Chronic pain has traditionally been defined by its duration (pain that persists for weeks to months). However, it is important to note that it can also characterized by changes in peripheral and/or central processing of afferent stimuli (see “The Pain Process” for a brief description of peripheral and central sensitization). These changes in the nervous system’s response to noxious (and sometimes innocuous) input contribute to the difficulty often encountered in treating chronically painful patients.

A variety of syndromes have been associated with chronic pain. In veterinary medicine, the most common causes include osteoarthritis, cancer pain and neuropathic pain (including post-amputation “phantom limb” syndrome and chronic IVDD). Because of the neurobiological changes that have occurred in the processing of noxious signals, the pain that is experienced is often increased in amplitude and duration. In all cases, the pain is maladaptive-- it serves no useful purpose, and significantly impairs the quality of life of the patient.

Treatment of chronic pain can be problematic. Many forms of it become “resistant” to single-agent therapy over time, necessitating a multimodal approach to therapy. Combining standard analgesics with adjuvant drugs can dramatically improve treatment success in some cases. The following is a brief discussion of some of the agents employed in the alleviation of chronic pain:

NSAID’s -- remain the mainstay of therapy for chronically painful patients. Their principal mode of action is to block prostaglandin production by binding and inhibiting cyclooxygenase (COX). While the result of this effect is mainly a reduction in inflammation and peripheral nociceptor sensitization, there is some evidence that NSAID’s have a central analgesic action as well, though the exact mechanism remains unclear.

Cyclooxygenase occurs in at least 2 isoforms: COX1 (constitutive), which mediates formation of prostaglandins responsible for GI mucosal protection, hemostasis and renal blood flow, and COX2 (inducible), which catalyzes production of prostaglandins that act as inflammatory mediators. This is a bit of an oversimplification, as COX2 can play a homeostatic role in some situations (ulcer healing, maintenance of renal blood flow during stress, prostacyclin production), but in general NSAID’s that inhibit COX2>COX1 are considered the drugs of choice in this class. These are often referred to as COX2 preferential or selective agents.

A variety of selective/preferential NSAID’s are available for use in dogs, including carprofen (2.2 mg/kg BID or 4.4 mg/kg SID), etodolac (10-15 mg/kg SID), deracoxib (1-2 mg/kg SID for chronic use) and meloxicam (0.2 mg/kg on day 1, then 0.1 mg/kg SID). Though in theory they should all be equally efficacious, some patients appear to respond better to one over another; thus, failure to achieve an analgesic effect with one drug does not imply lack of response to all NSAID’s.

Caution should be exercised when employing NSAID’s in cats. Differences in hepatic biotransformation can lead to prolonged half-lives and the potential for toxicity. Additionally, very few studies have been performed examining the feline response to these compounds. One of the few NSAID’s that appears to be well tolerated in cats is meloxicam (0.1 mg/kg SID for 3-5 days, then 0.1 mg/CAT/q 24-72 hours long-term). Because tremendous variation among individual cats exists in regard to metabolism, it’s recommended they be monitored closely for adverse effects when on long-term therapy.

In all patients, NSAID’s should be avoided in the presence of renal or hepatic dysfunction, coagulopathies, GI disorders, shock, hypotension/hypovolemia, hypoalbuminemia or pregnancy. Dogs and cats on chronic therapy should be monitored periodically for alterations in hematological or biochemical parameters, and their owners should be instructed to watch for signs of GI upset, alterations in appetite and PU/PD.

Opioids-- are the most powerful analgesics available, with actions at peripheral, spinal and supraspinal levels. There are 4 types of opioid receptors, with multiple receptor subtypes. Mu receptors produce the most profound analgesia, and can cause euphoria, respiratory depression, physical dependence and bradycardia. Kappa receptors trigger a lesser analgesic response, and may cause miosis, sedation and dysphoria. Delta receptors modulate mu receptor activity. Sigma receptors provide little to no analgesia. They are responsible for many of the adverse effects associated with opioids (dysphoria, hallucinations, respiratory and vasomotor stimulation). Some investigators classify sigma receptors as phencyclidine, rather than opioid, receptors.

Opioids can act as agonists (bind and stimulate receptors), antagonists (bind and block or inhibit activity), partial agonists (bind and stimulate, but with less than full activity at certain receptor subtypes), and mixed agonist/antagonists (stimulating some receptors while blocking others).

Opioids are useful in a variety of chronically painful conditions (though they may have limited effectiveness in some forms of neuropathic pain). For the purposes of chronic pain management, only the oral and transdermal versions of various opioids will be considered in the following discussion.

Pure mu agonists provide the best analgesia, but also have the potential to produce the most side effects (bradycardia, hypotension, sedation, respiratory depression, urinary retention, vomiting, defecation, constipation). Their use is best limited to short-term “rescue” analgesia, though certain disorders (especially cancer pain) may require continual usage in the later stages of the disease. With chronic use, tolerance often develops, necessitating progressively higher doses to achieve an analgesic effect.

