Intraocular pressure (IOP) is an important risk factor for the development or progression of glaucoma and is known to have diurnal rhythm variations.1–3 4
Many studies have examined the relationships between IOP and ocular parameters that can influence IOP or IOP fluctuation. David et al.5 6 7 8 9–14
METHODS
We retrospectively reviewed diurnal IOP data measured from May to December 2007 in Kangbuk Samsung Hospital, Seoul, Korea. All 115 subjects were patients who were admitted for cataract surgery. The research followed the tenets of the Declaration of Helsinki and was approved by the institutional review board of Sungkyunkwan University School of Medicine, Kangbuk Samsung Hospital, Seoul, Korea. Informed consent was obtained from all participants before IOP measurements were taken. Intraocular pressure was measured with a Goldmann applanation tonometer in habitual positions to evaluate the IOP during waking hours, and IOP was measured every 2 hours from 9 am to 11 pm. Patients were not confined to bed and were free to sit or walk, as they chose. Proparacaine 0.5% was applied before the measurements, and the right eye was measured before the left eye. The IOP was measured three consecutive times, and the mean value was used for statistical analysis. The diurnal IOP fluctuation was defined as the differences between peak and trough values in diurnal IOP readings.
Demographic and clinical history data were obtained. We examined ocular biometrics including the central corneal power (CCP) using a keratometer (Atlas; Carl Zeiss Meditec, Dublin, CA), CCT, axial length (AL), anterior chamber depth (ACD), and lens thickness using an ultrasound biometer (Axis II PR; Quantel Medical, Bozeman, MT). All subjects received a complete ophthalmic examination and none of the subjects had a narrow iridocorneal angle based on Zeiss four-mirror indentation gonioscopy. The refractive state was determined by manifest refraction and the spherical equivalent (SE) was used to calculate the refractive error.
Subjects who had previous ocular surgery or trauma, a shallow anterior chamber, peripheral anterior synechiae, any type of glaucoma, or poor cooperation with IOP measurement were excluded. Subjects with eyes shorter than 21 mm, assumed as a nanophthalmos, were also excluded. Patients with remarkable phacodonesis and zonular dialysis during cataract surgery that underwent intracapsular cataract extraction with scleral fixation were also excluded from this study.
We collected diurnal IOP data and analyzed diurnal IOP variations along with various biometric values such as CCT, CCP, AL, refractive error, ACD, lens thickness, and vitreous chamber depth (VCD). We divided all subjects into three groups according to the tertile of AL: group 1 is composed of subjects with an AL shorter than 22.85 mm, group 2 is made up of subjects with an AL between 22.85 and 23.63 mm, and group 3 is composed of subjects with an AL equal to or longer than 23.63 mm.
Statistical analyses were performed with SPSS for Windows version 18.0 (SPSS, Chicago, IL). Pearson correlation coefficients (r ) were calculated and linear regression analysis was applied to assign the effects of ocular biometrics on IOP. All models were adjusted for age and sex. One-way analysis of variance with Tukey honest significant difference test was used to compare study parameters between the three refractive state groups. p values of less than 0.05 were considered statistically significant in all statistical analyses.
RESULTS
The demographics of the subjects are summarized in Table 1 . One hundred fifteen subjects (214 eyes) participated in this study and were composed of 39 men (71 eyes) and 76 women (143 eyes). Four subjects (8 eyes) who had phacodonesis and zonular dialysis were excluded. The mean (±SD) age was 66 (±11.6) years (range, 21 to 86 years).
TABLE 1: Subject demographic information
The mean (±SD) IOP of all eyes from the entire subject population of this study was 12.33 (±2.55) mmHg. The mean (±SD) diurnal IOP fluctuation was 2.72 (±1.43) mmHg and diurnal IOP profiles are shown in Fig. 1 . The mean (±SD) CCT, CCP, AL, and SE were 542.37 (±38.32) μm, 45.33 (±1.24) diopters (D), 23.40 (±1.30) mm, and −0.33 (±3.21) D, respectively (Table 2 ).
