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In nine baboons following intracisternal injection of a suspension of their own washed platelets, biphasic spasm was demonstrated by vertebral angiography. Constriction of the intra-arachnoid portion of the vertebral artery and the basilar artery was severe 10 minutes after the platelet injection, had begun to relax after one hour, but had returned on day 2 or 3. It persisted for about 8 days. The animals showed clinical evidence of neurological damage. At autopsy the vertebral and basilar arteries showed stripping of the endothelium, with adherent platelets, protein and other cells. There was blockage of some small branches. The brains showed ischaemic changes. The clinical phenomenon of spasm after subarachnoid haemorrhage was thus fairly closely modelled by the injection of platelets alone. There was some evidence that 5HT was probably not responsible for the delayed phase of spasm. Some suggestions are made about the mechanism.

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