Where is the "Basal Skull"?

The skull bones surround the entire brain, extending underneath to create the base of the skull.  The base of the skull is identified by the red line in Diagram 1.

The occipital bone (identified in green at the back of the skull) continues underneath the brain to produce the posterior fossa of the basal skull (Diagram 2).  The temporal (pink) and sphenoid bones (yellow) make up the major structures of the middle fossa of the basal skull.  The anterior fossa consists primarily of the frontal bone (shown in brown), however, most of the frontal bone is covered in Diagram 2 by the maxilla (roof of the mouth, identified by blue).  A small component of the parietal bone (gray) extends under the brain to produce the lateral edge of the base of the skull.

Diagram 2

Diagram 1


How are basal skull fractures diagnosed?

Basal skull fractures are often not detectable with skull x-rays or even CT scan.   Basal skull fractures are most frequently diagnosed by clinical findings, making clinical assessment skills critical.  CT may reveal suspicious fluid collections near a fracture if bleeding has occurred, or if damage to the dura resulted in a leak of CSF.  The base of the skull contains a number of bony channels or foramen that permit the passage of blood vessels and nerves through the bottom of the skull.  Clinical findings consistent with basal skull fracture are generally the result of bleeding or CSF leaks into one or more of these foramen or into the sinuses, or due to damage of the nerve that traverses the bony canals.

What are the signs of basal skull fracture?

Anterior Fossa Fracture - anosmia, epistaxis, rhinorrhea, subconjunctival hemorrhage, periorbital hemorrage (raccoon eyes, visual disturbances, altered eye movement, ptosis, loss of sensation to forehead, cornea and nare
Middle Fossa Fracture - loss of sensation to lower face, ottorrhea, deafness, tinnitus, facial palsy, hemotympanium
Posterior Fossa Fracture - echymosis behind the ear (battle sign), impaired gag reflex
Catastrophic injuries can occur if there is a major disruption of the carotid artery (blood supply to middle and anterior cerebral cortex) or vertebral artery (blood supply to brainstem and posterior cerebral cortex), or if the brain stem is disrupted.

How do the symptoms correlate to the area of injury?

