Session 1 - Cranial Meninges Flashcards
What are the layers of meninges?
- three layers of tissue that envelop the brain and spinal cord.
• Dura mater – most external, lying against the skull. It is fibrous, thick and does not stretch.
• Arachnoid mater – the intermediate layer, it is much thinner and more flexible and resembles a spider’s web, hence the name.
• Pia mater – most internal, it lies on the surface of the brain. It is very thin such that it cannot be seen with the naked eye.
What is the duration of mater like?
The dura has two layers:
• The outer endosteal layer is adherent to the interior of the skull.
• The inner meningeal layer completely envelops the brain and spinal cord. The meningeal layer peels away from the endosteal layer in certain places and folds down into the brain to form a double layer of dura that separates certain parts of the brain.
o The falx cerebri is a double layer of folded dura lying in the longitudinal fissure that separates the two cerebral hemispheres.
o The tentorium cerebelli is a double layer of folded dura that separates the occipital lobe from the cerebellum.
o The falx cerebelli, like the falx cerebri, separates the two lobes the cerebellum. It is, however, much less pronounced.
What are the dural venous sinuses?
There are small channels where the outer endosteal layer and inner meningeal layer of the dura are briefly apart from each other, or where the inner meningeal layer folds back on itself leaving a small space. These channels are filled with venous blood and are called dural venous sinuses.
What are the different dural venous sinuses?
There are numerous dural venous sinuses:
• Superior sagittal sinus – formed in the space between the two layers of the dura, this sinus is located superiorly and runs the along the top of the brain in the sagittal plane.
• Inferior sagittal sinus – this is a smaller version of the superior sagittal sinus and runs in the same direction but is located inferior to it. It is formed as the meningeal layer of dura that forms the falx cerebri folds back on itself in the longitudinal fissure. It lies on top of the corpus callosum.
• Straight sinus – this dural venous sinus is found where the falx cerebri connects to the tentorium cerebelli posteriorly and allows venous blood to drain backwards from the inferior sagittal sinus.
• Transverse sinuses – found on both lateral aspects extending from the tentorium cerebelli around the side of the skull.
• Sigmoid sinuses – these s-shaped sinuses connect the transverse sinuses to the internal jugular veins outside the skull to drain venous blood from the brain.
• Confluence of sinuses – where the straight sinus meets the transverse sinuses and the superior sagittal sinus. It is found at the most posterior aspect of the skull and often leaves an impression in the internal aspect of the skull.
• Cavernous sinuses – these ‘cave-like’ sinuses are found anteriorly, either side of the sella turcica of the sphenoid bone. The internal carotid artery passes through it, along with some important nerves.
What is the arachnoid mater like?
The arachnoid is a single layer that loosely follows the contours of the meningeal layer of the dura. During dissection, it resembles a very thin, transparent layer of cling film. Between the arachnoid and the pia below is the subarachnoid space which contains cerebrospinal fluid (CSF). In some areas, the arachnoid spans between the gyri of the brain, leaving a covering over the sulcus. This forms a sealed space filled with CSF. These spaces are known as ‘cisterns’.
What is the pia mater like?
Pia Mater
As mentioned, the pia is so thin it cannot be seen without a microscope. It is tightly adhered to the brain and spinal cord and plays a vital role in forming the blood-brain barrier.
How is the blood-brain barrier formed?
The arteries that supply the brain are located within the subarachnoid space. When the vessels pass deep into the brain tissue, they take with them a layer of pia mater. At the capillary level, the pia fuses with the endothelial cells of the capillaries, forming a specialised layer of pia and endothelial cells known as the blood-brain barrier.
It limits the passage of certain molecules into the brain and spinal cord to protect them from harmful substances.
How does the BBB function?
It achieves this through four features:
1. The endothelial cells are tightly bonded together to prevent molecules passing between them.
2. The basement membrane of the capillaries in the brain and spinal cord lacks fenestrations (small holes) that are found elsewhere in the body.
3. Further specialised cells known as ‘pericytes’ wrap around the endothelial cells to regulate blood flow and permeability.
4. CNS cells called ‘astrocytes’ have specialised projections called ‘end feet’ that further wrap around the capillaries to restrict flow of certain molecules.
What is meningitis?
Meningitis
Inflammation of the meningeal layers is called meningitis. It is often caused by infection due to a viral or bacterial organism. Symptoms include headache, stiffness of the neck and photophobia (bright lights causing discomfort). Viral meningitis usually causes more mild symptoms and resolves on its own. Bacterial meningitis is much more serious and requires treatment with intravenous antibiotics. To confirm the diagnosis, a sample of CSF is required to be tested. To collect this, a small needle is inserted into the subarachnoid space in the lumbar region of the spine. This is known as a lumbar puncture or ‘spinal tap’.
What are the types of bleeding?
- extradural haemorrhage
- subdural haematoma
- subarachnoid haemorrhage
What is a extradural haemorrhage?
• Extradural haemorrhage – blood is located outside the dura, but inside the skull. The characteristic pattern on a CT scan is that of an oval or convex shape, often located at the lateral aspect of the brain. The reason the blood forms this shape is because the endosteal layer of the dura is tightly stuck to the inside of the skull, meaning blood trapped outside of it bulges inwards and cannot spread around the side of the brain. Extradural blood is arterial, and a common cause of an extradural haemorrhage is traumatic damage to the middle meningeal artery located on the lateral aspects of the skull. This region is sometimes referred to as the ‘temple’.
What is the subdural haematoma?
• Subdural haematoma – blood is located deep to the dura, but superficial to the arachnoid. The characteristic pattern on a CT scan is that of a crescent or concave shape, often located at the lateral aspect of the brain. Unlike in the extradural space, the arachnoid and dura are not adherent to each other, so blood can easily spread around the sides of the brain causing the crescent appearance. Subdural blood is venous, and bleeding is often caused because of stretching of bridging veins that are carrying venous blood from the brain to the dural venous sinuses. Any process or illness that causes the brain to shrink in size causes these bridging veins to become stretched, which is why subdural haematomas are more common in the elderly or patients suffering from alcoholism.
What is a subarachnoid haemorrhage?
• Subarachnoid haemorrhage – blood is located deep to the arachnoid, but superficial to the pia. Blood leaks into the subarachnoid cisterns, mixing with the CSF, sometimes causing a white star-shaped pattern on a CT scan. Subarachnoid blood is usually arterial and may be caused by a traumatic head injury or rupture of an aneurysm of one of the cerebral arteries. The classic symptom at presentation of spontaneous subarachnoid haemorrhage is sudden-onset severe headache, often termed a ‘thunderclap’ headache. Diagnosis may be confirmed by CT scan or by the presence of blood in the CSF. This can be sampled with a lumbar puncture.
What is a haemorrhage?
A haemorrhage is an active bleed, whereas a haematoma is a collection of blood without active bleeding. The main complication of bleeding inside the skull is the resultant increase in intracranial pressure. As the skull is a fixed size in adults, and cannot expand, if there is a build-up of blood inside it, the brain becomes compressed. Eventually, this will lead to impairment of functions of the brain, and ultimately, death.
What is the classic presentation of an extradural haemorrhage?
The classic presentation of an extradural haemorrhage, for example, is that of a patient who sustains a head injury, and then retains consciousness for a ‘lucid’ period, before becoming comatose. This is explained by the brain initially compensating for the bleed by becoming compressed, but eventually, it reaches its limit of compensation, and the patient loses consciousness. If a collection of blood or another space-occupying lesion, such as a tumour, is causing compression of the brain, you may be able to see a deviated falx cerebri or compressed lateral ventricle on at CT scan.
