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).

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.