Skull Osteology, Fossae, Meninges

Skull Osteology

• Skull is composed of the cranium and mandible.
• Cranium includes the calvarium, base or fossae, and facial bones.
• Bones are connected by fibrous joints called sutures.
• There are 22 bones in the skull, including paired and single bones.

Cranial and Facial Bones

• Cranial bones include the occipital, frontal, sphenoid, ethmoid, parietal, and temporal bones.
• Facial bones include the maxilla, mandible, palatine, zygomatic, vomer, nasal, lacrimal, and nasal conchae.
• The small bones of the ears are not included.

Views of the Skull

• Anterior view landmarks: frontal bone, glabella, nasion, piriform aperture, inferior nasal concha, zygomatic process, ramus of mandible, maxilla, angle and body of mandible, mental protuberance/tubercle, superciliary arch, supra-orbital notch, zygomatic process of frontal bone, nasal bone, frontal process of maxilla, infra-orbital foramen, nasal crest, anterior nasal spine, alveolar process, oblique line, alveolar part of mandible, mental foramen, and lesser wing of sphenoid.
• Lateral view landmarks: sphenoparietal suture, coronal suture, frontal bone, greater wing of sphenoid, lacrimal bone, nasal bone, zygomaticofacial foramen, zygomatic bone, maxilla, alveolar part of mandible, mental foramen, body of mandible, temporal process of zygomatic bone, sphenosquamous suture, squamous suture and part of temporal bone, parietal bone, parietomastoid suture, lambdoid suture, asterion, occipital bone, occipitomastoid suture, mastoid part of the temporal bone, tympanic part of the temporal bone, styloid process, condylar process, angle, ramus of mandible and coronoid process.
• Posterior view shows the sagittal suture, parietal bone, lambdoid suture, external occipital protuberance, mastoid notch/process, inferior nuchal line, external occipital crest, superior nuchal line, and inion.

Pterion

• The pterion is where the frontal, parietal, temporal, and sphenoid bones meet.
• The middle meningeal artery runs just behind the pterion.
• Damage may result in an intracranial bleed.

Sutures and Fontanelles

• The major sutures are coronal, sagittal, lambdoid, squamosal
• The Bregma and Lambda are also landmarks derived from sutures
• Anterior fontanelle (Bregma) closes around 18 months
• Posterior fontanelle closes between 2 and 3 months.

Calvaria

• The calvaria consists of the skin, dense connective tissue, aponeurotic layer (SCALP), diploe, bone, pericranium, external and internal tables.

Fetal Skull

• The fetal skull has a large cranium relative to the face.
• Cranium and face ossification occurs through intramembranous ossification.
• The base of the fetal the skull ossifies through endochondral ossification.
• The vault is cancellous bone, also known as diploe.
• At birth the bones are ossified but mobile.
• Sutures fuse later in life.
• Neonatal skulls have unossified gaps called fontanelles.

Fontanelle Clinical Signs

• Fontanelles that are depressed or sunken may indicate dehydration.
• Fontanelles that are bulging may indicate raised intracranial pressure, hydrocephalus, or meningitis.

Cranial Fossae

• The cranial fossae are divided into anterior, middle, and posterior regions.

Anterior Cranial Fossa

• The anterior cranial fossa consists of the frontal bone, ethmoid bone (crista galli and cribriform plate), and sphenoid bone (body and lesser wings).
• The foramina present are the foramen cecum and olfactory foramen.

Middle Cranial Fossa

• The middle cranial fossa includes sphenoid (body and greater wings) with the sella turcica and the squamous and petrous parts of the temporal bone.
• The foramina include optic canal, superior orbital fissure, foramen rotundum, foramen ovale, and foramen spinosum.

Posterior Cranial Fossa

• The posterior cranial fossa consists of the temporal bone (petrous part) and occipital bone (clivus and internal occipital protuberance).
• Foramina include the internal acoustic meatus (IAM), foramen magnum, jugular foramen, and hypoglossal canal.

