I enjoyed reading the article by Hara et al1 entitled “Increase of Peak Intraocular Pressure During Sleep in Reproduced Diurnal Changes by Posture.” I would like to add a thought to the discussion of the findings. Without a doubt, the intraocular pressure (IOP) increases in the supine position compared with the sitting position. As also pointed by Weinreb and Liu,2 one of the likely reasons for the IOP elevation in the supine position is the increase in episcleral venous pressure. When discussing the effect of an IOP elevation, one may consider the counterpressure against the IOP on the other side of the lamina cribrosa. The counterpressure is the cerebrospinal fluid pressure because the optic nerve as a fascicle of the brain is surrounded by meninges and cerebrospinal fluid. A change from the sitting position to the supine position may increase the cerebrospinal fluid pressure more or less parallel to the increase in the episcleral venous pressure. One may therefore discuss whether the “bad” increase in the IOP in the supine position may partially or completely be compensated for by the “good” increase in the cerebrospinal fluid pressure, leaving the trans–lamina cribrosa pressure difference and gradient unchanged. Another aspect in the physiology of the trans–lamina cribrosa pressure difference is the intra–lamina cribrosa blood perfusion, which may be decreased if the pressure in the lamina cribrosa is elevated even if the trans–lamina cribrosa pressure difference is unchanged.
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