THE MECHANISM OF INTRAOCULAR-PRESSURE RISE DURING CYCLOCRYOTHERAPY

Purpose. Intraocular pressure (IOP) spikes that occur during cyclocryo therapy for advanced glaucoma may further injure the already damaged g laucomatous optic nerve and be responsible for visual impairment that may occur after this treatment. The authors investigated the mechanism of pressure rise to see whether it can be avoided and thus prevent fu rther optic nerve injury. The authors postulated that intraocular ice forms during the cryo procedure and causes the pressure changes. Metho ds. Intraocular pressure was monitored using a pneumatonometer during 15 cryocycles of four patients with advanced glaucoma and 21 cryocycle s of five normal rabbits, A simple thermal model was developed to anal yze the relation between volume expansion and pressure rise in the eye . The physical effect of freezing rabbit eye structures was investigat ed in vitro. Results. The largest pressure spikes observed during the cryocycles in this work were increases of 32 mm Hg for humans and 25 m m Hg for rabbits. The mean value of the IOP immediately before and aft er the cryo freezing stage was 53 +/- 1 and 68 +/- 2 mm Hg, respective ly, for humans and 22 +/- 1 and 32 +/- 1 mm Hg for rabbits. The parame ters of the thermal model were determined from the observed IOP spikes . Calculated thaw times were consistent with measured times for return to precryo IOPs. In vitro cryoapplication (rabbit eye) showed the for mation of an ice ball internal to the eye. Conclusions. Volumetric inc rease of the intraocular content, related to the formation of an ice b all in the eye, is the mechanism of pressure spikes during cyclocryoth erapy. Because this complication is unavoidable, other cyclodestructiv e methods may be more prudent, particularly in patients with advanced glaucoma.