Base of Skull

• Notable features: palatine process of maxilla (hard palate), palatine bone, vomer, pterygoid plates, foramen ovale, foramen spinosum, spine of sphenoid, carotid canal, occipital condyles, foramen magnum, superior nuchal line, external occipital protuberance, foramen lacerum, zygomatic arch, condylar fossa for temporomandibular joint (TMJ), external acoustic meatus, and styloid/stylomastoid foramen.

Meninges of the Brain

• The meninges consist of the dura mater, arachnoid mater, and pia mater.

Cranial vs Spinal Meninges

• Bone, Dura, Arachnoid and Pia occur in that order
• In the brain, a subdural space exists
• In the spine, there is an epidural space

Cranial Dura Mater

• The cranial dura mater has two layers: an outer periosteal layer intimately attached to the bone and an inner meningeal layer.
• There is no space between the dura and bone.
• Venous sinuses lie between these two layers and lack valves, distinguishing them from veins.
• The cranial dura is outermost, thickest, tough, and fibrous
• The two connective tissue layers are an external periosteal layer and an inner meningeal layer
• The two layers adhere to each other except at the 'Dural folds' containing venous blood
• A component of the periosteum forms on the inner surface of the skull bone from the periosteal layer
• "Potential" epidural space exists in the skull, whereas "True" epidural space is present in the vertebral column

Meningeal Layer

• The meningeal layer is smooth, avascular, and lines by mesothelium.
• It is continuous with spinal dura at the foramen magnum.
• Folds/septa reduce or prevent displacement of the brain when the head moves.
• The falx cerebri separates two cerebral hemispheres.
• The tentorium cerebelli separates the occipital lobe from cerebellum.
• The falx cerebelli partially separates the cerebellar hemispheres.
• Diaphragma sellae forms a roof of pituitary across the fossa.

Dural Sinuses

• Dural sinuses are located between the two layers of dura.
• The dura has an outer periosteal and inner meningeal layer.

Superior Sagittal Sinus

• Runs in the upper part of the falx cerebri to the internal occipital protuberance.

Inferior Sagittal Sinus

• Runs in the free edge of the falx cerebri
• Forms the straight sinus when joined by the great cerebral vein (of Galen).
• The straight sinus runs in the tentorium cerebelli to the confluence of sinuses.

Confluence of Sinuses

• This can be variable, however the superior sagittal sinus typically drains to the right transverse sinus.
• The inferior sagittal sinus typically drains to the left transverse sinus.

Transverse Sinus

• Runs in the groove and receives the superior petrosal sinus, it then becomes the sigmoid sinus.

Sigmoid Sinus

• Leads to the jugular bulb, and it becomes the internal jugular vein at the jugular foramen.

Petrosal Sinuses

• Superior and Inferior petrosal sinuses run along the superior and inferior borders of the petrous temporal bone.
• Cavernous sinuses connect with sigmoid sinuses.

Cavernous Sinus

• Found on either side of the body of the sphenoid, surrounding the pituitary gland.
• Important relation to Internal Carotid Artery & cranial nerves located here.
• Superior petrosal sinus runs to the transverse/sigmoid sinus.
• Inferior petrosal sinus runs to the jugular bulb.
• Ophthalmic veins drain here, going to the superior orbital fissure.
• The sphenoparietal sinus is along the lesser wing of the sphenoid.

Cavernous Sinus Thrombosis

• Rare but life-threatening, can complicate facial, sinus, and orbital infections.
• Basilar plexus thrombosis can cause brainstem infarction.
• Symptoms include fever, headache, and periorbital swelling.

Arachnoid Mater

• Is an impermeable layer between dura and pia mater.
• It adheres to the dura.
• Subarachnoid space contains CSF and encircles arachnoid trabeculae (spider-leg-like connective tissue and blood vessel strands)
• Arachnoid villi and granulations drain CSF into dural sinuses.

