Painful neuropathy: altered central processing maintained dynamically by peripheral input

Abstract

We performed sensory assessments before and during diagnostic tourniquet-cuff and local anesthetic blocks in 4 patients diagnosed with reflex sympathetic dystrophy (RSD). All patients complained of mechano-allodynia; lightly touching the skin evoked an intense pain sensation. At detection levels, electrical stimuli were perceived as painful, suggesting that the mechano-allodynia was mediated by A beta low-threshold mechanoreceptor afferents. A beta-mediated allodynia was further supported by reaction time latencies to painful electrical stimuli at threshold for A-fiber activation and, in 1 patient, by differential cuff blocks which abolished A beta function and allodynia while thermal sensation (warm and cold) were preserved. Local anesthetic block of painful foci associated with previous trauma abolished mechano-allodynia, cold allodynia, and spontaneous pain in all patients and relieved the motor symptoms in 1 patient with tonic contractures of the toes. Tactile and thermal perception in the previously allodynic area was preserved. When the local anesthetic block waned, spontaneous pain, allodynia, and motor symptoms returned. We propose a model of neuropathic pain in which ongoing nociceptive afferent input from a peripheral focus dynamically maintains altered central processing that accounts for allodynia, spontaneous pain, and other sensory and motor abnormalities. Blocking the peripheral input causes the central processing to revert to normal, abolishing the symptoms for the duration of the block. The model accounts for sympathetically maintained (SMP) and sympathetically independent (SIP) pain. The peripheral input can be independent of sympathetic activity or driven completely or in part by activity in sympathetic efferents or by circulating catecholamines. The shared final common pathway may explain the common features of SMP and SIP.