Causes of an aneurysm

Cerebral arterial aneurysms are one of the most common causes of non-traumatic intracranial hemorrhages. According to V.V. Lebedev et al. (1996), the incidence of spontaneous subarachnoid hemorrhages ranges from 12 to 19 cases per 100,000 population per year. Of these, 55% are due to ruptured arterial aneurysms. It is known that about 60% of patients with ruptured cerebral arterial aneurysms die on the 1st to 7th day after bleeding, i.e., in the acute period of subarachnoid hemorrhage. With repeated aneurysmal bleeding, which can occur at any time, but most often on the 7th to 14th and 20th to 25th days, the mortality rate reaches 80% or more.

Arterial aneurysms rupture most often in individuals aged 20 to 40 years. The incidence of subarachnoid hemorrhage in women and men is 6:4 (WU Weitbrecht 1992).

Aneurysms of the cerebral arteries were known in ancient times. In the 14th century BC, the ancient Egyptians encountered diseases that are currently interpreted as "systemic aneurysms" (Stehbens W. E. 1958). According to R. Heidrich (1952, 1972), the first reports of aneurysm were made by Rufus from Ephesus around 117 BC, R. Wiseman (1696) and T. Bonet (1679) suggested that the cause of subarachnoid hemorrhage could be an intracranial aneurysm. In 1725, J. D. Morgagni discovered dilation of both posterior cerebral arteries during autopsy, which was interpreted as aneurysms. The first description of an unruptured aneurysm was given by F. Biumi in 1765, and in 1814 J. Blackall first described a case of a ruptured aneurysm of the terminal part of the basilar artery.

The diagnostics of cerebral arterial aneurysms gained qualitatively new possibilities after the introduction of cerebral angiography by Egaz Moniz in 1927. In 1935, W. Tonnis reported for the first time on an aneurysm of the anterior communicating artery detected by carotid angiography. Despite the long history of studying this issue, active surgery of arterial aneurysms began to develop only in the 1930s. In 1931, W. Dott performed the first successful operation on a ruptured segmental aneurysm. In 1973, Geoffrey Hounsfield developed and introduced a method of computed tomography, which significantly facilitated the diagnostics and treatment of subarachnoid hemorrhages of any etiology.

Over a period of more than sixty years, the theory of aneurysms has changed many times and has now reached a certain level of perfection. Aneurysm surgery has been developed to such an extent that it has reduced the mortality rate during surgical treatment from 40-55% to 0.2-2%. Thus, the main task at present is the timely diagnosis of this pathology, ensuring urgent specialized examination and treatment of patients.

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Theories explaining the causes of aneurysms

The most recognized theory explaining the causes of aneurysms is the Dandy-Paget theory, according to which aneurysms develop as a result of improper formation of the arterial wall in the embryonic period. Characteristic of the morphological structure of aneurysms is the absence of the normal three-layer structure of the wall of the altered section of the vessel - the absence of a muscular layer and an elastic membrane (or its underdevelopment). In most cases, an aneurysm is formed by the age of 15-18 and is a sac communicating with the lumen of the artery, in which the neck (the narrowest part), body (the most expanded part) and bottom (the thinnest part) can be distinguished. The sac is always directed along the blood flow, taking the main blow of the pulse wave. Due to this, arterial aneurysms are constantly stretched, increase in size, and its wall becomes thinner and, eventually, ruptures. There are other factors that lead to the development of aneurysms - degenerative diseases of humans, arterial hypertension, congenital developmental anomalies, atherosclerotic damage to the arterial wall, systemic vasculitis, mycoses, traumatic brain injury, which in total make up 5-10%. In 10-12% of cases, the cause of the disease cannot be determined.

In 1930, W. Forbus described the so-called media defects. In his interpretation, they are congenital malformations of the muscular membrane in the form of its absence in a small section of the artery, precisely in the area of branching. However, it soon turned out that media defects can be found in almost all people and in almost any fork of the arteries, while aneurysms are much less common.

In recent years, a team of scientists from the Russian Neurosurgical Institute named after A. Polenov (Yu. A. Medvedev et al.) have proven that the segmental (metameric) structure of the muscular apparatus of the arterial circle of the brain plays a decisive role in the development of an aneurysmal sac. The segments are connected by a specialized ligamentous apparatus, represented by a fibrous-elastic ring. An aneurysm is formed due to stretching of the articulation of segments due to hemodynamic reasons, which indicates their acquired nature. The rate of aneurysm formation is unknown.

By quantity, aneurysms are divided into single and multiple (9-11%). By size - miliary (2-3 mm), medium (4-20 mm), large (2-2.5 cm) and giant (more than 2.5 cm). By shape, aneurysms are millet-shaped, saccular, in the form of a fusiform expansion of the arterial wall, spindle-shaped. The predominant localization of arterial aneurysms is the anterior sections of the Willis circle (up to 87%).

