Limping is often orthopedic in origin however in some situations limping can be of neurologic origin. Nerve root signature (NRS) sign is observed clinically as a non-weightbearing flexion of one limb, during standing, which may also appear like limping when gaiting. The suspected causes for NRS include vascular compression, inflammation or compression of a nerve root. This is most commonly noted in the thoracic limbs but has been reported in the pelvic limbs also.
In a recent study, a group from NCSU evaluated dogs with cervical disc herniations to see what criteria were present when they had signs of a NRS. This study included 47 dogs. Not surprisingly, all 47 had signs of cervical hyperesthesia with signs like ataxia (n=14), intermittent lameness (n=7), tetraparesis (n=2) and tetraplegia (n=1) being observed less frequently. I personally am not sure how you can identify a dog with a root signature sign if they’re plegic…but apparently, they did! Interestingly (to me) the site of disc herniation was C2-C5 in 20 dogs (43%) and C5-T1 in the other 27 dogs (57%). The nerve roots are supposed to arise from the C6-T2 region which is why it would make the most sense that a NRS sign should involve those nerve roots. Why did the C2-C5 region have NRS signs? Perhaps there was tethering of the nerve roots from "tugging" secondary to a cranially located disc. Perhaps anatomically they had a neve root arise more cranial than typical.
Spinal cord compression was mild in the majority of cases but remember it takes a TON of disc material in the cervical spinal cord to result in compression. Mild spinal cord compression is often surgical because the canal is much more voluminous around the smaller spinal cord (which is different from the thoracolumbar spine). Therefore the canal can contain loads of disc material yet the spinal cord has room to scootch over away from the disc, thus resulting in mild compression. Disc material was almost 3x more likely to be located laterally, within the spinal canal, than medially. Additionally, dogs were 2x more likely to have disc material compressing a nerve root compared to dogs without NRS. The last interesting finding is that 75% of the dogs in this study were over 7 years old. Typical chondrodystrophic disc degeneration with resulting herniation occurs in ages 3-7 year old. Is there a relationship between age, site of disc herniation and NRS or was this coincidental? I’m not sure but felt it was worth of comment.
Take away message:
Thoracic limb limping is often orthopedic in origin, however NRS should be considered for cases in which orthopedic disease is not identified. Nerve root signature sign secondary to a cervical disc herniation is more likely to be lateralized and therefore referral for surgical correction is strongly recommended to alleviate the compression to the nerve root.
Thanks for reading! Are we through deep winter? I’m not sure but I sure hope you’re staying warm! Please reach out with any questions about root signature sign or other neurologic cases. Have a great week!
