Skull Base Anatomy Fatih Kökdere

Overview Forms the floor of the cranial cavity Separates the brain from other facial structures

Overview Formed by ethmoid, sphenoid, occipital, paired frontal, and paired temporal bones.

Overview Subdivided into 3 regions: the anterior, middle, and posterior cranial fossae.

Anterior Cranial Fossa Most shallow and superior of the three cranial fossae Lies superiorly over the nasal and orbital cavities Accommodates the anteroinferior portions of the frontal lobes of the brain

Anterior Cranial Fossa: Borders Anteriorly and laterally bounded by the inner surface of the frontal bone Posteriorly and medially bounded by the limbus of the sphenoid bone Posteriorly and laterally it is bounded by the lesser wings of the sphenoid bone The floor consists of the frontal bone, ethmoid bone and the anterior aspects of the body and lesser wings of the sphenoid bone

Anterior Cranial Fossa : Contents Frontal crest: acts as a site of attachment for the falx cerebri (a sheet of dura mater that divides the two cerebral hemispheres) Crista galli: midline of ethmoid bone, acts as another point of attachment for the falx cerebri Cribriform plate: on either site of the crista galli, supports the olfactory bulb and has numerous foramina that transmit vessels and nerves. Anterior clinoid processes: rounded ends of the lesser wings, serve as a place of attachment for the tentorium cerebelli (a sheet of dura mater that divides the cerebrum from the cerebellum)

Anterior Cranial Fossa : Foramina Cribriform plate: numerous small foramina, transmiting olfactory nerve fibers. 2 larger foramen; Anterior ethmoidal foramen transmits the anterior ethmoidal artery, nerve and vein Posterior ethmoidal foramen transmits the posterior ethmoidal artery, nerve and vein

Anterior Cranial Fossa : Clinical Relevance Cribriform plate: thinnest part, most likely to fracture Anosmia CSF rhinorrhoea: leakage of CSF into the nasal cavity

Middle Cranial Fossa As its name suggests, centrally in the cranial floor Butterfly shaped Anteriorly and laterally: lesser wings of the sphenoid bone Anteriorly and medially: limbus of the sphenoid bone Posteriorly and laterally: superior border of the petrous part of the temporal bone Posteriorly and medially: dorsum sellae of the sphenoid bone Floor: the body and greater wing of the sphenoid, and the squamous and petrous parts of the temporal bone.

Middle Cranial Fossa: Central Part Formed by the body of the sphenoid bone Contains the sella turcica(turkish saddle), acts to hold and support the pituitary gland Tuberculum sellae (horn of the saddle): anterior wall of the sella turcica, and the posterior aspect of the chiasmatic sulcus Hypophysial fossa or pituitary fossa (seat of the saddle): a depression in the body of the sphenoid, which holds the pituitary gland Dorsum sellae (back of the saddle): posterior wall of the sella turcica, separates the middle cranial fossa from the posterior cranial fossa.

Middle Cranial Fossa: Lateral Parts Formed by the greater wings of the sphenoid bone, and the squamous and petrous parts of the temporal bones Support the temporal lobes of the brain The site of many foramina

Foramina of the Sphenoid Bone Optic canals: optic nerves (CN II) and ophthalmic arteries, connected by the chiasmatic sulcus Superior orbital fissure: opens anteriorly into the orbit; contains the oculomotor nerve (CN III), trochlear nerve (CN IV), ophthalmic branch of the trigeminal nerve (CN V1), abducens nerve (CN VI), opthalmic veins and sympathetic fibers  Foramen rotundum: transmits the maxillary branch of the trigeminal nerve (CN V2). Foramen ovale: the mandibular branch of the trigeminal nerve (CN V3) and accessory meningeal artery Foramen spinosum: transmits the middle meningeal artery, middle meningeal vein and a meningeal branch of CN V3

Foramina of the Temporal Bone Hiatus of the greater petrosal nerve: transmits the greater petrosal nerve (a branch of the facial nerve), and the petrosal branch of the middle meningeal artery Hiatus of the lesser petrosal nerve – transmits the lesser petrosal nerve (a branch of the glossopharyngeal nerve). Carotid canal – located posteriorly and medially to the foramen ovale. This is traversed by the internal carotid artery, the deep petrosal nerve also passes through this canal Foramen lacerum: At the junction of the sphenoid, temporal and occipital bones, filled with cartilage

Clinical Relevance: Pituitary Surgery The pituitary gland lies in the sella turcica of the sphenoid bone In cases of a pituitary tumour, it may need to be removed surgically. usually by a endoscopic transsphenoidal approach The sphenoid sinus is opened and the endoscope passes through to the pituitary gland Complications of pituitary surgery include CSF rhinorrhoea, meningitis, diabetes insipidis, haemorrhage and visual disturbances

Posterior Cranial Fossa Most posterior and deep of the three cranial fossae Accommodates the brainstem and cerebellum Three bones: the occipital bone and the two temporal bones. Anteriorly and medially: dorsum sellae of the sphenoid bone Anteriorly and laterally: superior border of the petrous part of the temporal bone. Posteriorly: internal surface of the squamous part of the occipital bone Floor: mastoid part of the temporal bone and the occipital bone

Posterior Cranial Fossa - Foramina Internal acoustic meatus: transmits the facial nerve (CN VII), vestibulocochlear nerve (CN VIII) and labrynthine artery Foramen magnum: transmits the medulla of the brain, meninges, vertebral arteries, spinal accessory nerve (ascending), dural veins and anterior and posterior spinal arteries Jugular foramina:  transmits the glossopharyngeal nerve, vagus nerve, spinal accessory nerve (descending), internal jugular vein, inferior petrosal sinus, sigmoid sinus and meningeal branches of the ascending pharyngeal and occipital arteries.  Hypoglossal canal: Hypoglossal nerve

Clinical Relevance: Cerebellar Tonsillar Herniation Downward displacement of the cerebellar tonsils through the foramen magnum Produced by a raised intracranial pressure; causes include hydrocephalus, space occupying lesions, and a malformed posterior cranial fossa Cerebellar tonsillar herniation results in the compression of the pons and medulla, which contain the cardiac and respiratory centers

References TeachMeAnatomy. (2017). The Anterior Cranial Fossa. [online] Available at: http://teachmeanatomy.info/head/areas/cranial-fossa/anterior/ [Accessed 30 Mar. 2017]. Skull Base Anatomy. (2016, June 28). Retrieved March 30, 2017, from http://emedicine.medscape.com/article/882627-overview