Small Animal Medicine (SAM) Neurology Practice Exam

The main idea is that canine facial nerve paralysis is a peripheral nerve problem involving the facial nerve (CN VII) after it leaves the brainstem. When this nerve is damaged, the distal axons degenerate (Wallerian degeneration), causing loss of motor function to the ipsilateral facial muscles. In many cases, there is also demyelination of the affected nerve fibers as part of the injury and subsequent attempt at repair. So the dominant pathophysiology is axonal loss with some demyelination along CN VII.

Pure demyelination without axonal loss is less typical for canine facial paralysis and would present differently in terms of nerve conduction and recovery. Neuromuscular junction dysfunction would involve transmission at the muscle endplate rather than a primary problem in the facial nerve itself and would have a different clinical pattern. A central nervous system lesion of the facial nucleus implies a brainstem issue and would usually produce additional signs beyond isolated facial weakness, whereas peripheral CN VII injury is the usual mechanism in dogs.