A pain pathway approach to managing osteoarthritis (Proceedings)

Rationale

Whenever confronted with a painful animal, one should consider employing a considerable effort to control the pain as best as possible and as early as possible. This is true for both acute and chronic cases of pain.

     1. Many pet owners of painful animals come to see you with euthanasia already on their mind. Sometimes your first chance is your last chance.

     2. In cases of chronic pain, one has to consider that states of hyperalgesia or even allodynia exist secondary to plastic changes made in the dorsal horn of the spinal cord. Hence, unless these potential issues are addressed, the results of your pain treatment may be frustrating to both you and the client.

     3. In the case of acute pain, you have an opportunity to intervene early and avoid wind-up pain and central sensitization from even occurring

     4. In order to achieve these goals, you must approach the treatment of your case from a pain pathway approach: To treat pain effectively at every level of the pain path from periphery to the CNS, and to do so safely.

Opportunity

     1. As you start to treat cases from a multi-modal approach, you will see something start to happen: Your bottom line will get larger. I cannot think of one other area of veterinary medicine that has the opportunity for growth as does the treatment of chronic pain. I also cannot think of one other area of treatment where you can treat a condition that is never going to get better but still have both you and your client and your patient feel so good about it!

     2. You will find as you start to treat chronic pain cases from a multi-modal approach that you will want to employ non-pharmaceutical modalities such as physical therapy and acupuncture. I encourage everyone to pursue these lines of treatment...as the client community becomes more aware and sophisticated, they will soon demand these services from you or go elsewhere to find them.

     3. There are several programs out there for learning canine rehabilitation and acupuncture. My personal choices are also my recommendations: The Canine Rehabilitation Institute in Florida offers very hands-on training in their class setting. Colorado State University offers a science-based acupuncture course that will integrate seamlessly with your Western medicine practice. But where ever you go, it ultimately does not matter, what matters is that you advance your ability to treat pain in your patients.

Treatment of pain by pathway

     1. Besides the pharmaceutical pathways, we need to consider the 4 basic pathways within an animal's body.

          a. Perception of pain in the CNS

                i. Anesthetics

               ii. Opioids

               iii. Alpha-2 agonists

          b. Spinal Pathway

               i. Local anesthetics

               ii. Opioids

               iii. Alpha-2 agonists

               iv. Tricyclic antidepressants

               v. NMDA antagonists

               vi. NSAIDs

               vii. Calcium regulated voltaget/transmitter inhibitors

          c. Nerve transmission of impulses or conduction

               i. Local anesthetics

               ii. Alpha-2 agonists

          d. Nociceptive pathways in the transduction of initiating pain signals in the periphery

               i. Local anesthetics

               ii. Opioids

               iii. NSAIDS

     2. As you can see, many of the pharmaceutical agents we choose can affect pain in more than one of these body pathways

Chronic pain

We will follow a case of a dog with chronic pain secondary to osteoarthritis from chronic hip dysplasia. Let's assume that this dog is a 12 year old Labrador Retriever with no other ailments and normal blood work and urinalysis. Every one of the following treatments could be employed in the treatment of this dog

     1. COX inhibitors. NSAIDs are the mainstay of our pain medications in case of osteoarthritis. Never give more than one NSAID at a time. If one NSAID does not work, allow for a washout period and try another.

     2. Calcium regulated voltage inhibitors. Gabapentin is the most common used. Pregabalin is also available. This medication works especially well when given with an NSAID. Gabapentin stops some of the nerve signals at the dorsal horn of the spinal cord. Whenever you feel that there is central sensitization leading to hyperalgesia or allodynia, you should always add this class of drugs to your regimen

     3. NMDA Antagonists. These work within the NMDA track of the spinal cord. This pain pathway is often associated with hyperalgesia and allodynia. The only common oral form of the medication is amantadine.

     4. Tricyclic antidepressants help stop the reuptake of serotonin which can reduce the amount of pain.

     5. Opioids stop pain at many different levels. However, oral absorption is poor and there are few studies done on oral use of opioids in dogs. Morphine and tramadol (opioid-like with serotonin reuptake inhibition...do not use with tricyclic antidepressants) are commonly used in this country. Methadone is commonly used outside of the United States.

     6. Other.

          a. Adequan is beneficial to the treatment of OA in both dogs and cats. It's method of action is directly on the joint.

          b. Acetaminophen is also a drug that can be of benefit to part of the canine population, although not all dogs are able to metabolize and thus utilize this drug.

     7. Physical Modalities

          a. Acupuncture. Hip dysplasia and acupuncture were made for each other. Most dogs respond well after just a few treatments, often reducing the need for some drugs.

          b. Myofascial Trigger Point Therapy. All of the OA medication in the world will not treat trigger points. A complete success at treating OA may be overshadowed by muscle pain so severe, the animal is incapable of normal actions.

           c. Rehabilitation or "PT" is important in increasing joint use and mobility. It is also important to increase an animal's core strength to maintain proper posture and prevent accelerated degeneration of joints and muscles

Acute pain

Classically, the treatment of acute pain has been through the use of opioids. Rarely, in the advanced clinical setting is this the case anymore. They are often used in combination with alpha-2 agonists and NSAIDs. We will follow the case of an otherwise healthy spayed female beagle with a splenic tumor necessitating it's removal. This dog had normal pre-operative blood work. Once again, all of these medications can be used on the same animal, with doses adjusted for potentiation of their actions in the presence of other drugs. Many of these medications can be given in the post-operative period at home by the client for continued pain control

     1. Opioids are going to be important for the degree of pain this patient will encounter. I would consider opioid treatment for at least a 3 day, more likely a 5 day treatment. They can be used IV, IM, trandermally (but they have a delayed onset), combined with local anesthetics, or intrathecal use.

     2. NSAIDs are also quite potent in stopping pain of this caliber. This is compatible with general anesthesia as long as the dog is not hypovolemic. It can be given by the oral route prior to surgery or in the case of Metacam or Rimadyl, by injection

     3. Alpha-2 agonists work synergistically with opioids. I use them in greatly reduced doses in the presence of an Opioid when given IM. They can be given by the intrathecal route, but the results are sometimes disappointing, probably because of the affinity to adipose tissue. Dexmedetomidine is also a great rescue drug to be given in microdoses in the post-surgical setting.

     4. NMDA antagonists. Ketamine can be given IV or IM prior to surgery, or as part of an IV drip during surgery.

     5. Local Anesthetics can be given as a true local at the site of the incision, can be given by the intrathecal route, or in the case of lidocaine, by the intravenous route. Post-operatively, I like to put a diffusion catheter in the abdomen and inject diluted and buffered bupivacaine every six hours for pain control. With proper sterile technique and bacterial filters, this is something a client can do when overnight care is not affordable.

     6. Calcium regulated voltage/transmitter inhibitors. Human studies have shown that even a single dose of gabapentin prior to surgery will reduce the amount of pain medications needed for pain control in the immediate post-operative period.

     7. Physical Modalities

          a. Acupuncture. There are some specific acupuncture sites that can help control abdominal pain. For surgery elsewhere, there are other acupoints that can be employed

          b. Rehab. Icing, joint flexion and massage can help with pain relief in the immediate post-operative period.