaucoma simplex, (2) 25 patients with ocular hypertension, and (3) 26 patients with fibrillopathia epithe- liocapsularis (so-called pseudoexfoliation of the lens). Average data and computed correlations between the parameters are given in tabular form. Corneal thickness was normal in glaucoma simplex, but significantly increased in ocular hypertension. Corneal thickness was positively cor- related to applanation tension but not to indentation tension, nor to any other of the studied parameters. It is suggested that corneal thickness can be considered as a biornetric parameter which yields new information about the eye not covered by the conventional parameters. Key words: corneal thickness - ocular dimensions - ocular tension - glau- coma simpler - ocular hypertension - fibrillopathia epitheliocapsularis. While recent studies agree upon an average central corneal thickness (CCT) of about 0.52 mm (Kruse Hansen 1971, Mishima & Hedbys 1968, Lowe 1969), the significance of corneal thickness as a biometric parameter is still unclear. To help clarify this matter we report the data from a series in which, apart from central corneal thickness, measurements were also made of the axial length, depth of anterior chamber and thickness of crystalline lens by means ~~~~ ~ Received May 26, 1975. 652 Correlations of Corneal Thickness of ultrasonography. Finally, the intraocular pressure was measured both by applanation and by indentation tonometry. A positive correlation had previously been found between central thickness and applanation tension in a small normal material (Kruse Hansen 1971). We have also observed the correlation in a group of 45 conscripts, in 139 patients subjected to renal allografting (unpublished), and in the series in- cluded in this paper. By measuring applanation tension at various intraocular hydrostatic pressures in eyes with different corneal thicknesses, a correlation was established between corneal thickness and the error of the applanation tension (the difference between hydrostatic pressure and measured appla- nation tension) (Ehlers, Bramsen & Sperling 1975). Concordant with this, a preliminary study had shown an increased corneal thickness in patients with monosymptomatic ocular hypertension (Kruse Hansen & Ehlers 1971). This observation is substantiated by the data reported in the present paper. In low-tension glaucoma a reduced corneal thickness was found (Ehlers & Kruse Hansen 1974). Material and Methods Three groups of patients were selected for this study, (1) 16 cases of treated glaucoma simplex, (2) 25 cases of untreated ocular hypertension (suspicio glaucomatis), and (3) 26 cases of untreated fibrillopathia epitheliocapsularis (so-called pseudoexfoliation of the lens) without glaucoma. For each patient the investigation comprised the determination of central corneal thickness by optical measurement with the Haag-Streit pachometer (Ehlers & Kruse Hansen 1971), the axial length, depth of anterior chamber (after subtraction of corneal thickness), and lens thickness were determined by ultrasonography (Kretsch technik, 6 MHz). The axial length of the vitreous body was calculated by subtraction. The intraocular pressure was measured by Goldmann applanation tonometry and by a Schietz indentation tonometer. For the latter, scale readings as well as pressure readings from the Friedenwald (1955) calibration table were recorded. Ocular rigidity was determined from the applanation and the 5.5 g indentation readings. After an introductory graphical analysis of the material, statistical evaluation was made by computer following the methods of Sokal & Rohlf (1969). Values are given as mean f standard error of mean (s.e.m.). Significance levels were estimated from the t-distribution. Results Ocular dimensions. Table I summarizes the average data obtained in the studied groups of patients. Corneal thickness in glaucoma simplex does not differ from the normal thickness of the cornea, which for right eyes is 0.520 f 0.002, and for left 653 Niels Ehlers, Finn Kruse Hansen and Henry Aasved Table I . Ocular dimensions, pressures and rigidity