The base or bottom of the skull is continuous, with one large opening for the brainstem (called the foramen magnum) and several small foramens or canals that permit nerves and vessels to travel through the skull bones.  The approximate location of some of the major foramen are identified in Diagram 3 (note that in this diagram, the maxilla covers the frontal bone - the frontal bone is the true base of the anterior fossa).
Diagram 3
When the base of the skull is fractured, disruption of the dura can allow CSF to leak into the sinuses or foramen that are close to the injury.  Bleeding from surrounding blood vessels can also enter these channels.  Symptoms of basal skull fracture include CSF leaks or bleeding into sinuses and canals and/or injuries to adjacent nerves.  The following table summarizes the signs and symptoms.
Fossa Foramen Structures Contained in Foramen Function Signs/symptoms
Anterior Fossa
  • cribiform plate
  • CN I 
  • CN I - olfactory (ipsilateral sense of smell)
  • anosmia (loss of smell)
  • epistaxis (nose bleed)
  • rhinorrhea (CSF from nose)
  • optic foramen
  • CN II (optic  nerve)
  • ophthalmic artery
  • retinal artery
  • CN II - optic (vision)
  • visual loss or impairment
  • impaired pupillary light response (CN II carries the light message to the CN III) 
  • periorbital hemorrhage (raccoon eyes)
  • subconjunctival hemorrhage
  • superior orbital fissure
  • CN III
  • CN IV
  • CN V1
  • CN VI
  • CN III - oculomotor (ipsilateral up and down eye movement, eyelid opening, pupillary constriction)
  • CN IV - trochlear (contra lateral downward and medial eye movement)
  • CN V1 - 1st or ophthalmic division of the trigeminal nerve [V] (ipsilateral sensation of the cornea, nare and forehead)
  • CN VI - abducens (ipsilateral movement of the eye in the temperal or lateral direction)
  • impaired or dysconjugate eye movement
  • ipsilateral ptosis (eyelid droop)
  • ipsilateral pupillary dilation and loss of reaction
  • loss of sensation to forehead, cornea or nare (loss of corneal reflex or nasal tickle response)
Middle Fossa
  • foramen rotundum
  • CN V2
  • CN V2 - 2nd or maxillary division of the trigeminal nerve [CN V] (ipsilateral sensation of the maxillary region of the face)
  • loss of sensation to the mid face
  • foramen ovale
  • CN V3
  • CN V3 - 3rd or mandibular division of the trigeminal nerve [CN V] (ipsilateral sensation of the mandibular region of the face)
  • loss of sensation to the mid face
  • ipsilateral weakness of masticator muscles
  • foramen lacerum
  • internal carotid artery
  • sympathetic plexus
  • supply of blood to anterior and middle cerebral cortex and ophthalmic artery
  • cerebral cortex injury (upper motor neuron injury with contra lateral loss of motor function to face, upper and/or lower extremity; ipsilateral blindness)
  • foramen spinosum
  • middle meningeal artery and vein
  • blood supply to temporal lobe
  • temporal lobe injury (impaired hearing, comprehension, memory or seizure activity)
  • epidural hematoma
  • internal acoustic meatus
  • CN VII
  • labyrinthine artery
  • internal auditory artery
  • CN VII - facial nerve - (ipsilateral facial movement, lacrimation, salivation, taste to anterior 2/3 of tongue, sensation around ear)
  • CN VIII - vestibulocochlear nerve (hearing, balance)
  • blood supply to labyrinth
  • ipsilateral facial weakness 
  • ipsilateral inability to close the eye
  • ipsilateral dry eye 
  • mouth dryness 
  • hemotympanium (blood in the ear canal)
  • tinnitus
  • hearing loss
Posterior Fossa
  • jugular foramen
  • jugular vein
  • sigmoid sinus
  • CN IX
  • CN X
  • CN XI
  • drainage of blood from brain
  • CN IX - glossopharyngeal nerve (stimulates parotid gland, sensation to pharynx, soft palate, posterior third of tongue, auditory tube, tympanic cavity and carotid sinus)
  • CN X - vagal nerve (muscles of soft palate and pharynx, parasympathetic control of heart and smooth muscles)
  • CN XI - accessory (movement of neck and shoulders)
  • echymosis behind the ear (battles sign)
  • loss of gag reflex
  • bradycardias
  • inability to rotate neck
  • hypoglossal canal
  • CN XII
  • CN XII - hypoglossal nerve (movement of tongue)
  • inability to move tongue
  • foramen magnum
  • medulla oblongata
  • meninges
  • vertebral arteries
  • meningeal branches of vertebral arteries
  • spinal roots of CN XI
  • medulla - respirations, blood pressure
  • vertebral arteries - brainstem, occipital lobe and cerebellum
  • bradypnea, respiratory irregularity
  • hypertension and bradycardia
  • cerebellar infarction (impaired balance or fine motor coordination)
  • occipital lobe injury (loss of vision in the contra lateral visual field of both eye - e.g. right occipital lobe injury can cause loss of visual in the left field of the right and the left eyes)


Barr, M., and Kiernan, J. (1993). The Human Nervous System: An Anatomical Viewpoint. (6th Edition). Lippincott: Philadelphia. pp 105-113, 122-147.

Diamond, M., Scheibel, A., & Elson, L. (1985). Human Brain Coloring Book.  HarperPerennial: Toronto. pp 6-2.

Netter, F. (1997). Atlas of Human Anatomy. Novartis: New Jersey. pp 1-9.

Waxman, S. (1996). Correlative Neuroanatomy (23rd Edition).  Appleton & Lange: Connecticut. pp 166-171.


Brenda Morgan, Clinical Nurse Specialist
November 19, 1999

Updated: January 19, 2019