Arteriovenous aneurysm of the great vein of Galen.

The great cerebral vein of Galen extends from the junction of the two internal cerebral veins under the splenium of the corpus callosum to the commencement of the straight sinus. Arteriovenous aneurysms of this vein are rare congenital anomalies and only 22 cases have been found in a review of the literature. Other types of cerebral arteriovenous aneurysms are not so unusual. Their true incidence has become more apparent following the widespread use of angiography. Mackenzie (1953) estimated their occurrence as 1% of all neurological admissions. By way of contrast Paterson and McKissock (1956) found no example of great cerebral vein aneurysm in 110 cases of cerebral arteriovenous aneurysm, and the same is true of the series reported by Olivecrona and Riives (1948), Mackenzie (1953) and Potter (1955). These vascular malformations are predominantly of two main types (Hamby, 1952). In the first type, there is a direct end-to-end connexion between an artery and vein, and in the second a network of poorly differentiated, non-capillary vessels intervenes between an artery and several veins, forming a mass of abnormal, dilated and tortuous vessels which sometimes goes by the name of 'angioma' but is not generally considered to be neoplastic. In both types veins not directly involved in the malformation may become enlarged and tortuous in response to the raised venous pressure. The great cerebral vein, however, only shows aneurysmal dilatation when there are one or more abnormal arterial vessels opening into it, its immediate tributaries or continuation. Other vascular malformations of the brain may coexist. These abnormal arterial vessels have arisen, in the described cases, from the posterior cerebral, posterior communicating, callosal and superior cere-