Glaucoma Corneal thickness Depth of anterior chamber Lens thickness Length of vitreous Axial length Applanation tension Schietz tension Rigidity R L R L R L R L R L R L R L R L .521 f .003 526 f .003 2.61 f .147 2.72 f .I30 4.74 f .113 4.72 f .132 14.93 f 238 14.79 f 221 22.29 f 223 22.23 f ,212 14.69 f 1.03 15.21 f 1.13 14.44 f 1.02 14.05 f 1.03 .0223 f ,0014 .0227 k .0017 342 f .007 .557 f .007 2.97 f .071 2.82 f .081 4.38 f .067 4.49 f .056 14.71 f .I34 14.75 f .126 22.05 f .160 22.04 f .142 17.80 f 0.45 17.91 k 0.44 17.26 f 0.49 17.10 f 0.48 .0281 f .0021 .0294 f .0021 539 f .007 338 f .007 2.80 f .085 2.67 f .099 4.67 f .096 4.92 f .I07 15.03 f .I56 14.99 f .189 22.50 f .189 22.58 f ,204 14.50 f 0.68 16.26 f 0.62 14.50 f 0.67 15.62 f 0.63 .0226 f .00094 .0239 f .00098 536 543 2.82 2.74 4.58 4.7 1 14.89 14.85 22.28 22.29 15.78 16.65 15.51 15.82 .0246 .0258 All values are given as means f s.e.m., dimensions in mm, and tensions in mmHg. Glaucoma simplex, no. of eyes R = 16, L = 14. Ocular hypertension, no. of eyes R = 25, L = 23. Fibrillopathia epitheliocapsularis, no. of eyes R = 26, L 23. Aver- age age glaucoma simplex 66 years (49-76) ocular hypertension 63 years (51-76) and fibrillopathia epitheliocapsularis 72 years (59-86). eyes 0.524 f 0.002 (Kruse Hansen 1971). In patients with ocular hypertension the corneal thickness is significantly higher than i n a normal group (right eyes P < 0.01, left eyes P < 0.001), and i n the group with glaucoma simplex (right eyes P < 0.01, left eyes P < 0.001). In the group with fibrillopathy the corneal thickness is higher than normal, but not statistically significant (right eyes P < 0.02, left eyes P < 0.1). In the groups (2) and (3) the s tandard error of the mean is higher than i n both the glaucoma simplex group and the normal series, probably due to heterogeneity within the two former groups. 654 Correlations of Corneal Thickness As regards the chamber depth, lens thickness, length of vitreous and length of eyeball, no significant differences were found between the three groups. The average values for the depth of the anterior chamber agree with those given by Tornquist (1953) and Alsbirk (1974), although lower than those of Jansson (1963). The thickness of the lens, considering the age of patients, is also within the range given in the literature (see e.g. Jansson 1963). The values of length of vitreous and length of eyeball are lower than those of Jansson (1963) and Stenstrom (1946), and for some of the groups, statistically significant deviations from the data in the literature are found. Ocular tensions and rigidity. Table I shows the calculated average values for applanation tension, Schistz tension and rigidity. The applanation tensions tend to be higher than the Schistz tensions, although none of the differences are statistically significant. The tensions of the glaucoma simplex group were normal, following treatment. The group of ocular hypertension showed average tensions between 1 7 and 18 mmHg. Some of the differences between this group and the other groups are significant. The rigidity is also higher in the ocular hypertension group, the difference between this and the two other groups was probably significant (P - 0.05). Correlations of corn'eal thickness to other ocular dimensions. Possible correla- tions were studied graphically, and by computing the correlation and regres- sion coefficients for the three groups and for the total material, for right and for left eyes separately. The data are illustrated by the matrix in Table I1 which applies to the total material; right eyes, upper figures, and left eyes, lower figures. Similar matrices were made for the three subgroups (1-3) and the data were essentially similar. In Table I1 correlation coefficients above 0.24 may be regarded as significant (P < 0.05). It may be noted that no correlations could be demonstrated between corneal thickness and the other dimensions. Correlations between depth of anterior chamber, lens thickness, vitreous and axial length. Significant positive correlations were found between depth of anterior chamber and axial