Time for another terrible joke. Why did the neuron get in trouble at school?? (Answer at the bottom)
Let's review the cranial nerves and how we use them this week. First, there are 12 paired cranial nerves. Cranial nerve1 localizes to the olfactory bulb and cranial nerve 2 localizes to the thalamus, but the remaining 10 localize to the brainstem. You might even argue that CN 2 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 3-12 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/midbrain, metencephalon/pons, myelencephalon/medulla oblongata). 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? Let's review...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
Let's give this a try. The following findings are from a 6 year old FS Lab X
Mentation: normal
Cranial nerves: inability to perform blink reflex on LEFT, menace response or lip twitch with hemostat pinch on LEFT. Right side and all other remaining CN are normal.
Gait: normal
Postural reactions: normal paw replacement in all limbs
Reflexes: normal in all limbs
What is the neuroanatomic lesion localization? Let's go through the steps.
1. Identify the cranial nerve. The dog cannot MOVE it's lips and eyelid on the LEFT. Movement is caused by CN 7, so this is a CN 7 deficit.
2. What brainstem segment does this CN associate with? CN 7 comes from the medulla oblongata.
3. Any long-tract signs? No paw replacement deficits, no mention of hemiparesis and no mentation changes.
The neuroanatomic lesion localization for this pet should be peripheral CN 7 on the left.
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: it just couldn't control it's impulses! Thanks for reading and Happy February 2025!