How do you sort out a cat or dog with thoracic limb monoparesis following trauma?
This is an important question. First, let's review the innervation to the front leg. (Get back here - it's not that bad!) There are two important nerves that guide weight bearing and movement in the front leg:
1) Musculocutaneous - exits spinal cord segments C6-8. Important innervation is biceps muscle but it does a few others. The important action is flexion of the elbow and extension of the shoulder
2) Radial - exits spinal cord segments C7-T1 +/- 2. Innervates triceps and the muscles on the cranial distal limb that extend the carpus. The important action is extension of the elbow and carpus for weight-bearing.
So, to weight bear there must be an intact radial nerve. To move the limb forward there must be an intact musculocutaneous nerve. The other nerves (supra and subscapular, axillary, median and ulnar) are important too, but not as important. If you want to remember only two nerves, remember radial and musculocutaneous.
Now that we have that over with, let's put this to practice. Here is the scenario: You are presented with a 1 year old cat with a history of vehicular trauma a few days ago. The cat is presented dragging the left thoracic limb, unable to bear weight. When you watch it walk you can see advancement of the shoulder and elbow but it is minimal.
Question: What nerve is affected MOST?
Answer: Radial
Question: What spinal cord segment does the radial nerve arise from?
Answer: C8-T1 or 2.
Really good question: What is this cat's lesion localization?
Answer: Let's do a neurologic examination! :)
Neurologic exam:
Mentation: BAR
Cranial nerves: normal (note Horner's can be seen with thoracic limb injuries but isn't seen in this cat).
Reflexes: Absent triceps, absent withdrawal of the left thoracic limb. Unable to detect bicep or extensor carpi reflex (hey, it's a cat, give me a break!). All other limbs have normal withdrawal and pelvic limbs have normal patellar reflexes.
Palpation: non painful
Proprioceptive testing: absent tactile placing left thoracic limb, absent tactile placing left PELVIC limb, normal entire right side.
Gait: left thoracic limb monoparesis as previously described.
Now you can properly answer the previous question.
With absent radial nerve and diminished musculocutaneous innervation to the left leg, we KNOW the lesion must involve C6-T2, correct? (It's absent because of the missing withdrawal reflex - mediated through largely the radial nerve but also musculocutaneous too.) The real question here is this: is the injury in the plexus only or is there evidence of spinal cord involvement? To answer that question, you must look downstream from the affected segment. This means, look at the left pelvic limb. This limb has absent tactile placement so there has been disruption to the spinal cord tract going from the brain to the left pelvic limb, and back again.
Answer: This cat has a C8-T2 spinal cord lesion.
The differential list must now include things that affect the spinal cord such as avulsion and a disc herniation, hematoma or hemorrhage. Your diagnostic plan would include advanced imaging of the spinal cord. If the lesion localization had been peripheral plexus (not spinal cord) you would consider a brachial plexus avulsion only and advanced imaging would not be indicated. Knowing the lesion localization can markedly change your differential diagnoses, diagnostic plan and ultimately treatment and recovery!
Not sure about this case? Feel free to email me! This TidBit Tuesday is a slightly altered version of a real case seen recently. Keep those consults coming so we can share the knowledge folks! Please email/call/text me to schedule a consult or hop on my website and utilize the online scheduler to find the best time.
Have a good week!