Short-term IOP Fluctuation
As with long-term IOP fluctuation, there is no consensus on how to define short-term IOP fluctuation. One commonly used definition applied by Lee et al defines it as the difference between the highest and lowest IOP over 24 h or less, though the SD in IOP based on a curve has also been advocated.
Most studies of repeated IOP measurements throughout the day have determined that values tend to peak early in the morning and decline over the course of the day, though the exact timing of peaks and nadirs is not entirely uniform across reports.
Studies That Argue That Short-term IOP Fluctuation is Significant
Numerous reports demonstrate a correlation between increased short-term IOP fluctuation and pressure-related eye diseases. David et al assessed 690 IOP curves made from four to six measurements over the course of 10.5 h, and found that the difference in mean IOP peak and trough differed significantly between healthy eyes (5 mm Hg), eyes with OAG and chronic angle-closure glaucoma (ACG) (5.8 mm Hg), and eyes with OHT (6.8 mm Hg). A more recent study also showed that IOP fluctuation is significantly higher in eyes with primary chronic ACG (7.69±3.03 mm Hg) or POAG (8.31±2.58 mm Hg) compared to normal eyes (4.83±2.46 mm Hg). Similarly, Grippo et al followed the protocol of prior works and combined results to determine that daytime IOP variation—though not nighttime variation—was significantly greater in subjects with OHT and glaucoma than healthy individuals.
Baskaran et al determined that the difference between the peak and trough IOP in patients with primary angle closure (PAC) and PACG was significantly greater than that in PAC suspects and controls. Further, Kim et al noted that during waking hours, patients with NTG had a significantly greater degree of IOP fluctuation than controls.
Tajunisah et al reported similar results in a study of glaucoma suspects, noting that the mean magnitude in IOP variation for patients with POAG, NTG, glaucoma suspicion and OHT was significantly higher than that for controls. Saccà et al note that patients with POAG had a greater relative daily IOP fluctuation (7–9.6%) than patients with NTG (−4.7–6.4%) or healthy eyes (−3.4–6.9%).
Twenty-three patients of a cohort of 29 with OHT were available for final analysis in a 5-year follow-up study of IOP by Thomas et al, which found that four patients had progressed to glaucoma. Diurnal IOP profiles of these patients with readings taken every 2 h from 10:00 to 18:00 determined that the mean IOP variation was 8.6 mm Hg in the patients who progressed, compared to 5.4 mm Hg in those who did not.
One study used frequency doubling technology (FDT) perimetry to study 33 eyes with OHT while measuring IOP at 0830, 1030 and 1530. The authors discovered that office IOP fluctuation was significantly greater in patients with abnormal FDT perimetry results (median 5 mm Hg) compared to those with normal FDT perimetry results (median 2 mm Hg). Office IOP fluctuation was also correlated with the number of depressed points on FDT perimetry. Similarly, Sakata et al shows that 24 h IOP fluctuation in the habitual position in patients with NTG had a significant, negative correlation with mean VF deviation.
Analysis of daytime IOP curves from 149 patients with OHT found that 33 of the eyes studied had a difference of more than 5 mm Hg between the peak and trough IOP in their diurnal curve. Of these, 64% had VF defects within 4 years. By contrast, of those eyes with a fluctuation in IOP not exceeding 5 mm Hg, 84% had normal VFs after at least 5 years; these differences were significant.
In a prospective study, Asrani et al reported on 105 eyes from 64 patients with OAG who were monitored via VF and home tonometry five times daily. Patients in the highest quartile of home IOP range (at least 11.8 mm Hg) had a cumulative risk of VF progression of 88% over 8 years, compared to 57% for patients in the lowest quartile for IOP range (no more than 7.7 mm Hg).
A study of 3561 IOP profiles from 720 patients performed by Jonas et al analysed patients with healthy eyes, ocular HTN, preperimetric glaucoma (patients with normal VF data, but a glaucomatous-appearing optic nerve), POAG, secondary OAG and NTG. The authors found that mean IOP fluctuation was significantly greater in patients with OHT, preperimetric glaucoma, or secondary OAG as compared with healthy eyes. Interestingly, there was no significant difference in mean IOP fluctuation in those with perimetric POAG compared to those with preperimetric glaucoma, OHT, or normal eyes. Moreover, eyes with NTG had no significant difference in IOP fluctuation compared to healthy eyes, but a significantly lower IOP fluctuation than eyes with OHT or preperimetric glaucoma.
Studies That Argue That Short-term IOP Fluctuation is not Significant
Several reports suggest that short-term IOP fluctuations may not be significant. Sung et al examined 24 h IOP and VF data over approximately 6 years in patients with newly diagnosed NTG. Univariate analysis showed no correlation between VF progression and either 24 h or follow-up IOP fluctuation, among other findings.
Similarly, Lee et al monitored diurnal IOP in 177 patients with NTG. In univariate and multivariate analysis, the authors found no significant correlation between IOP fluctuation and either pattern standard or mean deviation on VF. Additionally, a series of 14 patients with newly diagnosed, untreated POAG, by Sehi et al found no significant association between diurnal changes in IOP and optic nerve head appearance.
Likewise, Wang et al state that they found no significant association between 24 h IOP fluctuation and mean VF deviation in patients with POAG. Further, in patients with unilateral POAG, they found no difference in SD of diurnal IOP variation or IOP fluctuation between fellow eyes.
As indicated above, in their analysis of patients with POAG, Jonas et al found no association between glaucoma progression and IOP amplitude. They also reported no significant association between IOP fluctuation and glaucoma progression in an analysis of patients with NTG and POAG as a single group. Also alluded to previously, Fogagnolo et al found no significant difference between patients with POAG who progressed and those who remained stable in regard to short-term IOP fluctuation.
Finally, Smith measured IOP every 2 h from 0500 to 1500 and attained VFs in patients with OHT, suspected glaucoma and glaucoma. Analysis found mean diurnal IOP variation in 400 eyes with VF defects was not significantly different compared to 400 eyes without VF defects.