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Intraocular pressure (IOP) is subject to cyclic fluctuations through the day. Diurnal variation in glaucoma was first reported in 1898.1 Duke-Elder and others reported high IOP on awakening.2–6 There is therefore a chance of missing a pressure elevation with single readings.
Phasing is mainly carried out from 0900 to 1800 hours, thereby missing any early morning spikes of IOP. Any delay in the first measurement will miss variations in IOP that are present immediately on wakening.4,7 We carried out a retrospective study, where patients under the care of the senior author (CL) are routinely admitted for phasing in order to obtain an early morning recording of IOP immediately on awakening.
A total of 93 patients (mean age 73.2 years) were admitted for phasing under the care of the senior author between December 1997 and August 2002. The first measurement of IOP took place between 0800–1000 hours. Further measurements were taken every 2 hours until 2000 hours. The final measurement was taken early morning (between 0600–0800 hours) immediately on awakening. Goldmann applanation tonometry was carried out by experienced staff.
In all, 17 patients requiring treatment or change in management for their glaucoma were identified. Of these, 13 were identified as having POAG. Four patients out of 13 (30.8%) showed peak IOP (23–28 mm Hg, mean 24.5) in the early morning (0600–0800 hours) immediately on awakening (fig 2). These cases would have been missed if routine outpatient measurements of diurnal variation only had been carried out.
The evaluation of IOP is usually based on measurements performed during office hours. As IOP is considered a major risk factor for glaucoma, an undetected IOP spike could be the missing link that has not been taken into account.8 Various investigators have shown that IOP just after awakening is increased.4,7 Phasing during routine office hours only would miss this. In our series we detected four patients with high IOPs in the early morning.
Goldmann tonometry was done in the sitting position, which might have reduced our chance to detect more cases, as IOP could be higher in the supine position.9 We did our best to take measurements immediately on awakening although there was a time delay in some patients because of reduced mobility. These limitations could have been avoided to some extent by using a portable tonometer such as the Perkins tonometer.
The disadvantages of our study were its retrospective nature, small population of study, and different members of staff involved in IOP measurements, thereby introducing variability. Staff members were not blind to the results but they do not have any vested interest in whether the early morning result is important. IOP could be overestimated in patients with thicker corneas, which we have not controlled for. We believe this factor, and its own diurnal variation, should be taken into consideration in future studies.
Based on our study we believe that more cases of glaucoma could be identified by early morning measurement of IOP during phasing.
The authors gratefully acknowledge the assistance of Clare Jenkins, clinical audit coordinator, Queen Victoria Hospital, East Grinstead, and Matthew Hankins, senior lecturer, Post Graduate Medical School, University of Brighton, Brighton, UK.
No proprietary interest or research funding.
Ethics committee approval: As this was a retrospective study, ethics committee approval was not required.
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