Chronic pain affects more than 100 million people in the United States and accounts for 20 percent of outpatient visits, 12 percent of all prescriptions, and over 100 billion dollars in direct and indirect healthcare expenses. Pain-related expenditures (direct costs and lost wages) in the United States exceed those for cancer, heart disease, and diabetes combined. The use and misuse of opioids for management of chronic pain is a major concern due to problems that arise from their multiple adverse side effects including drug dependency and drug diversion. There is often under-treatment of chronic pain symptoms because of fear of opioid abuse. Thus, chronic pain is a major medical and social issue that affects the quality of life of individual patients, their friends and families, the work force, and society in general.

A comprehensive pain evaluation is essential to developing an effective plan for treatment. Although there are unique aspects to every individual's pain complaints, there are many common elements to the pain evaluation, irrespective of the pain complaint. An appropriate history and physical examination are vital to a proper evaluation. Laboratory studies, imaging, and other diagnostic testing may be appropriate in selected situations.

COMMON CAUSES OF CHRONIC PAIN — Although pain is one of the most common presenting symptoms to the primary care clinician, only a percentage of patients ultimately develop a chronic pain syndrome. In a study based on a survey of representative population in the United States from 1999 to 2002 (the National Health and Nutrition Examination Survey, NHANES), chronic pain (defined as >three months of pain) was reported as follows: back pain 10.1 percent, leg/foot pain 7.1 percent, arm/hand pain 4.1 percent, headache 3.5 percent, chronic regional pain 11.1 percent, widespread pain 3.6 percent; the majority of patients who reported chronic pain reported more than one type of pain.

Chronic pain can be considered to be in one of four categories. Identifying which of these categories the patient falls into is helpful in designing an appropriate treatment plan, although multifactorial causes of chronic pain are not uncommon. These pain categories can be considered to be:

●Neuropathic pain (either peripheral, including post-herpetic neuralgia, diabetic neuropathy; or central, including post-stroke pain or multiple sclerosis)
●Musculoskeletal pain (e.g., back pain, myofascial pain syndrome, ankle pain)
●Inflammatory pain (e.g., inflammatory arthropathies, infection)
●Mechanical/compressive pain (e.g., renal calculi, visceral pain from expanding tumor masses)

Note: These categories are not entirely mutually exclusive; back pain might be considered both musculoskeletal and mechanical/compressive if it results from nerve root compression.

Complex regional pain syndrome (CRPS) is defined as a disorder of the extremities characterized by regional pain that is disproportionate in time or degree to the usual course of any known trauma or other lesion. The pain is not restricted to a specific nerve territory or dermatome and usually has a distal predominance of abnormal sensory, motor, sudomotor, vasomotor, and/or trophic findings. The syndrome shows variable progression over time.

●To prevent the development of CRPS in patients with acute distal limb fractures or patients having limb surgery, we recommend treatment with supplemental vitamin C (500 mg daily) for 50 days.
●A multidisciplinary approach is suggested for the management of CRPS. Interventions appropriate for all patients with CRPS include patient education, physical therapy (PT), and occupational therapy (OT).
●Patients with CRPS who have pre-existing or suspected psychologic or psychiatric issues and those who have insufficient improvement with physical and pharmacologic therapies may benefit from psychosocial and behavioral management.
●Pharmacologic and invasive procedures for pain control are utilized in an escalating fashion, beginning with those that are relatively safe and for which there is some evidence of effectiveness, and progressing to more risky interventions if a desired response is not achieved after a few weeks of therapeutic trial. The goals of pain management are to allow active participation in a rehabilitation regimen and to restore movement and strength of the affected limb.
●For patients with early CRPS who require treatment for pain, we suggest starting with one (or more) of the following agents.
•Ibuprofen 400 to 800 mg three times a day or naproxen 250 to 500 mg twice daily.
•Amitriptyline or nortriptyline (10 to 25 mg at bedtime as initial dose for both).
•Gabapentin (starting dose of 100 mg at bed time for older adults and 300 mg at bed time for the rest, titrating the dose up as tolerated and needed).
•A bisphosphonate (eg, oral alendronate 70 mg weekly) for patients with early CRPS who have pain and abnormal uptake on bone scan.
•Topical lidocaine cream (2 to 5 percent) or topical capsaicin cream 0.075 percent.

Ketamine infusion – Systematic reviews published in 2013 [1] and 2015 [2] have found that there is only low- to moderate-quality evidence supporting the use of ketamine for CRPS. In one of the higher-quality randomized trials, ketamine infusion was compared with placebo in 60 patients with type I CRPS [3]. Patients assigned to five-day ketamine infusions had a statistically significant decline in pain scores from weeks 1 through 11 of follow-up compared with the placebo group, but the reduction was no longer statistically significant by week 12. Frequent side effects of ketamine in this trial included psychomimetic symptoms (eg, hallucinations, delirium), nausea, and vomiting.

1. O'Connell NE, Wand BM, McAuley J, et al. Interventions for treating pain and disability in adults with complex regional pain syndrome. Cochrane Database Syst Rev 2013; :CD009416.
2. Connolly SB, Prager JP, Harden RN. A systematic review of ketamine for complex regional pain syndrome. Pain Med 2015; 16:943.
3. Sigtermans MJ, van Hilten JJ, Bauer MC, et al. Ketamine produces effective and long-term pain relief in patients with Complex Regional Pain Syndrome Type 1. Pain 2009; 145:304.