Fig 3.4 Arteriovenous malformation surgically resected from the left occipital lobe of a 24 year old female patient (see Fig 3.78). Note the considerable variation of vessel size with dilated, partially arterialized veins (V) and occasional small arteries (arrow). In the lower half malformed compact vessels with little or no intervening parenchyma prevail, thus resembling a cavernous angioma (Elastica van Gieson, x10). By courtesy of Prof. P. Kleihues, Zurich.
From such a dysplastic vascular plexus may arise all known and angiographically observable types of “malconnection”. Persistence of the embryonal plexus will lead to a pure plexiform type containing vessels without direct arteriovenous fistulae. A gradual but incomplete destruction of the embryonal plexus will result in a mixed type of malformation, composed of both plexiform convolutions and direct arteriovenous fistulae. The preponderance of plexiform or fistulous vessels depends on the degree of destruction of the plexus. Gradual complete destruction of the plexiform parts will ultimately result in pure, direct arteriovenous fistulae (Table 3.1).
It is also evident that vascular resistance will be highest in the pure plexiform and lowest in the pure fistulous types, explaining the angiographic observation that the flow through plexiform lesions is slower than through fistulous lesions.
The different types of arteriovenous malformations may be demonstrated angiographically. Based on their angiographic appearance, arteriovenous malformations may therefore be divided into three main types (Table 3.2).
Traditionally, descriptions of cerebral vascular malformations used in classifications include 1. the composition of the vascular wall, 2. the presence or absence of an intervening brain parenchyma between the vascular spaces of the malformation, and 3. the state (normal or gliotic) of the intervening neural tissue. Based on these morphological parameters vascular malformations are divided into four main types: 1. arteriovenous malformations 2. venous malformations 3. cavernous malformations and 4. capillary malformations (or telangiectasias) (McCormick 1966).
Despite this attempt to separate various different forms, certain observations support the hypothesis of a single underlying primary lesion.
Transitional forms exhibiting the histologic characteristics of more than one of the above mentioned types are sometimes encountered within the same malformation. It is, in fact, difficult to distinguish histologically between telangiectasia and venous angioma. Also telangiectasias have been reported to be a component of venous angiomas (McCormick 1966, Manuelidis 1950). Combinations of cavernous and telangiectasias (Roberson et al. 1974), as well as venous angiomas and arteriovenous malformations (Huang et al. 1984), have been reported to occur within the same malformation. Also multiple lesions of different histologic types can occur in the same individual (McCormick 1966).
Although absence of capillaries has usually been described as the hallmark of arteriovenous malformations, abnormal proliferation of capillaries may be observed within the malformation or even in adjacent tissue. Hamby (1958) in a unique histologic study of a specimen of an arteriovenous malformation of the brain, demonstrated not an agenesis or absence of capillaries, but a multitude of different types of capillary-like vessels, clearly distinguishable from the entering arteries and the draining thin-walled tortuous veins. These capillary-type vessels found in the central core of the malformation form a complex of coiling and intercommunicating vessels (see Fig 3.2).
Dilated capillaries or capillary-like spaces are found in telangiectasias, which are therefore also called capillary malformations, as well as in cavernomas (Huang et al. 1984). By the same reasoning certain vascular malformations of the subcutaneous tissue are also called capillarovenous malformations (Merland et al. 1983). In histologic studies of Cabanes et al. (1979) cases of venous angioma with a clear participation of capillaries are demonstrated.
We should also, perhaps, remember Virchow’s statement of 1851 – that “one type of angioma can transform into another by changes in flow and pressure or by cellular proliferation”.
Histologically, the presence or absence of intervening neural parenchyma, as well as its state (normal or gliotic) are used as parameters for classifying vascular malformations. Usually, arteriovenous malformations surround gliotic tissue, venous malformations and telangiectasias have normal intervening tissue, and cavernous malformations contain no intervening parenchyma. Both histologic studies and intraoperative observations show, however, that an intervening neural parenchyma and even gliosis within it may occur with all types of cerebral vascular malformations.
Cavernous malformations are classically described as being compact, with the vascular spaces being contiguous with one another and lacking intervening tissue. During operation on such lesions, however, one may observe through the operating microscope, small cavernous spaces located at the periphery of the mass and being clearly separated from it by brain parenchyma.
In a histologic study, Manuelidis (1950) clearly demonstrated neural tissue between the vascular spaces of an otherwise typical case of cavernous angioma.
A finding common to all types of cerebral vascular malformation is spontaneous thrombosis, occurring most frequently in the venous space of the lesion. Although such spontaneous thromboses have been most often reported in cases of true AVM, they clearly also occur with the other types, especially venous and cavernous malformations.
The histological character of the resected lesions and the relative frequency are given in Table 3.3.
Table 3.3 Histological findings in 398 AVMs
| Mixed type | 374 cases | (94.0%) |
| More arterial | 12 cases | ( 3.0%) |
| More venous | 12 cases | (3.0%) |
| 398 cases |
Not investigated 16 galenic and 2 fistulous lesions.
Hemorrhage, which most frequently occurs in arteriovenous malformations, may also be observed with other types of vascular anomalies. Microscopic hemorrhages with foci of hemosiderin laden macrophages are frequently found in arteriovenous malformations, but may also be seen in venous, cavernous and even capillary-type malformations.
From these pathologic-anatomic observations it becomes evident that cerebral vascular malformations have characteristics in common with respect to their histologic nature, their vascular composition, and regressive changes, irrespective of their type.
Using cerebral angiography, the different morphologic types of vascular malformation described above can usually be distinguished (Tables 3.2, 3.4). Arteriovenous malformations typically appear during the arterial phase of the angiogram and are characterized by large feeding arteries, a more or less compact conglomeration of coiled vessels and prominent draining veins. Venous angiomas most frequently appear during the venous phase and are characterized by numerous dilated, linearly arranged medullary veins, producing an umbrella-shaped configuration and converging towards a markedly dilated central parenchymal vein. Cavernous angiomas may cause an avascular mass effect, but remain invisible with usual angiographic techniques, owing to their slow circulation and the lack of prominent feeding arteries. A blush, representing pooling of contrast material within the vascular spaces of the lesion, may however appear, if either prolonged injection angiography (Numaguchi and Nishikawa 1979) or a repeated injection series (Huang et al. 1984) is performed. In telangiectasias, angiography is usually negative, owing to their small size and their slow circulation time. Occasionally,