Subarachnoid Space and CSF

• The subarachnoid space is filled with CSF and surrounds the brain and spinal cord, providing a protective function.
• The subarachnoid space is in direct continuity with the fourth ventricle.
• It is where CSF flows in from the IV ventricle.

Subarachnoid Cisterns

• These are enlargements of the subarachnoid spaces in the cranial cavity.
• They include the cerebellomedullary cistern, cisterna magna, and chiasmatic cistern.
• Suspected pathological sites in the brain can be visualized through these cisterns using radiological procedures.

Arachnoid Granulations (Villi)

• These are projections of arachnoid mater protruding into venous sinuses.
• The largest ones are along the superior sagittal sinus and are visible on specimens.
• Cerebrospinal fluid is reabsorbed into venous circulation here.

Pia Mater

• Delicate layer adheres to the surface of the brain
• Dips into gyri and sulci, being macroscopically invisible, except terminal filum

Meningitis

• Inflammation of the pia
• Permeable to water & small solutes
• May secrete small amount of CSF
• Mainly produced by choroid plexuses

Intracranial Bleeding Types

• Extradural (EDH)
• Subdural (SDH)
• Subarachnoid (SAH)
• Intracerebral/intraparenchymal (ICH)
• Intraventricular (IVH)

Extradural (Epidural) Hemorrhage

• High-pressure arterial bleeding in a tightly closed space.
• A skull fracture tears arterial vessels in the extradural space.
• Commonly occurs in the temporal/temporoparietal region.
• Involves the middle meningeal artery (anterior branch) behind the pterion.
• The bleed lies extra-dural - runs in the groove or a tunnel of bone.
• Can result from damage to the sagittal/transverse sinus.
• Does not cross skull suture line.
• Creates a biconvex hematoma in the extra-dural 'space' between 2 layers of cranial dura.
• It is marked by an area of increased density and limited spread due to dural adhesion to the skull
• One clinical feature is a lucid interval followed by rapid deterioration
• Other clinical signs can be a short period of LOC, and loss of consciousness
• Leads to compression signs

Subdural Hemorrhage

• An area of increased density that becomes hypodense at 1-2 months.
• Exhibits a crescent or sickle shape.
• Causes cerebral or ventricular shift.

Subarachnoid Hemorrhage

• Appears as blood in the subarachnoid space.
• May be widely distributed throughout cisterns and fissures.
• May be caused by an aneurysm in the circle of Willis (Berry Aneurysm).
• Symptoms related to the severity of the bleed include sudden-onset headache ("blow to head"), transient/prolonged LOC, and nausea/vomiting.

Meningeal Bleedings Summary

• Extradural:
  • Arterial source
  • Trauma to middle meningeal artery (ant. branch) at pterion
  • Presentation: Blow to temple, loss of consciousness, followed by lucid interval, then deterioration.
• Subdural:
  • Venous source
  • Cerebral atrophy with stretched cerebral veins
  • Minimal trauma needed to shear or tear these
  • Presentation: Elderly patient with stumble/fall, gradual increase in confusion over 2-3 days.
• Subarachnoid:
  • Arterial source
  • Ruptured berry aneurysm on Circle of Willis
  • Presentation: Sudden, severe headache (during episode of raised blood pressure) – like a blow to the back of the head.

Herniation

• Occurs in bleeding, tumors, or other space-occupying lesions.
• Can cause compression of the cerebral cortex, leading to confusion.
• Herniation (coning) of structures results in compression at various levels.

Uncal Herniation

• Results from uncus of the temporal lobe being pushed through the tentorial notch.
• Compresses the oculomotor nerve (III), mainly parasympathetic fibers, which will result in a sluggish light relfex.

Cerebellar Tonsil Herniation

• Happens when the cerebellar tonsil is pushed into the foramen magnum.
• This then leads to compression of the medulla.
• Abnormal function of the cardiorespiratory center causes BP to rise and the pulse to fall. Cycles of apnoea and tachypnea/hyperventilation may occur.
• Cheyne-Stokes respiration results.