Monitoring IOP pressure during sleeping hours, two doctors test it out for themselves
To find the proper diagnosis, the intraocular pressure (IOP) is usually measured 3–4 times during the day (office hours) in a sitting position with Goldmann's applanation tonometer. The results will be a peak, a trough, the mean and the fluctuations of the values.
Very rarely the IOP is measured in a supine position. It is well known, that the IOP will increase in the supine position due to the rise of the episcleral venous pressure and changes of the compartments of fluids.1–4 These measurements can be done by a pneumo-tonometer (Tonopen) or by an applanation tomometer (Perkins).
However, it is not the recumbency alone that leads to higher pressure. In 90% of healthy individuals and in two thirds of patients with glaucoma, who were supine during the entire day time, the peak IOP was during the night, in the very early hours of the morning.5 This is an astonishing finding in that the production of aqueous humour, the flow, is reduced by half during the night-time.6,7 The sophisticated night-time measurements took place in a sleep laboratory, using infrared goggles so as not to disturb the humoral rhythms by switching on bright lights.
Day-time IOP curves provide us with very short-time information about the IOP and with no information about its behaviour during the night-time, despite glaucoma being a 24-hour disease. Several attempts have been made to develop devices for continuous IOP measurements.8 It was envisaged that this might succeed with a microchip and pressure sensitive sensor embedded in an intraocular lens or in a contact lens.
Triggerfish – contact lens sensor
In 2008 the Swiss start-up company Sensimed achieved CE certification for its product, Triggerfish. This followed extensive experimental trials with rabbits around safety, validity and reproducibility.9,10 The device (see Figure 1) for measuring the IOP consists of:
All activities, including sports, lying flat in a supine position, playing wind instruments, sleeping, using the eye drops etc., should be recorded in a diary to provide feedback about why there may be changes in the profile of the curve.