CORRELATION BETWEEN INTRAOCULAR PRESSURE, PAQUIMETRY AND KERATOMETRY IN A NORMAL POPULATION

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1 ARCH SOC ESP OFTALMOL 2007; 82: ORIGINAL ARTICLE CORRELATION BETWEEN INTRAOCULAR PRESSURE, PAQUIMETRY AND KERATOMETRY IN A NORMAL POPULATION CORRELACIÓN ENTRE PRESIÓN INTRAOCULAR, PAQUIMETRÍA Y QUERATOMETRÍA EN UNA POBLACIÓN NORMAL SÁNCHEZ-TOCINO H 2, BRINGAS-CALVO R 1, IGLESIAS-CORTIÑAS D 2 ABSTRACT Purpose: To study the intraocular pressure (IOP), the corneal thickness and the corneal curvature distribution in a normal population. To investigate the relation between these variables, and to determine if the IOP and the paquimetry will be modified by the refractive error and age. Methods: A population of 273 subjects, 545 eyes, without ocular pathology was selected. The refractive error, the corneal curvature, the corneal thickness and the IOP [with a non-contact tonometer (NCT) and with a Goldmann tonometer (GT)] were evaluated. Results: The average age of the population was (SD 7.23) years (27-68). The average corneal thickness was (SD 33) mm in females and (SD 29) mm in males. An inverse correlation was found between corneal thickness and age but this was not significant statistically. A mildly statistically significant correlation was found between corneal thickness and intraocular pressure with both methods of measurement of IOP: r = (p < 0,001) and r = (p < 0,001), with NCT and GT respectively. No correlation was found between RESUMEN Objetivo: Estudiar la distribución de la presión intraocular (PIO), paquimetría y queratometría en una población normal. Establecer las relaciones existentes entre estas variables y conocer la influencia de la edad y el estado refractivo. Métodos: En una muestra de 545 ojos de 273 sujetos, sin patología ocular previa, se ha evaluado el estado refractivo, curvatura corneal, grosor corneal central, PIO mediante tonometría de no contacto (TNC) y tonometría Goldman (TG). Resultados: La media de edad de la población fue de 49,34 DE 7,23 años (27-68). La paquimetría media fue de 544,3 DE 33 Ìm en mujeres y de 543,1 DE 29 Ìm en hombres. No se ha encontrado una asociación relevante entre paquimetría y edad. Se ha obtenido una correlación leve, pero significativa entre la paquimetría y la cifra de PIO para ambos métodos r = 0,316 (p < 0,001) y r = 0,264 (p < 0,001), con TNC Y TG respectivamente. No encontramos relación de la paquimetría ni con el estado refractivo ni con la K media. En nuestro modelo de regresión múltiple la paquimetría y la K media Received: 24/3/06. Accepted: 18/4/07. Ophthalmology Dept. Rio Hortega Hospital. Valladolid Spain. 1 Graduate in Medicine, ophthalmology specialist 2 Ph.D. in Medicine, ophthalmology specialist. Correspondence: Hortensia Sánchez Tocino Servicio de Oftalmología Hospital Río Hortega Avda. Cardenal Torquemada, s/n Valladolid Spain hsanchez@auna.es

2 SÁNCHEZ-TOCINO H, et al corneal thickness and refractive error, or with mean corneal curvature. In the regression model, corneal thickness and mean corneal curvature were significant in the IOP prediction: R = (p < 0.001). Conclusion: Mean corneal thickness was very similar to that reported in other studies of a latin population. A statistically significant inferior corneal thickness has been found in people older than 54 years. The relationship between the corneal thickness and the corneal curvature with the IOP has been calculated by linear regression analyses (Arch Soc Esp Oftalmol 2007; 82: ). aparecieron como variables predictoras del cálculo de la PIO: R = 0,341 (p < 0,001). Conclusiones: La medida de espesor corneal central fue similar a la obtenida por otros estudios sobre población latina. Existe una paquimetría significativamente menor en el grupo de mayores de 54 años. Se ha definido la relación del grosor y la curvatura corneal con la PIO mediante un análisis de regresión lineal. Palabras claves: Presión intraocular, paquimetría, queratometría, refractometría, estudio poblacional. Key words: Intraocular pressure, pachimetry, keratometry, refractometry, population based study. INTRODUCTION Variability is inherent to human beings in many measurable aspects. The eye globe is no exception, and subsequently there are different parameters which may potentially exhibit wide differences within a given population, such as the axial length, the corneal