Author
Lars Grimm, MD, MHS
House Staff
Department of Internal Medicine
Duke University Medical Center
Durham, North Carolina
Lars Grimm, MD, MHS, has disclosed no relevant financial relationships.
Editor
Consuelo T Lorenzo, MD
Consulting Staff, Department of Physical Medicine and Rehabilitation
Alegent Health Care
Immanuel Rehabilitation Center
Omaha, Nebraska
Consuelo T Lorenzo, MD, has disclosed no relevant financial relationships.
Pain is a universally understood sign of disease and the most common symptom bringing people to seek medical attention. Physicians have been struggling throughout history to better understand pain and provide true relief for their patients. The image shown is from Rene Descartes' 1664 Treatise of Man, demonstrating his theory of how the human body processes painful stimuli. Image courtesy of Wikimedia Commons.
Pain measurements are scored on the Numeric Rating Scale (NRS). The NRS ranges from zero, meaning no pain at all, to 10, meaning the worst possible pain. Each patient's pain score should be documented in their medical record. The benefit of the NRS is that it provides a systematic and consistent means of documenting of pain. It is also an important mechanism for identifying unrelieved pain in the individual patient. The under-treatment of pain in hospitalized patients led to the development of "pain as the fifth vital sign" policy, in which pain scores are routinely assessesd during all patient encounters. This policy has been adopted by the US Department of Veterans Affairs. Despite the good intentions of the program, it has been criticized because pain is a subjective symptom, not an objective sign. Image courtesy of the US Department of Veterans Affairs. Image courtesy of the National Institute of Health.
The components of peripheral nerves responsible for carrying pain impulses are the primary sensory afferent: A-beta, A-delta, and C fiber axons. A-beta are responsible for light touch and moving stimuli, whereas A-delta and C fiber axons are responsible for pain sensation. A-delta are small-diameter myelinated nerves and C fibers are small-diameter unmyelinated nerves. Because of the differences in myelination, A-delta fibers produce fast pain responses and C fibers produce slow pain responses. Primary afferent nociceptors respond maximally to pain, and if blocked, the ability to detect pain is lost. Image courtesy of the National Institutes of Health.
The 2 major categories of pain are nociceptive and neuropathic. Nociception is a normal physiologic response to stimuli, initiated by nociceptors that detect mechanical, thermal, or chemical changes. It may be subdivided into 3 subtypes. Superficial somatic pain is from cutaneous nociceptors on the skin or superficial tissues. Deep somatic pain is from somatic nociceptors on ligaments, bones, blood vessels, and muscles. Visceral pain is from visceral nociceptors within body organs. The colonoscopy image shown demonstrates severe colitis inducing visceral nociceptive pain.
Neuropathic pain is pain induced by damage to the nerves themselves. Hyperpathic symptoms of burning, tingling, or electrical sensations are classic for neuropathic pain. Unfortunately, neuropathic pain is not traditionally responsive to standard pain medications. Herpes zoster (shown) can cause neuropathic pain via growth and inflammation within dermatomal nerves.
The axons of primary afferent nociceptors enter the spinal cord through the dorsal root ganglion. Multiple sensory nerves converge onto ascending spinal nerves of the spinothalamic tract on their way to the thalamus. This convergence gives rise to the concept of referred pain, whereby pain signals originating in one part of the body may be felt in the dermatomal distribution of another nerve (shown). For example, patients with ischemic chest pain feel pain in their left shoulder because the sympathetic afferent nerve fibers of the heart are concentrated in the dorsal root ganglion of the T2-T6 spinal segments. Image courtesy of National Institutes of Health.
Sensitization is an adaptive process in which innocuous stimuli produce an excessive response. Repeated intense stimuli to damaged tissue lowers the threshold and frequency of firing of afferent nociceptors. Local inflammatory mediators contribute by recruiting additional nociceptors, which normally remain silent to routine stimuli. For example, patients with bad sunburns will experience severe pain and discomfort to even very light touches because of sensitization of the pain fibers. Sensitization may also be partly responsible in patients with chronic pain syndromes.
Pain modulation can both enhance and dampen pain signals. Placebo can have a significant analgesic response, and anxiety can magnify the perceived stimuli. Descending signals from the frontal cortex and hypothalamus help modulate the ascending transmission of the pain signal by opiate receptors. The image shown demonstrates the visceral afferent and modulatory pathways responsible for the pain felt by patients with irritable bowel syndrome. The plus sign denotes pain facilitation and the minus sign denotes pain inhibition. A6 = locus coeruleus, ACC = anterior cingulate cortex; NTS = nucleus tractus solitarius; PAG =periaqueductal gray matter; PFC = prefrontal cortex; RVM = rostra ventral medulla
Laboratory tests, imaging, and nerve or muscle conduction studies may help identify the root cause of a patient's pain, as well as provide important information about therapeutic planning. Magnetic resonance imaging of a patient with cervical radiculopathy is shown, demonstrating a C6-7 disk herniation responsible for the patient's symptoms. Depending on the extent of the injury, patients may be eligible for injury-specific procedural interventions.
