The Effect of Experimental Silicone Encircling on Intraocular Pressure and Electroretinogram

ABSTRACT

The purpose of this study is to evaluate the course and prognosis of secondary glaucoma due to silicone encircling surgery for retinal detachment. The experiment was performed by observing the changes of elevated intraocular pressure and electroretinogram produced by silicone encircling in 20 rabbits. The rabbits were divided into 4 groups according to the elevattd level of intraocular pressure which was obtaintd by tightening of #240 silicone bands around the equator of the eyeball. The intraocular pressure was measured with a Schiotz tonometer and electroretinogram was recorded with a set of preamplifier, (freqwoncy reSponse 0.3-240 Hz, gain 80 decibel). Tektronix 5A18N dual trace amplifier, 5B12N dual time base, and 5103N oscilloscope. From the experiments, fo1lowing findings were obtained. 1. The effect on intraocular pressure. a) The elevated intraocular pressure returned to the preoperative level within 5-15 minutes in 35-45 mmHg group, 3-4 hours in 55-65 mmHg group, 4-6 hours in 75-85 mmHg group, and 7-8 hours in 95-105 mmHg group. b) In all the groaps, rapid fall of the intraocular pressure was observed within 10 minutes and relatively slow decrease was followed. From the second postoperative day, the intraocular pressure was maintained at a lower level than the preoperative intraocular pressure. 2. The effect on electroretinogram. a) In 35-45 mmHg group, supernormal wave was recorded during the 3 postoperative hours and then the wave returned to the preoperative amplitude. b) In 55-65 mmHg group, the electroretionogram reduced immediately after operation to 70% of the preoperative amplitude in 'a' wave and to 60% of the preoperative amplitude in 'b' wave' The reduced amplitude recovered to the preoperative level on the postoperative 1-2 days. c) In 75-85 mmHg group, both 'a' and 'b' wave were abolished immediately after the operation. The 'a' wave started to recover when the intraocular pressure decreased to 57.6-67.2 mmHg and 'b' wave started to recover when the intraocular pressure decreased to 49.9-57.6 mmHg. d) In 95-105 mmHg group, both 'a' and 'b' wave were abolished immediately after the oreration. The 'a' wave started to recover when the intraocular pressure decreased to 49.9-66.2 mmHg and 'b' wave started to recover when the intraocular pressure decreased to 37.2-40.2 mmHg. Consequently, it was found that the intraocular pressure over 75 mmHg caused irreversible damage to the function of retina, though the increased intraocular pressure due to silicone encircling returned to the preoperative level within 8 hours, and the visual cell layer of retina was more resistant to the increased intraocular pressure than the inner nuclear layer of retina.