Although the different morphologic types of cerebral vascular malformation are distinguishable on angiography, certain observations support the concept of a single underlying cause, common to all types of vascular malformation. One may occasionally demonstrate both transitional forms as well as the coexistence of two or more different types of lesion within the same vascular malformation. A pure venous type of malformation was demonstrated angiographically in 1958 by Krayenbühl and Yaşargil. The histological examination showed no arterial component in the lesion (Fig 3.5). According to Huang et al. (1984) 14% of cases of venous angioma contain fine arteries which form a reticular blush in the arterial phase of the angiogram. This indicates the presence of an arterial or low-flow arteriovenous component in certain venous angiomas. It clearly contradicts the classical definition, according to which venous angiomas lack arteriovenous shunts or an arterial component and become visible only in the late venous phase of the angiogram. Similar observations on venous angiomas with an arterial component were reported by Moritake et al. (1980). The close embryological relationship between apparently different pathologic entities such as venous angioma and arteriovenous malformation is demonstrated in a case reported by Huang et al. (1984), in which a typical venous angioma contained within it, two small arteriovenous malformations. Furthermore, all three coexistent vascular malformations had a common venous drainage! In cases of telangiectasia, Rosenbaum (1974) has observed an early appearing blush and early draining veins, suggesting the presence angiographically of small or cryptic arteriovenous malformations.
Fig 3.5A–C This may be the first angiography demonstration of a venous angioma. (54 year old male presenting with subarachnoid hemorrhage. From Krayenbühl, H., M. G. Yaşargil: Series Chirurgia Geigy 4: 76 1958.)
A Normal arterial phase of carotid angiography.
B Venous phase of carotid angiogram after a SAH shows the lesion in the right temporal lobe. It drains into the dilated basal vein of Rosenthal. In 1958 this malformation was called “Arteriovenous malformation visible only in the venous phase”.
C Histological examination shows venous malformation with arterial components.
It is interesting to note, that during operation for cavernous angiomas, one regularly observes with the help of the operating microscope, slightly dilated arteries entering the cavernous space of the lesion and thus indicating an arterial participation in their supply.
In cases of venous angioma it is a frequent angiographic finding that the medullary or subependymal veins adjacent to the angioma are hypoplastic or even absent and that the adjacent superficial or cortical veins may be poorly developed. Also hypoplasia of the internal cerebral veins, poor development or even absence of certain major subependymal veins and a paucity of superficial cortical veins have occasionally been observed (Huang et al. 1984). Veins pursuing an unusual course, most probably representing persistent fetal or intrauterine venous structures, are frequently observed angiographically in such cases (Huang et al. 1984).
Similar anomalies of the venous system may also be observed in cases of arteriovenous malformation. Unfortunately, angiographic study of the venous drainage patterns of cerebral vascular malformations has been generally neglected (see below). Review of our own angiographic material disclosed an unsuspected 30% incidence of associated anomalies in the venous drainage system of AVMs similar to those reported to occur with venous angiomas (see Vol. III B, Table 9.2).
Our own operative findings have also demon strated clear overlaps of histological types of malformation within single lesions. There have been AVMs with a predominance of arterial or venous components, cavernous malformations with definite feeders bearing aneurysms, capillary cavernomas with no visible arterial or venous connections and virtually isolated from surrounding tissue by firm encapsulation, and venous malformations with arterial components found at operation and confirmed histologically but which could not be demonstrated angiographically.
Upon comparing the clinical features of the different types of cerebral vascular malformation, it becomes evident, that with the exception of a bruit and some symptoms associated with steal phenomena, which exclusively occur with certain high-flow arteriovenous malformations, all other symptoms such as epileptic seizures, hemorrhage, progressive neurological deficit, and headache, may occur with any type of vascular malformation albeit with some variations in incidence (Table 3.5).
There is therefore pathological, anatomical, angiographic, surgical and clinical evidence for a common underlying pathogenesis of all forms of cerebral vascular malformation, based upon a disease of capillaries. The seemingly distinct forms of cerebral vascular malformation described by pathologists, diagnosed angiographically by neuroradiologists and operated upon by neurosurgeons represent nothing more than different manifestations of the same disease.
This concept supports the theory of van Bogaert (1935), who doubted that the different types of cerebral vascular malformations represent different disease entities and expressed the opinion that there is only one Angioma-Disease (maladie angiomateuse) with a variety of subgroups. He was able to explain the pathogenesis of this disease by assuming a disturbance in the development of small vessels as the underlying mechanism.
Classification of Vascular Malformation
“The classification of the vascular malformations of the brain has been the subject of considerable discussion and the extensive literature on this topic reflects a varying and, at times, confusing nomenclature.” (Bebin and Smith 1982, p. 13). The confusion continues and applies not only to vascular malformations of the brain but also to those of all other organs. We agree with Mulliken (1983) “the words to describe the common vascular birthmarks reflect our ignorance of their pathogenesis”. There are majors problems with both nomenclature and classification.
Nomenclature
A. Both Greek and Latin roots are used: Vascular malformation (Latin roots), Angiodysplasia (Greek roots).
B. The suffix oma (= neoplasm) is commonly used not only for true vascular tumors such as hemangioblastoma, but also for vascular malformations. The use of the suffix osis (e. g. “angiomatosis”) has sometimes been inappropriate. The term should be reserved for diffuse or multiple lesions only.
At the present time the English version of “malformation” has found general acceptance and there is little point in entering further into sophisticated linguistic struggles.
Classification
As ever more sophisticated means of studying vascular malformations have developed, systems of classification have diversified from the early descriptive terms based purely on gross morphological observation. In some instances, old terminology has been retained, in others changed and in yet further (often simultaneous) publications regrouped under different headings.
Noran presented and discussed all the proposed classifications in the literature up to 1945 and concluded: “a comprehensive evaluation of the literature is warranted in order that one may arrive at some correlation between these various nomenclature and classification.”
Within the last 40 years further new concepts have been proposed. Table 3.6a contains some of the more notable historic and modern classifications, and shows the development of thinking regarding the malformations.
Virchow (1863) conducted his own thorough studies and described 4 main types of malformation and stated, as early as 1851: “one type of angioma can transform into another by changes in flow and pressure or by cellular proliferation.”
The venous anomalies, and plexiform angioma