Experimental transscleral freezing at temperatures ranging from −20 to −160°C

The ophthalmoscopic and histologic aspects of cryocoagulation in 264 rabbit eyes were studied. Temperatures ranged from -20° to -160° C, times of freezing varied from 5 to 60 seconds. The periods of observation ranges from 30 minutes to 3 months after cold treatment. 1. Scleral thinning or scarring was not observed and there were no signs of overdosage: scleral lesions (except pigment migration) disappeared within two to four weeks after cold application. The condition of the sciera two weeks after intense cryocoagulation was invariably good enough to permit reoperation. 2. Choroidal lesions were reversible irrespective of temperature and time of freezing, with the exception of pigmentary changes and atrophy of the choriocapillaris. At -100°, -130° and -160° hyperaemia, haemorrhages and plasmatic exudation in the choroid were more intense than at higher temperatures. At -130° and-160° the vessel walls were damaged to such an extent that many choroidal and subretinal haemorrhages developed immediately after freezing. 3. After freezing at -40° and lower temperatures an irreversible atrophy of all the retinal layers developed. At -60°, and in particular at -100° and lower temperatures, retinal haemorrhages and oedema occasionally developed, which were sometimes so intense as to cause a rupture of the internal limiting membrane. 4. The early subretinal oedema located in the area of freezing developed most rapidly and was most intense after freezing at -100°; it caused numerous retainl tears in the treated area during the first few hours after cold application but was rapidly resorbed in the course of one day. At -130° development and resorption of the oedema occurred more slowly; it was still present in the stage of incipient chorioretinal adhesion. At -160° the late subretinal oedema developed one week after freezing, leading to retinal tears around the area of coagulation where a good chorioretinal adhesion had already been attained. The above mentioned complications interfered with a good chorioretinal adhesion. 5. Cold-induced lesions of the vitreous developed only at temperatures of -130° and -160°. Small vitreous haemorrhages were resorbed and vitreous clouding which was also ophthalmoscopically visible frequently cleared. Vitreous strands would sometimes interfere with good chorioretinal adhesion. 6. The intensity of the lesions induced by the Linde cryode is influenced by various factors :