Myths and misconceptions in veterinary pain medicine (Proceedings)

The major barrier to good pain management is the mythology that has persisted around animal pain behavior, physiology of pain, owner psychology and analgesic drugs and techniques. Veterinarians and technicians must work constantly to dispel the myths for themselves, colleagues, other staff members and pet owners.

Myth #1. Animals do not feel pain as people do.

From a physiologic standpoint, mammals and humans process pain in the same way; by nociception and cognition. Nociception, derived from the Latin word nocere (to injure), uses specific pathways — transduction, transmission, and modulation —for its physiologic processes. The pain pathway begins at the point of tissue trauma, such as a site of inflammation, injury, or surgical incision, where nociceptors (pain receptors) are stimulated. These specialized nerve endings convert mechanical, chemical, and/or thermal energy into electrical impulses (transduction). If the noxious stimulus is large enough to exceed the nociceptor's threshold, a nerve impulse is generated and transmitted along peripheral nerves to the spinal cord (transmission). Once at the spinal cord, a nerve impulse is either projected upward to the thalamus and then to other parts of the brain, including the cerebral cortex or it may be transmitted to an a nerve cell located entirely within the central nervous system that modifies nerve signals and links sensory and motor neurons that in turn activates sympathetic reflexes damping the pain sensation (modulation).

This understanding of pain physiology gives rise to the concept of multimodal analgesia. That is, attacking pain from many angles is more effective than from only one. Since the pain pathway has distinct phases, pain can be interrupted at various points. For example, we may want to do a local ore regional nerve block (transduction or transmission) in addition to pre-emptive NSAIDs (transduction and modulation) as well as postoperative opioid administration (modulation and perception). Using drugs from three different classes provides better pain control and has the added benefit of allowing us to use lower doses of individual agents thereby reducing side effects. Effective analgesia can also reduce the amount of gas anesthesia required for a procedure.

Myth #2. Animals tolerate pain better than people do or Tail wagging = comfort

In many cases animals do "appear" to tolerate pain better than humans. There may be several explanations for this. In contrast to pain-detection threshold, pain tolerance — the greatest intensity of pain that is voluntarily tolerated — varies widely between species and individuals within a species. Like humans, animals likely tolerate pain to a particular level before showing changes in behavior. Knowing that patients may exhibit a wide range of pain tolerances as well as a broad spectrum of behaviors can improve pain recognition and treatment.

In the past much attention has been given to recognizing the signs of pain in animal patients. This approach focuses on the individual and requires the patient "prove" he/she is in pain in order to receive treatment. By consensus we have concluded what we believe may be clinical signs of pain in our patients. Increased HR, increased RR, restlessness, increased temp, increased BP. abnormal posturing, inappetence, aggression, frequent movement, facial expression, trembling, depression and insomnia have all been listed among the top signs of pain by veterinary professionals. Anxiety, nausea, pupillary enlargement, licking/chewing/staring at site, poor MM color, salivation, decreased CO2 and head pressing are also sited by some observers. We see that many behaviors and clinical signs may be evidence of pain. Ultimately, we must conclude that any abnormal sign(s), in a veterinary patient, which cannot be attributed to another cause, are suspect of indicating pain. In actual practice, the clinical manifestations may be quite different between species and even among different members of the same species.

When evaluating animal pain we must always bear in mind that, much to their detriment (in a setting without predators) animals, in fact, attempt to hide pain from us. Science has shown us dogs observed by closed circuit camera post spay surgery. These dogs displayed many pain behaviors that abruptly abated when a human researcher entered the room. This type of research has led us to become less reliant on pain signs. Recently, the focus of much research in pain management has shifted toward identifying and even predicting known painful events. For example: Severe pain is expected with cervical disc herniation, extensive inflammation, medical or surgical, fracture repair, limb amputation, declaw, TECA etc... Moderate to mild pain is expected with cruciate repair, laparotomy, mass removal, castration, dental procedures, etc... This approach encourages us to treat patients who undergo painful procedures or diseases processes without requiring proof. It does not however, consider the vast variation in pain tolerance in the individual. It seems reasonable to incorporate both concepts to develop a truly effective analgesic plan. That is, to have direction given by what are known to be painful events and be prepared to provide adequate analgesia for the expected level of pain but also to look a the individual and tailor analgesic protocols accordingly.

Myth # 3. Animals should be given post-operative analgesia only when they show evidence of pain.

This thinking requires the patient to prove he/she is in pain at the time the next dose is scheduled. It is far better to reach a sustained plane of analgesia than to allow pain to return before administering additional drugs. The length of time that pain relief is provided is determined by several factors, including the onset, duration and source of pain. Acute pain is usually most intense during the first 24 hours after tissue injury and routinely responds to treatment.

In addition, different drugs have varying durations of action. Opioids, a diverse group of natural and synthetic analgesic drugs widely used in veterinary medicine, are generally reserved for perioperative or short-term in-hospital use because they tend to have short duration of action. For example, the analgesic properties of butorphanol, the only opioid approved for veterinary use in dogs, cats and horses, may only last about 45-90 minutes... Effective pain management involves perioperative analgesia, ongoing evaluation and use of drugs with different modalities. We must think in terms of managing pain for at least 24 hours with even simple procedures.

