Neuropathic Pain Syndrome in dogs, sciatic and radial nerve presentations

Pain emanating from the lower back or neck can be a very debilitating condition and is hugely under diagnosed, the signs of pain can be very varied and may include behavioural changes such as hiding away, changes in temperament (showing signs of aggression to other dogs is common), sudden pain, increased thirst (psychogenic polydipsia) and general depression (lower back pain in man is commonly associated with psychosocial problems such as depression) but can also present as lameness, skipping and other signs of localised pain such as biting and chewing feet or just simple loss of performance in working and agility dogs. This syndrome can affect any dog of any age but it is not uncommon in very active and sporting dogs.

The source of the pain can also be just as varied ranging from pain associated with muscular spasms and “trigger point pain” common with altered gait patterns to clear, physical nerve root impingement by soft tissues or disc protrusions (slipped disc). However more subtle discogenic pain, neuralgia (diffuse nerve pain possibly caused by long term pain elsewhere) or facet joint changes can also be causes, the list is potentially endless and we are only just starting to get top grips with this syndrome. The primary underlying cause can be one sudden event that leads to an acute injury, a complex sequence of events caused by a continuous low grade lameness and changes in limb carriage and gait or associated with a repetitive strain type injury with a gradual build- up of “micro injury” over time that fails to heal, this is presumably the reason for a higher incidence in working dogs and is likely to be similar to the high incidence of back injuries and debilitating pain in human athletes, in some cases an anatomical variant that they were born with may also predispose for a repetitive strain type injury. Because the cause of pain can be so varied trying to determine the cause can be very complicated and in some cases is not possible. That doesn’t mean that there is no back pain it simply means that we don’t have the tests sophisticated enough to determine what is causing the pain. 10-15% of people suffering long-term lower back pain with sciatica have no identifiable abnormalities on MRI scans; in these patients, stretching of the nerve roots that contribute to the sciatic nerve may cause the pain or even just hypersensitization of the nerves causes by pain elsewhere (central wind up).

Who suffers from this type of pain?
As already eluded this is a common problem in sporting and highly active dogs, and in these dogs the pain may manifest itself “before” it would in a pet dog, for example refusing certain obstacles in agility or showing less ability to perform for working dogs. Lower back pain or radial nerve pain can also be seen in dogs with other primary orthopaedic problems and in these cases the combination of the neuropathic pain and the primary orthopaedic problem can then be doubly debilitating when either in isolation may be manageable. A good example of this would be a dog with chronic low grade elbow arthritis that then starts to show problems generally because the elbow problem is exacerbating the pre existing low grade back problem. Both conditions need to be recognised and treated for the best outcome. Other recognised causes of lower back pain include being born with abnormal vertebrae (transitional vertebrae), having cartilaginous defects (OCD), having large and wide intervertebral discs and breed; for example, GSD appear to have very angled facet joints which contribute to the development of disc problems. Specific causes of radial nerve and neck pain may include structural abnormalities of the cervical vertebrae. Certain breeds may be over represented by particular presentations for example Terrier breeds can present with a skipping hindlimb gait and will often also have other signs of forelimb pain. Often however there is no clear physical reason for the nerve pain.

So where on the wide spectrum can your dog be and how far do we take investigations to determine this?
So this has to be the first question asked once a diagnosis has been made. In a case where the presenting signs are clear, unremitting and extreme neurogenic pain (usually intense pain often associated with vocalisation (barking or screaming) and on occasion severe lameness in one leg) or if there are neurologic deficits (weakness to the legs and or loss of nerve reflexes) then prompt investigation is required in order to avoid risking permanent loss of function and these cases justify prompt onward referral to a neuro surgeon as physical decompression of nerve roots needs to be considered. MRI is the gold standard in assessing for direct nerve root compression.

However the majority of the dogs we are talking about are less clearly defined cases of more diffuse pain without nerve deficits. Investigations are not therefore as urgent and in many cases if there is another primary orthopaedic cause of lameness and altered gait such as hip dysplasia or cruciate ligament failure or elbow dysplasia then investigation and management of the primary condition will then help resolve the secondary pain. Physiotherapy to aid retraining of gait and especially relieve muscular pain after management of the primary condition is fundamental and in some cases acupuncture may also benefit. In other words in these cases investigation and treatment of the primary condition is required and specific investigations into the nerve pain may not be required.
The grey area in between these two extremes is the challenge and it is imperative to understand that many of the very subtle primary causes of pain (such as neuralgia, discogenic pain, complex regional pain syndrome and muscular pain) cannot be “imaged” that is to say no manner or number of pictures of any form will show a problem, that is because these are physiologic pathologies NOT physical. This doesn’t mean that there isn’t a problem it just means that we can’t see it and in these cases we have to manage the pain.

