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The effect of ambient illumination and time in the dark on force required to percieve a pressure phosphene

1 May 2005


Purpose: Intraocular pressure has been correlated with the force required to allow perception of a pressure phosphene. The Proview tonometer is based on this principle and can be used to self monitor intraocular pressure. This study explores the question of whether dark adaptation affects the force required and thus the intraocular pressure indicated with the Proview tonometer.

Methods: Fifteen healthy subjects measured and recorded intraocular pressure with the Proview tonometer at five minute intervals for 25 minutes in a dark room. They were also monitored throughout this time with the Tonopen tonometer. In addition, intraocular pressures were measured prior to and following the 25 minutes in the dark using the Goldmann tonometer.

Results: After ten minutes in the dark the intraocular pressure indicated by the pressure phosphene tonometer had increased significantly compared to baseline measurements and concurrent Tonopen measurement. The force required to allow perception of the phosphene increased with each successive five minute interval until the data collection was stopped at 25 minutes and the subjects returned to normal room illumination. The Goldmann and Tonopen measurements did not vary, indicating there had been no actual change in intraocular pressure.

Discussion: Intraocular pressure as measured with the pressure phosphene method is independent of ambient illumination as there was no difference after just five minutes in the dark, but length of time in the dark is highly correlated with an increase of measured pressure. Aththough this doesn't match the dark adaptation curve, it indicates that the force needed to create the phosphene perception is affected by some ocular change that occurs with prolonged period in dim illumination. Patients using the Proview tonometer should be advised to avoid taking measurements under such conditions in order to avoid spurious high readings.


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