diameter and depth of the anterior chamber. Furthermore, each individual may show certain variations with respect to a number of age-dependent parameters (1). Likewise, the instruments used may be influenced in terms of results by the use of physical principles which assume ex-ante certain constant ocular variables (2,3). Even the fact of belonging to one race or another could influence decisively many of these measurements, as several studies have shown in the past (4-6). Some authors have reported this link in patients with previous eye pathology (2). Therefore, it is advisable to know the distribution of certain ocular variables and the existing relationships between them in populations not suffering from ocular pathologies (6,7). This would allow later on for comparison with groups with different pathologies in our environment. This study has a double aim. First, to determine the average and distribution of intraocular pressure (IOP), central corneal thickness (CCT) and corneal curvature radius (CR) were measured in a normal Caucasian population. And, secondly, to find the correlation between these variables, as well as the influence exerted by age and their refractive condition. SUBJECT, MATERIAL AND METHODOLOGY In total, the study surveyed the eyes belonging to 273 individuals who worked in our hospital. All of them were subjected to ophthalmic examination including: biomicroscopy of the anterior segment in order to discard corneal pathologies, IOP measurement via Goldmann tonometer (GT) and non-contact tonometer (NCT) (neumotonometer TX-10, Canon Inc, Japan), central pachymetry (OLCR pachymeter, Haag-Streit AG, Switzerland), keratometry and refractometry (Keratorefractometer RK- 3, Canon Inc., Japan). Anamnesis included: age, gender, and body mass index (BMI) by means of a formula based on height and weight; BMI = weight/height in square maters, family history of glaucoma and personal history of diabetes and/or high blood pressure. When analyzing relationships between the subject s non-intrinsic variables, central corneal thickness, IOP, corneal curvature, the two eyes for each subject were taken as independent data, whereas only the right eye was taken into account when dealing with variables such as gender or age. Individuals with a history of past ocular trauma or eye surgery were excluded, together with those patients already diagnosed and/or treated for ocular hypertension or optic neuropathy of glaucoma. Non-medical health personnel performed a noncontact tonometry, refractometry (the spherical equivalent was defined as the spherical error plus 268 ARCH SOC ESP OFTALMOL 2007; 82:

3 IOP, pachymetry and keratometry in a normal population half of the cylindrical error) and keratometry. Medical personnel performed anamnesis, pachymetry and applanation tonometry. A Goldmann tonometry was performed fifteen minutes after the non-contact tonometry without knowing the corresponding IOP result. Three IOP and corneal thickness measurements were undertaken for each eye, using the arithmetic mean for statistical estimations. The principle of non-contact tonometers, and among them the one used herein, is to achieve the corneal applanation via a known force. The force is achieved using an air stream and the corneal applanation is verified with an electronic system which casts a beam of collimated light and detects the parallel rays reflected on the cornea when the same is flattened. The IOP result in mmhg is estimated based on the time it takes to flatten the cornea (9). All functions are automated. The device itself discards erroneous measurements and points out the less reliable ones. Two measurement ranges may be used, 0-30 mmhg or 0-60 mmhg, depending on the strength of the air stream used by the device, either 30 mmhg or 60 mmhg (usually, the first range is used; if IOP exceeds 30 mmhg, it switches automatically to the following range). Measurement increments are set at 1 mmhg. In order to mitigate the effects associated with IOP measurements at different points in time during the cardiac cycle, manufacturers recommend performing three valid measurements and the average obtained is the IOP result provided by the device. The pachymeter used, mounted on a slit lamp, resorts to an optical low-coherence reflectometry to obtain these measurements. To do so, two.1 mm guide laser beams, color red, one perpendicular to