Chronic pain characteristics

Opioids maybe administered in a variety of routes including oral (tablet and liquid), sublingual, rectal, transdermal, transmucosal, intravenous, subcutaneous, and intraspinal. While the oral and transdermal routes are most common, the method of administration is based on patient needs (severity of pain) and characteristics (swallowing difficulty and preference). Oral opioids have an onset of effect of 45 minutes, so intravenous or subcutaneous administration maybe preferred if more rapid relief is desired. Intramuscular injections are not recommended because of pain at the injection site and wide fluctuations in drug absorption and peak plasma concentrations achieved. More invasive routes of administration such as PCA and intraspinal (epidural and intrathecal) are primarily used postoperatively, but may also be used in refractory chronic pain situations. PCA delivers a self-administered dose via an infusion pump with a preprogrammed dose, minimum dosing interval, and maximum hourly dose. Morphine, fentanyl, and hydromorphone are commonly administered via PCA pumps by the intravenous route, but less frequently by the subcutaneous or epidural route. 

Jamison, R.N., Kauffmann, J., Katz, N.P. Characteristics of methadone maintenance patients with chronic pain, J. Pain Symptom Manage. 2000, 19, 53-62. 

Capsaicin is the compound responsible for the characteristic spicy flavor of jalapeho and habahero peppers. Although it first produces a burning sensation on contact with the mouth or skin, repeated application desensitizes the area to pain. This property has made it the active ingredient in several topical creams for treatment of chronic pain. Capsaicin has also been used as an animal deterrent in pepper sprays, and as an additive to make birdseed squirrel-proof In Chapter 1, we discuss the structure, bonding, and properties of organic molecules like capsaicin. 

The exact mechanisms of pain transmission are unknown. Pain is a complex interaction involving receptor stimulation, pain transmission, and response. Pain is generally divided into two categories acute and chronic. These categories have different characteristics, which affect the approach to treatment. Patients may suffer from both acute and chronic pain at the same time. 

The obvious way to relieve pain is to eliminate the underlying cause. This is often not possible, however, and symptomatic relief usually TABLE 58—3. Characteristics of Acute and Chronic Pain ... 

In the last decade a number of newer anticonvulsants have been Introduced, and are being used in a variety of treatment settings, including treatment of partial and generalized seizure disorders, chronic pain syndromes, bipolar disorders, and migraine prophylaxis. Characteristics of several of these drugs are provided in Table 11-6. 

The best-known vanilloids are capsaicin and resiniferatoxin (RTX). Capsaicin is the compound in various types of hot peppers that gives them their characteristic hot and spicy flavor. It is used in a variety of products ranging from personal-defense pepper sprays to squirrel-proof birdseed. RTX is a powerful irritant extracted from a cactuslike plant. Euphorbia resinifera. The milky latex obtained from the plant is referred to in medical literature dating as far back as the first century. In these writings, it was described as a nose and skin irritant and as a treatment for chronic pain. In powdered form, RTX causes sneezing, a characteristic used by some practical jokers. 

When either of the compounds is applied to the skin or mouth, a burning sensation results. This sensation is caused by the compounds bonding to receptors of small sensory neurons associated with the transmission of pain. Following repeated application of capsaicin or RTX, these neurons, or C sensory nerve fibers, become desensitized to the initial irritation or pain the compounds cause. This characteristic allows the compounds to be used topically to relieve chronic pain. Capsaicin is now used as an ingredient in a few OTC pain-reheving creams such as Capzasin-P, Meet, Menthacin, and Pain Doctor. 

Although acute and chronic pain have distinguishing characteristics, there is often overlap, making the diagnosis and management of pain challenging. Differences between acute and chronic pain are outlined in Table 2.2. 

Finally, it is well recognized that certain acute traumatic and chronic pain conditions are associated with a mixture of noiciceptive inflammatory and neuropathic pain. For example, tissue injury and a marked inflammatory response following laparotomy or thoracotomy initiates a somatic nociceptive component responsible for incisional and muscular pain, while peritoneal or pleuritic irritation is responsible for a visceral nociceptive component. Neural injury related to retraction or transection initiates a neuropathic component. Clinical pain complaint, intensity of symptoms, pain characteristics and choice of analgesic are related to the extent of inflammation, visceral versus somatic nociception, and neural tissue injuries. 

Bouhassira D, Lanteri-Minet M, Attal N, et al. Prevalence of chronic pain with neuropathic characteristics in the general poptdation. Pain 2008 136(3) 380-387. 

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