1. Introduction

    Intraocular pressure is determined by the balance between production and drainage of aqueous humor and by changes in choroidal blood volume. Resistance to outflow of aqueous humor in the trabecular tissue maintains IOP within physiologic range. Normal IOP is 12 to 16 mmHg in the upright posture and increases by 2 to 3 mmHg in the supine position.

    When the globe is open, the IOP is equal to ambient pressure. If the volume of choroid and vitreous should increase while the eye is opened, the vitreous may be lost. Any deformation of the eye by external pressure on the globe will cause an apparent increase in intraocular volume. This discussion is of open-eye procedures.

2. Preoperative assessment and patient preparation