22 Oral Analgesics for Chronic Low Back Pain in Adults
KEY POINTS
Introduction
The most common type of pain for which patients seek medical attention is back pain. Chronic back pain is associated with a reduced quality of life, depression, loss of sleep, and reduced psychosocial and physical function. Analgesic medications are used to treat chronic pain, in conjunction with traditional physical modalities and therapy. Treatment with analgesic medications can help to restore physical and social function, while improving sleep,mood and concentration. Side effects should be minimized as best as possible.1 Acetaminophen and nonsteroidal antiinflammatory drugs (NSAIDs), along with pain modulators, are on the first step of the analgesic ladder to treat mild to moderate pain, as recommended by the World Health Organization (WHO). Opioid analgesics are added as a second-line treatment for moderate to severe pain. 2 Adjuvant medications are used to treat conditions contributing to pain such as altered neuronal function or muscle spasm. An example would be an antidepressant or anticonvulsant medication to treat neuropathic pain from a lumbar radiculopathy. Muscle relaxants can be used to treat muscle spasm.2,3
When treating the elderly, it is important to use a medication with the shortest possible half-life as well as the lowest effective dose, as the elderly population is more prone to side effects. Sedative effects of medications can cause significant confusion and thereby impair function and increase the risk of falling. In the elderly, metabolism of medications is reduced with reduced renal clearance. Therefore, buildup of medication is more likely, placing patients at a higher risk of side effects, especially when combined with other drugs. However, when pain is continuous, a long-acting analgesic medication should be considered5. Elderly and medically ill patients are more prone to toxic renal and hepatic side effects. Also, drugs have variable effects on the liver’s cytochrome P450 system, which vary between individuals; therefore it is important for medications to be obtained from one pharmacy, which can then monitor for medication interactions.
Nonopioid Analgesic Agents: Acetaminophen, Nsaids, Aspirin
Acetaminophen
Acetaminophen (Tylenol) is one of the most commonly used medications both by prescription and over the counter, either by itself or in combination with other products. Acetaminophen is used for the treatment of mild to moderate pain. It is also an antipyretic, but does not have significant antiinflammatory properties. Although considered a safe drug, acetaminophen has a very narrow therapeutic index, thus making a safe dose and a dose able to contribute to liver toxicity very close to each other. The once recommended 1000 mg per dose or 4000 mg a day are being reconsidered. The FDA now recommends a lower dose of a max of 3250 mg per day, which can be divided into 5 doses of 650 mg a day. Studies have shown that a 650 mg dose is almost as effective as the 1000 mg for pain and may be a lot safer for patients.17 Due to acetaminophens effects on the liver, chronic use of acetaminophen should be avoided in patients with hepatic impairment, and patients should limit usage of alcohol products (1-2 drinks per day). Long-term overdosing of acetaminophen may result in hepatic necrosis and can be fatal. Patients should be made aware of the possible ease of overdose if they are on many pain medications, such as Percocet®, Vicodin®, and Darvocet®, all of which contain varying doses of acetaminophen.3,5,6
Nonsteroidal Antiinflammatory Drugs (NSAIDs)
All NSAIDs have antiinflammatory, analgesic, and antipyretic properties.3,4,6,7 They are commonly used to treat inflammatory conditions, and are used for analgesia in low back pain as well as for the pain and inflammation of many types of arthritis. NSAIDs are characterized as either selective or nonselective COX-2 inhibitors.
The inflammatory cascade can be triggered by a number of causes such as a physical injury or response to infection. In response, at the cellular level, prostaglandins are synthesized by cyclooxygenase. NSAIDs inhibit the cyclooxygenase (COX) enzyme and therefore reduce production of prostaglandin, usually nonselectively via COX-1 and COX-2 pathways.12 Prostaglandins are thought to sensitize the pain receptors and are involved in peripheral and central pain transmission. The enzyme COX-1 (cyclooxygenase-1) is prevalent, located in most normal cells, and involved in many aspects of physiologic function. COX-2 is more involved in the inflammatory process. COX-2 is expressed in parts of the kidney, brain, and endothelial cells. COX-1 is found in the gastric epithelium and is involved in the formation of cytoprotective prostaglandin. The COX-2–specific inhibitory NSAIDs were formulated to be gastric-protective; however, they do not reduce the risk of other effects of traditional NSAIDs.3, 4, 6, 7, 12 The NSAIDS and aspirin do not affect the lipoxygenase pathway, therefore they do not reduce the formation of an important inflammatory mediator, the leukotrienes.3,4,6,7,12,18
The NSAIDs have a black box warning that includes cardiovascular disease, myocardial infarction, stroke, and hypertension. The nonselective NSAIDs, such as ibuprofen at low doses, also can interfere with the cardioprotective effect of aspirin. NSAIDs, such as ibuprofen at doses as low as 400 mg, should be taken either 8 hours before or 30 minutes after aspirin, if the aspirin is taken on a daily basis for its cardioprotective effect. If they are taken together then the cardioprotective effect of aspirin may be negated.12 Also, NSAIDs are contraindicated for the perioperative pain of coronary artery bypass grafts, because platelet adhesion and aggregation can be disrupted.4, 5, 6
NSAIDs may increase the risk of renal and hepatic impairment, as well as of gastrointestinal irritation including bleeding and ulceration. GI bleeding risk is increased in the elderly; in smokers; when NSAIDs are used with aspirin; with corticosteroid use, ethanol use, anticoagulation, debilitation, and history of peptic ulcer. The risk of gastric ulceration can be reduced with the use of a proton pump inhibitor, or a prostaglandin analog such as misoprostol. However, despite that there is still an important risk of developing a severe life threatening gastrointestinal bleed.NSAIDs should be taken with food or a large volume of milk or water to reduce GI side effects. 4–6
Congestive heart failure can be aggravated by the increased action of antidiuretic hormone due to NSAID use. Renal circulation is reduced due to reduced prostaglandin synthesis. This can affect the electrolytes at the renal level and lead to fluid retention and edema, a common side effect. Central nervous system side effects include headache and dizziness. NSAIDs should be discontinued if a rash develops from their use, due to the risk of Stevens-Johnson syndrome.4,6
Contraindications to NSAID use include hypersensitivity or allergic reaction to the drug used or any component of its formulation, aspirin, NSAIDs, as well as perioperative use for coronary artery bypass grafting. There is a cross-sensitivity to NSAIDs in those patients who have bronchial asthma, aspirin intolerance, and rhinitis. Avoid NSAIDs in severe hepatic and renal failure. Use caution for uncontrolled hypertension. See Table 22-1 for individual drug descriptions.4,6
