Adjuvant analgesics (co-analgesics) are not analgesics in the true pharmacological sense. They are developed primarily for treating conditions other than pain. However, adjuvant analgesics may contribute significantly to pain relief when used either alone or in combination with other analgesics.
Adjuvant analgesics comprise a various group of medications with different initial indications and include antidepressants, anticonvulsants, cannabinoids, bisphosphonates, antiarrythmics2. These adjuvants may be used alone or in combination with a conventional analgesic to treat persistent pain conditions of different origin, particularly neuropathic pain (a form of chronic pain caused by injury to or disease of the peripheral or central nervous system).
Antidepressants have been used for decades as primary analgesics. They may provide analgesia for different types of pain, but not all and not to the same degree.
The tricyclic antidepressants (TCAs) have been well studied and are most likely to be effective. TCAs (such as amitriptyline, nortriptyline, and desipramine) have demonstrated efficacy for pain caused by:
The most evidence exists for amitriptyline but some patients cannot tolerate its numerous sedating and anticholinergic effects. Desipramine or nortriptyline may be better tolerated in such cases.
Controlled studies of the tricyclic antidepressants have shown that effective analgesic dose is often lower than that required to treat depression, and the onset of analgesic action usually is earlier.
TCAs are probably more effective at relieving neuropathic pain than the newer antidepressants, but frequently induce poorly tolerated side effects. The newer antidepressants, selective serotonin and norepinephrine reuptake inhibitors (SNRIs) and serotonin-selective reuptake inhibitors (SSRIs) have fewer side effects than TCAs.
Unfortunately, SSRIs lack efficacy in pain relief. Paroxetine is the only SSRI that has some evidence for modest efficacy in treating chronic pain.
SNRIs appear to be more effective analgesics than the SSRIs. There has recently been some published evidence for the effectiveness of Venlafaxine for neuropathic pain. Duloxetine, the newest of the SNRIs has been approved by the FDA as a treatment for pain due to diabetic neuropathy.
Bupropion, which is predominantly noradrenergic, has also shown evidence to alleviate pain.
The precise painkilling mechanism of antidepressants remains unknown. They may increase neurotransmitters in the spinal cord that reduce pain signals. Antidepressants may also exert adjunctive therapeutic influences through histamine receptors as well as modulation of sodium channels3. They don't work immediately. You may have to take an adjuvant for several weeks before it starts reducing the pain.
Anticonvulsants have been used in the management of pain since the 1960s. There is strong evidence that anticonvulsants are useful in reducing neuropathic pain, especially when the pain is lancinating or burning. They act by suppressing the spontaneous neuronal discharges and neuronal hyperexcitability that occur after nerve injury and may also have a central effect.
Carbamazepine was the first representative from this class to be studied in clinical trials. It has been used for the treatment of trigeminal neuralgia, painful diabetic neuropathy and postherpetic neuralgia.
Gabapentin is currently widely used for neuropathic pain. It is FDA approved for the treatment of diabetic painful polyneuropathy and postherpetic neuralgia. Its mechanisms of action are still not fully defined, but it is effective in relieving indexes of allodynia and hyperalgesia in animal models.
Pregabalin, which is a distinct compound but has the same mechanism of action, was recently approved for the same indications. It has more stable pharmacokinetics and should be simpler to use. Lamotrigine has shown some efficacy for carbamazepine-resistant trigeminal neuralgia.
Other antiepileptics, such as phenytoin, clonazepam, valproic acid, topiramate, tiagabine, and oxcarbazepine, also have antihyperalgesic and antinociceptive effects, and are used for neuropathic pain.
Recently anticonvulsants have been examined for adjuvant use for postoperative pain. Similar to nerve injury, surgical tissue injury is known to produce neuroplastic changes leading to spinal sensitization and the expression of stimulus-evoked hyperalgesia and allodynia.
Alpha-2-adrenergic agonists are known to have a spinal antinociceptive effect via alpha-2 receptor subtypes. Animal studies and clinical experience indicate some usefulness of clonidine, tizanidine and dexmedetomidine for several painful states.
Alpha-2 agonists can induce analgesia by acting at three different sites: in brain, spinal cord and in peripheral tissues.
Clonidine potentiates the analgesic effect of opioids. Additionally, clonidine effect may be enhanced by amitriptyline. Studies have clearly shown that addition of clonidine in preoperative, perioperative or postoperative situation leads to greater analgesia.
Ketamine is a non-competitive antagonist of the N-methyl-d-aspartate (NMDA) receptors. It produces antinociceptive action via inhibition of NMDA receptors and activation of descending inhibitory monoaminergic pain pathways. However, ketamine is rarely used as a sole analgesic because it has a narrow therapeutic window and can cause intolerable side effects.
Low-dose ketamine is often used as an adjuvant to opioid analgesia. Ketamine (used as an adjuvant to opioids) appears to increase pain relief by 20% to 30% and allows opioid dose reduction by 25% to 50%.
References & Resources
Published: May 05, 2007