Text from: Guidelines for recognition, assessment and treatment of pain. J Small Anim Pract. 2014, with permission http://onlinelibrary.wiley.com/doi/10.1111/jsap.12200/abstract
This is pain of long duration. In humans, chronic pain is often accompanied by fear, anxiety, depression and anger, which can exacerbate pain and its negative impact on the patient's quality of life. It is estimated that at least 30% of pet dogs and cats that are seen by veterinarians can be classified as 'senior' and this population is likely to have a high incidence of chronic pain. However, chronic pain is often not diagnosed as it is often mistaken for 'getting old.' Veterinarians treating animals with chronic disease should consider the potential for accompanying chronic pain, even in the absence of immediately obvious signs. The changes in behaviour that accompany chronic pain may be insidious in onset and subtle.
The approach to treatment depends on the underlying cause of pain, its duration at presentation, and how well it has been managed previously. Chronic disease may not be static and acute exacerbations of previously well-controlled pain can occur and these can be especially challenging to treat. A multimodal approach (combination therapy) is likely to be most effective and owner education is essential. The mainstays of treatment of chronic pain are the NSAIDs; however, adjunct drug therapies, physical and other approaches (e.g., acupuncture, surgery) may play an important role in management. There is a wide range of NSAIDs licensed for long-term use in dogs; they are most commonly given orally and long-acting injectable preparations are available. In cats, the only NSAID currently approved for long-term use is meloxicam.
Although many non-drug therapies are suggested to be effective for the management of chronic pain, there is little evidence behind their efficacy, and almost nothing known about potential side effects. In addition, drug side effects, disease progression or comorbidities can be mistaken for worsening pain, leading to additional treatments that are at best non-efficacious, or at worst detrimental. One example is the dog with chronic OA which then develops neurological disease and is prescribed additional drugs in an attempt to alleviate what is thought to be only pain-related difficulty in mobility. In all chronic pain cases, non-drug treatments should be used alongside drug treatments, and there should be regular evaluation to detect beneficial and unwanted effects, and regular reassessment of the patient's pain.
Neuropathic Pain
Neuropathic pain requires several classes of medications and procedures as it cannot be adequately managed with a single pharmacological or non-pharmacological therapy. Prior to, and during any surgical procedure, various different analgesic drugs and modalities can be used to reduce the inciting nociceptive afferent impulse. Many of these are continued postoperatively to reduce both peripheral (PNS) and central (CNS) sensitization.
NSAIDs
There is evidence to support an inflammatory response driving the pathophysiological changes of the peripheral and central nervous systems resulting in neuropathic pain and augmentation of pain processing by spinal prostanoids. While no studies are reported at this time, human clinical trials are currently underway investigating various modalities to target specific components of the neuroinflammatory process. It is advised that NSAIDs be used in the treatment of neuropathic pain.
Opioids
Opioids may be included in a multimodal regimen to manage neuropathic pain, but not as a standalone analgesic. Opioids may have reduced effectiveness, where tactile allodynia (A beta stimulus) is a component of neuropathic pain and where opioid receptors in the descending inhibitory pathway are reduced or inactivated, which may occur in neuropathic pain. Also, the closer the nervous system lesion is to the CNS, the less effective opioids may be; peripheral nerve injuries tend to respond better to opioid therapy than nerve root injuries, which respond better than spinal cord injuries. The shorter half-life of fentanyl is an advantage in patients with acute CNS or PNS pain/injury as withdrawal for assessment is more easily planned. Opioids with less propensity to cause emesis (e.g., fentanyl, methadone, butorphanol) should be titrated cautiously in any trauma patient to avoid potential vomiting and retching, which will cause a marked and sudden increase in intracranial pressure in patients with known, suspected or occult brain injury. The naloxone titration technique to reverse side effects of opioids is recommended (see Table 1; VIN editor: Table 1 was not provided). Buprenorphine OTM may be suitable for continuing home management for cats and small dogs.
NMDA Antagonists
Low-dose ketamine is frequently used pre-, intra, and postoperatively to prevent and treat neuropathic pain. Following the administration of an opioid and an NSAID (when not contraindicated), an IV loading dose > 0.5 mg/kg (to effect) of ketamine is administered, followed by a CRI 0.1–0.5+ mg/kg/h. Amantadine (3–5 mg/kg once daily orally) may be continued after ketamine is discontinued for longer-term therapy at home.
Local Anaesthetics
Lidocaine systemically administered has been shown to be effective in the treatment of several neuropathic pain disorders. Lidocaine infusions should not be used in cats. Lidocaine 5% dermal patches may be of benefit where pain originates. Pharmacokinetic studies of the lidocaine patch in dogs are reported; however, no analgesic efficacy studies have been reported in dogs or cats for IV infusions or transdermal patches for neuropathic or chronic pain.
Antiepileptics
Studies in humans and laboratory animals indicate that perioperative administration of gabapentin to animals with nerve injury may reduce the potential establishment of, or ongoing, neuropathic pain. Based on blood levels in dogs, dose at 10 mg/kg PO q 8 h (5 mg/kg PO in cats), increasing as needed to effect (dose range 10–15 mg/kg). The dose-limiting side effect is sedation. Some animals need several weeks to months for resolution of pain, or longer. A benefit of long-term administration of gabapentin following trauma was reported in three cats; however, to date there are no prospective veterinary studies investigating the long-term effects of multimodal analgesia including gabapentin.
Alpha2 Adrenoceptor Agonists
Medetomidine and dexmedetomidine may be added to a multimodal regimen. As an example, dexmedetomidine (1–2 µg/kg/h), in addition to low-dose fentanyl (3–4 µg/kg/h) and corticosteroids, can be effective for management of the severe pain associated with meningitis in the dog. Intra- and postoperative pain management for intervertebral disc herniation is another example. No observed adverse effects are noted at this low dose other than potential for increased urinary output.
Acupuncture and Medical Massage
These should be included in the analgesic regimen as soon as possible. Neuropathic pain is difficult to manage with pharmaceutical agents alone; therefore, the use of acupuncture and other integrative techniques should be included as adjuncts to a multimodal pharmaceutical regimen.
Serotonin and Norepinephrine Reuptake Inhibitors
These (e.g., amitriptyline, dogs: 1–2 mg/kg orally q 12–24 h; cats: 2.5–12.5 mg/cat orally q 24 h, gabapentin [see above] may be beneficial as a home medication in combination with those listed above, as the descending inhibitory system appears to be dysfunctional in neuropathic states.