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R. A. Barraza, A. J. Sit; Investigation to Determine a Relationship Between Scleral and Corneal Tonometry. Invest. Ophthalmol. Vis. Sci. 2010;51(13):580.
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Most devices used to determine intraocular pressure (IOP) perform measurements on the corneal surface. In some patients, such as those with keratoprosthesis, corneal measurements of IOP are impossible. Alternative approaches, such as digital palpation of the globe, are crude and prone to subjective variation. Although scleral tonometry has been reported as an unreliable method to estimate IOP, this method has not been extensively or precisely explored. The purpose of this study was to determine whether a predictable relationship between corneal- and scleral-derived IOP measurements could be determined
Subjects aged 18 years and older were recruited from the glaucoma clinic at the Mayo Clinic. Healthy subjects, glaucoma suspects, and patients with glaucoma but without prior glaucoma surgery were included. We obtained IOP measurements from 2 locations on the sclera (1-2 mm, and 4-5 mm temporal to the limbus) and compared them to corneal tonometry measurements using two tonometers: the Reichert TonoPen XL and Model 30 Classic pneumatonometer. The mean corneal-derived IOP was plotted against mean scleral-derived pressure for each anatomic site. Regression analyses were performed to determine a relationship between scleral- and corneal-derived tonometry.
Fifty eyes from 34 subjects (20 females, 14 males) were included in the study. The average age of the subjects was 72 years (range 33 to 89). The mean IOP derived by corneal tonometry with TonoPen and pneumatonometer was 17.80 mmHg (range 9.3 to 40.0) and 17.84 mmHg (range 10.8 to 42.0) respectively. We found no predictive value in scleral tonometry using the TonoPen. However, there was a significant relationship between scleral and corneal tonometry using a pneumatonometer. This relationship was strongest at the 4-5mm scleral location with R2 =0.40 (P=0.66) for a logarithmic curve. Residual values ranged from -9.48 to 22.91 mmHg. A weaker relationship was observed at the 1-2mm scleral location with R2 =0.25 (P<0.01). There was no correlation between outliers and other variables such as type of glaucoma, history of laser therapy or cataract surgery, or central corneal thickness.
Scleral tonometry using the TonoPen did not provide accurate or predictable IOP measurements compared to corneal tonometry. Use of the pneumatonometer for scleral tonometry may be of limited but useful predictive value for IOP. Further testing will be required to determine whether other scleral sites may be of improved predictive value, and whether this predictive value holds for eyes with prior corneal or glaucoma surgery.
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