What is the dura mater?

The dura mater, also known as the hard meninges, is a rough protective layer that surrounds the brain.Together with the deeper layers arachnoid mater and pia mater, it forms an important system of membranes for the central nervous system (CNS) and the adjacent spinal cord.It is the outermost layer of the three meninges, is located on the inside of the skull bone and represents its periosteum.

The dura mater also provides an anatomical arrangement of different parts of the human brain through septa and folds.Another special feature of the dura mater is that it is equipped with numerous pain receptors.Due to the high proportion of peptidergic axons (CGRP, substance P) in the vicinity of the meninges, it is associated with certain forms of headache such as migraines, even if the exact pathomechanisms for this clinical picture have not yet been finally clarified.The high density of pain receptors also explains the severe headaches caused by irritation of the meninges when blood spreads between them during a subarachnoid hemorrhage.Meningeal inflammation is also frequently associated with severe headaches and neck pain.Neck stiffness, or the inability to move the chin toward the chest, is a typical symptom of this disease.This is referred to as meningism.

The structure, function, and characteristics of these encapsulating layers are explained below.

What function do the meninges have?

Meninges protect the brain and spinal cord from mechanical influences, and from changes in temperature and volume.

The first of the dural folds to be mentioned is the falx cerebri, the cerebral crescent, a dural plate located midsagittally between the two cerebral hemispheres.The falx divides the two hemispheres into the right and left cerebral hemispheres.If a hemorrhage causes the brain to shift to the opposite side, this can lead to subfalcine herniation – this is referred to as brain entrapment.The result is massive cerebral damage that can lead to death.

Furthermore, the cerebellum is separated from the occipital lobe by the tentorium cerebelli, the cerebellar tent.This dural layer separates the infratentorial from the supratentorial space and is attached laterally to the upper edges of the petrous bones, posteriorly to the edges of the transverse sinus sulci and anteriorly to the clinoid process.In the direction of the clivus there is a gap known as the tentorial notch for the passage of the brain stem.At the occipital point of attachment, the tentorium cerebelli surrounds the paired transversal sinuses.When a lesion is operated on in the cerebellum, these thick veins are the proximal boundary and lead to massive bleeding that is often difficult to stop in the event of injury.

When there is a volume shift below the tentorium cerebelli, this is referred to as an upper incarceration.Here, too, it can be assumed that increased intracranial pressure, e.g. due to swelling, bleeding or tumor growth, is the cause.

In contrast to an upper incarceration, a lower incarceration occurs when the cerebellar tonsils are pressed into the foramen magnum.This is caused by an increase in volume due to swelling, hemorrhage or a tumor in the posterior fossa.This can be prevented by timely surgical removal of the mass and restoration of cerebrospinal fluid flow.In some cases, the bone flap removed for this purpose is not reattached and a dura dilatation procedure is performed to give the swollen brain more space to expand.

Another very variable dural plate is the falx cerebelli, or cerebellar crescent.It lies medial to the tentorium cerebelli and contains another venous blood vessel, the occipital sinus.

The diaphragma sellae should also be mentioned.This dural plate extends between the anterior and posterior clinoid processes over the hypophysial fossa, with a hole in the middle for the pituitary stalk.This is where the pituitary gland, an endocrine organ in the central nervous system, is located.Numerous hormones are synthesized and released here.

Tumors in this gland, usually benign adenomas, can cause hormone deficiency or overproduction.If surgery is required, the most common route is the transsphenoidal route, i.e. through the nose or paranasal sinus to the pituitary gland, which is located at the base of the skull.

One of the other folds of the dura at the base of the skull is the cavum trigeminale, also known as Meckel’s cave.It is located on the anterior surface of the petrous bone at the base of the middle cranial fossa and surrounds the trigeminal ganglion.It has an opening for the trigeminal nerve, the 5th cranial nerve.

The structure of meninges.

The structure: The brain is surrounded by three different meninges, the pia mater, the arachnoid mater and the dura mater.

The outermost layer, the dura mater, lines the inner surface of the skull and is also its periosteum.It is made up of dense, partly fibrous connective tissue and itself consists of two layers: the outer stratum fibrosum and the inner stratum neurotheliale.

Between the dura and the skull bone are the meningeal arteries, the anterior, posterior and middle meningeal arteries, which leave furrows on the inner surface of the bone.If the middle meningeal artery is injured in a TBI, it bleeds into the epidural space.Bleeding in this space can increase rapidly, leading to increased intracranial pressure, which can cause brain damage and death.From a therapeutic point of view, a space-occupying epidural hematoma must be quickly relieved by surgical removal.For this purpose, the skull is opened using a trephine such as the evoDrill from evonos and a subsequent craniotomy is performed.The blood on the dura is removed and the meninges are fixed to the bone with high sutures.The removed bone flap can then usually be reinserted and fixed with bone plates.With evoFix from evonos, evonos offers a wide range of plates for cranial fixation.Because of their good pliability and adaptability, they lead to less irritation of the septum.

The arachnoid mater encephali, or arachnoid membrane, lies beneath the dura.It follows the contour of the brain without extending into the furrows and does not have its own blood supply.Between these two superficial meninges is the subdural space.Bleeding can also occur in this capillary space, which is known as a subdural hematoma.It can develop over weeks if small bridging veins are injured, but it can also occur acutely as part of a head injury.Here too, rapid surgical relief is necessary, as otherwise the shifting of the brain can lead to an entrapment (see above).The skull must be opened by a craniotomy, then the underlying dura mater and then the visible blood can be removed.The skullcap is often not immediately refitted due to the swelling of the brain, but is initially positioned outside the body.After the swelling of the brain has subsided and the patient has recovered, it can then be reinserted and fixed to the skull with bone plates and small screws.If the autologous bone flap is no longer available, a so-called ‘bone graft’ can be used instead.It is individually reconstructed and fabricated in advance with the help of a thin-layer CT scan so that a good cosmetic result can be achieved.The evoShape from evonos is a perfectly fitting implant, both anatomically and in terms of surgical technique, which also shortens the operation time and improves patient safety thanks to its precise fit and prefabricated drill holes.

