IOP Physiology & Risk Factors

Aqueous Production & Outflow

The ciliary body produces aqueous humor into the posterior chamber. It flows through the pupil into the anterior chamber, then drains primarily through the trabecular meshwork into Schlemm's canal and out via episcleral veins, the conventional pathway, with a smaller uveoscleral (unconventional) route as well. Normal IOP sits between 10 and 21 mmHg; above that is considered statistically elevated. Glaucoma can still occur at pressures within this normal range, a distinction covered in more detail on the classification page.

Central Corneal Thickness: A Risk Factor, Not Just an Artifact

Thin central corneal thickness (CCT) is an independent risk factor for glaucoma, and the reason is two-part. Thinner corneas artifactually underestimate true IOP on Goldmann applanation tonometry: the device is calibrated assuming a standard corneal thickness (~520 µm), and a thinner, more easily-deformed cornea requires less force to flatten, so the reading undershoots (see tonometry methods and artifacts for the mechanics of applanation itself). But the OHTS trial showed that thin CCT (<555 µm) independently predicts conversion from ocular hypertension to POAG even after correcting for its effect on tonometric accuracy. Thin corneas carry real biological risk beyond the measurement artifact.

Diurnal Variation & Target IOP

IOP is not fixed through the day. It typically follows a diurnal rhythm, peaking in the early morning, driven by circadian variation in aqueous production and outflow. A normal daytime clinic reading does not rule out an undetected morning peak, which is why some patients benefit from diurnal IOP curves or home tonometry.

Target IOP is the individualized pressure goal set for a given patient to minimize further optic nerve damage. It is set from baseline IOP, degree of existing optic nerve damage, and disease stage, not a single population-wide cutoff, and more advanced damage generally demands a lower target.