Positive vitreous pressure: Pathophysiology, complications, prevention, and management.

Positive vitreous pressure occurs during anterior segment intraocular surgery associated with acute hypotony and is characterized by forward displacement of the lens-iris diaphragm with shallowing of the anterior chamber resistant to reformation, repeated iris prolapse, and, in severe cases, zonular rupture and vitreous or lens prolapse that can lead to a cascade of intraoperative complications. Positive vitreous pressure is particularly common during penetrating keratoplasty, conventional nuclear expression cataract extraction, and repair of anterior open-globe injury. Hypotony resulting from aqueous loss leads to elevated vitreous pressure from 3 possible causes: external scleral compression, acute intraocular intumescence, or rarely acutely increased vitreous volume. Understanding the pathophysiology of positive vitreous pressure helps in its prevention and management.