The innermost layer of the meninges is the pia mater.It is separated from the arachnoid membrane by the subarachnoid space.This is filled with cerebrospinal fluid (CSF) and is traversed by bundles of connective tissue collagen fibres, the arachnoid trabeculae, which connect the two layers.Where the subarachnoid space is not unfolded, the pia mater and arachnoid form a common membrane.

If a hemorrhage occurs in this space, this is referred to as a subarachnoid hemorrhage.Classically, it is caused by a burst aneurysm, a bulge in the arterial vessel wall.Clinically, this results in a devastating headache and patients can lose consciousness immediately.Again, rapid intervention is required.Firstly, a CSF drainage device in the form of an EVD must be placed to treat the accumulation of cerebral fluid.Secondly, the source of the hemorrhage must be eliminated.In this case, surgical treatment is possible by removing the aneurysm through the insertion of a clip.Depending on the anatomy and localization of the aneurysm, it may also be possible to eliminate it by interventional means.In this case, the aneurysm is filled with coils so that it is no longer supplied with blood and no longer poses a risk of bleeding.Overall, an aneurysmal subarachnoid hemorrhage is a serious, life-threatening condition.

What special features do the meninges have?

The meninges have a special blood supply.The vessels involved are divided into the vasa privata and vasa publica.

The vessels of the vasa publica are venous and transport the blood from the brain into the jugular veins.If they are blocked by a thrombosis, this can lead to serious neurological complications, such as sinus vein thrombosis.

The vasa privata are the meningeal arteries that supply the dura itself.They can typically cause epidural hematomas if the skull is injured.

The most important function of the dura mater is to protect the brain.Because it lies against the base of the skull from the inside, it can be damaged in the event of a fracture.If this happens, a CT scan can show air inside the dura (intradural).This is known as an open TBI, which is associated with a much higher risk of infection than a closed TBI where the dura is not damaged.

Trauma to the head can also affect the facial skull in the area of the paranasal sinuses or the orbit, through which the optic nerve passes.If fluid leaks from the ear or nose, it may be cerebrospinal fluid.A CSF fistula often forms spontaneously, but if it does it may need to be closed surgically.Artificial products can be used, but even better results can be achieved using human tissue, such as fascia from the thigh.

Tumors that originate from the meninges are called meningiomas.They arise from cells of the arachnoid membrane and usually lie over a wide area of the dura.They do not grow infiltratively, but displace the surrounding brain tissue.In order to reduce the risk of a subsequent recurrence during surgery, the point of attachment, i.e. the dura, should be removed, which is usually successful at the convexity.A dura replacement is then inserted instead, which is usually not possible at the base of the skull.Coagulation of the attachment point is performed here.

Is only the brain surrounded by dura mater?

Not only the brain, but also the central nervous system and the spinal cord are surrounded by dura mater. Collectively, these structures form a closed membrane system that is filled with cerebrospinal fluid.The structure of the cerebrospinal system is comprised of three layers, which also surround the spinal cord and the nerves of the anterior and posterior roots, as well as the spinal ganglia.

The outermost layer is the dura mater spinalis, which is the hard skin of the spinal cord.It divides at the foramen magnum into an outer sheet, the periosteum of the spinal canal, and an inner sheet.A cavity, filled with fat and veins, is located between the two layers.An anaesthetic is injected within this space during the epidural anesthesia process.This procedure involves the palpation of the spinal column to identify the appropriate location for puncture, which is then performed between two vertebrae under sterile conditions through the ligamenta flava into the epidural space.

The layer that is situated deeper within the structure is referred to as the arachnoid mater spinalis, a term that is analogous to the structure of the brain, and it is positioned against the inner layer of the dura.The subdural space is located between the two.The innermost layer, the pia mater spinalis, is located beneath the arachnoid mater spinalis.The subarachnoid space, which is filled with cerebrospinal fluid, is located between these two layers.Below the conus medullaris, this space is enlarged and is referred to as the cisterna lumbalis, in which the cauda equina is located.The extraction of cerebrospinal fluid from this space is a relatively straightforward procedure.This procedure includes the insertion of a needle between the spinous processes of the lower vertebrae under sterile conditions.The analysis of a cerebrospinal fluid sample can yield valuable insights into the presence of chronic neurological diseases, while the composition of the fluid itself can provide indications of inflammation, such as meningitis.This condition, frequently accompanied by symptoms such as headaches, fever and neck stiffness, necessitates prompt antibiotic or antiviral therapy.

It is also feasible to drain the cerebrospinal fluid from the cisterna lumbalis.The temporary or permanent drainage of cerebrospinal fluid via a tube is a procedure that can yield significant diagnostic information in cases of hydrocephalus.Testing before and after lumbar puncture with drainage of cerebrospinal fluid plays a particularly important role in the special form of normal pressure hydrocephalus, which is not an acute obstruction of drainage and congestion, but a chronic disease associated with symptoms such as memory impairment, urinary incontinence and gait disturbance.The MOCA test and a gait test can provide indications of the presence of hydrocephalus.The results obtained before and after the drainage of cerebrospinal fluid can be displayed and compared in a diagram.

The spinal meninges are supplied sensitively via the meningeal rami of the spinal nerves.The histology of the tissue is similar to that of the cerebral meninges.