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Editor,

For almost 50 years, the Goldmann applanation tonometer (GAT) has been the international ‘gold standard’ for the measurement of intraocular pressure (IOP). It is commonly noted that IOP pressure measurements obtained with the use of a GAT are affected by different biometric ocular parameters, including central corneal thickness (CCT), corneal curvature (keratometry readings K1, K2) and axial length (AL; Whitacre & Stein 1993; Kohlhaas et al. 2006). CCT in a population varies widely and ranges from 440 to 640 μm (Doughty & Zaman 2000). A thick cornea has been calculated to lead to higher, and a thin cornea to lower, IOP reading values. This effect is not always taken into account in practices and hospitals where the measurement of CCT is not routine in patients presenting for glaucoma assessment. A reduction in IOP of a little as 1 mmHg evokes a 10% reduction in the risk for visual field deterioration and a 10% improvement in outcome for patients suffering ocular hypertension (Chihara 2008). Thus, the bias in IOP measurement with a GAT may have significant effects on the results of glaucoma treatment. Myopia is a well-known risk factor for primary open-angle glaucoma (Chihara 2008). A number of studies have attempted specifically to address whether CCT in myopes is different from CCT in hyperopes, but the overall results are inconclusive. Owing to possible errors between CCT and IOP readings, different correction formulas have been created to furnish true IOP readings (Whitacre & Stein 1993; Kohlhaas et al., 2006; Chihara 2008).

The aim of this study was to evaluate the relation of biometric profile of eye with IOP and the validity of the Orssengo, Whitacre and Dresdner nomogram (Whitacre & Stein 1993; Kohlhaas et al., 2006) in correcting GAT IOP in healthy, non-operated myopic patients (one or both eyes myopic). In Jyväskylä Central Hospital 230 eyes of 115 non-glaucomatous patients (53 men and 62 women, mean age 34.5 ± 1.51 years, range 29.2–37.4 years) were examined. Scheimpflug imaging (Pentacam®; Oculus, Wetzlar, Germany) was used to measure the following ocular parameters: CCT, anterior chamber depth (ACD), anterior chamber volume (ACV), anterior chamber angle (ACA) and corneal curvature (K1 and K2 diopters). IOL master® (Carl Zeiss Meditec, Jena, Germany) was used to measure the AL of the eye and K values. Pearson’s correlation test was used to evaluate the relationships between IOP and the ocular parameters.

Table 1 shows the mean spherical equivalents (SE), IOP, CCT, ACD, ACV, ACA, AL and K values for the right and left eyes. All the measured parameters show considerable variation.

Table 1.   Refraction and ocular biometric parameters of 115 myopes.
 SE (D)IOP (mmHg)CCT (μ)ACD (mm)ACV (μl3)ACA (°)AL (mm)
DXSINDXSINDXSINDXSINDXSINDXSINDXSIN
Mean−4.82−4.8716.2316.39528.8530.43.213.23193.2196.338.139.425.3425.32
SD2.242.152.832.8035.30 36.080.280.2732.5031.785.675.680.990.96
Range11.25–+1.1310.50–+0.389–259–23437–645444–6512.50–3.952.54–3.93111–293114–28524.2–50.524.1–53.522.91–28.3522.75–28.62
 IOL MasterPentacam
K1K2Avg KK1K2Avg K
DXSINDXSINDXSINDXSINDXSINDXSIN
  1. SE = spherical equivalent, IOP = intraocular pressure, CCT = central corneal thickness, ACD = anterior chamber distance, ACV = anterior chamber volume, ACA anterior chamber angle, AL = axial length, DX = right eye, SIN = left eye, SD = standard deviation, K1 = diopters in vertical meridian, K2 = diopters in horizontal meridian, Avg = average.

Mean43.4243.4444.4844.4943.9543.9743.2443.2843.9744.0143.6143.65
SD1.211.271.321.341.221.261.211.611.391.401.201.38
Range40.66–46.6840.47–46.2341.56–47.8041.51–47.4741.11–46.8941.07–46.7640.60–46.1040.50–54.1040.80–47.4041.00–47.2040.75–46.4540.85–50.00

No significant correlations between non-corrected GAT IOP and the measured biometric variables were found. Orssengo, Whitacre and Dresdner corrections to the GAT IOP values increased the mean GAT IOP values by 0.81, 0.20 and 0.75 mmHg in the right eye and by 0.73, 0.17 and 0.77 mmHg in the left eye. After correction of the IOP values, the Orssengo- and Dresdner-corrected IOP values showed a significant association with CCT (p < 0.0001 and p < 0.01, respectively). The non-corrected GAT IOP measures or Whitacre-corrected IOP values show no significant correlations with CCT, ACD, ACV or ACA. Corneal curvature corrections were not statistically significant.

Our findings suggest that in myopic subjects, when CCT values are within normal limits, the influence of anterior segment parameters or AL of the eye is not reliable related to the IOP readings for different IOP correction formulaes. However, a difference of up to 208 μm was observed between the thinnest and thickest cornea in the present sample of myopes. It is estimated that a difference in CCT of 25 μm may cause a change of about 1 mmHg in the GAT IOP readings. Therefore, the 208 μm variance measured in the myopic population might evoke a variation of about 8 mmHg in the true IOP value. Previous findings indicate that CCT is not systemically altered in myopia and different instruments may affect on biometric parameters that disturb results of correction formulaes (Pedersen et al. 2005). When myopia per se is a risk factor for glaucoma, this variation in CCT should be taken into account when measuring IOP values in myopes. Further studies are required for IOP correction formulaes in myopes.

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