Resident/Fellow Research Day 2006

Diurnal Fluctuation and Concordance of Intraocular Pressure in Glaucoma Suspects and Normal Tension Glaucoma Patients

L.M. Shuba, MD, PhD
M.K. Maley, MD, M.B. Zimmerman, PhD, A.P. Doan, MD, PhD, R.B. Dinn, MD,
E.C. Greenlee, MD, W.L.M. Alward, MD, Y.H. Kwon, MD, PhD, sponsors

Purpose: The one-eye drug trial for intraocular pressure (IOP) lowering medications assumes the concordant fluctuation of the IOP between the fellow eyes. Our objective was to determine the diurnal variation and concordance of IOP in glaucoma suspects (GS) and normal tension glaucoma (NTG) patients.

Methods: A retrospective review was performed on GS and NTG patients with diurnal curves performed at the University of Iowa .  No subject had IOP greater than 21 mmHg. We defined GS patients as having suspicious optic nerves (ON) with normal visual fields (VF), and NTG patients as having glaucomatous ON with associated VF loss.  Subjects were excluded with: glaucoma surgery, trauma, use of anti-glaucoma or steroid medications, uveitis or secondary glaucoma. The IOPs were measured using Goldmann applanation tonometry at 10:00, 13:00, 16:00, 19:00, 22:00 and 07:00. The linear association of OD and OS IOP over the 6 time points was estimated using the average Pearson correlation coefficient (r).  Linear mixed model analysis for repeated measures, with Eye and Time as the within subject fixed effects, was used to test if the mean change in the IOP over time differs between OD and OS. For the analysis of concordance, the diurnal period was divided into seven time intervals of 3, 6, 9, 12, 15, 18 and 21 hours and the absolute difference in change in IOP between fellow eyes and probability that it was within 3 mm Hg were calculated. 

Results: Sixty eight GS subjects (29 males and 39 females; age 56.8±18.5 (SD), range 12-86 years) and 95 NTG subjects (32 males and 63 females; age 62.2±14.9 (SD), range 27-89 years) were included in the study. In GS subjects, the pooled mean IOP was 14.9 mmHg with the inter-eye SD of 1.4 (95% CI: 1.4, 1.6) and coefficient of variation (CV) of 9.7% (95% CI: 9.1, 10.4).  In NTG subjects, the pooled mean IOP was 15.5 mmHg with the inter-eye SD of 1.3 (95% CI: 1.2, 1.4) and CV 8.3% (95% CI: 7.8, 8.8). The average correlation of OD and OS IOP over the six time points was r=0.8 (95% CI: 0.7, 0.8) for GS, and r=0.8 (95% CI: 0.8, 0.8) for NTG subjects.  The mean absolute difference in IOP change between OD and OS over the six time intervals ranged between 1.4 and 1.9 mmHg for GS, and 1.3 and 1.5 mmHg for NTG subjects.  The probability that this difference was within 3 mmHg ranged between 87% and 94% for GS, and 86% and 93% for NTG subjects.

Conclusions: We observed high correlation and similarity of the mean IOP diurnal curves of fellow eyes in GS and NTG subjects. The current results show that the diurnal variation in IOP between the fellow eyes is mostly concordant within the limits of measurement variability in GS and NTG patients. Assuming no cross-over effect, we conclude that the one-eye therapeutic trial should help determine the effectiveness of a topical agent in approximately 90% of the time in GS and NTG patients.

Support: Residents and Fellows Research Program, Research to Prevent Blindness, Lew Wasserman Award, McLaughlin Fellowship.