Meninges: Dura Mater and Epidural Space

Dura Mater

• In the skull, the dura is structured like a double-layered sandwich
• The outer layer, known as the periosteal layer, adheres to the skull bones
• The inner layer constitutes the actual meningeal dura
• These layers separate in certain areas, thus forming dural sinuses for blood drainage
• In the spine, the dura consists of a single layer and does not attach to the vertebrae
• The epidural space exists between the bone and dura, filled with fat and veins
• There is naturally no epidural space in the brain, unless a condition like bleeding occurs

Epidural Space Differences

• The epidural space in the spine is a legitimate, open area employed during labor and surgery for anesthesia
• The space is filled with fatty cushioning and veins, including Batson's plexus, facilitating the spread of pelvic infections to the brain
• In the head, the epidural space is not naturally present
• The outer layer of the dura is fused to the skull
• The presence of a lens-shaped hematoma on a scan suggests bleeding, pointing to a torn middle meningeal artery resulting from a skull fracture

Subdural vs Subarachnoid Spaces

• Sandwiched between the dura and arachnoid lies the subdural space
• Damage to bridging veins, which connect the brain surface to the dura, can lead to a subdural hematoma
• This hematoma appears as a crescent moon shape on CT scans, crossing skull sutures but not the midline, typically observed in older patients with brain atrophy
• The subarachnoid space is located beneath the arachnoid, immediately above the pia
• It contains CSF and major brain arteries
• Experiencing a thunderclap headache can indicate a subarachnoid hemorrhage, as seen with a ruptured aneurysm
• Meningitis occurs in this space, necessitating CSF analysis for white cells

Spinal Cord Anchoring

• The spinal cord is held in place by two structures made of pia mater
• Denticulate ligaments, numbering 21 pairs, extend from the cord and attach to the dura, acting as seatbelts to keep the cord centered
• The filum terminale, a slender pia thread, connects the bottom of the cord, or conus medullaris, to the tailbone
• It prevents cord bouncing during movement, originating from embryonic pia tissue

Lumbar Punctures

• To prevent spinal cord injury, lumbar punctures are performed below L2
• In adults, the spinal cord typically extends to L1-L2
• Accessing the subarachnoid space for CSF collection is the aim
• Initially encountering blood during needle insertion indicates a "traumatic tap"
• This signals the puncture of an epidural vein
• In contrast, a true subarachnoid bleed results in blood that does not clot
• Xanthochromia or a yellow tinge becomes apparent in the fluid after processing

Arachnoid Granulations

• Arachnoid granulations act as crucial one-way valves extending into the dural sinuses, notably the superior sagittal sinus
• CSF flows through them into the bloodstream
• If the drainage route becomes obstructed, such as after meningitis, CSF accumulates, leading to hydrocephalus
• Thin trabeculae within the arachnoid support and stabilize blood vessels

Head Injury with Lens-Shaped Bleed

• A lens-shaped bleed visible on CT is indicative of a classic epidural hematoma
• This occurs when arterial blood, usually from the middle meningeal artery, separates the dura from the skull, forming a high-pressure pocket needing quick surgical intervention
• Epidural hematomas do not cross skull sutures because the dura is fused in these regions

Epidural vs Subdural Hematomas

• Epidural Hematomas are bright, biconvex (lens-shaped), stay within suture lines, and imply artery plus skull fracture with young patients and temporal bone fracture energy
• Subdural Hematomas are crescent-shaped, cross sutures but not the midline, are caused by bridging veins, and are common in elderly or chronic alcoholics even from minor trauma

Bloody Tap

• A bloody tap during lumbar puncture is when CSF starts bloody but clears during collection indicative when a vein in the epidural space is poked
• The needle probably hit something on insertion and shows fresh blood
• If the CSF is uniformly bloody and doesn't clot subarachnoid hemorrhage should be considered
• The presence of xanthochromia or yellow supernatant means RBCs broke down indicative of old blood

Pia Mater and Blood-Brain Barrier

• The pia mater is like saran wrap stuck to the brain's surface, following curves and dives into sulci
• The blood-brain barrier stems mostly from tight junctions in capillary endothelial cells and astrocyte foot processes, and the pia helps keep things sealed

Subarachnoid Space Scenarios

• Bugs or bacteria/viruses cause meningitis when they inflame the leptomeninges and arachnoid plus pia
• CSF becomes cloudy with white cells, requiring lumbar punctures
• Aneurysm rupture involves sudden explosion of a berry aneurysm filling the subarachnoid space with blood, causing severe headache and vasospasm

Falx Cerebri

• The falx cerebri is a sickle-shaped curtain of dura that splits the brain into left and right halves
• It's from the meningeal dura layer, not the periosteal one
• A midline shift on a scan shows the falx being pushed aside by a mass

Spinal Epidural Hematomas

• Spinal epidural hematomas are emergencies because the spinal canal is a tight space
• A small bleed can crush the cord causing sudden back pain, leg weakness or paralysis
• The need for treatment of the spinal cord is urgent, and may require surgical decompression within hours

CSF Circulation and Issues

• CSF drains through arachnoid granulations into the dural sinuses
• If drainage is blocked due to a subarachnoid hemorrhage or infection CSF builds up causing hydrocephalus
• Blockage within the ventricles from a tumor is non-communicating while blockage at the granulations from meningitis is communicating. Either way, a shunt may be needed

Crescent-Shaped Bleed after Trauma

• After trauma a crescent-shaped bleed on CT is a subdural hematoma
• Subdurals result from slow venous oozing rather than epidurals' arterial, rapid nature
• They are prevalant in older people whose brains have shrunk with age
• Epidurals are frequent among young people with skull fracture

The meninges are protective layers covering the brain and spinal cord. The dura mater, the outermost layer, has distinct structures in the skull and spine. Differences in the epidural space between the spine and brain have clinical significance.