Step 1: Lecture Notes
Step 1: Lecture Notes
Step 1: Lecture Notes
®
USMLE
Anatomy
Lecture Notes
2017
Cranial Meninges
The brain is covered by 3 meninges that are continuous through the foramen
magnum with the spinal meninges. There are several similarities and differences
between spinal and cranial meninges.
• Pia mater tightly invests the surfaces of the brain and cannot be dis-
sected away, having the same relationship with the brain as spinal pia
mater.
• Dura mater (thickest) unlike the spinal dura, consists of 2 layers (peri-
osteal and meningeal) that are fused together during most of their
course in the cranial cavity.
–– Periosteal layer: outer layer lines the inner surfaces of the flat bones
and serves as their periosteum; can easily be peeled away from the
bones
–– Meningeal (true dura) layer: innermost layer that is mostly fused
with the periosteal dura mater throughout the cranial cavity. At
certain points in the cranium, the meningeal layer separates from the
periosteal layer and forms the dural venous sinuses and connective
tissue foldings or duplications: falx cerebri, diaphragma sellae and
tentorium cerebelli. These duplications separate and support differ-
ent parts of the CNS.
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Anatomy Immunology
• Arachnoid is the thin, delicate membrane which lines and follows the
inner surface of the meningeal dura. Projections of arachnoid called
arachnoid granulations penetrate through the dura mater and extend
into the superior sagittal dural venous sinus. Arachnoid granulations
Pharmacology Biochemistry
are where CSF returns to the systemic venous circulation.
Deep vein
Physiology Arachnoid
Medical Genetics of scalp Emissary vein
granulations
Diploic vein Skin
Pericranium
• Epidural space is a potential space between the periosteal dura and the
bones of the skull: site of epidural hematomas (described later).
• Subdural space is the potential space between the meningeal dura and
the arachnoid membrane: site of subdural hematomas (described later).
• Subarachnoid space lies between the arachnoid and pia mater contain-
ing CSF: site of subarachnoid hemorrhage (described later).
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The primary tributaries that fl w into the sinuses are the following:
• Cerebral and cerebelli veins form bridging veins, which pass across
the subdural space to drain into the sinuses.
• Emissary veins are valveless channels that course through the bones
of the skull and allow dural sinuses to communicate with extracranial
veins.
• Diploic veins drain the spongy (diploe) core of the flat bones.
• Arachnoid granulations are where CSF returns to the venous circula-
tion.
• Meningeal veins drain the meninges.
1
A
1. Superior sagittal* 6
Orbit (ophthalmic veins)
2. Inferior sagittal 3 7
B
3. Straight* 4
4. Transverse* (2) 4
6
Deep face veins
5. Sigmoid (2)
6. Cavernous (2) 5
Confluence of sinuses
5
7. Superior petrosal (2)
* Drain into the confluence of sinuses
located at the inion.
Jugular foramen
FigureFigure
II-6-12.II-6-12. Dural Venous
Dural Venous Sinuses Sinuses
239
Anatomy Immunology
• The straight sinus is formed by the union of the inferior sagittal sinus
and the great cerebral vein. It usually terminates by draining into the
confluens of sinuses (or into the transverse sinus).
Pharmacology Biochemistry • The occipital sinus is a small sinus found in the posterior border of the
tentorium cerebelli. It drains into the confluens of sinuses.
• The confluens of sinuses is formed by the union of the superior sagit-
tal, straight, and occipital sinuses posteriorly at the occipital bone. It
Physiology Medical Genetics drains laterally into the 2 transverse sinuses.
• The transverse sinuses are paired sinuses in the tentorium cerebelli
and attached to the occipital bone that drain venous blood from the
confluens of sinuses into the sigmoid sinuses.
Pathology Behavioral Science/Social Sciences
• The sigmoid sinuses are paired and form a S-shaped channel in the
floor of the posterior cranial fossa. The sigmoid sinus drains into the
internal jugular vein at the jugular foramen.
Microbiology
• The paired cavernous sinuses are located on either side of the body of
the sphenoid bone.
–– Each sinus receives blood primarily from the orbit (ophthalmic
veins) and via emissary veins from the deep face (pterygoid venous
plexus). Superficial veins of the maxillary face drain into the medial
angle of the eye, enter the ophthalmic veins, and drain into the cav-
ernous sinus.
–– Each cavernous sinus 1 via the superior and inferior petrosal sinuses
into the sigmoid sinus and internal jugular vein, respectively.
–– The cavernous sinuses are the most clinically significant dural sinuses
because of their relationship to a number of cranial nerves. CN III
and IV and the ophthalmic and maxillary divisions of the trigemi-
nal nerve are located in lateral wall of the sinus. CN VI and internal
carotid artery are located centrally in the sinus.
Nasopharynx
Figure
Figure Coronal
II-6-13.
II-6-13. Section
Coronal Through
Section Pituitary
Through PituitaryGland
Glandand
andCavernous Sinuses
Cavernous Sinuses
240
Clinical Correlate
INTRACRANIAL HEMORRHAGE
Epidural Hematoma
An epidural hematoma results from trauma to the lateral aspect of the skull
which lacerates the middle meningeal artery. Arterial hemorrhage occurs rap-
idly in the epidural space between the periosteal dura and the skull.
• Epidural hemorrhage forms a lens-shaped (biconvex) hematoma at the
lateral hemisphere.
• Epidural hematoma is associated with a momentary loss of conscious-
ness followed by a lucid (asymptomatic) period of up to 48 hours.
• Patients then develop symptoms of elevated intracranial pressure such
as headache, nausea, and vomiting, combined with neurological signs
such as hemiparesis.
• Herniation of the temporal lobe, coma, and death may occur rapidly if
the arterial blood is not evacuated.
Subdural Hematoma
A subdural hematoma results from head trauma that tears superficial (“bridg-
ing”) cerebral veins at the point where they enter the superior sagittal sinus. A
subdural hemorrhage occurs between the meningeal dura and the arachnoid.
• Subdural hemorrhage forms a crescent-shaped hematoma at the lateral
hemisphere.
• Large subdural hematomas result in signs of elevated intracranial pres-
sure such as headache and nausea.
• Small or chronic hematomas are often seen in elderly or chronic alco-
holic patients.
• Over time, herniation of the temporal lobe, coma, and death may result
if the venous blood is not evacuated.
241
Anatomy Immunology
Microbiology
Subarachnoid Hemorrhage
A subarachnoid hemorrhage results from a rupture of a berry aneurysm in
the circle of Willis. The most common site is in the anterior part of the circle of
Willis at the branch point of the anterior cerebral and anterior communicating
arteries. Other common sites are in the proximal part of the middle cerebral
artery or at the junction of the internal carotid and posterior communicating
arteries. Typical presentation is the onset of a severe headache.
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