Determining the best treatment course for pain management begins with identification of the intensity and duration of pain. Acute pain is typically treated with short courses of medication therapy, whereas chronic pain may require long-acting medications or other interventional modalities. Medical management of pain proceeds in a stepwise fashion, as shown in the 3-step analgesic ladder produced by the World Health Organization. For mild to moderate pain, non-narcotic analgesics are used: aspirin, acetaminophen, ibuprofen, naproxen, indomethacin, ketorolac, and celecoxib. For moderate to severe pain, narcotic regimens are typically used: codeine, oxycodone, morphine, hydromorphone, methadone, meperidine, fentanyl, and tramadol. Combination regimens that contain opioids and non-narcotic analgesics provide additive pain control. Adjuvant medications therapies include tricyclic antidepressants, antihistamines, and anticholinergics. Image courtesy of World Health Organization.
The pharmacology of pain control is based on influencing one of several biochemical pathways. Many non-narcotic analgesics inhibit the cyclooxygenase enzyme, which is responsible for the formation of prostaglandin, prostacyclin, and thromboxane. Opiate medications mimic endogenous opioid peptides. Opioids bind to one of 3 principle classes of opioid receptors (mu, kappa, delta) to produce centrally mediated analgesia. Tricyclic antidepressants are thought to potentiate the effect of opiates. The image shown is of an opium poppy, from which natural and semisynthetic opiates are derived. Image courtesy of Wikimedia Commons.
Patient-controlled analgesia (PCA) allows patients to self-titrate their intravenous pain medication. It allows for a more consistent administration of analgesia with a reduced duration between when the patient feels pain and when analgesia is administered. It reduces the chances for medication errors, reduces nursing workload, increases patient autonomy, and provides objective data about the amount of medication a patient needs. It is traditionally used for postoperative patients and those with serious oncologic or hematologic diseases. CNS = central nervous system
Transdermal patches provide controlled drug delivery with a lower potential for abuse than oral analgesics. Patches can be applied once every 12 to 24 hours. Conditions like postherpetic neuralgia and chronic cancer pain are routinely treated with transdermal patches.
Opiate-infused lollipops and buccal lozenges are other alternative forms of drug delivery used to treat patients with malignant pain.
Regional blocks with therapeutic injections can provide excellent relief for patients with localized pain and inflammation. Depending on the clinical scenario, therapeutic, sympathetic, diagnostic, prognostic, or prophylactic blocks may be used. Therapeutic injections allow for a return to normal function, preventing the development of compensatory injuries. The exact procedural technique is dependent on the nerve involved, but the general principle involves the direct injection of local anesthetic or corticosteroid into the perineural space. The diagram shown is for the lateral injection technique of a sural nerve block.
Depending on operator familiarity and the difficulty of accessing injection sites, image guidance may be used for direct visualization. Computed tomography, ultrasound, or fluoroscopic guidance allows for more precise needle placement, thus reducing the amount of injected drug and reducing complications. The procedure is especially useful in patients with distorted native anatomy. The computed tomography-guided image shown demonstrates an injection needle in good position in the outer aspect of the neural foramen.
Surgical interventions are limited for patients with discrete deficits who fail conservative management. Depending on the location of pain, patients will typically undergo a stepwise treatment course involving non-interventional management before being eligible for invasive therapy. Surgically implanted devices such as morphine pain pumps and spinal cord stimulators are available for use on a case-by-case basis. The x-ray shown is of a spinal cord stimulator implanted in the thoracic spine. Image courtesy of Wikimedia Commons.
Transcutaneous electrical nerve stimulation, or TENS, units are adjuvant pain control devices that provide pulsatile electric impulses. The proposed mechanisms by which they reduce pain are presynaptic signal inhibition, endogenous pain control, direct inhibition of abnormally excited nerves, and restoration of afferent inputs. TENS units have been used for low back, arthritic, sympathetically mediated, neurogenic, visceral, and postsurgical pain. Although they are widely utilized and there is a great deal of anecdotal and observation-based evidence, there is a paucity of randomized controlled trials confirming the effectiveness of TENS units. Image courtesy of Wikimedia Commons.
Chronic, refractory pain is best managed with a multidisciplinary team approach that includes psychology, occupational therapy, physical therapy, vocational rehabilitation, and relaxation training. Patients in these situations frequently seek complementary and alternative medicine treatment options as well. Acupuncture (shown), dietary supplements, and hypnosis are commonly used methods of pain control in the nonmainstream medical community. As with most forms of complementary and alternative medicine, however, evidence from large-scale trials is lacking. Image courtesy of Wikimedia Commons.
Author
Lars Grimm, MD, MHS
House Staff
Department of Internal Medicine
Duke University Medical Center
Durham, North Carolina
Lars Grimm, MD, MHS, has disclosed no relevant financial relationships.
Editor
Consuelo T Lorenzo, MD
Consulting Staff, Department of Physical Medicine and Rehabilitation
Alegent Health Care
Immanuel Rehabilitation Center
Omaha, Nebraska
Consuelo T Lorenzo, MD, has disclosed no relevant financial relationships.
