Neurologic Causes of Lameness
Joseph Eagleson, DVM, DACVIM
Veterinary Specialty & Emergency Center
There are multiple neurologic causes for thoracic limb lameness in the dog. Differentiating orthopedic versus neurologic causes of thoracic limb lameness can be challenging. There are certain exam findings that can help point you down the right direction; however, there are certain cases where it can be impossible to distinguish what the cause is based on the exam alone.
Some of the more reliable abnormalities on the neurologic exam that would support the cause of the lameness is neurologic in nature would include a decreased withdrawal reflex, postural reaction deficits, neck pain and a gait abnormality in the pelvic limbs. Patients with a musculoskeletal cause of lameness will typically have a normal withdrawal reflex. Exceptions to this rule would include a limited range of motion due to joint disease, severe pain in a joint, or a mechanical issue (e.g. triceps tendon rupture). Postural reactions should also be normal if the cause of lameness is musculoskeletal.
Exceptions to this rule would include a mechanical issue with the limb (e.g. tendon rupture). If the only source of pain on the lame patient is in the neck, then go look in the neck. If pelvic limb abnormalities with either the gait or postural reactions accompany the thoracic limb lameness, this would be a dead giveaway that the cause of the lameness is at the level of the spinal cord.
Other helpful exam findings that could suggest the cause of lameness is neurologic in origin would include profound muscle atrophy and pain on palpation of the soft tissue within the axilla. Muscle atrophy will either be secondary to denervation or disuse. Typically, denervation atrophy will happen quickly as opposed to disuse atrophy. The severity of the atrophy with disuse or denervation can be similar. So the chronicity and severity of the lameness should help you determine if the cause of atrophy is disuse or denervation. As one can tell, differentiating the two types of atrophy based on exam or history alone can be difficult. Pain on palpation of the axilla could just represent any soft tissue mass that is causing pain or soft issue trauma. A more common mass in this region that will cause lameness and often a decreased withdrawal is a nerve sheath tumor.
There are patients that can have subtle or profound thoracic limb lameness where no other abnormality can be found. These patients have no neurologic deficits or identifiable joint, muscle, neck, or tendon pain. Sometimes the signalment can help. If the patient is a dachshund or a beagle, then there is a decent chance the cause is coming from the spinal cord (and likely a disc herniation). But often times it is not that easy. Radiographic evaluation of the limbs and neck is a good place to start. If no abnormality is found on radiographs, then MRI of the cervical spinal cord and possibly the brachial plexus would be the next step.
The more common neurologic causes of thoracic limb lameness in dogs include a lateralized disc herniation, neoplasia either lateralized at the level of the canal or within the brachial plexus, brachial plexus injuries, neuritis, and meningomyelitis.
A common term that is used when a patient shows either weight bearing or non-weight bearing lameness associated a nerve root problem is a “root signature.” As one could guess, root signature typically infers that there is some type of pathology that is affecting a nerve root and causing nerve pain in the limb. Root signature can occur secondary to a problem at any of the nerve roots in the neck and not just at the cervical intumescence (C6-T2).
Figure 1 shows a lateralized C3-C4 disc herniation causing compression of the nerve root in a beagle. This patient presented with a weight bearing lameness and mild neck pain which improved with conservative management. If the severity of the compression is severe or conservative management fails, patients with lameness secondary to a disc herniation typically have an excellent prognosis with surgery.
Any neoplasia affecting a nerve root or peripheral nerve can cause lameness. A more common neoplasia affecting the nerves of the cervical spine and brachial plexus is the nerve sheath tumor. Nerve sheath tumors are locally aggressive and very slow to metastasize. Aggressive treatment typically involves surgery, radiation, or both. Prognosis with treatment is dependent on the location of the tumor. If the tumor is at the level of the spinal cord the prognosis is worse than if the tumor is located more distal on the limb. If the tumor is very distal, sometimes a cure could be achieved with amputation alone. If the mass is already at the level of the spinal cord, prognosis without treatment is typically weeks to months. Median survival with surgery or radiation when the tumor is at the level of the spinal cord has been reported to be 5 months.
Figure 2 shows a nerve sheath tumor of C3. This tumor was considered a mix-compartment mass because there was pathology that was extradural, intradural/extramedullary and intramedullary. This is not uncommon to see with nerve sheath tumors because the neoplasia will follow the nerve as it enters into the spinal cord.
Inflammatory conditions of the nerves (neuritis) or even the spinal cord (myelitis) can result in lameness.
In Figure 3, there appears to be a lateralized disc herniation of the C5-C6 disc. The patient had thoracic limb lameness, neck pain, a mild spastic paraparesis, and general proprioceptive ataxia to the pelvic limbs. The patient was taken to surgery and the compressive lesion seen in Figure 3 was actually a very enlarged nerve root. A rhizotomy was performed and the nerve root was submitted for histopathology. The results of the histopathology revealed a lymphocytic/plasmacytic neuritis.
The patient was initially weaker after surgery (which was not surprising since a nerve root was transected that contributes to the brachial plexus), but eventually the lameness resolved and the patient had only a mild weakness in that limb.
A good neurologic exam is the best and most important tool in trying to determine if the cause of lameness is neurologic or orthopedic in origin. The MRI then helps you to determine the details behind the lameness and best course of treatment.
Dr. Joseph Eagleson is a board-certified veterinary neurologist at the Veterinary Specialty & Emergency Center. The Veterinary Specialty & Emergency Center operates state-of the-art emergency and specialty veterinary hospitals that are open 24/7/365 in Levittown PA, Philadelphia PA and Conshohocken PA. For more information about our world-class emergency and specialty care, please visit VSEC on the web at www.VSECVET.com.