Aims To evaluate postural fluctuations (PFs) and diurnal variation (DV) of intraocular pressure (IOP) in patients with untreated glaucoma, glaucoma suspects and healthy volunteers, and study their relationship, if any, to the extent of glaucomatous damage.

Methods This prospective, observational cross-sectional study was carried out in a tertiary care referral institution. The patient population included five groups of patients comprising the following: 19 with ocular hypertension (OHT), 26 with optic discs suspicious for glaucoma (DS), 18 with normal tension glaucoma (NTG), 19 with primary open angle glaucoma (POAG) and 20 normal subjects. The IOP was measured at four time periods using Perkins tonometer, in sitting and supine positions. The main outcome measures were change in IOP with posture, the DV in both postures and the relationship between PF, DV and the extent of visual field damage.

Results The supine IOP was significantly higher than the sitting IOP, at all time points of the day, in all groups (p<0.001). The PF at 04:30 was significantly higher in POAG, OHT and NTG. The PF at 09:00 correlated significantly with the mean deviation (MD) on visual fields in the NTG group (r=0.735; p=0.001). The DV did not correlate with the MD in any of the five groups studied.

Conclusions The significantly higher supine IOP is frequently missed in routine glaucoma practice. An early morning supine IOP measurement may reveal a peak IOP hitherto not picked up during routine office IOP measurements, and may be a useful measurement in unexplained progressive glaucoma.