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Kernohan's notch

(James Watson Kernohan, born 1897, Irish-born American pathologist), grooved wedge of degeneration demonstrated at necropsy in the case of protrusion of part or the whole of the hippocampal gyrus on one or both sides through the tentorial opening due to a supratentorial space-occupying lesion (tumour, haematoma). As a result the upper part of the brain stem is deformed and compressed, the aqueduct being reduced to a linear slit. In unilateral herniation the thrusting of the opposite cerebral peduncle against the rigid edge of the tentorium may cause sufficient damage to cause homolateral pyramidal signs as a "false localizing sign".