TABLE 2: Subject biometric information
FIGURE 1: Diurnal IOP profiles according to the AL groups. There was no difference between the three groups; one-way analysis of variance with Tukey honest significant difference was used.
The subjects were divided into three groups based on tertile of AL values: 72 eyes were in group 1 and had an AL that was shorter than 22.85 mm, 71 eyes were in group 2 with an AL between 22.85 and 23.63 mm, and 71 eyes were in group 3 with an AL equal to or longer than 23.63 mm (Table 5 ). No significant difference was identified in the biometric data analyses of the three groups with the exception of the refractive states, ACD, and VCD. The mean SE was +0.06 D, +0.22 D, and −1.36 D in group 1 (AL < 22.85 mm), group 2 (22.85 mm ≤ AL < 23.63 mm), and group 3 (AL ≥ 23.63 mm), respectively.
Central corneal thickness was associated with mean IOP; thicker CCT has higher IOP (r = 0.217, p = 0.001). The following equation shows the correlation between mean IOP and CCT: IOPmean = 0.06 × CCT − 22. No relationship was found between CCT and diurnal IOP fluctuation (r = −0.013, p = 0.859) (Fig. 2 ). The AL was not associated with the mean IOP (r = 0.049, p = 0.476) and the diurnal IOP fluctuation (r = 0.058, p = 0.395) (Fig. 3 ).
TABLE 5: Intraocular pressure profiles by AL groups
FIGURE 2: (A) Linear regression that showed a correlation between CCT and mean IOP (p < 0.01, r 2 = 0.047). (B) Linear regression that showed no correlation between CCT and diurnal IOP fluctuation (p = 0.859).
FIGURE 3: (A) Linear regression that showed no correlation between AL and mean IOP (p = 0.476). (B) Linear regression that showed no correlation between AL and diurnal IOP fluctuation (p = 0.395).
The results did not identify any relationship between the examined IOP parameters (mean, time of peak, trough, and daily fluctuation) and the following values: CCP, refractive error, ACD, lens thickness, and VCD (Table 4 ). Mean ocular perfusion pressure was calculated as 2/3 of the mean arterial BP − IOP. The mean ocular perfusion pressure showed no correlation with IOP parameters, ACD, lens thickness, and AL (all p values > 0.05).
TABLE 4: Correlations between IOP and variables
DISCUSSION
In this study, we investigated the relationship between IOP profiles and ocular biometry. Ehlers et al.15 16 17
Increases in glaucoma in elderly patients occur like in cataract.18 8,19 8
There were some limitations to this study. One limitation was the absence of IOP measurement in the supine position and nocturnal period. Nocturnal IOP elevation has been noted in some studies8,19 20 21 22–25 25 Table 3 ). Six of eight excluded eyes had phacodonesis with zonular dialysis and had a long AL (25.88 to 28.54 mm) and we assumed that the diurnal IOP of those eyes would not have been within normal range. Although there were some limitations, our study was able to confirm the zonular state of all subjects and exclude subjects with zonular dialysis.
TABLE 3: Axial length distribution
In summary, mean IOP was associated with CCT, but not with AL. Based on the results, we propose that CCT should be considered in clinical practice to obtain a good approximation of the true IOP value. Additional studies, including more subjects with wide ranges of ocular biometric values and a wider range of ages, should be conducted to determine the relationship between ocular biometry and IOP. Our results can be used to correct errors, which may occur during IOP readings and interpretation, and to better understand the effects of ocular biomechanical properties on IOP as well as IOP fluctuation.
Joon Mo Kim
Department of Ophthalmology
Sungkyunkwan University School of Medicine
Kangbuk Samsung Hospital
108 Pyeong-Dong, Jongno-Gu Seoul
Korea
e-mail: [email protected]
ACKNOWLEDGMENTS
This study was supported by a grant from the Korea Health Technology R&D Project, Ministry of Health and Welfare, Republic of Korea (A101727).
Received July 20, 2014; accepted October 7, 2014.
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