For my "old-timers" this is a repeat TidBit Tuesday from 2021 (wow! time flies!). This is a really cool case about a cranial nerve deficit that we don't see very often. Enjoy this week's installment of our Cranial Nerve Summer Bonanza!
Signalment: 12 year old MC Mixed breed dog, 45 kg
History: 1 month history of change in bark, with a 1-2 week history of difficulty eating and drinking. The owners also identified difficulty walking in the last few days and a decrease in the dog's interaction with them.
Physical Examination: Grade II/VI left heart murmur, previously noted and not progressed. The remainder of the exam was unremarkable.
Neurologic Examination
Mentation: Mildly obtunded. The pet interacted when asked, but otherwise seemed content to stare at the floor.
Cranial nerves: Decreased to absent gag reflex, tongue atrophy (see the photo above), all remaining cranial nerves were normal.
Gait: Ambulatory mild proprioceptive ataxia in all four legs
Reflexes: Normal spinal reflexes including c. trunci and perineal.
Palpation: Non painful spinal palpation however pain elicited with cervical ventroflexion
Postural reactions: absent right thoracic and right pelvic limb paw replacement test, normal left paw replacement thoracic and pelvic.
Neuroanatomic Lesion Localization??
To do this, we need to break it down and identify all of the possible anatomic localizations each neurologic deficit could be noted. I prefer table form, but flow charts are also great.
DeficitWhere are ALL the places this abnormality could localize?Likely localization for THIS patient and whyObtundedCerebrum, diencephalon, midbrain, pons, medullaMedulla due to cranial nerve deficits noted.Gag reflexAbsent CN 9 and/or 10. This localizes to the medulla OR the peripheral nerves of 9 or 10Unable to determine if it has affected 9 or 10 however a central (not peripheral) lesion localization is suspected due to the concurrent paw replacement deficits and obtunded mentation.Tongue atrophy (right)Right CN 12 or right medullaSuspect medulla due to concurrent right sided paw replacement deficits and obtundationProprioceptive ataxia in all 4 legsC1-C5, medulla, pons, midbrain, prosencephalonSuspect medulla due to other findings but this does not localize all by itselfPain on cervical ventroflexionIncreased intracranial pressure (also called referred spinal pain), C1-C5, C6-T2 myelopathyLikely intracranial referred spinal pain due to concurrent cranial nerve deficitsAbsent right thoracic and pelvic paw replacementTo cause ataxia in both front and back legs, we need to assume the lesion is cranial to C6. Therefore, possible locations include C1-C5, medulla, pons, midbrain, prosencephalon.Likely medulla based on concurrent CN 9, 10 and 12 deficits noted above.
Do you see what I did here? I listed all of the possible locations that the affected deficit might involve and then narrowed down the lesion localization two ways:
Find the common denominator. In this case, the medulla. OR
Find the cranial nerve(s) affected and determine if the pet also has: a) abnormal mentation, b) hemiparesis ipsilateral to the affected cranial nerve or c) paw replacement deficits ipsilateral to the affected cranial nerve. If they do, it is central. If not, it is a likely a peripheral neuropathy.
Neuroanatomic Lesion Localization
Medulla, right side.
Differential Diagnoses
Neoplasia, meningoencephalitis (infectious or inflammatory)
Case Conclusion
This dog had normal CBC, serum biochemistry, UA, chest radiographs and abdominal ultrasound. Brain MRI identified a discrete contrast enhancing extra-axial mass in the right caudal fossa affecting the right side of the medulla. This finding was most consistent with a meningioma. A spinal tap was not performed due to the proximity of the mass to the cerebellomedullary cistern. Based on the working diagnosis surgical decompression, radiation therapy or supportive care were discussed with the owners and they elected supportive care.
Thanks for reading! I hope you have a great week. If you have a new colleague in your clinic please ask them if they'd like to sign up - the more the merrier!