Nasociliary nerve

The nasociliary nerve is a branch of the ophthalmic nerve. It is intermediate in size between the two other main branches of the ophthalmic nerve, the frontal nerve and the lacrimal nerve, and is more deeply placed.

Path
The nasociliary nerve enters the orbit between the two heads of the lateral rectus muscles and between the superior and inferior rami of the oculomotor nerve (CN III). It passes across the optic nerve (CN II) and runs obliquely beneath the superior rectus muscle and superior oblique muscle to the medial wall of the orbital cavity. After giving the anterior ethmoidal nerve, it terminates as the infratrochlear nerve on the medial margin of the orbit.

Tests
Since the long ciliary nerves carry the afferent limb of the corneal blink reflex, one can test the integrity of the nasociliary nerve (and, ultimately, the trigeminal nerve) by examining this reflex in the patient. Normally both eyes should blink when either conjuntiva is irritated. If neither eye blinks, then either the ipsilateral nasociliary nerve is damaged, or the facial nerve (CN VII, which carries the efferent limb of this reflex) is bilaterally damaged. If only the contralateral eye blinks, then the ipsilateral facial nerve is damaged. If only the ipsilateral eye blinks, then the contralateral facial nerve is damaged.

Branches
The nasociliary nerve gives off the following branches:
 * posterior ethmoidal nerve
 * long ciliary nerves
 * infratrochlear nerve
 * communicating branch to the ciliary ganglion (long root of the ciliary ganglion)
 * anterior ethmoidal nerve

PLICA is a mnemonic often used to remember these branches.