What is a Brachial Plexus Avulsion?
The brachial plexus (aka the nerves that innervate the thoracic limb muscles) is located at spinal cord segments C6-T2. Vehicular trauma is the most common cause of injury. When struck by a vehicle, the limb is not only crushed but also pulled, or stretched (aka avulsed), resulting in separation of the peripheral nerve from the cell body at the nerve root in the spinal cord.
What are Common Clinical Signs?
Common signs include monoparesis or monoplegia, absent paw replacement in the affected limb, loss of ipsilateral cutaneous trunci reflex and partial or complete ipsilateral Horner's syndrome. A recent study by Marika Menchetti, et al, used a common grading scale to describe the clinical signs. The grading scale is as follows:
Grade 0: normal
Grade 1: weight-bearing monoparesis
Grade 2: non-weight bearing monoparesis, with possible flexion of elbow or shoulder
Grade 3: non-weight bearing monoparesis or plegia with intact pain perception
Grade 4: non-weight bearing monoplegia with absent pain perception
What is the Outcome of Dogs and Cats with Brachial Plexus Avulsion?
According to the study by Menchetti, 30 dogs and 19 cats were retrospectively evaluated for outcome factors. Of these, 80% of dogs (24/30) and 63% of cats (12/19) had a poor outcome as defined as limb amputation or lack of a function recovery of the limb. They report functional recovery in 20% of the dogs, and 37% of the cats, overall. The single prognostic indicator for amputation was a loss of pain sensation in the limb. Dogs with absent deep pain had a 9x higher likelihood of amputation compared to those with intact deep pain. Does this mean they wouldn't have recovered? No it doesn't, it only means that in this study dogs with limb amputation were more likely to have a lack of pain sensation which we have previously linked to a poor outcome.
What is the Time line for Recovery?
If the nerve has been severed, functional recovery is nearly impossible. If the nerve has been stretched, partially torn, but the basement membrane remains intact, re innervation is possible. The only way to uncover if the nerve is intact, or not, is through electrodiagnostic testing such as motor nerve conduction velocities. Typically nerves are thought to grow 1 inch/month for a total of 6-8 inches. In the previously referenced study, the dogs that showed recovery did so within the first 3 months after the injury. This differed slightly from cats in which 3 cats showed recovery within the first 3 months, 3 more recovered by the 6 month follow-up and 1 additional cat was noted to have functional recovery by the 12 month follow-up. Perhaps cats are slower to heal? My recommendation has historically been to wait at least 6 months before considering amputation but perhaps I should lengthen that for cats up to 1 year!?
Did you know?
Peripheral nerves continue to have normal, or near normal, nerve conduction distal to the injury up to 7 days after avulsion. Weird, but true!
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