the corneal surface and the other lower, in the center of the cornea. The measurement head detects through the reflection of light from an infrared laser changes in the reflection speed between the rear and anterior surface of the cornea. In order to manage keratometric measurements, the mean K was obtained ex ante using the K1 and K2 figures provided by the refractometer. For the statistical study, an SPSS 12.0 software for Windows (SPSS Inc, Chicago, Illinois, USA) was used. Statistical differences were assessed for the different IOP subgroups based on the pachymetry or age, using non-parametric tests such as the Kruskal-Wallis and U-Mann Whitney tests. Correlations were determined between variables using the Spearman correlation test and IOP predictions were assessed via a multiple linear regression study with respect to those factors showing a significant association in correlation tests: corneal thickness, age, corneal curvature and refractive status. A statistical significance was considered for p <.05 bilateral. RESULTS IOP and Pachymetry Relation with Age Table I shows the distribution by age of pachymetries and IOP for our study group. The average age was SD 7.23 years (range 27-68) percent were female. The mean pachymetry among women was SD 33 µm and among men SD 29 µm. The mean IOP among women was 15.5 SD 3 mmhg and 15.5 SD 3 mmhg with the noncontact tonometer and Goldmann, respectively. Among men, it scored 15.2 SD 3 mmhg and 14.6 SD 3 mmhg. No significant differences were found in either group nor through the pachymetry and IOP obtained using both methods. No relevant correlation exists between corneal thickness and age r=.131 (p=.003). In the study by groups two to two, significant differences were found between the age group between with respect to the range (p=.05) and in the group with respect to the group (p=.008) (table I). No significant Table I. Distribution by age for the Pachymetry and IOP, NCT (Non-Contact Tonometer), GT (Goldmann Tonometer) Pachymetry NCT GT a SD 31 µm 15.2 SD 2 mmhg 15.2 SD 2 mmhg a SD 31 µm 15.4 SD 3 mmhg 15.4 SD 3 mmhg a SD 36 µm 15.5 SD 3 mmhg 15.4 SD 3 mmhg p value* * Kruskal-Wallis for the study of differences in all groups. SD: Standard Deviation. ARCH SOC ESP OFTALMOL 2007; 82:

4 SÁNCHEZ-TOCINO H, et al Table II. Mean IOP by Corneal Thickness Groups and Statistical Survey on Differences between Groups. NCT (Non-Contact Tonometer), GT (Goldmann Tonometer) Pachymetry NCT mmhg GT mmhg No. of Eyes µm 13,9 DE 3 14,2 DE 2, µm 13.9 SD SD µm 15.3 SD SD µm 17.3 SD SD 3 82 p value* ** < ** <.001 <.001 ** <.001 <.001 * Kruskal-Wallis Test: differences among groups. ** Mann-Whitney differences 2 to 2. SD: Standard Deviation. differences were found at the IOP level among the different age groups. Relation between IOP and Pachymetry There is a significant correlation between corneal thickness and IOP levels, both through the non-contact tonometer and Goldmann tonometer, r =.316 (p <.001) and r =.264 (p <.001), respectively. Table II shows IOP measurements by corneal thickness groups and the existence of statistically significant differences among the different groups. There is a tendency to the increase of corneal thickness with the increase of IOP, so that the average pachymetry increases from SD. 30 µm for the IOP group < 13 mmhg to SD. 34 µm for the IOP group between mmhg and SD 28 µm in the IOP group > 17 mmhg. These differences are significant both with respect to total numbers in both groups and intergroups (p<.001). Relation of IOP and Pachymetry with Refractive Status The mean spherical equivalent was 0.66 SD 3 diopters. No significant correlation was found between IOP and refractive error. As for IOP with neumotonometer and Goldmann, it was r=.037 (p=.402); r=.068 (p=.120), respectively. There is a positive significant correlation between the refractive status and age r=.268 (p<.001). No correlation exists between corneal thickness and refractive status r=.021 (p=.630). Relation of IOP and Pachymetry with Corneal Curvature The mean corneal curvature was K1: 7.8 SD.2 mm, K2: 7.6 SD.2 mm; the range for K1 and K2 was (7.2-8) and (7-8.5), respectively; average K: 7.70 SD.2 mm (fig. 1). No relevant correlation exists between the different IOP levels and corneal curvature. The average K was r=.10 (p=.04), r=.115 (p=.014) for the neumotonometer and Goldmann, respectively. No correlation was found between the corneal thickness and the value of the average K. The correlation between the corneal curvature and age cannot be considered significant for the average K r=.148 (p=.003). Multiple Regression Study Fig. 1: Line chart illustrating the mean corneal curvature by intraocular pressure groups and pachymetry groups. The multivariate linear regression study was performed including the IOP prediction model via a Goldmann tonometer: corneal thickness, age, ave- 270 ARCH SOC ESP OFTALMOL 2007; 82:

