Introductory Chapter: New Models of Cerebral Circulation

1. Introduction

The cerebrovascular system is very complex in its macro and micro-anatomy.

We can anatomically divide the cerebrovascular system into an arterial and a venous compartment.

2. The arteries

Blood flow enters the brain mainly through internal carotid arteries and vertebral arteries. These vessels subsequently create the so-called “Circle of Willis” which is constituted by anterior cerebral artery, middle cerebral artery, anterior choroidal artery, posterior communicating artery, anterior communicating artery, posterior cerebral artery, basilar artery.

Each artery is responsible for the vascularization of a specific cerebral area [1].

Arteries are characterized by anatomical variations and are responsible for several diseases such as aneurysms, arterio-venous malformations (AVMs), restrictions in blood flow that may occur from vessel narrowing (stenosis), clot formation (thrombosis), blockage (embolism), or blood vessel rupture (hemorrhage). These conditions cause a lack of sufficient blood flow to the brain parenchyma and may cause a stroke [2, 3, 4].

Standard diagnostic tools for this kind of disease are:

Cerebral Angiography: a catheter for dye injection is usually inserted in the femoral artery and then threaded through the main vessels of the abdomen and chest until reaching the carotid or vertebral artery. Through the use of a fluoroscope after the contrast dye injection, it is possible to have dynamic pictures of cerebral vessels. This is considered the gold standard diagnostic tool for studying cerebral circulation.

CT Angiography: a contrast dye is endo-venous injected, and CT images are subsequently acquired. Visualization of cerebral vessels injected by the contrast dye is possible together with the brain parenchyma. 3D reconstruction of vessels is possible.

Doppler ultrasound: it is a non-invasive, low cost, and rapid diagnostic test able to study the velocity of flow into the cerebral vessel.

Magnetic Resonance Angiogram: it is similar to CT angiography, with the difference that no contrast dye is required, and no ionizing radiations are used.

Indeed, all these diagnostic tools are constantly updated and improved, and this also allows to discover new pathological entities and helps in finding new treatments.

The aim of this book is to help the readers discovering new tools and models for cerebrovascular disease diagnosis and treatment such as augmented reality, 3D animated anatomical models, or new measurements of cerebral circulation.

3. The veins

The veins of the brain have no muscular tissue in their thin walls and possess no valves.

Veins constitute a deep and superficial circulation [1].

The deep parenchymal circulation drains blood from the deep white matter of the cerebral hemisphere, the basal ganglia, and the mesencephalon. Part of this circulation is the Basal Veins of Rosenthal, the internal cerebral veins, and the great cerebral vein (of Galen).

The superficial system comprises dural sinuses and cortical veins.

The superficial cerebral veins can be divided into three collecting systems:

• the mediodorsal group draining into the superior sagittal sinus (SSS) and the straight sinus (SS);

• the lateroventral group draining into the lateral sinus;

• the anterior group draining into the cavernous sinus.

The veins of the posterior fossa are also divided into three groups:

• the superior group which drains into the Galenic system;

• the posterior group which drains into the torcular Herophili and transverse sinuses.

• the anterior group which drains into the petrous sinus.

Venous sinuses are located between two rigid layers of the dura mater.

Venous blood finally leaves the brain through the internal jugular veins or the vertebral system.

If compared to the arteries, veins have always been less considered and studied for cerebrovascular diseases.

For a long time, they have been considered only a drainage system of the brain parenchyma. Indeed, new models and theories have proven a more complex function and their indispensability for maintaining balanced homeostasis of all brain parenchyma [5].

For example the balance of pressure between inflow and outflow in the brain, is mainly maintained thanks to bridging veins.

Moreover, in the past 10 years the idea of a “glymphatic system” has been proposed [6]. This system consists of a sort of “circulation” where cerebro-spinal fluid (CSF) is filtered by the glia limitans and enters the interstitium. It subsequently collects waste products and flushes them to the paravenular space to be discharged. Here, thanks to a balance between venular pressure and paravenular space, it passes into the veins and left the brain. The balance between cerebro-spinal fluid, interstitial fluid, and venous pressure is fundamental to allow this process of “cleaning” the brain parenchyma.

Studies regarding neurodegenerative diseases (such as multiple sclerosis or Alzheimer disease) are being carried on taking into account this new theory [7].

The “occlusive” venous diseases are also becoming more considered and studied.

Cerebral vein thrombosis is a rare cause of stroke (about 0.5–1%), however, it can affect young patients and it is associated with several risk factors, including the SARS-Cov2 infection that since 2019 is burdening the world population [8].

Moreover, new diseases have been associated to vein occlusion, in particular internal jugular vein occlusion.

The JEDI (Jugular Entrapment, Dilated ventricles, intracranial hypertension) syndrome has been described in 2019, consisting of a novel form of high-pressure hydrocephalus that can be successfully treated without a CSF shunt and depending on a bilateral internal jugular vein entrapment at the neck [9].

The Eagle jugular syndrome is characterized by a compression of the internal jugular vein at his passage between the styloid process and the arch of C1. It can be a rare cause of idiopathic intracranial hypertension (IIH) and it can be characterized by headache, tinnitus, insomnia, visual disturbances, or hearing impairment [10, 11].

In this book, a deeper look into new theories and diseases regarding the venous compartment will be discussed.

We hope readers will find interesting hints to improve their daily practice and fields of research.