The best way to treat pain is to prevent it!

This is the concept of pre-emptive analgesia. All research in human and veterinary medicine shows that preventing pain is unquestionably the best approach to treatment. Imagine that tomorrow at 5 PM you were going to have an excruciating headache, guaranteed. What would you do at 4:30? Probably you thought, "I would take something" like Ibuprofen or another NSAID. You would make that choice because the looming headache is an example of a planned painful event. Knowing the pain is coming allows you the opportunity to stop that pain before it starts. Elective surgery is also a planned painful event. It makes sense to pre-empt the pain that is associated with ALL surgical procedures. It is an easy concept to grasp but not so easy to remember to implement. That is because we have become used to treating animal pain on "request", that is, when we see overt signs of pain even though we rationally know that once we see the signs it is already too late. We have already missed the opportunity to effectively manage pain in that patient.

Myth #4. Labradors are made of steel and Yorkies are wimps or huskies are just hopeless whining idiots who do not require pain meds.

This myth illustrates our preconceived notions about variations in breed response to painful stimuli. The inherent danger associated with this belief system is that we may fail to treat patients who are appear stoical or we may under treat patients whose overt signs seem exaggerated. (We also discriminate against puppies, older animals and aggressive animals). Because pain is an individual, subjective experience, assessment depends on a combination of good examination skills; familiarity with species, breed and individual behavior; knowledge of the degree of pain associated with particular surgical procedures and illnesses; and recognition of the signs of stress and pain.

Myth # 5. Pain does not persist beyond hospitalization.

The message is "don't quit till the pain quits". Send pain relief home with the patient. Many professionals agree that most soft tissue procedures such as spay or neuter require 3-4 days of postoperative analgesia while orthopedic procedures probably require a week's supply. Of course individual patients may vary and owners should be advised to request additional analgesia if they perceive their pet to be in pain beyond the anticipated period. Non steroidal anti-inflammatory drugs provide the backbone of take-home analgesia in dogs. Tramadol, oral opioids and fentanyl patches are used to provide additional analgesia when needed. In the cat, buprenorphine is the main take home analgesic although Tramadol has been used with positive results

Myth # 6. Pain is beneficial in limiting a recovering animal's activity.

Although one of the mostly widely held myths about pain, the type of pain produced by tissue injury, inflammation or direct damage to the nervous system is never beneficial. Aside form being morally questionable, there appears to be little evidence to support this idea. In fact, studies demonstrate that animals in pain tend to be restless, changing position frequently and bite/chew/lick at painful sites whereas analgesed animals tend to rest quietly. Perhaps the most compelling reason to abandon this myth is the deleterious effects of pain itself that far outnumber any possible benefit, real or imagined. Although the nervous system is the main target of painful stimuli and provides the means by which the animal responds to that information, the body's response is not limited to the nervous system. Most, if not all, of the body's major systems; the cardiopulmonary, gastrointestinal, neuroendocrine, and immune systems are affected by unrelieved or improperly treated pain. Pain is a stressor, triggering autonomic reflexes governed by the sympathetic nervous system. This response sets the body's defenses in motion, creating the appropriate internal environment for tissue repair and healing. Unfortunately, the increased cortisol levels that can accompany pain can interfere with tissue healing and reduce the immune system's ability to work effectively.

In addition, the classic "stress response" to pain, regardless of the underlying cause, ultimately results in a catabolic state or state of metabolic breakdown. Pain-induced distress that is prolonged suppresses the immune system and promotes inflammation, thereby predisposing patients to infection. Other adverse effects of the stress response include increased blood pressure (which affects organ perfusion and can lead to abnormal heart rhythm), fluid retention, decreased gut motility, and increased likelihood of sepsis.

Myth # 7. The clients won't pay for pain management.

Owners of ill animals can present a great source of frustration to veterinary staff. But, they can also provide a great sense of accomplishment, mutual trust and emotional satisfaction. Pain management is an area where strategies for success must include the client right from the beginning. We have grown accustomed to viewing the client as the guardian of his or her wallet. In reality, although many clients do have financial concerns, the vast majority are people who love their pets and generally want what is best for them. When our behaviors illuminate pain management as an essential aspect of good medicine clients will agree. It is not a good idea to make pain management optional at the owner's discretion because it gives the wrong message. We do not want to say," we only manage pain if we are asked to". Rather we want to message to be" we care about your pet and that includes our commitment to manage his pain". In most practices this burden will rest largely with technicians and support staff as they provide most of the client contact during patient admissions and discharges. In fact, it is advisable to broach the topic of pain management even as early as the initial phone call to schedule services. The topic is revisited on admission and again at discharge. When handled appropriately, clients will ultimately choose veterinary hospitals because of their reputation for providing good pain management. Taking the following actions fully engages the client in proper pain management:

• The receptionist includes "pain management" when quoting prices over the phone

• There is a sign about pain management in the waiting room or staff is wearing buttons when the client arrives

• The admitting technician talks about expected level of pain associated with the condition or surgery and explains hospital policy/protocol regarding pain management

• Pain/comfort level is discussed in phone calls during hospitalization

• Expected level of "at home" pain and its recognition is discussed at discharge

• Appropriate take home pain relief is routinely provided at discharge

• Clients are encouraged to call if pain appears to persist

Myth #8. Local blocks are unnecessary if the patient is under general anesthesia.

