14 How Does Cyclodestruction Affect Diurnal Intraocular Pressure Variation?

It is well accepted that intraocular pressure (IOP) fluctuates through the 24-hour light/dark cycle. Some argue that IOP peaks in the morning because of the morning rise in cortisol,1 whereas others assert that IOP peaks in the evening due to the supine position during sleep.² IOP fluctuations may explain why some patients continue to experience visual field decline despite seemingly adequate IOP control during clinic appointments. Some studies have found that the range of IOP fluctuation is greater in patients with glaucoma versus normal subjects,³ and that it may even be an independent risk factor for glaucoma progression.⁴

Several studies have looked at the effect of medical, laser, and surgical therapies on IOP fluctuation. Over a 24-hour period, prostaglandins appear superior to timolol, brimonidine, and dorzolamide in decreasing IOP.⁵ Timolol showed good diurnal IOP reduction but poor nocturnal IOP reduction.⁶ Both argon laser trabeculoplasty (ALT)⁷ and selective laser trabeculoplasty (SLT)⁸ effectively decrease mean IOP and the range of IOP. Interestingly, in some cases in which SLT did not achieve significant IOP reduction during clinic hours, nocturnal IOPs were diminished after laser treatment.⁹ Compared with medical therapy, surgical therapy with trabeculectomy may be more successful in blunting IOP fluctuation.^10,11

Studies that measure IOP fluctuation have several limitations. Due to the logistical difficulties, they typically have small sample sizes, IOPs may be measured for only 8 to 20 hours during a 24-hour period, or IOPs may be measured every 2 to 6 hours instead of every hour. Also, study protocols may differ in how IOP measurements are made (sleep center versus home monitoring), in what body position nocturnal IOPs are measured (supine versus sitting), or how much time elapses between awakening and IOP measurement. It has been suggested that there may be a transient IOP spike immediately after awakening.¹²

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