Mechanisms of neuropathic pain: nerve, brain, and psyche: perhaps the dorsal horn but not the sympathetic system

Peripheral Nerve Stimulator for Pain After Surgery for Ulnar Neuropathy at the Elbow

Neuropathy secondary to ulnar nerve entrapment is a painful condition that often persists following surgical decompression. We present the case of a 43-year-old female with Ehlers-Danlos syndrome and left ulnar neuritis refractory to surgical management. Peripheral nerve stimulation of the ulnar nerve proximal to the elbow resulted in a significant reduction in pain and improvement in disability post-implantation. This case suggests that peripheral nerve stimulation is a promising minimally invasive technique that should be considered for treating non-operative upper extremity neuropathic pain.

Involvement of the Sympathetic Nervous System in Complex Regional Pain Syndrome

Complex regional pain syndrome (CRPS) occasionally develops as a complication of limb trauma. Sympathetic neurotransmitter release is compromised in the affected limb of at least a subgroup of patients throughout the course of the disorder, whereas signs of sympathetic deficit (a warm flushed limb) often evolve into signs of sympathetic overactivity (a cool moist limb) due to the development of adrenergic supersensitivity. Cross-talk between sympathetic neurotransmitters and the sensory neurons that signal pain appears to contribute to CRPS in a subgroup of patients. In addition, sympathetic activity may retard normal healing by aggravating the vascular disturbances associated with inflammation. Sympathetic dysfunction seems to originate from within the central nervous system in patients without peripheral nerve injury, possibly in association with chronic activation of the “defeat” response associated with inhibitory opioid-mediated pain modulation. Fatigue of this inhibitory process may unmask a facilitatory influence of arousal on nociceptive transmission in the thalamus and cortex that contributes to stress-induced pain.

Hyperexcitable polymodal and insensitive nociceptors in painful human neuropathy

Six patients with chronic pain, mechanical and thermal hyperalgesia/allodynia, and cutaneous vasodilatation starting distally in their extremities, were evaluated using clinical and neurophysiological methods and microneurography. Evidence of small-fiber polyneuropathy was documented in all, but the etiology remained cryptogenic in several. Different forms of hyperexcitability were detected by microneurography in both common polymodal and mechanically insensitive C nociceptors, which explain all the somatosensory abnormalities. Signs of hyperexcitability included reduced receptor threshold (accounting for mechanical and heat allodynias), spontaneous C nociceptor discharge (explaining spontaneous "burning" pain and antidromic vasodilatation), and multiplied nociceptor responses to stimulation (accounting for hyperalgesia). The clinical and electrophysiological profiles of these patients resemble the experimental syndrome evoked by application of capsaicin to the skin. This similarity, and the striking heat dependence of the spontaneous pain, suggest that a common feature may be altered expression or modulation of vanilloid 1 receptor, provoking abnormal nociceptor discharges.

Spectrum of cutaneous hyperalgesias/allodynias in neuropathic pain patients

OBJECTIVES

The aim of this study was to discern the pathophysio-logical bases for neuropathic hyperalgesias.

METHODS

In this study, neurological and neurophysiological evaluation of 132 consecutive hyperalgesia patients using rigorous clinical and laboratory protocols were carried out.

RESULTS

Two discrete semeiologic entities emerged: classic neurological vs atypical, fulfilling taxonomically complex regional pain syndrome (CRPS) II and I, respectively. The classic group (34.9%) exhibited sensorimotor patterns restricted to nerve distribution and documented nerve fiber dysfunction. Among them four (3.03%) had sensitization of C-nociceptors, seven (5.3%) had central release of nociceptive input, and 35 (26.52%) probable ectopic nerve impulse generation. The atypical group (65.1%) displayed weakness with interrupted effort; non-anatomical hypoesthesia and hyperalgesia; hypoesthesia or paresis reversed by placebo, or atypical abnormal movements, and physiological normality of motor and sensory pathways.

CONCLUSIONS

Spatiotemporal features of neuropathic hyperalgesia constitute key criteria for differential diagnosis between CRPS II and I and, together with other behavioral sensorimotor features, signal psychogenic pseudoneurological dysfunction vs structural neuropathology. 'Neuropathic' hyperalgesias may reflect neuropathological or psychopathological disorders.