Morphine sulfate (CII) is available in oral tablet, capsule and liquid preparations. A suggested dose range in dogs is 0.5-2.0 mg/kg QID (some dogs experience unacceptable constipation at doses exceeding 1 mg/kg). Cats have been dosed with the liquid form at 0.2-0.5 mg/kg TID-QID, but most cats strongly dislike the taste.

Codeine has also been used as an oral mu agonist, though it is usually less efficacious than morphine. It is most commonly available in combination with acetaminophen as a CIII preparation, and is generally dosed in dogs at 1-2 mg/kg of the codeine portion TID-QID (it should NOT be used in cats in combination with acetaminophen due to the risk of fatal methemoglobinemia).

Fentanyl is available as a transdermal patch (Duragesic -- CII) in 25, 50, 75 and 100 ug/hour strengths. Some studies suggest the 75 and 100 ug patches may not provide consistent plasma levels; combinations of 25 and 50 ug patches can be used to achieve the appropriate dose (2-4 ug/kg) in dogs. In cats > 2.5 kg, a whole 25 ug patch is used; if < 2.5 kg, half of the plastic backing covering the gel is removed (DO NOT CUT patches).

Duragesic patches provide very good background analgesia, though they occasionally need to be supplemented with oral mu agonists. In dogs, the onset time is 12-36 hours, with a 72 hour duration of effect. Cats tend to have a faster onset time (5-8 hours) and a longer (up to 120 hours) duration. Side effects may include inappetance, agitation/dysphoria, sedation and hyperthermia (cats).

Partial mu agonists bind at the mu receptors but only partially activate them. Buprenorphine (CIII) is the prototypical drug in this class; it’s moderately expensive but VERY safe, producing few side effects and minimal sedation. Buprenorphine has tremendous affinity for the mu receptors, and will competitively inhibit pure mu agonists from binding. This property makes it useful for “reversing” the effects of morphine or fentanyl if adverse consequences arise, while still maintaining a level of analgesia.

A ceiling effect on analgesia exists with partial agonists, making them less useful for severe pain. Buprenorphine is interesting in that increasing the dose prolongs the duration of analgesia, while the degree of pain relief remains essentially unchanged. Doses of 30 ug/kg (0.030 mg/kg) will provide ~ 8-10 hours of analgesia, and 40 ug/kg (0.040 mg/kg) may produce as much as 12 hours of pain control. The onset of action is fairly slow (~ 30 minutes when given IV, 60 minutes IM, transmucosal or transdermal).

Buprenorphine is not available as an oral preparation (significant first-pass effect renders it inactive), but its lipophilic nature lends itself to absorption across skin or mucous membranes. Compounding pharmacies can produce a PLO (pleuronic lecithin organogel, or transdermal gel) for application on the inner surface of the pinna or shaved skin on the neck in dogs and cats. Alternatively, the alkaline salivary pH of cats allows for excellent transmucosal absorption when the injectable drug is given in the mouth (it should not be mixed with flavored syrups, as swallowing will inactivate it; the injectable form is tasteless and well-tolerated by cats). No studies have been performed on transmucosal usage in dogs, though the pH of their saliva is closer to that of humans, where bioavailability after mucosal administration is only ~ 30%.

Mixed agonist/antagonists like butorphanol (CIV) are not considered useful in the management of chronic pain. First-pass effect destroys some of the drug, and the analgesia is considered to be relatively short-lived (1-2 hours). Because these drugs are kappa agonists and mu antagonists, the pain relief is often less than optimal for chronic discomfort. However, visceral nociception is considered to be more responsive to kappa agonism, leading some urologists to advocate butorphanol’s use in chronic bladder pain (FLUTD).

Tramadol is one of the most useful drugs available to veterinarians for treating chronic pain. It has a dual mode of action: mu agonism (though it’s not technically an opioid, and is not a controlled substance) and monoamine reuptake inhibition (principally serotonin and norepinephrine), which enhances the endogenous spinal inhibitory mechanisms and produces mild anti-anxiety effects.

The degree of mu agonism produced is relatively weak (the parent compound has very little affinity for the mu receptors; most of the mu effects come from the M1 metabolite). However, in conjunction with the monoamine reuptake inhibition a powerful synergistic action occurs, leading to analgesia comparable to meperidine or codeine. It is helpful in a variety of acute and chronic pain syndromes, including neuropathic pain and allodynia. Combining tramadol with other analgesics (NSAID’s, mu agonists) further enhances tramadol’s efficacy, producing a multimodal pain relieving action. Because of tramadol’s monoamine reuptake inhibition, it should not be given with TCAs, SSRIs, or MAO inihibitors due to the risk of serotonin syndrome.

