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 found in the pons, the cerebral cortex, the cortical white mater and rarely in the spinal cord. Capillary telangiectasias rarely show pathologic alterations such as hemorrhages, thromboses or ruptures. Therefore, they are usually clinically silent and most are found by chance at autopsies. Embryologically, these are vascular malformations which involve only the capillaries of the intraneural vascular territory of the CNS and should be enclosed within the perivascular glial tissue compartment.
Cavernous angiomas are large vascular malformations composed of cystic vascular spaces lined by a single layer of endothelial cells. These vascular spaces vary greatly in size and are often very irregular suggesting secondary changes. The vascular spaces of these malformations probably represent abnormal capillaries since no recognizable arteries or veins are found in them. These vascular spaces are structurally similar to those found in capillary telangiectasias. These vascular malformations may be circumscribed but not encapsulated and could be lobulated. Like telangiectasias, they lack either abnormal arterial supply or abnormal venous drainage. They are found in the cerebral cortex, the pons and rarely in the spinal cord.
Cavernous angiomas invariably show areas of thromboses with subsequent organization, recent and old hemorrhages with hemosiderin laden macrophages, fibrosis and or gliosis, focal calcification and even areas with bone formation. The overlying pia mater and arachnoids are stained, thickened and fibrosed. All of these changes are obviously the result of pathological (secondary) alterations. There is no normal nervous tissue between the abnormal vessels in these malformations, probably because it has been progressively destroyed. Eliminating the prominent acquired pathologic changes, cavernous angiomas could represent large telangiectasias, an idea which has been often expressed previously (Russell 1931, Russell and Rubinstein 1977).
Embryologically, cavernous angiomas can only be large vascular malformations involving the capillaries of the intraneural vascular territory of the CNS since no distinct arteries or veins have been recognized in them. They could represent large capillary telangiectasias with a greater propensity to undergo pathologic alterations. These alterations will result in the complete obliteration of the perivascular glial compartment and in the progressive reactive fibrosis and gliosis of the intervening nervous tissue causing its complete destruction. The neurosurgical treatment of cavernous angiomas will necessarily involve the removal of some of the normal nervous tissue around the malformation.
Venous and Arteriovenous Malformations
Venous malformations are vascular anomalies composed solely of abnormally dilated and tortuous veins. They may be composed of a single greatly dilated and tortuous vein or of small number of them. They involve primarily the pia-arachnoidal veins and few of its intramedullary tributaries. They can be located in the spinal cord, and occasionally in the drainage territories of the vein of Galen and of the cerebellum.
Secondary pathologic alterations including muscular hypertrophy and or hyalinization (fibrosis) of the vessel wall, and thromboses with subsequent organization are frequent findings in these malformations. However, more important alterations are those caused by the compression of the spinal cord by the anomalous veins, and the compression of the nervous tissue by the dilated intramedullary tributary veins. Cord atrophy and ischemic changes are often sequelae in long standing cases.
Embryologically, venous malformations are developmental anomalies which involve primarily the veins of the perineural vascular territory of the meningeal (pia-arachnoid) tissue compartment, and also some tributary veins of the interneural vascular territory of the Virchow-Robin tissue compartment. Therefore, uncomplicated venous malformations should be entirely outside the nervous tissue proper and hence liable to their complete microneurosurgical removal. On the other hand, secondary pathological alterations affecting these venous malformation will result in the obliteration of their perivascular tissue compartments making their neurosurgical treatment more difficult requiring the removal of some of the surrounding unaffected nervous tissue.
Arteriovenous malformations are large and complex vascular anomalies composed of abnormally developed arteries and veins. They involve primarily the vessels of the leptomeninges with extension into the fissures and sulci. They could also involve deeper vascular territories of the cortex, midbrain, cerebellum and choroid plexuses. These vascular malformations are also characterized by the participation of regional perforating vessels which could also be abnormally developed, tortuous and dilated. The presence and location of the abnormal perforating vessels should always be explored because they could be the cause of serious damage to the nervous parenchyma and must be treated adequately during the microneurosurgical removal of the malformation. The main arteries and veins leading to and from the malformation are usually dilated and a secondary collateral circulation could be prominent in some of them.
Arteriovenous malformations are subject to pathologic alterations including: ruptures, hemorrhages, thrombosis, atrophy, and progressive reparative fibrosis and or gliosis. The arachnoid around the malformation as well as the underlying or adjacent nervous tissue show rusty pigmentation and fibrosis or gliosis. Microscopic examination of both the arteries and the veins of these malformation show abnormalities involving their elastic and muscular elements. Some vessels also show atheromas, organized thrombus, focal calcifications and postinflammatory fibrosis or gliosis.
Embryologically, these malformations involve: a) vessels (arteries and veins) from the perineural vascular territory of the CNS within the meningeal tissue compartment; and b) some perforating or emerging vessels (arterioles or venules) from the interneural vascular territory within the Virchow-Robin tissue compartment. Therefore, unaffected arteriovenous malformations should lie outside of the nervous tissue proper and should be liable to complete removal by microneurosurgery. On the other hand, in arteriovenous malformations which have undergone pathologic alterations, the meningeal and Virchow-Robin perivascular compartments might be obliterated making their microneurosurgical treatment more difficult since it must involve the removal of the surrounding nervous tissue.
Sturge-Weber-Dimitri’s Disease
This congenital vascular malformation is characterized by an increase in the number of capillaries and of few small veins throughout the affected pia mater and underlying surface of the cerebral cortex. This extensive capillary-venous cerebral malformation is associated with a homolateral cutaneous angioma over the trigeminal nerve distribution. This vascular malformation is also characterized by significant pathologic alterations, severe damage to the nervous parenchyma, and abundant mineral granular deposition of calcium and iron.
While
