Subdural Space and Arachnoid Space an Overview

The subdural space (or subdural cavity) is a potential space that can be opened by the separation of the arachnoid mater from the dura mater as the result of pathologic process, trauma, or the absence of cerebrospinal fluid as seen in a cadaver. It can also be the site of trauma, such as a subdural hematoma, causing abnormal separation of the dura and arachnoid mater. Furthermore, in the cadaver, due to the absence of cerebrospinal fluid in the subarachnoid space, the arachnoid mater falls away from the dura mater. Therefore, it may also be the site of trauma, such as a subdural hematoma, causing abnormal separation of the dura and arachnoid mater. Therefore, the subdural space is remarked or referred to as “artificial space” or “potential”.

The subdural space is a potential space that exists between the meningeal layer of the dura mater and the inner arachnoid mater of the leptomeninges which are adherent to each other.

Medical Definition of Subdural Space

A Fluid-filled space or potential space between the arachnoid and the dura mater.

Gross Anatomy

The meningeal layer of the dura mater is usually adherent to the underlying arachnoid mater via a series of tight junctions. The subdural space does not exist under normal circumstances and is appreciable only when there is underlying pathology.

Furthermore, bridging veins drain from the underlying brain to the superior sagittal sinus and the dura mater. They have a relatively straight course and they don’t allow for much movement of the underlying the brain tissue. Therefore, they are easily placed under high tension. it is important in the elderly where the underlying the atrophic brain places these vessels under higher than normal tension
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Therefore, the bleeding from bridging veins can strip the dura from the arachnoid mater. Therefore, this collection of blood is also known as a subdural hematoma.

Subdural Haematoma

Normally the subdural space is potential rather than actual and therefore the cerebral hemispheres are kept apposed to the dura with a pressure somewhat below 10 mmHg within the lying position. In elderly people, cerebral atrophy decreases this pressure and it’s also reduced in infants by the soft surrounding skull. Therefore, there’s a spontaneous fall in the intracranial pressure in the upright position because of a relative drop in the venous component of the intracranial vascular volume and, conversely, an increase in the pressure with maneuvers causing venous distension like coughing and straining.

Severe direct or indirect trauma to the head can tear veins (or arteries) on the cerebral surface, where the cortical veins pass from the hemispheres to the venous sinuses, and therefore, cause an accumulation of blood from venous (or arterial) bleeding. furthermore, this kind of hemorrhage soon forms a solid clot but if operated upon in the very early stage manifests itself as liquid blood, furthermore, its sometimes mixed with CSF from disrupted subarachnoid cisterns (i.e. acute subdural hygroma).

An acute subdural haematoma is typically related to contusion and swelling of the underlying brain, and therefore, the combination of those two increasing volumes raise the intracranial pressure and therefore, displace the brain away from the side of a haematoma. furthermore, the transtentorial herniation of the brain rapidly follows.

Less severe trauma, that is usually indirect rather than direct, could cause a subdural haematoma that clots and therefore, the increased volume are accommodated by compression of the underlying brain and ventricular system in the more gradual manner.

This subacute haematoma begins to liquefy about a week or 10 days once its formation and produces a surrounding membrane which consists of granulation tissue and fine blood vessels. Therefore, despite compensation for the increased volume, and the intracranial pressure is generally raised. furthermore, in this type of haematoma, resolution may occur, as an alternative, another episode of trauma, transitory airway obstruction, straining, or overhydration could precipitate an extra rise in intracranial pressure and cause a transtentorial herniation.

The chronic subdural haematoma may occur more normally in the very old and very young. Therefore, the injury, particularly in the former, maybe minimal but as a result of the relatively low pressure in the subdural space the blood accumulates with a little bit compression, the haematoma is accommodated by the displacement of underlying the atrophic hemisphere in the elderly or furthermore, by the expansion of the soft cranial vault in the infant. Furthermore, after 3 weeks the haematoma membrane is well defined and the contents are completely liquid and dark brown in color.

Thereafter the volume of haematoma is increased periodically either by rebleeding into the cavity from the membrane or by an osmotic gradient, across the inner membrane from the CSF to the high protein haematoma, therefore, which exceeds the rate of haematoma reabsorption. Therefore, in infants, there is usually a direct addition to the subdural haematoma by the extracerebral CSF which escapes from basal cisterns and therefore, this produces the chronic post-traumatic subdural effusion of infancy.

REFERENCES

1. Subdural space From Wikipedia https://en.wikipedia.org/wiki/Subdural_space
2. article taken from the reference https://radiopaedia.org/
3. Reference Taken From the Sciencedirect
4. At the US National Library of Medicine Medical Subject Headings (MeSH)
5. Haines, Duane E.; Harkey, H. Louis; Al-Mefty, Ossama (1993). “The “Subdural” Space”. Neurosurgery.
6. This article incorporates text in the public domain from page 875 of the 20th edition of Gray’s Anatomy (1918)