style="font-size:15px;"> The results of recent electron microscopic studies are in accordance with our proposed concept. Meyermann and Yaşargil (1981) found that the ultrastructural composition of small vessels of 41 surgically obtained arteriovenous malformations could be divided into two distinct types; vessels with a closed and vessels with a fenestrated endothelial cell layer. This second type of vessel, characterized by a fenestrated endothelial coat is clearly abnormal, since fenestrations of endothelial cells do not occur in the normal brain vasculature with exception of the area postrema, choroid plexus, pineal and pituitary glands, intercolumnar tubercle, and certain nuclei within the hypothalamus (Lee 1971). Another observation of this study was the sprouting of new capillaries in the fibrotic arachnoid surrounding superficial pathological vessels. This finding supports the concept of a proliferative capillaropathy (Figs 3.3, 3.4).
Depending on the extension and distribution of the capillary disease involving the primitive vascular plexus, vascular malformations may therefore be defined as localized, multiple or diffuse collections of metamorphotic vessels, abnormal in number, in structure and in function.
The result of this primary disease of capillaries is a mal-production and therefore a mal-formation of both arteries (or arterioles) and veins (or venules), i.e. a metamorphotic angiodysplasia or capillaropathy.
Fig 3.3A–B A Sinusoid-type vessels of AVMs are coated by fenestrated endothelial cells. The fenestrae are indicated by arrows. In the normal cerebral vasculature this type of endothelial coat is only present in certain distinct areas of the CNS. The cytoplasm of the endothelial cells is filled with cross-sectioned filaments and some vacuoles. The arrowhead indicates a so called Weibel-Palade body. This organelle can only be found in endothelia, and is a rare feature in a normal cerebral vessel wall. Bar = 1 μm B Although some gaps in the endothelial cell layer of AVM are demonstrated as in A, some cell contacts of adjacent endothelia are tight as seen in normal cerebral vessels. Bar = 1 μm. By courtesy of Dr. R. Meyermann.
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
