Cranial nerves pearls

Cranial nerves I & II: (olfactory and optic)

The first two nerves (olfactory and optic) arise from the cerebrum, whereas the remaining ten emerge from the brain stem.

In head trauma, the most commonly damaged nerve is Cranial Nerve I, the olfactory nerve

Cranial nerve III: (Oculomotor Nerve)

In Cranial nerve III (CN III) legion, the eye looks depressed and abducted, and pupil dilated (mydriasis).

Pupillary constrictor fibers run along outside of the oculomotor nerve (III CN), whereas vasculature is contained within nerve

For Cranial nerve III palsy with intact pupil, that means the lesion is from inside the nerve (ischemic cause)

For Cranial nerve III palsy with mydriasis, think of lesion from outside (compressive lesion)

Cavernous sinus lesions can cause unilateral and isolated third, fourth, or sixth cranial nerve palsy, as well as orbital pain and proptosis

Cranial nerve IV: (Trochlear Nerve):

Cranial nerve IV has long intracranial course, because of this, it is at risk of trauma during neurosurgical procedures involving the midbrain

Cranial nerve V: (Trigeminal nerve):

3 branches:

-          Ophthalmic: Scalp, forehead and nose

-          Maxillary: Cheeks, lower eye lid, nasal mucosa, upper lip, upper teeth and palate

-          Mandibular: Anterior 2/3 tongue (posterior 1/3 from Glossopharyngeal nerve, CN IX), skin over mandible and lower teeth

Cranial nerve VI: (Abducens Nerve):

Cranial nerve VI has the longest intracranial course; it may be affected in case of increased intracranial pressure

Cranial nerve VII: Facial Nerve

LMN lesion: ipsilateral facial weakness (with flattening of forehead)

UMN lesion: contralateral facial weakness with forehead sparing (due to bilateral frontalis innervation)

Cranial nerve VII palsy is the most common single cranial nerve involvement in sarcoidosis

In temporal bone fractures, there is usually injury to cranial nerves VII (Facial Nerve) or VIII (vestibulocochlear nerve)

Cranial nerve X: (Vagus nerve):

Xth cranial nerve, Vagus, is the longest among all cranial nerves

Cranial nerve XI: (Accessory Nerve)

LMN lesion: paralysis of ipsilateral sternocleidomastoid and trapezius

UMN lesion: paralysis of ipsilateral sternocleidomastoid and contralateral trapezius