but at operation only a red draining vein is present, or there may be no evidence of the AVM at all as it is deep within the sulci. The lesion is still a cortical + subcortical one but hidden deep within a sulcus which may be two or three centimeters or more below the actual surface of the brain. Dissection through the arachnoid of the sulcus will expose the lesion (Fig 3.10A–D).
Fig 3.10A–D Cortical-subcortical AVM located in the depth of sulci (with [D] and without [C] subependymal extension) (B) and therefore not visible on the surface (A).
II 2 Within the Deep Fissures
Similarly, an AVM may lie deep within the interhemispheric (median surface of frontal, parietal, occipital lobe), Sylvian (insula and adjacent operculum) or transverse fissures respectively or in the sulci of these fissures. These AVMs are deeply located but actually are still superficial in relation to the corpus callosum, cingulate gyrus, insula, parahippocampus, and pulvinar thalami (see Fig 3.13B).
Exposure of these lesions can be gained with minimal retraction of the adjacent lobuli and by opening the subarachnoid cisterns.
II 3 Within the Deep White Matter
(see Fig 3.8B)
These often small lesions may be found in every part of the brain, but more frequently around the paraventricular area and within the internal capsule. An approach can be made through the fissures or sulci, or sometimes transcallosally. In some cases it is necessary to employ a transcortical incision or stereotactic or ultrasound techniques or to use stereotactic irradiation.
II 4 Within the Deep Gray Matter (see Fig 3.8)
These AVMs are localized within the amygdala, putamen, pallidum, nucleus caudatus, thalamus, hypothalamus, nucleus ruber and substantia nigra, nucleus dentatus and other nuclei. They are supplied primarily by perforating arteries of the ACA, MCA, PCA, PcoA, anterior and posterior choroidal arteries, AICA, PICA, SCA, vertebral and basilar arteries.
II 5 Cisternal (Subarachnoidal)
(see Fig 3.13B)
Although angiography of an AVM of the vein of Galen shows the lesion in the very center of the brain, surgical experience has shown that they lie entirely within the cisternal system. Surgical explorations have further demonstrated that there exist pure cisternal (subarachnoidal) AVMs which may be paramesencephalic (ventral or dorsal), parapontine (ventral or ventrolateral within the cerebellopontine angle) and parabulbar (around the medulla oblongata).
These paramedullary superficial AVMs of the brain stem seem to be an intracranial equivalent to some paramedullary spinal AVMs.
II 6 Intraventricular (see Fig 3.11)
A few totally intraventricular AVMs of the choroid plexus within the trigonum are known. These lesions may be approached through the corpus callosum. We have seen 2 cases with an AVM of the tela chorioidea of IIIrd ventricle, three cases of lateral ventricle, and two cases of an AVM of the plexus chorioideus of the IVth ventricle.
Fig 3.11A–C Surgically observed locations of para- and intraventricular AVMs with obstruction of ventricular system. A Anterior callosal and septal (1), posterior callosal (2), choroid plexus of Illrd ventricle (3), IVth ventricle (4).
Fig 3.11B, C B Paraventricular. C Varix, 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
