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Last but certainly not least, we’ve made it to the last of our cranial nerve review. I bet you’re super bummed. Don’t worry, this sparked a whole slew of other neuro posts that need to happen since none of us can stand the super technical garbage.
So the hypoglossal nerve, its CN XII, aka TWELVE. This is the only cranial nerve that does not provide any sensory function, solely motor function. The hypoglossal nerve provides motor innervation to all intrinsic and extrinsic tongue muscles except the palatoglossus (which you totally remember is CN X).
CN XII is mostly involved with the oral phase. You are able to assess its function at the bedside by having the patient protrude the tongue and push against the cheek. The tongue will deviate to the side that is WEAK. If this is an upper motor neuron (UMN) lesion, the weak side will be contralateral (opposite) to the lesion. If this is a LMN lesion, then weakness is ipsilateral to the lesion (same side). Tongue fasciculations are also an indicator of a neurological impairment of LMN only – so loss of bulk, and fasciculations, both suggest a LMN lesion.
Side note: I was not prepared for all of the questions about UMNs/LMNs, I will do a seperate post about those, because those dudes are tricky.
If you suspect an impairment to CN XII, the patient may present with poor bolus manipulation, prep, and transfer, meaning oral residue after the swallow, and/or poor tongue base retraction to the posterior pharyngeal wall, resulting in residue in the valleculae post swallow. The hypoglossal nerve also provides lingual pressure to drive the bolus through the PES/UES. If that tongue isn’t wagging quite the way it should be, potential treatment strategies may include: exercises to lingual musculature, Masako, Effortful swallow, gargling, super-spraglottic exercises, the IOPI, or NMES in the facial placement.
Now, there are 2 other nerves that we have to address that may slightly confuse the hell out of you, but are important nonetheless. Some people consider the infrahyoids to be innervated by CN XII, however unless you want to fight your way through the massive layers of cobwebs to figure out exactly which loop is what, heres the deal.. The cervical plexus runs right next to CN XII at the level of C1 and provides innervation to the thyrohyoid and geniohyoid, (Yes I know the geniohyoid is a suprahyoid, remember this paragraph is meant to confuse you.) There is also a loopy mess formed at C1-C3 that innervates the sternohyoid, omohyoid, and sternothyroid, and is called the Ansa Cervicalis. Again, stick that in your cranial nerve back pocket the next time you’re nerding out trying to figure out how the hell the infra hyoids move.
I hope this cranial nerve review was as fun for you as it was for me, This shit sucks, there’s no way to sugar coat it, but it’s very important nonetheless. We have to understand why things are happening so that we can treat them appropriately. If this review really hit a nerve (no pun intended haha), that perhaps you should learn a little bit more about this jazz, there are a few great online CEU courses available now:
Disclaimer: I don’t make a dime off these courses, I just think they were written by pretty phenomenal people.