Causes of development of arteriovenous malformations

The pathomorphology of arteriovenous malformations is characterized by a disruption in the embryogenesis of brain vessels at the earliest stages of fetal development (4 weeks). Initially, only the capillary system is formed. Then, some of the capillaries are resorbed, and the rest, under the influence of hemodynamic and genetic factors, are transformed into arteries and veins. The development of vessels occurs capillary-fugal, i.e. arteries grow in one direction from the capillary, and veins in the opposite direction. It is at this stage that AVMs are formed. Some of them arise from capillaries that are subject to resorption, but for some reason remain. From them, a tangle of pathological vessels develops, only vaguely resembling arteries and veins. Other arteriovenous malformations are formed due to agenesis of the capillary system or a delay in direct primordial connections between arteries and veins. They are mainly represented by arteriovenous fistulas, which can be single or multiple. Both described processes can be combined, giving a wide variety of AVMs.

Thus, three variants of morphogenesis are possible:

1. preservation of embryonic capillaries from which the plexus of pathological vessels develops (plexiform AVM);
2. complete destruction of capillaries with preservation of the connection between the artery and vein results in the formation of a fistula AVM;
3. partial destruction of capillaries leads to the formation of mixed AVMs (plexiform with the presence of arteriovenous fistulas).

The latter type is the most common. Based on the above, all AVMs can be characterized as local sets of numerous metamorphotic vessels, abnormal in quantity, structure and function.

The following morphological variants of malformations are distinguished:

1. AVM itself is a tangle of pathological vessels with multiple fistulas, having a spider-like or wedge-shaped form. Between the loops of vessels and around them is gliotic brain tissue. They are localized in any layer of the brain and in any place. Wedge-shaped or cone-shaped AVMs are always directed with their apex toward the ventricles of the brain. They are also called spongy. In 10% of cases, they are combined with arterial aneurysms. Fistula AVMs or racemose AVMs are distinguished separately. They look like vascular loops penetrating the brain substance.
2. Venous malformations occur due to agenesis of the connecting venous segment. They look like an umbrella, jellyfish or mushroom. The veins are surrounded by normal brain tissue. Most often, such malformations are localized in the cerebral cortex or cerebellum.
3. Cavernous malformations (cavernomas) arise as a result of sinusoidal changes in the capillary-venous system. They resemble honeycombs, mulberries or raspberries in appearance. In the enlarged cavities, blood may circulate or may be practically motionless. There is no brain matter inside the cavernomas, but the surrounding brain tissue undergoes gliosis and may contain hemosiderin due to diapedesis of blood cells.
4. Telangiectasias occur due to capillary dilation. They are most often localized in the pons Varolii and macroscopically resemble petechiae.

In addition, some authors consider Moya-Moya disease (translated from Japanese as "cigarette smoke") as a variant of arterial malformation. This pathology is a congenital multiple stenosis of the main arteries of the base of the skull and brain with the development of multiple pathological collateral vessels that have the form of spirals of various diameters on the angiogram.

Actually, AVMs are macroscopically vascular tangles of different sizes. They are formed as a result of the disorderly interweaving of vessels of different diameters (from 0.1 cm to 1-1.5 cm). The thickness of the walls of these vessels also varies widely. Some of them are varicose, forming lacunae. All AVM vessels are similar to both arteries and veins, but cannot be classified as either.

AVMs are classified by location, size and hemodynamic activity.

By localization, AVMs are classified according to the anatomical parts of the brain in which they are located. In this case, they can all be divided into two groups: superficial and deep. The first group includes malformations located in the cerebral cortex and underlying white matter. The second group includes AVMs located deep in the convolutions of the brain, in the subcortical ganglia, in the ventricles and the brainstem.

By size, there are: micro AVMs (up to 0.5 cm), small (1-2 cm in diameter), medium (2-4 cm), large (4-6 cm) and giant (more than 6 cm in diameter). AVM can be calculated as the volume of an ellipsoid (v=(4/3)7i*a*b*c, where a, b, c are the semi-axes of the ellipse). Then small AVMs have a volume of up to 5 cm ³, medium - up to 20 cm ³, large - up to 100 cm ³ and giant or widespread - over 100 cm ³.

AVMs differ in hemodynamic activity. Active AVMs include mixed and fistula AVMs. Inactive AVMs include capillary, capillary-venous, venous, and certain types of cavernomas.

Hemodynamically active AVMs contrast well on angiograms, while inactive ones may not be detected with conventional angiography.

From the point of view of the possibility of radical surgical removal, AVMs are divided by localization into silent zones of the brain, functionally important zones of the brain and the midline, which include AVMs of the basal ganglia, the sheath of the brain, the pons and the medulla oblongata. In relation to the brain, its membranes and the bones of the skull, AVMs are divided into intracerebral, extracerebral (AVMs of the dura mater and AVMs of the soft tissues of the skull) and extra-intracerebral.