Thirteen monkeys with catheter reservoir systems for cerebral spinal fluid pressure measurements were exposed to 3500 rads of x-irradiation at 250 kVp, 30 mA, in a single dose to the right visual cortex. In eight, there was abrupt brain swelling 18-36 weeks after irradiation, reflected by cerebral spinal fluid pressure 3-4 times baseline, with absence of visual evoked response and behavioral blindness in four, and unilateral visual evoked response depression in the other four. After the intravenous injection of Evans Blue, seven monkeys, sacrificed between 20 and 36 weeks, showed proliferative and degenerative changes in the right occipital lobe which was deeply stained with dye. The swelling extended throughout the right hemisphere with distortion in brain structures. One sacrificed at 52 weeks, after resolution of increased pressure, had a fibrotic, nonstained occipital lesion with atrophic changes in both hemispheres. In five there were less pronounced functional and structural changes. Two nonirradiated controls remained within normal limits. The cause for the delayed brain swelling was a breakdown in the blood brain barrier accompanying the delayed radiation lesion, with extravasation of fluid into brain parenchyma. The impaired function was related to local destruction in the right visual cortex and swelling or compression of the visual pathways by the vasogenic edema.

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