Removing consciousness (general anesthesia) should never be regarded as a substitute for analgesia. Blocking the transmission of pain signals to the spinal cord before stimulation results in lower requirements for general anesthesia and a marked reduction in post operative pain.

Myth #9. Box 'em down! Inhalant gas is the safest drug to use in geriatric or "sick" animals.

Gas anesthesia has marked cardiovascular effects and should be minimized in all patients particularly those who are older or debilitated. Addition of analgesic drugs and safe sedatives allows a profound reduction in gas inhalants and is always preferential over full gas procedures. In addition, the act of masking or boxing a patient down from full consciousness to unconsciousness is considered extraordinarily stressful, physiologically as well has psychologically.

Myth #10 A few drug myths or misconceptions

Opioids

• Mixing opioids is ok. As a rule, opioids do not play well together unless they belong to the same class (e.g. pure agonist). Some opioids act as antagonists reversing effects as specific receptors. For example butorphanol is a mu antagonist and as such will reverse most of the analgesic activity of morphine, fentanyl and other mu agonists. In fact, butorphanol can be used to electively reverse other opioids when adverse effects occur. In addition to reversal, some opioids bind tightly at receptors inhibiting the ability of other opioids to exert their effect. This is not true reversal but rather competitive binding. A common example is buprenorphine a partial mu agonist which has a great affinity for the mu receptor, binding tightly there. This explains the long lasting effects of buprenorphine compared to other opioids but also why administration of buprenorphine likely results in inability of other mu agonist opioids to have effect.

• Opioids cause respiratory distress. Opioids cause extreme respiratory depression in human beings, especially small children and infants. Fortunately this effect is rarely seen in small animals who do not seem to be as sensitive to opioids. Interestingly cats who receive fentanyl by transcutaneous route, often receive a 25 μg patch, the starting dose for an adult human male.

• Morphine epidurals might cause motor impairment. Morphine has so effect on motor function, only sensory. Concurrent use with local anesthetics may cause temporary motor blockade attributed to those agents.

NSAIDs

• When it comes to NSAIDs, COX 2 mediated effects are all bad. COX 2 mediated prostaglandins are responsible for inflammation and pain resulting from tissue injury and are targeted by COX 2 inhibitors. More and more is being revealed however, about the important role COX 2 enzymes play in regulating body function in the face of disease states particularly repair of existing GI lesions and regulation of fluid balance in the face of hypotension. The full role of COX 2 is still being discovered.

• NSAIDs cause hypotension and should not be given pre-operatively. NSAIDs do not cause hypotension however, in the face of prolonged hypotension COX 2 enzyme is required for fluid balance and therefore should not be inhibited in patients who are already hypotensive or are suspect of becoming hypotensive during surgery.

• Cats should not get NSAIDs. Cats get inflamed and respond very well to NSAIDs however due to their unique and variable metabolism of this drug class, safe dosing regimes are difficult to establish. NSAIDs labeled for use in cats are for one time dose only. Although there are recommended doses for chronic treatment, extreme caution must be taken to avoid renal failure, the most common adverse NSAID event in cats.

Alpha2 agonists

• Dexdomitor causes animals to get hypotensive. Unlike other sedatives, Dexdomitor actually causes transient hypertension not hypotension. This is secondary to peripheral vasoconstriction and should result in reflexive bradycardia. All of these effects are expected and considered normal with administration of alpha2 agonists but require changes in thinking and monitoring behaviors. Aside from blood pressure support, the advantages Domitor include dose flexibility for desired effect, analgesic properties, synergism with opioids and reversibility.

References

1. Cryer B, Feldman M. Cyclooxygenase-1 and cyclooxygenase-2 selectivity of widely used nonsteroidal anti-inflammatory drugs. Am J Vet Med 1998; 104:413–21.

2. Davis LE. Species differences in drug disposition as factors in alleviation of pain. In: Kitchell RL, Erickson HH, editors. Animal Pain: Perception and Alleviation. Bethesda (MD): American Physiological Society; 1983. p 161–78.

3. Hansen B. Through a glass darkly: Using behavior to assess pain. Semin Vet Med Surg (Small Anim) 1997 May;12(2):61–74.

4. Muir WW, III. Physiology and pathophysiology of pain. In: Gaynor JS, Muir WW, III, editors. Handbook of Veterinary Pain Management. St. Louis: Mosby; 2002. p 13–45.

5. Tranquilli WJ, Grimm KA, Lamont LA. Pain Management for the Small Animal Practitioner. Jackson (WY): Teton NewMedia; 2000. Pain management for specific conditions and procedures; p 73–103.