Critical perfusion of brain and retina.

Four patients with extensive occlusive disease of the extracranial arteries are described. All suffered attacks of visual loss due to transient retinal ischaemia and all developed the haemorrhagic changes of low-flow retinopathy. Resting cerebral blood flow was within normal limits in all patients. The attacks were provoked by falls in systemic blood pressure or, in one patient, by facial heating. The probable mechanism in this last case is thought to be diversion of blood to a dilated external carotid vascular bed. Temporary cerebral and brachial symptoms occurred but no permanent changes developed. Successful surgical reconstruction was achieved in two patients with relief of symptoms.