| Affiliation |
(AGL) Chairman, Department of Ophthalmology, The Methodist Hospital, Houston, Texas; Professor of Ophthalmology, Weill Cornell Medicine, New York City, New York; (KJ) Class of 2022, Baylor College of Medicine, Houston, Texas |
| Description |
Summary: •The skull base is divided into the anterior, middle and posterior cranial fossa. • Anterior cranial fossa o The optic nerve is located extremely close, so it is important to know anterior cranial fossa meningiomas. -Can be located at bony landmarks-frontal, olfactory, planum sphenoidale, tuberculum sellae, anterior clinoid can all produce optic neuropathy -Optic nerve crosses at the chiasm near sellae turcica and pituitary gland. • Pituitary adenomas cause chiasmal defects-bitemporal hemianopsia. • Middle cranial fossa o Foramen lacerum near cavernous sinus carries internal carotid artery -Intercavernous lesions can produce ophthalmoplegia here o Foramen spinosum carries middle meningeal artery -Epidural hematoma after trauma in the skull base o Superior orbital fissure connects cavernous sinus with the orbit -CN III, IV, V1, and VI travel through SOF o CN V2 goes through foramen rotundum o CN V3 goes through foramen ovale • Posterior fossa o The posterior clinoid is the superior-most extension of the clivus, the bone that slopes into foramen magnum o The pons carries CN VI, which travels up the clivus into the cavernous sinus and into the orbit through the superior orbital fissure o Tonsillar descent can occur through foramen magnum. This is called Chiari malformation and usually presents with downbeat nystagmus. |
| Transcript |
We are going to be covering all of the pieces of the skull base. We will start with the anterior cranial fossa, move to the medial cranial fossa, and finish in the posterior cranial fossa. Because the optic nerve is very close to the anterior cranial fossa, anterior cranial fossa meningiomas, including frontal, olfactory, planum sphenoidale, tuberculum sellae, and anterior clinoid, can all produce optic neuropathy. A tuberculum sellae or planum meningioma can press on both optic nerves. This saddle-like structure is the sellae, or sellae turcica, and the pituitary gland lives here. Because the optic nerve crosses at the chiasm, pituitary adenomas and any suprasellar lesion produce a chiasmal field defect, usually bitemporal hemianopsia. However, they can also produce optic neuropathy, bilateral or unilateral, or affect the tract. These small foramina carry the internal carotid artery, so intercavernous aneurysms produce ophthalmoplegia in this location. This oval-shaped foramen is ovale. Behind ovale is spinosum, which carries the middle meningeal artery, known most commonly for epidural hematoma after trauma across the skull base. In addition, everything in the cavernous sinus has to travel to the orbit, and the way things get there is through the superior orbital fissure. The superior orbital fissure carries cranial nerves III, IV, V1, and VI, from the cavernous sinus directly into the orbit. V3 goes through the foramen ovale, and V2 goes through foramen rotundum. So, we've got the superior orbital fissure, rotundum, and ovale carrying divisions 1, 2, and 3 of the trigeminal nerve. This back part here is the posterior clinoid and is the superior-most extension of this sloping bone, which is called the clivus. The pons carries the CNVI, which rides right up the clivus, into the cavernous sinus, and then into the superior orbital fissure. You can notice the hypoglossal canal is at the base of the clivus. CNVI and CNXII normally do not run together, but if they do run together, you should be thinking of a lower cranial nerve clival lesion. As you go to the petrous apex, that is where CNVII and CNVIII are exiting towards your ear. VII is going to be exiting out the stylomastoid foramen. In the posterior fossa, this giant magnum foramen is notable because we can have tonsillar descent through here, and that is called Chiari malformation that usually presents to us as downbeat nystagmus. You should know each of the structures of the skull base, particularly meningiomas, which can come to us as optic neuropathies, bitemporal hemianopsias or homonymous hemianopsias, ophthalmoplegias from cranial nerves III, IV, and VI, trigeminal dysfunctions in V1, V2, and V3, and isolated VIth dysfunctions from the clivus or from cerebellar pontine lesions affecting VI, VII, and VIII. A VI-XII dysfunction is a lower clivus lesion, and by the time we get to foramen magnum we are really talking about cervico-meduallary junction lesions, resulting in Chiari malformation and downbeat nystagmus. |