There are differences in opinion as to whether all these cases should have an MRI scan or not and the ultimate decision needs to be made on a cases by case assessment, if extreme changes in lifestyle are potentially going to be required (such as permanent retirement from work or agility) then an MRI may help make this decision. CT and dynamic plain radiographs (X rays) can be of some benefit in cases where there are major structural changes but imaging of nerves is limited.
As MRI remains the cornerstone of neuro imaging what does this involve?
MRI or Magnetic Resonance Imaging uses extremely powerful magnets to create a picture of hydrogen ions in tissues and so provides detail to tissues that cannot be seen with either X rays, CT or ultrasound. MRI scanning takes a long time and so patients have to be anaesthetised and typically
may be under general anaesthesia for between 50 minutes and 2 hours for some extended scans. This is a considerable length of time to be under GA and so the risk of this does need to be considered. Gadolidium may be used to provide extra contrast and there is also some risk involved with giving this both anaphylaxis and renal failure have been reported. More recently dynamic MRI, that is to take more than one MRI scan with the patient positioned differently, has been proposed, however this is still controversial and specific protocols for positioning have not been agreed and specific findings have not yet been recorded and accepted widely. Just who reads the MRI scan and determines whether the changes seen OR NOT SEEN are clinically significant or not is another question and this is best done by the clinician responsible for the investigations as opposed to a remote reporting service that can only see the images and not be able to apply the clinical appreciation of signs from the patient. The lower back or lumbosacral region will often have a poor correlation between clinical findings and advanced imaging. MRI will not define a cause of pain in all cases of lower back pain in dogs and estimates vary amongst specialists but up to 20% of dogs with lower back pain will have no specific cause seen on MRI.

How can my dog be treated for its back pain?
For cases that have clear neurologic deficits or continuous and unremitting pain immediate referral to a neurosurgeon is required, however this is not the group of dogs we are typically seeing. For this group of dogs the corner stone for treatment will be pain killers (of a variety or combination), periods of rest and controlled exercise and avoidance of precipitating or aggravating activities. Physiotherapy, acupuncture and addressing any other orthopaedic cause are important but rest and restricted exercise for a period of time is essential, that period of time is likely to be a minimum of 10 weeks. In human athletes particularly at a professional level, back pain is a common cause of retirement from sport, it is not uncommon for working or sporting dogs to miss an entire season in order to overcome the back pain and in some cases we have to accept that the activity of work or sport is the cause of the problem and that patient has to be retired from that activity for its own good.
Other treatments if the fundamental cornerstones are not working can include epidural steroids and surgery, either to decompress the nerves (dorsal laminectomy or lateral foraminotomy) or to distract and fuse the vertebrae, both of these carry some degree of risk especially with surgery. Epidural stem cells have also been proposed but there is currently no evidence for their use and the cost is significant, until better evidence is available this is not a treatment that Ridge Referrals support.

So who should I be seeing to treat my dog with back pain?
We have developed and are continuing to develop as we improve our understanding, a comprehensive protocol to treat and manage most of these cases that involves a number or different people, all with particular skills, these include myself as an orthopaedic surgeon, the physio here and elsewhere, Kate Rew as a rehabilitation vet and pain clinic expert and neurosurgeons with
access to MRI.

Every case is subtly different and so being completely set to a rigid protocol is not always possible however as a guide to management we may take the following path:

• Diagnosis made on clinical signs with no indication of neurologic impairment
o Address any primary orthopaedic issues
o Restrict exercise to light lead walking or less as dictated by any orthopaedic problem
o Start Gabapentin initially in evening only and then extended to twice daily
o Continue with other medications started
• Review after 3 weeks or 6 weeks if primary orthopaedic condition being treated
o If no significant improvement then:
o Add Amantadine to pain killer regime and continue restrictions as appropriate
o If improved considerably then continue Gabapentin and arrange to see the physio and pain clinic for long term management
In cases where further investigations are deemed necessary then we send them to see Tom Cardy at Cave Vet Specialists in Wellington with whom we are working with on this condition. Tom is a neurology specialist with a particular interest in this field and he has on site MRI and other specialist investigatory equipment.
This protocol is a basic plan and must be tailored to an individual’s needs.