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Pressure Phosphene Self-Tonometry: A Comparison with Goldmann Tonometry in Glaucoma Patients

¹From the Department of Ophthalmology and Visual Sciences, The Chinese University of Hong Kong, Hong Kong Eye Hospital, Kowloon, Hong Kong, People’s Republic of China; and the ²Department of Ophthalmology, National University of Singapore and Singapore Eye Research Institute, Singapore.

PURPOSE. To evaluate whether the pressure phosphene tonometer (PPT) is suitable for self-tonometry in patients with glaucoma or ocular hypertension.

METHODS. This was a prospective comparative study of 102 eyes of 102 patients with chronic glaucoma or ocular hypertension. Intraocular pressure (IOP) measurements by the Goldmann tonometer (GT) were compared with self-measured readings with the PPT. Patients evaluated the ease of home use of the PPT. The last 15 patients were asked to stop their glaucoma medications, and the ability of the PPT to detect an elevated IOP during self-tonometry was studied.

RESULTS. The mean ± SD difference between PPT and GT readings was –0.24 ± 1.57 mm Hg. Comparing the PPT with the GT, 86% of the readings were within ± 2.0 mm Hg, and 91% were within ± 3.0 mm Hg. Spearman’s correlation coefficient was 0.91. A Bland-Altman plot showed that the 95% limits of agreement between the two methods lay between 2.90 and –3.38 mm Hg. Within-subject coefficients of variation for the GT and the PPT were 4.4% and 7.3%, respectively. In detecting an elevated IOP of more than 21 mm Hg, the sensitivity and the specificity of the PPT were 92.3% and 98.6% respectively. The mean satisfaction score for home use of the PPT was 87.4 ± 16.3 (maximum 100).

CONCLUSIONS. With proper training and technique, self-tonometry with the PPT appears to be accurate up to at least 25 mm Hg and is reproducible. The PPT was sensitive and specific in detecting an elevated IOP of more than 21 mm Hg. As patients were expected to seek ophthalmic care before the self-measured IOP reaches 25 mm Hg, the PPT may have a value for self-monitoring. Patients rated the PPT as satisfactory for home use. Because the PPT is portable and relatively inexpensive and requires no topical anesthesia or direct contact with the eyeball, it may have potential as an instrument for home self-tonometry.

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