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M. A. Kapamajian, G. M. Bang, T. Vajaranant, J. de la Cruz; Correlation Between Corneal and Scleral Pneumotonometry Measurements in Adults with Healthy Eyes. Invest. Ophthalmol. Vis. Sci. 2009;50(13):1503.
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To determine if a correlation exists between corneal and scleral pneumotonometry and to assess the accuracy of scleral measurements so as to provide an alternate method of measuring IOP when the cornea cannot be used (i.e. in patients with keratoprostheses).
Subjects were recruited from the General Eye Clinic at the University of Illinois Eye and Ear Infirmary. Inclusion criteria were age 18 years or older and no history of ocular pathology/surgery/trauma. Exclusion criteria were myopia > 6 diopters (D), hyperopia > 3 D, astigmatism > 2 D, pregnancy, or history of prisoner status. Measurements on each eye included 1 central corneal pneumotonometry measurement, 3 scleral pneumotonometry measurements (2 mm from the limbus in the inferotemporal quadrant), 5 axial length (AL) measurements, and 3 central corneal pachymetry (CCT) measurements. An average of the repeated measurements was used for final analysis. Statistical data was calculated using the paired t-test and Pearson’s correlation.
Fifty eight eyes from 30 subjects (range = 18 to 82 years) were included. Mean SE refraction was -0.74 ± 2.06 D, mean AL was 24 ± 1 mm, and mean CCT was 528 ± 34 µ. Mean corneal pneumotonometry reading was 17 ± 3 mmHg (range = 12 to 26). Mean scleral pneumotonometry reading was 25 ± 8 mmHg (range = 13 to 46). Scleral IOP was consistently higher than that of corneal IOP (mean 7 ± 6 mmHg, p < 0.01). There was good correlation between scleral and corneal IOP (r = 0.78, p < 0.01). The difference between scleral and corneal IOP correlated well with scleral IOP (r = 0.88, p < 0.01), but less with corneal IOP (r = 0.39, p < 0.01) and CCT (r = -0.40, p < 0.01). The difference between scleral and corneal IOP showed no statistically significant correlation with AL (r = -0.10, p = 0.50).
While scleral pneumotonometry readings appear to be consistently higher than their corneal counterparts, both are highly correlated with one another. Additionally, scleral IOP measurements are more accurate at lower values. To our knowledge, these are the first findings that provide a basis with which scleral pneumotonometry may be applied and interpreted when the cornea cannot be used, such as in eyes implanted with keratoprostheses.
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