Microneurosurgery, Volume IIIA. Mahmut Gazi Yasargil

intraventricular.

      As can be proven angiographically, AVMs in groups I 1a, 2a, 3a and II 1a, 2a, 3a (see page 64) are supplied by cortical arteries rather than perforators whereas in groups I 1b, 2b, II 1b, 2b, 3, 4, 5, 6 the reverse is true. Cisternal AVMs, particularly vein of Galen malformations, appear to be supplied about equally by cortical and perforating vessels (Figs 3.12–13).

      Fig 3.12A–D The cerebral and cerebellar AVMs are similarly composed. Angiographically the flow to the AVM can be visualized from mainly 1 (A), 2 (B) or 3 (C) sources (ACA, MCA, PCA or PICA, AICA, SCA) or only from perforators (D). a = anterior cerebral artery, m = middle cerebral artery, p = posterior cerebral artery, d = deep perforators.

      Fig 3.13A-D A Cerebral convexial AVM supplied mainly by cortical branches of ACA, MCA, PCA. Possible participation of dural branches. The perforating feeders very often participate in the supply of the AVM even though they may be invisible angiographically.

      B AVM of vein of Galen is usually supplied by both cortical and perforating branches.

      Fig 3.13C Deep, e.g. thalamic, parathalamic AVMs, are mainly supplied by perforating arteries.

      Fig 3.13D Infratentorial convexial AVMs are composed similarly to cerebral convexial AVMs. These are mainly supplied by cortical (SCA, PICA, AICA) and perforating feeding arteries, from basilar artery and its branches.

The Nidus

      The term “nidus” was introduced by Doppman (1971) when describing the structure of spinal AVMs as demonstrated by his innovative techniques of selective spinal angiography.

      This terminology has been subsequently adopted by interventional neuroradiologists. They are now able to study the exact composition of extracranial AVMs with the help of superselective angiography. They are also able to describe the different compartments of an AVM, information which is extremely useful for the neurosurgeon in understanding the angioarchitecture of an AVM as well as in determining if and how it can be treated microsurgically. Despite this significant process, it still remains difficult to identify the actual nidus or the core of an AVM as one cannot always distinguish between the hemodynamic effects upon normal vascular channels (whether they are arteries or veins) and true embryonic remnants. Since the nidus represents the area of arteriovenous shunting within the AVM, it is probably best to consider it as that part of the malformation which is interposed between the recognizable feeding arteries and the larger terminal draining veins.

      The nidus (epicenter) of the AVM is composed of a conglomerate of vascular loops, whose precise origin remains a source of controversy. Some feel these represent abnormal vascular channels, others that they are embryonic veins or normal veins arterialized by high blood flow and pressure. It has been noted at operation that the nidus of an AVM contains two types of connections. The first is a tangle of loops which appear to have some interconnections although the number and extent of these is generally not clear.

      Histologic sections through AVMs do not show definite connections between loops. It is therefore possible that these represent branches of the arterial feeders which eventually reanastomose in order to form the draining vein or veins. A second type of connection is the direct (small to large) arteriovenous fistula which has long been recognized (Dandy 1928, Olivecrona 1957).