On your schedule today is a 4 year old FS Beagle-X with a 3 day history of difficulty walking in the back legs. The owners described swaying, falling and occasional vocalization as if in pain. She has a history of a stifle injury about 1 year ago, but no other important medical history. Today is a lesion localization practice case, so grab a pencil and let's get cracking!
Physical examination: Mild thickening of the right stifle but no evidence of drawer, or instability in the stifle or any other joint. The remainder of her physical examination is unremarkable, other than the neurologic examination.
Neurologic examination:
Mentation: BAR, anxious
Cranial nerves: normal
Gait: Ambulatory, paraparesis with moderate proprioceptive ataxia in pelvic limbs only.
Reflexes: normal withdrawal in all four limbs, normal patellar reflexes bilaterally and normal anal reflex. The cutaneous trunci reflex stops at L2 bilaterally.
Postural reactions: Absent paw replacement testing in both pelvic limbs, normal in both thoracic limbs. Normal hopping in both thoracic limbs, absent hopping in both pelvic limbs.
Palpation: Spinal pain at TL junction, the remainder is non-painful. Normal cervical ROM and tail ROM.
The first questions we ask ourselves is "does this dog have evidence of neurologic disease?"
The answer, of course, is abnormal, so let's break it down. At this point in the exam you could draw a "stick figure" dog to help you navigate the lesion localization. I find this trick quite useful for visual learners so don't be shy! Label the stick figure with the spinal cord segments below. Carrying on...
This dog has normal mentation, no cranial nerve deficits and no history of behavior changes or seizures so I think we can safely assume the lesion is NOT intracranial. Cross off the head on your stick figure. This leaves spinal cord, peripheral nerve, neuromuscular junction, or muscle to choose from. Let's start by assuming it's spinal cord in origin but if the lesion doesn't localize to ONE spot on the spinal cord you should then move on to considering the neuromuscular system. When looking at the spinal cord, you have four localization segments to choose from:
C1-C5
C6-T2
T3-L3
L4-S3
The C6-T2 and L4-S3 segments are where the lower motor neuron cell bodies are housed and when a reflex is performed on the examination, these lower motor neuron cell bodies are used. That means that a DECREASED reflex must have a lesion in either C6-T2 (if a front leg reflex is affected) or L4-S3 (if a back leg reflex is affected).Look at the reflexes listed on the neurologic examination. No spinal reflex deficits are noted, except for c. trunci, correct? We'll get to the c. trunci reflex in a minute. This means you can consider C6-T2 and L4-S3 "free" of disease, or normal. Cross off C6-T2 and L4-S3 on your stick figure. This leaves us C1-C5 and T3-L3 for possible lesion localization. To do this, we must look at the gait description.
What is paraparesis? Paraparesis is a weakness in the pelvic limbs. Monoparesis = one limb weakness, tetraparesis = all four limb weakness. Make sense?
What is proprioceptive ataxia? There are 3 forms of ataxia, and proprioceptive ataxia is the most common one. This gait deficit occurs when the sensory nerves running from the toes --> peripheral nerve --> spinal cord --> brainstem --> forebrain become disrupted. When the nerves are disrupted, anything "downstream" or caudal to that disruption may show ataxia. In this case, it is just the pelvic limbs, therefore the lesion is caudal to the thoracic limbs. Caudal to the thoracic limbs is T3. We've already decided that we don't have reflex deficits therefore the lesion must be in front (cranial to) L4. Voila! The neuroanatomic lesion localization for this case is T3-L3 by process of elimination (and by doing a thorough neurologic examination).
What about the cutaneous trunci reflex? This reflex has sensory nerves that run from about T1 or T2 to L6. When stimulated (pinch the skin), the impulse travels UP the spinal cord to synapse on the lateral thoracic nerve that originates C8-T2. This motor nerve then activates the panniculus muscle to "twitch". If you pinch the skin at L6 and don't get a reaction (a "twitch") move cranially vertebra by vertebra until you DO get a response. In this dog, that response was at L2. Because of the pathway of the sensory nerves, we typically count 1-2 spots cranially from where we see the twitch and assume the lesion is there. Therefore, this dog has a lesion T12-T13 or T13-L1. Most importantly, when this reflex is reduced, the lesion is in the T3-L3 segment. If it is normal, it doesn't mean that the lesion CAN'T be in this segment. To make it more confusing, this reflex is completely unreliable in cats!
DDx: The most common differential diagnoses for this dog with spinal pain and acute, progressive T3-L3 myelopathic signs would be an intervertebral disc herniation, meningomyelitis,or trauma. I wouldn't exclude neoplasia or discospondylitis however they are less likely based on her history.
Plan: Spinal radiographs can be used to diagnose discospondylitis but cannot be used to diagnose disc herniations or meningomyelitis. If vertebral neoplasia is present, spinal radiographs may be helpful. If spinal cord neoplasia is present, spinal radiographs will not be diagnostic. 3D imaging is needed to look at the spinal cord which would be a myelogram with CT, a CT alone or an MRI (my personal favorite). If the client is willing to pursue imaging and possibly surgery, if indicated, consider a referral. If the client is not willing to pursue these findings, a referral may still be useful (second opinion, confirm lesion localization or for assistance with medical management). Spinal injection of steroids may be an option for clients unwilling to pursue surgical management so reach out if this describes one of your cases!
How did you do? Was this easy-peasy or more challenging? I'd love to know! Please feel free to email me your comfort with the localization on this case so I can introduce either more or less challenging localization practice in the future.
Thanks for reading! I hope you have a great week and I look forward to working with you soon!