Brainstem Aneurysms & Cavernous Sinus Disease

CN 3, 4, 6 neuropathies from Aneurysms & Cavernous Pathology

Regional Anatomy

• Brainstem from the midbrain to the pontomedullary sulcus.
• Cerebellum.
• Inferior hypothalamus (the pituitary body, anteriorly, and mammillary body, posteriorly).
• Clivus (the skull base).
• Cavernous sinus – filled with venous blood. Venous drainage, makes the cavernous sinus susceptible to infection to facial infection.
• Superior orbital fissure.

Specific anatomy

• Oculomotor complex of cranial nerve 3 within the dorsal, center of the midbrain, lies at the level of the superior colliculus.
  • CN 3 emerges from the anterior, medial midbrain, passes through the basal cisterns, cavernous sinus, and superior orbital fissure to enter the orbit.
• Trochlear nucleus of cranial nerve 4, lies at the level of the inferior colliculus.
  • CN 4 passes dorsally out of the posterior aspect of the midbrain and courses around the outside of the opposite cerebral peduncle through the basal cisterns (specifically, the ambient cistern), through the cavernous sinus and through the superior orbital fissure to enter the orbit to innervate the superior oblique muscle on the side opposite its nucleus of origin.

CN 4 is the only cranial nerve to exit posteriorly from the brainstem and the only cranial nerve to make a decussation.

• Abducens nucleus of cranial nerve 6 in the dorsal, inferior pons.
  • CN 6 exits the brainstem at the pontomedullary sulcus, climbs the clivus, passes through
    Dorello's canal: a dural channel within a basilar venous plexus, and then passes through the cavernous sinus and superior orbital fissure to enter the orbit.

At Dorello's canal, CN 6 is fixed where it pierces the dura and can be stretched when there is downward herniation of the brainstem, such as from increased intracranial pressure.

CN 6 is in close proximity to the CN 5 (1) at this site and so a combined injury of these two nerves points to this localization site; the triad of otitis media, CN 5(1) and CN 6 injury is called Gradenigo's syndrome, due to spread of infection to the petrous apex of the temporal bone from mastoiditis.

The neurophysiology of the six cardinal positions of gaze, so when we add in the cranial pathologies, we can understand their extraocular manifestations.

• In brief, CN 6 innervates the lateral rectus, CN 4 innervates the superior oblique, and CN 3 innervates the rest.
  • Lateral rectus directs the eye laterally (called abduction).
  • Medial rectus directs the eye medially (called adduction).
  • When the eye is abducted, the superior rectus directs the eye superiorly and the inferior rectus directs the eye inferiorly.
    -When the eye is adducted, the superior oblique directs the eye inferiorly and the inferior oblique directs the eye superiorly.

Exam Findings

CN 6 palsy

• The lateral rectus fails to activate.
• The remaining muscles activate.
• The eye is medially rotated (adducted); it fails to rotate out.
• The patient notices this as double vision when attempting lateral gaze or when looking at a far image.
• Can be a warning of brain herniation, because CN 6 is tethered to the dura.

CN 4 palsy

• Superior oblique fails to activate.
• The remaining muscles activate.
• The eye is elevated (aka hypertropic).
• CN 4 is long and thin and is the only cranial nerve to make a decussation; so its innervation originates from the opposite side of the brainstem.
• Patients are often unaware of this deficit, because they produce a head tilt to counter it: they tilt their head to the side opposite the affected eye in order to bring their eyes into alignment. To understand why, do the following:

Hold your fists with your index fingers straight out to demonstrate the direction of the eyes. To demonstrate a right fourth nerve palsy, elevate your right hand. Now, tilt your head both ways; tilting your head toward the elevated (affected) side worsens the disconjugate lines of vision whereas tilting it the opposite way (towards the normal side) brings the lines of vision

into alignment.

CN 3 palsy

• Only the lateral rectus and superior oblique activate.
• The remaining muscles fail to activate.
• The eye is "Down & Out" and the patient has near-constant diplopia.
• In diabetic 3rd nerve palsy, a common cause of isolated CN3 injury, these eye movements often slowly recover over the course of several weeks to months.

Cerebral Aneurysms as a cause of CN 3, 4, 6 neuropathies.

The basilar artery ascends the brainstem – its branches are in close contact to CNs 3, 4, and 6.

• The posterior cerebral artery (PCA) wraps around the outside of the midbrain; PCA aneurysm can compress CNs 3 and 4.
• The superior cerebellar arteryy (SCA) passes around the upper pons; SCA aneurysm can also compress CNs 3 and 4.
• The anterior inferior cerebellar arteryy (AICA) wraps around the lower pons; AICA aneurysm can compress CN 6 as it exits the pontomedullary sulcus.
• The posterior communicating arteryy most commonly connects the ICA and PCA. P. comm. aneurysm is the most common aneurysmal cause of CN 3 compression.

The internal carotid arteryy (ICA) ascends into the cavernous sinus in a reverse "C": it juts forward and then backward again and ascends out of the sinus. Here, arteriopathies, such as carotid dissection, can cause vessel wall expansion with resultant compression of multiple cranial nerves.

Cavernous Sinus Pathology as a cause of CN 3, 4, 6 neuropathies.

Regional Anatomy

• The sella turcica of the sphenoid bone
• The pituitary body
• The base of the brain and underlying optic nerves
• The medial edge of the temporal lobe
• The sphenoid sinuses within the sphenoid bone – they are filled with air (unlike the venous cavernous sinus).

Cavernous sinus Anatomy

• The cavernous sinuses lie between the sella turcica and the temporal lobe.
• They are filled with venous blood, which drains the face.
• Cavernous sinus contents:

Along the lateral wall, from superior to inferior, are:

• CN 3
• CN 4
• CN 5 (1)
• CN 5 (2)
• CN 6 medial to CN 5 (1).
• The ICA lies within the medial aspect of the cavernous sinus.
• The carotid artery doubles back across the top of the cavernous sinus.

Within the cavernous sinus, sympathetic fibers run across CN 6 and join CN 5 (1), thus cavernous sinus ICA arteriopathy is a cause sympathetic nervous system injury to the face (Horner's syndrome).

Cavernous sinus thrombosis can cause neuropathies of all of the above cranial nerves, most commonly CN 6, because of its proximity to the carotid artery.

Causes of cavernous sinus thrombosis are:

• Cavernous sinus tumors:
  • Meningiomas (as we can predict from the presence of the lateral dural wall – remember meningiomas grow from the dura (such as at the dural convexity)
  • Pituitary adenomas, which we can guess from the proximity to the pituitary gland.
  • Trigeminal schwannomas – the trigeminal ganglion lies within Meckel's cave, adjacent to the cavernous sinus.
• Carotid-cavernous fistula, in which either from trauma or spontaneously the carotid artery forms fisutlas with the cavernous venous channels.
• Infectious invasion of the cavernous sinus, most commonly from staph aureus because of the venous drainage from the face, but notably from such causes as apergillus and mucormycosis.

Painful ophthalmoplegia is the most common presenting symptom of cavernous sinus disease.

• Cavernous sinus diseases (such as thrombosis, which we addressed, and non-thrombotic causes, such as Tolosa-Hunt syndrome) often manifest with ocular pain, redness, and proptosis (bulging).
• Tolosa-Hunt syndrome is the clinical eponym of granulomatous inflammation of the cavernous sinus. As an over-simplification, we can think of it as sarcoidosis restricted to the cavernous sinus.