When is Lameness not Orthopedic?

Lameness is typically orthopedic, as we know, but there are times when this isn’t true. How do we know when this is true? To answer this question, I’d love to go through a case with you today.

Signalment: 11 year old FS Labrador

History: She has a 4 week history of right thoracic limb lameness. She is a hunting dog, and signs were noted after a busy weekend in the field. On initial evaluation, mild discomfort was appreciated in her elbow therefore carprofen was started. Clinical signs did not improve; they worsened. She became intermittently non-weightbearing lame in the right limb. All other limbs appeared normal. Additionally, she started helping when laying down or rising from laying. At this point, I stepped into the game. What am I looking for to suggest this might be neurologic in origin?

Neurologic Examination:

Mentation: BAR

Cranial nerves: miosis OD, did not dilate in a dark space. Remaining CN normal.

Gait: Ambulatory, moderate right thoracic limb lameness. When toe touching, the limb was touching on tiptoe only.

Reflexes: Reduced withdrawal distal to the elbow on the right (unable to flex the carpus or elbow completely during withdrawal but able to mechanically flex both joints when I flex them), unable to detect triceps reflex on either side, remainder normal. Absent cutaneous trunci reflex on right, normal on left. Normal reflexes both pelvic limbs.

Postural reactions: Normal paw replacement in left thoracic limb and both pelvic limbs. Unable to assess paw replacement in the right thoracic limb due to non-weightbearing status. Reduced hopping on right thoracic limb compared to left thoracic limb.

Palpation: No spinal pain, normal cervical ROM.

What is the neuroanatomic lesion localization?

The first question is always, is the problem neurologic in origin? She has evidence of reduced reflexes so I would suggest, yes, it is. Essentially, our abnormalities are focused on that right thoracic limb with the addition of miosis OD and absent c. trunci right side.

Reduced withdrawal in the thoracic limb suggests a radial nerve injury/damage. Even though signs are distal to the elbow, the lesion could still be proximal, and neurodegeneration has occurred distally. Is the damage inside the spinal canal (central) or in the peripheral nerves (brachial plexus)? To answer this question, look at the right pelvic limb. Do you see any paw replacement deficits or weakness in the right pelvic limb? These are the long tracts going from the limb to the brain and the must pass right through C6-T2 and C1-C5 as they do so. If the lesion is in the canal, we should see paw replacement deficits in the right pelvic limb. We don’t, so the lesion is more likely in the radial nerves after they have exited from the canal.

What do we do with the miosis? The sympathetic pathway to the eye is as follows: thalamus à brainstem à cervical spinal cord à exit T1-T3 à join vagosympathetic trunk and ascend to the head à ganglion  à go through bulla to jump on CN 5 (trigeminal) à end in the iris for dilation. The radial nerve exits from segments C6-T2, which would cross over with T1-T3 and therefore we could make an argument that the miosis is secondary to involvement of this segment of the radial nerve.

What about the c. trunci? This one is pretty simple to understand once we refresh our memory of anatomy. Cutaneous trunci reflex is a simple spinal cord reflex that uses the dorsal sensory spinal nerves as afferent (sensory), and lateral thoracic nerve as efferent (motor). The lateral thoracic nerve arises from C8-T1. This fits with our assumption of a C6-T2 right sided neuropathy because loss of the lateral thoracic nerve on one side would cause an absent cutaneous trunci reflex.

Neuroanatomic lesion localization: Peripheral C6-T2 neuropathy/radiculopathy.

The differential diagnoses for this case were a lateralized intervertebral disc herniation, neoplasia, much less likely neuritis (infectious or inflammatory) and hypothyroidism. As it turns out this dog was diagnosed via MRI with a mass in the right brachial plexus. The owners elected amputation and it was determined to be a nerve sheath tumor. Why did the signs appear acutely? It’s not completely clear but my guess is that the dog was already decompensated slightly, and the heavy work caused inflammation or maybe even hemorrhage around the mass that resulted in an acute exacerbation.

That’s it for now! Reach out if you have any questions about this case, or any other case! I look forward to working with you soon. Have a great week!

Brachial Plexus Injury and Recovery

Brachial Plexus Injury and Recovery

Nerve injury can take 3 different forms, from least to most severe:
1) Neurapraxia: transient function loss (ex: conduction block) with no loss of nerve anatomy.
2) Axonotmesis: disruption of axons with some (mild) degree of myelin loss. Importantly the surrounding perineurium and epineurium are intact.
3) Neurotmesis: complete rupture of the nerve (axon, myelin and all surrounding structures).This injury does not lend itself well to recovery.
**Important, but trivial sounding, note: Nerve root avulsion is irreversible.

A recent study evaluated traumatic nerve injury and outlined their findings. The most clinically applicable key points are listed below:

• 226 animals were included (175 dogs, 51 cats)

• 46% were injured before age 2

• Horner's syndrome was seen in 68 animals (42%of dogs and 38% of cats) with miosis ipsilateral to the affected thoracic limb. Note: It should always be ipsilateral unless there is spinal cord damage.

• Cutaneous trunci reflex (which has it's motor origin between C8-T2) was lost in 81% of dogs, again ipsilateral to the affected limb. This reflex doesn't count in cats. :)

Prognostic factors in this study were largely related to electrodiagnostic studies, which are not clinically useful markers in practice. (Unless you have access to electrodiagnostic testing...which I don't, anymore!) That said, we know that animals with neurotmesis do not recover and prior studies have identified that animals with axonotmesis can recover but it may take awhile. Nerves may regrow 1 inch per month for a total of 8 inches. Therefore distal injuries resulting in axonotmesis may result in some functional recovery of the limb.
In the study referenced below, the only important clinical prognostic indicator from this study was the absence of cutaneous trunci reflex. When, absent, it was linked to a lesser chance of recovery, but no statistical analysis was performed to assess this trend. This study also reinforced the finding that electrodiagnostic testing is a valuable testing tool for providing a prognosis for dogs and cats following brachial plexus injury.

Do you have a patient with a traumatic limb injury with possible neurologic involvement? A neurologic examination may be able to better identify chances of recovery and direct treatment towards appropriate physical therapy, acupuncture and other management during the recovery phase. Reach out if I can help!

Have a great week and stay warm in this chilly turn towards fall!

Referenced article: https://doi.org/10.1111/jvim.16254

Monoparesis following Vehicular Trauma

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!