Most C&L questions regarding pain relate to chronic pain. Acute pain is usually fairly well-managed through patient-controlled anesthesia. ★Patients with chronic pain have a 2-5 fold greater risk of depression that the general population. Increasingly, patients with chronic pain are being managed with opiates, and dosages appear to be increasing.
Major depressive disorder is present in up to 30% of patients with pain. In addition, anxiety can intensify pain.
Nociceptive pain
Nociceptive pain is peripheral and activated by the stimulation of nerve endings. The pain is typically localized and sharp and the source is often easy to find.
Neuropathic pain
Neuropathic pain is not really that well defined, though has some general characteristics. It can include chronic pain, pain from a variety of sources (diabetic neuropathy, post-herpetic neuralgia, etc), and a variety of other etiologies. Generally, neuropathic pain involves neuronal structures that are proximal to nociceptors. It is usually poorly localized and leads to poor descriptions. It is thought that neuropathic pain can result from damage or changes in somatosensory pathways.
Approximately 1.5% of the population in US suffers from neuropathic pain.
Depression is a fairly common comorbidity of neuropathic pain and affects approximately 57% of individuals. Such comorbidity complicates both the assessment of pain and the assessment of depression.
Postherpetic neuralgia. Occurs at least three months after the onset of varicella zoster in about 10% of patients. If the patient is over 65, then the number increases to over 50%. Postherpetic neuralgia is more likely in patients with cancer, diabetes or taking immunosuppressants. Most cases immprove and only 25% of patients experience pain at one year. The most common treatments are tricyclic antidepressants, anticonvulsants, and opioids.
Peripheral neuropathy. Pain from peripheral neuropathy occurs when sensory neurons are damaged by a disease process. Thus, about 25% of patients with diabetes will contract peripheral neuropathy. The treatments are pretty much the same as for postherpetic neuralgia, but duloxetine may also help.
Chronic pain
Chronic pain has persisted for at least 6 months and the original stimulus for the pain is now gone. The prevalence of chronic pain is not well established and estimates in the general population range from 10-55%. ★Patients with chronic pain tend to accommodate the pain both neurophysiologically and psychologically. Thus, health care providers may conclude that the person does not really have pain because they do not appear to be in acute pain.
Pharmacology
Opioids. Chronic opioid intake has several effects on different mechanisms. Importantly, nerve cells involved in pain transmission will adapt to chronic opioid use, either through upregulation of cAMP or upregulation of NK1 receptors. Thus, patients on high doses of methadone will often report extreme pain. ★During a surgical procedure, patients on methadone do not need less medication, but more (sometimes up to 50% more). The use of chronic high dose opioids to treat pain seems unsafe and ineffective.
Antidepressants. Like many things in psychiatry, the exact mechanism by which antidepressants decrease pain is unknown. Presumably, they have an effect on serotonergic and noradrenergic transmission related to descending spinal pain pathways. It is also possible that modulation of sodium channels plays a role.