S2) Meningeal Layers, Dural Folds and Dural Venous Sinuses Flashcards
There are 3 membranous sacs that surround the brain.
Identify them
- Dura
- Arachnoid
- Pia
Describe the structure of the three membranous layers around the brain
- Dura: tough fibrous membrane
- Arachnoid: soft translucent membrane
- Pia: microscopically thin, delicate closely adherent to surface of brain
What additional feature is found between the membranous layers of the brain?
Potential spaces:
- Subarachnoid space – between pia and arachnoid mater
- Subdural space – between dura and arachnoid mater
The dura consists of two layers.
Identify and describe them
- Periosteal: endosteum lining inner bones of skull (cranial floor)
- Meningeal: layer adjacent to arachnoid
The periosteal and meningeal layers are closely adhered but separate in some areas.
What is the result of this?
- Dural folds
- Dural venous sinuses (spaces which become venous channels)
Identify the two dural folds
- Falx cerebri
- Tentorium cerebelli
Describe the structure of the tentorium cerebelli
- Tentorium cerebelli is a strong fold of dura mater roofing over the posterior cranial fossa
- It has a tentorial notch and is attached at the midline to the falx cerebri
Describe the structure and function of the falx cerebri
Falx cerebri is a large fold of meningeal layer of dura mater that descends vertically between the cerebral hemispheres
Generally, dural folds stabilise the brain.
Explain how these folds can contribute to herniation
- A rise in pressure inside the skull leads to herniation
- Can be secondary to a bleed, compressing and displacing the brain against rigid dural folds and/or through foramen magnum
What are dural venous sinuses?
Dural venous sinuses are venous blood filled spaces created by the separation of meningeal from periosteal layer of dura
Describe the drainage of blood into dural venous sinuses
Venous blood from cerebral veins (in the subarachnoid space) traverses the subarachnoid space and drains into the sinuses
Describe the drainage of blood from the dural venous sinuses
Dural venous sinuses drain into internal jugular vein (eventually)
Describe the location of the dural venous sinuses
Major dural venous sinuses lie at the margins of borders of falx cerebri and tentorium cerebelli and on cranial floor
Identify the following dural venous sinuses:
- Superior sagittal sinus
- Inferior sagittal sinus
- Transverse sinus
- Sigmoid sinus
- Cavernous sinus
What is an intracranial haemorrhage?
An intracranial haemorrhage is bleeding in ‘spaces’ between meningeal layers due to head trauma or the rupture of a blood vessel (aneurysm)
Bleeding from blood vessels in the meningeal layers will cause accumulation of blood in which potential spaces?
- Extradural
- Subdural
- Subarachnoid
Bleeding can also occur within the brain tissue itself.
What is this called?
Intracerebral haemorrhage
E.g. contusions, tearing of white matter
Why are intracranial haemorrhages so dangerous?
Addition of ‘volume’ to an already fixed space (the skull) leads to rise in pressure and damage to brain tissue and other structures e.g. cranial nerves
Briefly, what type of bleeding occurs in an extradural haemorrhage?
Arterial bleed – middle meningeal artery
Briefly, what type of bleeding occurs in a subdural haemorrhage?
Venous bleed – bridging veins
Briefly, what type of bleeding occurs in a subarachnoid haemorrhage?
Arterial bleed – circle of Willis
How does a subarachnoid haemorrhage occur?
- Secondary to trauma or spontaneous rupture of blood vessel e.g. aneurysm
- Blood leaks into subarachnoid space, mixing with CSF
What is the common cause of subarachnoid haemorrhages?
A branch of ‘Circle of Willis’ ruptures (the arterial circuit responsible for supplying brain structures)
Describe the CT imaging of the head for a subarachnoid haemorrhage
- 93% picked up if within 24 hours and 100% if within 6 hours
- Lumbar puncture if CT inconclusive: sample CSF to identify presence of blood