5 IOP, pachymetry and keratometry in a normal population rage K and the interaction between pachymetry and age. The pachymetry and average K were included in the regression model with very good tolerance limits for prediction. The remaining variables showed low significance and tolerance issues. With a r=.304 (p<.001) a regression line is obtained (fig. 2). IOP Goldman = (.23 x pachymetry) (1.2 x average K). This means that for each 100 µm increase in the pachymetry, the IOP increases by 2.5 mm. Even though this model only predicts 10 percent of the IOP value, there is an obvious relation between the later and corneal thickness. Using the IOP resulting from the neumotonometer, results are very similar. R=.341 (p<.001). The line obtained is as follows: IOP neumotonometer = (.30 x pachymetry) (1.8 x average K). DISCUSSION Fig. 2: Illustration of a regression curve to estimate IOP with a Goldmann tonometry dependent upon the pachymetry and the average corneal curvature. The study found a mean corneal thickness of µm. This mean is similar to that found in Latin individuals by Hahn et al (9), of µm, and is smaller than the one found in the La Rosa s study on the American Caucasian population (10). Nevertheless, caution is advised when performing comparisons, since minimal differences may respond to distribution based on age or gender, or to the use of different types of pachymeters. Certain studies found measurement errors and interobserver differences when using optical pachymeters due to a poor eye alignment. Such errors may be rectified using laser pachymeters (11,12), which is the case in the present study. A relation was found between corneal thickness and age by groups, with older individuals presenting thinner corneas. Hahn et al (9) found among Latins that corneas were thinner in older people and thicker among younger subjects. Similar findings were reported by Foster (11) and La Rosa (10). This may be due to the fact that age leads to a decrease in the density of keratocytes and likely to a rupture of the collagen fibers in older corneas. Nevertheless, Nemesure (4), Shimmyo (5) and the studies by Reykjavik (6) and Rotterdam (13) did not find such relation. Nemesure (4) found a relation between thickness and age, refractive status and a history of diabetes. In the present study, no relation was found with the refractive status. ARCH SOC ESP OFTALMOL 2007; 82: In the same fashion as Shimmyo (5), the present study found a statistically significant relation between the IOP and the corneal curvature both in the correlation test and in the multivariate study. The study authored by Cho and Lui (14) found that IOPs measured with a Goldmann tonometer and the neumotonometer may be comparable. The present study aimed at using both data in order to assess whether there were any differences between the measurement of IOP with both methods and its relation with the remaining variables. The IOP measured by the applanation tonometer or the neumotonometer is correlated with the corneal thickness (15). This relation, already described by Goldman and Shimmyo (5), was confirmed in other studies, thus pointing at the need of performing an IOP adjustment in relation with the corneal thickness (5,16). The study revealed an IOP difference of 2.5 mm for each 100 µm of difference in corneal thickness. Similar findings with IOP variations ranging from 1.1 and 3.2 mm have been described by other authors (6,9,16,17). Although corneal thickness barely accounts for 10 percent of the IOP, it seems to be a relevant factor to be taken into account in the patients daily examinations. The relation between corneal thickness and IOP may have implications in the diagnosis of ocular hypertension, open angle glaucoma or normal-tension glaucoma. The IOP is still a very important 271

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