length and between vitreous and axial length. A negative correlation was found between depth of anterior chamber and lens thickness (Table 11). Other tested correlations were insignificant. Correlations of dimensions to tension and rigidity. There is a positive correla- tion between corneal thickness and applanation tension (P - 0.05). The same tendency may be noted for the Schistz tension, although it is not statistically significant. The following equations have been obtained from the total series: Appl. (mmHg) = 2.43 + 24.90 . CCT (mm) 655 Table 11. Correlation matrix depth Lens Vitreous Axial APPl. Schietz Schietz Rigidity thickness length length Age tension tension scale Corneal thickness -0.128 0.032 0.129 0.117 0.067 0.228 0.174 -0.180 0.137 0.007 -0.163 0.228 0.122 -0.030 0.251 0.180 -0.187 0.251 AC depth Lens thickness Vitreous length ch Q, Axial lengthAppl. tension Schietz tension Schietz scale -0.505 0.173 0.409 -0.111 0.129 0.191 -0.189 -0.044 -0.640 0.273 0.380 -0.120 0.074 -0.018 0.043 -0.023 -0.202 0.061 0.319 -0.032 -0.038 0.019 -0.060 -0.281 -0.057 0.429 -0.101 -0.073 0.040 -0.1320.867 0.244 -0.081 -0.157 0.148 -0.329 0.904 0.113 0.045 -0.046 0.042 -0.2450.321 -0.012 -0.062 0.044 -0.345 0.247 0.033 -0.091 0.083 -0.296-0.151 -0.124 0.101 -0.303 -0.086 -0.096 0.050 -0.284 0.700 -0.703 0.299 0.865 -0.847 0.457 -0.983 0.514 -0.978 0.475 -0.498 -0.440 Correlation coefficients above 0.31 are significant at the 1 O/o level, coefficient above 0.24 at the 5 O/o level. Upper figures, 67 right eyes; lower figures, 60 left eyes. Correlations of Corneal Thickness (right eyes, N = 67). Computed t-value for regression coefficient 1.89 (P < 0.05). Schietz (mmHg) = 5.55 + 18.59 * CCT (mm) (right eyes, N = 67). Computed t-value for regression coefficient 1.42 (P < 0.15). There were no correlations between depth of anterior chamber or lens thickness, and tensions or rigidity (Table 11). A negative correlation exists between vitreous length and axial length on the one hand and rigidity on the other. Correlations to age. Lens thickness and axial length are positively correlated to age, whereas there is a negative correlation between age and rigidity, (Table 11). I t must be emphasized that the material comprises mainly old patients, and is therefore rather unsuitable for a study of age correlations. Multiple correlations. The measured applanation tension is correlated to the corneal thickness. Although the present material is heterogeneous and not composed of normal eyes it was decided to utilize it in an attempt to determine whether other parameters are of importance for the measurement of tension. This has been done by computing several multiple correlations, of which only a few will be discussed. Appl. (mmHg) = 2.67 + 24.93 * CCT (mm) - 0.012 . Axial length (mm) The t-value for the regression coefficient with respect to corneal thickness is 1.86 (P < 0.05); the regression with respect to axial length is not significant. (t = - 0.02). The present material suggests therefore that the axial length (size of eyeball) is of no importance in applanation tonometry. A similar computation for the Schietz tension gives Schietz (mmHg) = 12.23 + 19.63 . CCT (mm) - 0.32 * Axial length (mm) t-Values for the regression coefficients are 1.49 and - 0.68, both statistically insignificant. Thus from the material, the axial length would also seem to be of little importance for indentation tonometry. Discussion The present study was performed after it had been found that the central corneal thickness in a small series of patients with monosymptomatic ocular hypertension was significantly higher than normal (Kruse Hansen & Ehlers 657 Acta ophthal. 