What is the neuroanatomic lesion localization for the following case?
Signalment: 4 year old FS DSH
History: Acute onset inability to blink one eye. No history of trauma. She is an indoor only cat.
To answer this question, of course you must start with a cranial nerve exam. At its most basic level, the cranial nerve examination is a process of elimination. Let's start with the blink reflex. If you touch the medial and lateral canthus, what cranial nerves are you testing? (CN 5 and CN 7) How do you know which nerve is affected? To do this, we try to isolate each cranial nerve in the reflex to see which one misbehaves. How can you isolate these two nerves from each other to see which is the affected nerve? Lets's try a corneal reflex! When done correctly, the cotton swab touches the cornea and the eye retracts into the socket. Doing this tests CN 5 (sensory) and 6 (motor). Voila! If the cat does not blink when you touch the medial or lateral canthus, but DOES retract the globe when you do corneal reflex which nerve is affected? Think you know... scroll (or read!) to the bottom to see the answer.
But wait! That is only part of the question. We have now localized which cranial nerve is affected but we don't know if this is a central or peripheral nerve lesion localization, right? To look at the brainstem we focus on the nerve pathways running towards and from the forebrain and determine if they're affected. The nerve pathways that are easiest to test are proprioception and motor/tone. Watching the animal walk you may be able to detect a toe drag or delayed placement but ultimately we have to test proprioception through paw replacement testing or my personal favoriate for cats: tactile placing. Similarly, when watching the patient walk you may see a hemiparesis (weakness on one half of the body). Often this is more obvious in the pelvic limb but both ipsilateral limbs can be affected. The last piece to the puzzle is an evaluation of level of mentation. If the animal is obtunded, stuperous or in a coma, we have an effect to the brainstem RAS. If you have 1 or more of these signs, the animal has a brainstem disease. If we DON'T have delayed proprioception, evidence of hemiparesis or a change in mentation, we are more likely to be dealing with a peripheral neuropathy.
Remember:
1) cranial nerve deficit + delayed paw replacement/tactile placing, weakness or decreased mentation = brainstem.
2) cranial nerve deficit without the above = peripheral
Answer: Cranial nerve 7 is affected. (5 is normal in corneal reflex therefore it is not the problem in the blink reflex either.)
Thanks for reading and have a great week! Do you have a case that is puzzling you? Please reach out - I'd love to help. Did you know I also do onsite or virtual private CE for hospitals? Reach out for more details, if you're interested.