18 Does Subconjunctival Filtration Affect Intraocular Pressure Fluctuation?

Greater short-term and long-term fluctuation of intraocular pressure (IOP) have been linked to glaucomatous progression in several studies.1–9 As such, it is important to understand the effects of various treatment modalities on circadian IOP fluctuation. Konstas et al,¹⁰ in a prospective observational study, reported that post-trabeculectomy patients have reduced mean, peak, and range of IOP compared with baseline IOP-matched glaucoma patients on maximally tolerated medical therapy. Mansouri et al¹¹ and Medeiros et al¹² demonstrated reduced peak IOP in response to a water drinking test in post-trabeculectomy patients compared with medically treated patients. A limitation to these and similar studies¹³ is that IOP is often measured at only a few time points in a 24-hour period, and usually with the subject in the sitting position. The sitting position results in a lower IOP than the habitual sleeping position,¹⁴ and we do not fully know the effect of wakefulness on nocturnal IOP measurements. Most importantly, having only a few measurements in a day means that peak or trough IOP values may be missed.

Why would patients with subconjunctival filtration have reduced diurnal or nocturnal IOP fluctuation? These patients have a new nonphysiological route for aqueous outflow, a drainage pathway directly from the anterior chamber into the subconjunctival space. Liu et al^15–19 demonstrated that, for most patients, IOP is highest during sleep, as measured in the habitual position. However, because aqueous production also decreases significantly during this time period,^20–22 the elevation in nocturnal IOP is hypothesized to be due to changes in aqueous outflow. Sit et al²³ explored whether changes in outflow facility, episcleral venous pressure, or uveoscleral flow account for the higher nocturnal IOP. Based on their modeling results, they concluded that changes in episcleral venous pressure or uveoscleral flow were responsible for elevated nocturnal IOP. Assuming this is true, it follows that post-trabeculectomy patients would have reduced IOP fluctuation because trabeculectomies bypass the physiological outflow mechanisms responsible for the circadian variation of IOP.

Furthermore, the fact that post-trabeculectomy patients also withstand the water drinking test with less of an IOP spike¹¹ suggests that not only does subconjunctival filtration offer a circadian-independent mode of aqueous outflow, it also seems to provide a lower resistance pathway for dealing with aqueous loads compared with medically treated patients. This supports the idea that a post-trabeculectomy patient would be able to better withstand stressors from aqueous overproduction and thus would have reduced long-term fluctuation in IOP.

Clinical data have shown that patients with subconjunctival filtration have reduced IOP fluctuation^10–13 and that patients with reduced IOP fluctuation have reduced rates of glaucomatous progression.^1–9 The greatest limitation to understanding all of this lies in our extremely limited data points of IOP throughout any given day, a problem that new continuous monitoring technologies aim to address. If drinking a liter of water in 15 minutes can cause significant IOP elevations in unoperated glaucoma patients, what other activities and habits among them are causing IOP spikes that we have never detected in the clinic? Until we can advance from isolated snapshots of IOP to a continuous around-the-clock assessment, our understanding will be limited by our numerous assumptions about what is truly happening.

References

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