53, 4 43 Niels Ehlers, Finn Kruse Hansen and Henry Aasved 1971). This observation has been confirmed by the present study where a statistically Significant higher thickness was found in the group with ocular hypertension than in either the glaucoma simplex group or our normal series (Kruse Hansen 1971). No difference between the glaucoma simplex group and the normal series was found, in accordance with the statement of Tomlinson & Leighton (1972). The thickness in the fibrillopathy group is slightly, but not significantly higher than normal. This might be due to the fibrillopathy and then be correlated with the fact that the mean intraocular pressure in eyes with fibrillopathy is higher than in eyes without fibrillopathy (Aasved 1971). No significant differences were found between the three subgroups for the other dimensions. A general comment on the other parameters is outside the scope of this paper. The flatter anterior chamber in simple glaucoma is in accordance with Tornquist & BrodCn (1958) and Tomlinson & Leighton (1972). The average values for applanation and indentation tension are normal, again excepting the group with ocular hypertension in which it is increased. The ocular rigidity is also higher in this group than in the other groups. The statistical analysis confirmed the previously found (Kruse Hansen 197 1 ) correlation between corneal thickness and applanation tension. There was a tendency to a correlation between corneal thickness and Schietz tension, although not statistically significant. It appears from the calculated correlation coefficients that the central corneal thickness is a parameter which is mainly independent of other ocular dimensions. This independence of corneal thickness, and the correlation be- tween thickness and pressure would seem to indicate that the thickness is a factor of clinical importance in the evaluation of intraocular pressure. This ccinclusion is in agreement with the findings of reduced thickness in low tension glaucoma and increased thickness in monosymptomatic ocular hyper- tension. From the recently reported experimental study (Ehlers et al. 1975) it seems that the influence of the thickness on the applanation tonometer reading is an additional factor of clinical importance. References Aasved, H. (1971) Intraocular pressure in eyes with and without fibrillopathia epithe- liocapsularis (so-called senile exfoliation or pseudo-exfoliation). Acta ophthal. (Kbh.) Alsbirk, P. H. (1974) Anterior chamber depth in Greenland Eskimos. Acta ophthal. 49, 601-610. (Kbh.) 52, 565-580. 658 Correlations of Corneal Thickness Ehlers, N., Bramsen, T. & Sperling, S. (1975) Applanation tonometry and central corneal thickness. Acta ophthal. (Kbh.) 53, 34-43. Ehlers, N. & Kruse Hansen, F. (1971) On the optical measurement of corneal thickness. Acta oplithal. (Kbh.) 49, 65-71. Ehlers, N. & Kruse Hansen, F. (1974) Central corneal thickness in low-tension glau- coma. Acta ophthal. (Kbh.) 52, 740-746. Hansen, F. Kruse (1971) A clinical study of the normal human central corneal thick- ness. Acta ophthal. (Kbh.) 49, 82-89. Hansen, F. Kruse & Ehlers, N. (1971) Elevated tonometer readings caused by a thick cornea. Acta ophthal. (Kbh.) 49, 775-778. Jansson, F. (1963) Measurement of intraocular distances by ultrasound and comparison between optical and ultrasonic determinations of the depth of the anterior chamber. Acta oghthal. (Kbh.) 41, 25-61. Lowe, R. (1969) Central corneal thickness. Ocular correlations in normal eyes and those with primary angle-closure glaucoma. Brit. /. Ophthal. 53, 824-826. Mishima, S. & Hedbys, B. 0. (1968) Measurement of corneal thickness with the Haag- Streit pachometer. Arch. Ophthal. (Chicago) 80, 710-713. Sokal, R. R. & Rohlf, F. J. (1969) Biometry. The Principle and Practice of Statistics in Biological Research. Freeman, San Francisco. Stenstrom, S. (1946) Untersuchungen iiber die Variation und Kovariation der optischen Elemente des menschlichen Auges. Acta ophfhal. (Kbh.) Suppl . 26, pp. 103. Tomlinson, A. & Leighton, D. A. (1972) Ocular dimensions in low tension glaucoma. Compared with open-angle glaucoma and the normal. Brit. /. Ophthal. 56, 97-104. Tornquist, R. (1953) Shallow anterior chamber in acute glaucoma. Acta ophthal. (Kbh.) Tornquist, R. & BrodCn, G. (1958) Chamber depth in simple glaucoma. Acta ophthal. supp1. 39, pp. 74. (Kbh.) 36, 309-323. Authors address: Niels Ehlers, Department of Ophthalmology, Arhus Kommunehospital, DK-8000 Arhus C, Denmark. 659

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