Evaluation of the pressure phosphene tonometer as a self-tonometer

Different from conventional tonometers, the pressure phosphene tonometer (FPT) measures intraocular pressure (IOP) through the upper eyelid without corneal applanation. We evaluated the usefulness of the FPT as a self-tonometer by comparing FPT IOP readings with those obtained with the Goldmann applanation tonometer (GAT). We also evaluated the influence of central corneal thickness (CCT) on IOP measurements obtained with the two different devices.

We confirmed the repeatability of FPT measurements in a preliminary study. The main investigation formed part of a prospective clinical trial, in which IOP was measured in 101 eyes of 101 participants (55 normal and 46 glaucomatous eyes) using GAT and FPT. FPT measurements were self-acquired by each participant. CCT was measured with an ultrasonic pachymeter. The agreement between FPT and GAT measurements was evaluated by the method of Bland and Altman. Using individual IOP and CCT values, we determined the correlation coefficients and performed regression analysis.

FPT met the British Standard criteria for reproducibility. Among 101 participants, seven patients with glaucoma were unable to detect the pressure phosphene and to measure IOP using the FPT. By the statistical method of Bland and Altman there was a significant difference between FPT readings self-measured by the remaining 94 participants and GAT readings obtained by an ophthalmologist. There was no correlation between FPT and CCT readings in 65 participants with no prior history of anti-glaucoma medications or glaucoma surgery.

Self-tonometer FPT readings differed from GAT readings. However, we suggest that the FPT may be a clinically acceptable device because it enables patients to self-measure their IOP easily and safely. As FPT readings are not affected by CCT, this tonometer may be of clinical and practical value for the at-will measurement of IOP in patients with corneal changes.