In dogs, a starting dose of 1-2 mg/kg BID (up to 5 mg/kg BID) works well, though more frequent administration (TID-QID) can be used if needed. Cats are dosed at 2-4 mg/kg (generally ¼ of a 50 mg tablet) BID. Metabolism is principally via hepatic biotransformation, with a small amount excreted unchanged by the kidneys. Side effects, though rare, may include GI upset and sedation. (Webmaster's note: be aware that this is a bitter medication and it is not always easy administered to cats via the oral route.)

NMDA receptor antagonists are used as adjunctive drugs (i.e. in combination with other analgesics) to improve the control of pain. Intense and/or chronic painful stimuli result in changes in the central nervous system’s response to input, leading to an “amplification” of pain intensity. This process of “central sensitization” is mediated in part by activation of NMDA receptors (see the section on “Analgesic CRI’s) for more details). By blocking the activation of these receptors, a reduction in CNS hyperresponsiveness can be achieved, allowing other analgesics to function more effectively. Additionally, NMDA receptor antagonists act to increase opioid receptor sensitivity, reduce opioid tolerance and minimize rebound hyperalgesia (the phenomenon of markedly increased pain that occurs when an opioid wears off).

While the use of intravenous infusions of microdose ketamine has been the principal application of this concept, effective oral medications are available that achieve similar results. It’s important to note that the use of oral antagonists often results in a slower onset of action than that of CRI ketamine; it may take up to a week for oral compounds to produce noticeable results.

Amantadine is the most commonly used oral NMDA receptor antagonist. It was originally developed as an antiviral compound, and has also been used to treat extrapyramidal drug reactions and Parkinson’s disease in humans. The standard dose used to block receptors in dogs and cats is 3-5 mg/kg SID. It may be given on a continual basis if needed, though in most cases it can be given daily for 7-14 days and then discontinued until pain worsens again. Amantadine is available as 100 mg capsules and a 10 mg/ml oral liquid. Elimination is almost exclusively via the kidneys, so dose reductions should be considered in cases of severe renal failure. Side effects are rare, but can include agitation or diarrhea.

Dextromethorphan, commonly used as an anti-tussive in humans, has also been advocated as an antagonist of NMDA receptors. A dosage of 0.5-2.0 mg/kg TID-QID is considered effective. Metabolism is by hepatic biotransformation. . Robitussin CoughGels are an OTC gelcap that contains 15 mg dextromethorphan per capsule and no other drugs. Coughgels cost approximately $4.50/20 gelcaps. Dexalone is an OTC gelcap that contains 30 mg dextromethorphan per capsule and no other drugs. Dexalone costs approximately $14.00/30 gelcaps. Vicks Formula 44 Cough Relief is a dextromethorphan only liquid OTC product that contains 2 mg/ml and comes in 118 ml bottles. The Vicks syrup costs approximately $4.50.

Gabapentin (Neurontin) is an anti-convulsant medication with purported adjunctive analgesic action. Its mechanism of action is unclear, though it may involve inhibition of post-synaptic neuron firing. Gabapentin has been used for many forms of chronic pain, though its best application may be for neuropathic pain. A suggested dose is 10 mg/kg BID, though doses as low as 1.25 mg/kg SID have been reported effective. It is metabolized by the liver and excreted by the kidneys. Possible side effects may include sedation and weight gain. This is a very expensive drug, which limits its usefulness as a first-line drug for chronic pain.

Tricyclic antidepressants (TCA’s) have been used in humans and animals as adjuncts to other analgesics (especially opioids) for chronic pain. They act to inhibit serotonin and norepinephrine reuptake, though they may have other analgesic effects as well (including possible actions at opioid receptors and on nerve transmission). Amitriptyline is the most commonly used drug in this class. Dogs and cats are usually dosed at 1-2 mg/kg SID-BID; side effects can include sedation and anticholinergic effects.

Other adjunctive drugs employed for the relief of chronic pain can include glucocorticoids, chondroprotectives, anxiolytics, doxycycline, omega-3 fatty acids, magnesium, immunonutritional modifiers and bioflavinoids. Non-pharmacologic therapies include acupuncture, electroacupuncture and various electrical nerve stimulation procedures, laser therapy and pulsed magnetic field therapy.

As can be seen from the multiplicity of drugs and alternative treatments utilized for chronic pain, management of these patients can be difficult and complex. It’s important to emphasize to the owners of these pets that multiple trials may be required to find the right combination of analgesics, and that not all patients can be effectively managed. Animals with chronic pain should be rechecked frequently to assess response to therapy and monitor for side effects, and the owners should be consulted with closely as to the effectiveness of treatment. With patience, determination and a willingness to try new techniques, the lives of many of our patients with chronic pain can be dramatically improved.

Dr. Dave Thompson