Conclusions
The embryonic vascularization of the CNS is characterized by the sequential development of three independent, though interrelated, vascular territories. In order of appearance and development they are: the perineural, the interneural and the intraneural vascular territories (Table 2.2). Embryologically, each vascular territory evolves and remains within a distinct and specific tissue compartment, namely: the meningeal, the Virchow-Robin and the perivascular glial tissue compartment, respectively. In the course of embryonic development, the vasculature of each of these territories undergoes an integrated metamorphosis, continuously adapting to the growing structural and functional needs of each particular region of the developing CNS. This progressive vascular metamorphosis is the result of continuous remodelling of the original anastomotic plexuses in which both capillary angiogenesis and capillary reabsorption are active processes. The three vascular territories and the different and specific types of vessels that characterize each one can be easily recognized at any time in the course of embryonic development (Fig 2.14).
The separation of the CNS vasculature into three different and specific vascular territories and associated tissue compartments implies significant structural as well as functional differences among them. Undoubtedly, a clear understanding of these structural and functional differences is important and relevant both to the clinician and the neurosurgeon. In this context, it is interesting that only the intraneural capillaries and associated perivascular glia (intraneural vascular territory) are actually involved in the so-called blood-brain barrier, since the meningeal and the Virchow-Robin vasculatures actually evolve and remain outside of the CNS substance.
B. Vascular Malformation of the Central Nervous System. Embryological Considerations
A study of the early embryogenesis of the CNS vasculature would be incomplete without some comments concerning the possible origin and nature of the malformations affecting its different vessels. In spite of considerable individual variations most vascular anomalies of the CNS can be classified in one of the following three basic types (Russell and Rubinstein 1977):
– Capillary telangiectasias and cavernous angiomas.
– Venous and arteriovenous malformations.
– Sturge-Weber-Dimitri’s disease.
Most investigators agree that these vascular anomalies represent developmental malformations rather than angiomas or true vascular neoplasms (Aronson 1971, Dudley 1982, Larroche 1983, McCormick 1966, 1983, Russell and Rubinstein 1977).
Morphologically, these vascular anomalies often resemble the embryonic vessels and the early anastomotic plexuses formed during the early stages of the vascularization of the CNS (Streeter 1918, Padget, 1948, 1957; see Chapter: Embryogenesis of the Early Vascularization of the Central Nervous System). However, few attempts have been made to correlate the various types of vascular malformations with the vasculogenesis of the CNS. The three basic types of vascular malformations will be correlated herein with the embryonic development of specific types of vessels, vascular territories, pericellular tissue compartments; and, regions of the CNS.
Prior to undertaking such a developmental study it is necessary to establish some general facts concerning the natural history and clinical behavior of the vascular malformations of the CNS. First of all, it is important to recognize that these vascular malformations are not static entities but quite dynamic ones. They are constantly subject to circulatory mechanical forces and to a variety of pathologic alterations which progressively transform them into complex vascular anomalies which are often difficult to understand or to explain adequately. Secondly, most vascular malformations of the CNS become clinical entities and hence, brought to the attention of neurosurgeons and neuropathologists when they have either bled, ruptured, or thrombosed. Therefore, most of them are already transformed by the time they are removed surgically or studied pathologically. It should be of great significance in the study of these malformations to be able to distinguish and to separate their primary or original features from the secondary or acquired ones.
Any pathologic alteration (rupture, hemorrhage or thrombosis) of a vascular malformation will not only transform the affected vessels but the surrounding tissue as well. A reparative inflammatory process will take place around the affected vessels resulting in fibrosis and or gliosis and more importantly in the obliteration of the perivascular tissue compartment which originally surrounded the vascular malformation. The secondary obliteration of the perivascular tissue compartment could lead to confusion about the original location of the malformation and the CNS vascular territory originally involved. Furthermore, the inflammatory process around injured vessels of the original malformation will result in the formation of many new or secondary vessels. The presence of postinflammatory vessels should be recognized because they could also obscure the original architecture of the vascular anomaly.
The recognition of these facts is important because in the interpretation any type of vascular malformation of the CNS the following aspects must be clearly established: a) type of vessels originally affected; b) vascular territory and perivascular tissue compartment originally involved; and, c) original location of the anomaly. The establishment of these facts is sine qua non for the understanding of the nature of these vascular malformations as well as for the selection of the most adequate method for their neurosurgical removal.
During the early vascularization of the CNS three distinct vascular territories evolve sequentially. These have been named: perineural, interneural, and intraneural vascular territories respectively, because of their specific relationship to the nervous tissue (see Chapter: Embryogenesis of the Early Vascularization of the Central Nervous System). Each of these three vascular territories is characterized by distinct types of vessels and most importantly by a specific perivascular tissue compartment. These compartments are: the meningeal, the Virchow-Robin, and the perivascular glia, respectively. The three basic types of vascular malformations of the CNS will be analyzed separately and correlated embryologically with the vessels of its different vascular territories and tissue compartments.
Capillary Telangiectasias and Cavernous Angiomas
Capillary telangiectasias are small vascular malformations composed solely of abnormally dilated capillaries. They vary greatly in caliber and saccular or fusiform dilatations are common. They lack an anomalous arterial supply and their venous drainage may be dilated but not abnormally so. The actual number of capillaries may not be increased in these malformations. The overlying pia mater and arachnoid are normal. The intervening tissue between the dilated capillaries is normal and both glial and neuronal elements are recognized in it. These vascular anomalies are frequently