The subarachnoid space is a space between two meninges. Cerebrospinal fluid circulates in it.
What is the subarachnoid space?
The subarachnoid space forms a cleavage zone between the pia mater as well as the arachnoid mater, which are part of the meninges. It is also known as cavitas subarachnoidea, cavum leptum meningicum, spatium subarachnoideum, or cavum subarachnoideale. Because cerebrospinal fluid (CSF) circulates in the subarachnoid space, it is also called the external CSF space. There is a connection between the outer CSF space and the inner CSF space, which is the ventricular system. The most common diseases of the subarachnoid space include subarachnoid hemorrhage.
Anatomy and structure
As mentioned earlier, the subarachnoid space is located between the pia mater and arachnoid mater. It connects to the internal CSF space through the apertura mediana (foramen magendii) as well as the apertura lateralis (foramen luschkae). The inner CSF space gets its shape from the cerebral ventricles. Its continuation occurs as spatium perivasculare (Virchow-Robin space) along the vessels pulling in the internal direction. In some places, the subarachnoid space reaches a particular width. These sections are called cerebrospinal fluid cisterns (cisternae subarachnoideae). Among the most important cisterns is the cisterna cerebellomedularis, also known as the cisterna magna. It is located on the side of the neck between the spinal cord (medulla spinalis) and cerebellum (cerebellum). At this point, a medical puncture between the first cervical vertebra atlas and the occiput is possible through the gap to extract cerebrospinal fluid. However, it is performed only in exceptional cases. Another cistern is the cisterna fossae lateralis cerebri. It is also called Cisterna valleculae lateralis cerebri and is located at the cerebrum. There it is located between the frontal lobe, parietal lobe and temporal lobe of the cerebral cortex. Also included in the cisterns is the cisterna chiasmatica, which is located on the lower side of the diencephalon in the region of the optic chiasm (optic nerve junction). At the midbrain, the cisterna interpeduncularis is located. More specifically, it is positioned in the cerebral crura cerebri. Together with the cisterna chiasmatica, it bears the designation cisterna basialis. At the midbrain, the location of the cisterna quadrigeminalis is at the four-hill plate (lamina tecti). Together with the cisterna interpeduncularis, it comprises the midbrain and is also called the cisterna ambiens. Other cisterns of the subarachnoid space include the cisterna pericallosa between the bar surface (corpus callosum) and the inferior portion of the cerebral crescent, the cisterna pontocerebellaris inferior within the cerebellopontine angle, and the cisterna pontocerebellaris superior, located at the border with the cerebellum at the lateral portion of the bridge (pons).
Function and Tasks
The subarachnoid space surrounds the spinal cord in humans. It acts as a buffer between the bony spinal canal and the soft spinal cord. It also has cerebrospinal fluid flowing through it, which serves as a protective barrier for the spinal cord. Thus, the cerebrospinal fluid envelops the brain like a water cushion. Furthermore, the human brain receives important nutrients from the CSF. It also removes metabolic waste products from the tissue of the nerves. The subarachnoid space is crisscrossed by trebeculae. These are covered by connective tissue cells. The cells have the properties of mononuclear phagocytes and can form macrophages. In the course of cerebrospinal fluid punctures, the macrophages can be detected, which in turn allows diagnostic conclusions to be drawn. Due to the aggregation of pia cells and arachnoid cells above the convoluted crests, the subarachnoid space occasionally shrinks. Conversely, however, its severe expansion may also occur.
Diseases
The most common disease of the subarachnoid space is subarachnoid hemorrhage (SAB). This refers to arterial bleeding that enters the subarachnoid space. Subarachnoid hemorrhage is considered a neurologic emergency that occurs relatively often. Women are particularly affected by the hemorrhage. In most cases, subarachnoid hemorrhage presents between the ages of 40 and 50. Every year, about 20 out of 100,000 people suffer from such a hemorrhage.Death occurs in a proportion of patients before they are treated in hospital. One third die in hospital or suffer permanent brain damage. Only in one third of patients does the subarachnoid hemorrhage take a positive course. In about 85 percent of all affected individuals, the subarachnoid hemorrhage results from the rupture of an aneurysm in the brain. An aneurysm is a sac-like malformation in a vessel wall. Because this vessel wall has less stability in the area of the bulge, there is an increased risk of rupture, which in turn leads to a subarachnoid hemorrhage. Even in the absence of other symptoms or disease, the aneurysm may rupture. Some people are physically active and lift heavy loads before rupture. In some cases, an abrupt rise in blood pressure is responsible for rupture of the aneurysm. Rather rare causes represent injuries of the craniocerebral region, poisoning, infections, blood clotting disorders, vascular inflammations or tumors. In some patients, no specific cause can be found at all. There are some factors that increase the risk of hemorrhage in the subarachnoid space. These include the use of tobacco or cocaine, excessive consumption of alcohol, and high blood pressure. A subarachnoid hemorrhage is noticeable by a severe headache. These spread from the forehead or neck further towards the back. In addition, those affected often suffer from neck stiffness, nausea, vomiting, sensitivity to light and impaired consciousness. Overall, the prognosis is considered unfavorable, with up to 40 percent of all patients dying and approximately 25 percent experiencing severe disability.
