As we sit here at the head of the year, I cannot think of a better time to review the cranial nerves. (Yes, I like cheesy jokes. I have kids. Here's another: what's the best present? See the bottom for the answer.)
First important thing to remember about cranial nerves: all cranial nerves except I and II have cell bodies associated with a specific brainstem segment. You might even argue that CN II localizes to the thalamus, which is an embryological part of the brainstem but...well, then we should have a coffee and get to know each other better! Let's stick to the idea that CN III-XII have cell bodies in specific brainstem segments, thus making it possible (dare I say easy??) for us to localize a lesion to either a specific brainstem segment OR the peripheral nerve.
What are the names of the brainstem segments, again? (Mesencephalon, metencephalon, myelencephalon). Okay, whew! Now that that is over, how do you decide if a deficit is coming from the nerve nucleus in the brainstem OR the peripheral nerve?
Take the following steps:
Identify the cranial nerve affected (i.e. facial nerve = CN 7).
Identify the segment of brainstem associated with the nucleus of this cranial nerve. Don't remember what cranial nerves are associated with which brainstem segment? Midbrain = CN 3,4; Pons = CN 5, Medulla = CN 6-12
Are any ipsilateral long tract deficits (postural reaction deficits, hemiparesis) or mentation changes (obtunded, stupor, coma) present?
If yes, the lesion is in the brainstem segment associated with the cranial nerve (i.e. medulla).
If no, the lesion is affecting the peripheral portion of the affected nerve
Guess what? You can apply these steps to any deficit affecting CN III-XII. Yay!
Do you need help performing the neurologic exam? I'd love to help! Looking for the answer to the joke? The answer is: a broken drum! You just cannot beat it. Thanks for reading and Happy New Year!