A Review of Neuropathic Pain Treatments for the Older Adult
Neuropathic pain (NP) results from injury or dysfunction in the processing of sensory information in the nervous system. It occurs in a wide array of disease processes and may involve complex management strategies. A comprehensive approach utilizing proven pharmacologic and nonpharmacologic therapies can be used to return function and improve quality of life that has been lost because of pain. In the older population, age-related physiologic and pharmacodynamic alterations, coexisting diseases, and the prevalence of polypharmacy must be considered when selecting therapies for neuropathic pain.
Key words: neuropathic pain, older adults, neuropathy, pain, analgesics.
Introduction
The International Association for the Study of Pain defines pain as an “unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage.” Neuropathic pain (NP) “arises as a result of a lesion or dysfunction of the nervous system.”1 The most common causes of neuropathic pain in the older adult include low back pain (LBP), postherpetic neuralgia (PHN), and diabetic peripheral neuropathy (DPN) (Table 1). In patients without a known diagnosis, a thorough diagnostic evaluation or neurologic referral may be necessary to elucidate the cause of the pain.
Clinical Features
Across the various disorders, the description of neuropathic pain by individuals is quite similar. Patients complain of burning, tingling, itching, stabbing, or electric sensations. Numbness and other neurologic deficits may also be present. There may be pain from innocuous stimuli such as sheets or clothing (allodynia), or an exaggerated response to mildly painful stimuli (hyperalgesia).
Pathophysiology of Neuropathic Pain
The normal transmission of painful impulses is summarized in Figure 1.
Significant advances have been made in our understanding of the pathophysiology underlying neuropathic pain.2 The principal theme is the development of abnormal sensory processing within the central and peripheral nervous system. Altered gene expression, upregulation of excitatory neurotransmitter receptors, downregulation of inhibitory neurotransmitter receptors, neuron-glial interactions, and neuroplastic changes within the nervous system have been described as potential mechanisms contributing to the development of NP (Figure 2).
Physiologic Changes in the Older Adult
Changes in the older adult population that may result in altered responses to drugs include decreased lean body mass, gastrointestinal motility, cardiac output, renal clearance, protein binding, and increased central nervous system (CNS) effects of drugs.3 The net effect is susceptibility to constipation, delayed redistribution and clearance of drugs, increased availability of active drugs, and increased CNS side effects. Physicians must keep these physiologic changes in mind when placing patients on medication trials for NP.
Analgesics are now the second most frequent new class of medications prescribed to older patients.4 Dosing starts at the minimum dosage and is allowed to reach steady state before further upward titration commences. Delayed redistribution may result in slower achievement of steady state concentration. Care should be taken to monitor for cognitive dysfunction. Furthermore, disease-specific relative contraindications for particular analgesics and the increased susceptibility of many older adults to systemic side effects must be carefully considered in the selection of medications. For example, opioids and tricyclic antidepressants (TCAs) both delay micturition and may have synergistic effects in patients with bladder outlet obstruction. Opioids, gabapentin, or TCAs alone or in combination can impair bowel motility and worsen constipation or obstipation. TCAs can also induce arrhythmias and should be