Painful sensory neuropathy

Involvement of the PKA pathway and inhibition of voltage gated Ca2+ channels in antihyperalgesic activity of Lippia grata/β-cyclodextrin

Neuropathic pain (NP) is a difficult condition to treat because of the modest efficacy of available drugs. New treatments are required. In the study we aimed to investigate the effects of the essential oil from Lippia grata alone or complexed in β-cyclodextrin (LG or LG-βCD) on persistent inflammatory and neuropathic pain in a mouse model. We also investigated Ca²⁺ currents in rat dorsal root ganglion (DRG) neurons. Male Swiss mice were treated with LG or LG/β-CD (24 mg/kg, i.g.) and their effect was evaluated using an acute inflammatory pleurisy model and nociception triggered by intraplantar injection of an agonist of the TRPs channels. We also tested their effect in chronic pain models: injection of Freund's Complete Adjuvant and partial sciatic nerve ligation (PSNL). In the pleurisy model, LG reduced the number of leukocytes and the levels of TNF-α and IL-1β. It also inhibited cinnamaldehyde and menthol-induced nociceptive behavior. The pain threshold in mechanical and thermal hyperalgesia was increased and paw edema was decreased in models of inflammatory and neuropathic pain. PSNL increased inflammatory protein contents and LG and LG-βCD restored the protein contents of TNF-α, NF-κB, and PKA, but not IL-1β and IL-10. LG inhibited voltage gated Ca²⁺ channels from DRG neurons. Our results suggested that LG or LG-βCD produce anti-hyperalgesic effect in chronic pain models through reductions in TNF-α levels and PKA, and inhibited voltage-gated calcium channels and may be innovative therapeutic agents for the management of NP.

Functional exploration for neuropathic pain

Neuropathic pain (NP) may become refractory to conservative medical management, necessitating neurosurgical procedures in carefully selected cases. In this context, the functional neurosurgeon must have suitable knowledge of the disease he or she intends to treat, especially its pathophysiology. This latter factor has been studied thanks to advances in the functional exploration of NP, which will be detailed in this review. The study of the flexion reflex is a useful tool for clinical and pharmacological pain assessment and for exploring the mechanisms of pain at multiple levels. The main use of evoked potentials is to confirm clinical, or detect subclinical, dysfunction in peripheral and central somato-sensory pain pathways. LEP and SEP techniques are especially useful when used in combination, allowing the exploration of both pain and somato-sensory pathways. PET scans and fMRI documented rCBF increases to noxious stimuli. In patients with chronic NP, a decreased resting rCBF is observed in the contralateral thalamus, which may be reversed using analgesic procedures. Abnormal pain evoked by innocuous stimuli (allodynia) has been associated with amplification of the thalamic, insular and SII responses, concomitant to a paradoxical CBF decrease in ACC. Multiple PET studies showed that endogenous opioid secretion is very likely to occur as a reaction to pain. In addition, brain opioid receptors (OR) remain relatively untouched in peripheral NP, while a loss of ORs is most likely to occur in central NP, within the medial nociceptive pathways. PET receptor studies have also proved that antalgic Motor Cortex Stimulation (MCS), indicated in severe refractory NP, induces endogenous opioid secretion in key areas of the endogenous opioid system, which may explain one of the mechanisms of action of this procedure, since the secretion is proportional to the analgesic effect.

Vitamin B for treating peripheral neuropathy

BACKGROUND

Vitamin B is frequently used for treating peripheral neuropathy but its efficacy is not clear.

OBJECTIVES

The objective of this review was to assess the effects of vitamin B for treating generalised peripheral neuropathy.

SEARCH STRATEGY

We searched the Cochrane Neuromuscular Disease Group Trials Register (searched August 2005), MEDLINE (January 1966 to September 2005), EMBASE (January 1980 to September 2005), Philippine databases (searched September 2005) and reference lists of articles. We also contacted manufacturers and researchers in the field.

SELECTION CRITERIA

Randomised and quasi-randomised trials where vitamin B was compared with placebo or another treatment in generalised peripheral neuropathy.

DATA COLLECTION AND ANALYSIS

Two authors independently assessed trial quality and extracted data. We contacted study authors for additional information.

MAIN RESULTS

Thirteen studies involving 741 participants with alcoholic or diabetic neuropathy were included. In the comparison of vitamin B with placebo, two small trials showed no significant short-term benefit in pain intensity while one of the trials showed a small significant benefit in vibration detection from oral benfotiamine, a derivative of thiamine. In the larger of two trials comparing different doses of vitamin B complex, there was some evidence that higher doses resulted in a significant short-term reduction in pain and improvement in paraesthesiae, in a composite outcome combining pain, temperature and vibration, and in a composite outcome combining pain, numbness and paraesthesiae. There was some evidence that vitamin B is less efficacious than alpha-lipoic acid, cilostazol or cytidine triphosphate in the short-term improvement of clinical and nerve conduction study outcomes but the trials were small. There were few minor adverse effects reported.

AUTHORS' CONCLUSIONS

There are only limited data in randomised trials testing the efficacy of vitamin B for treating peripheral neuropathy and the evidence is insufficient to determine whether vitamin B is beneficial or harmful. One small trial in alcoholic peripheral neuropathy reported slightly greater improvement in vibration perception threshold with oral benfotiamine for eight weeks than placebo. In another small study, a higher dose of oral vitamin B complex for four weeks was more efficacious than a lower dose in reducing symptoms and signs. Vitamin B administered by various routes for two to eight weeks was less efficacious than alpha-lipoic acid, cilostazol or cytidine triphosphate in short-term improvement of clinical and nerve conduction study outcomes. Vitamin B is generally well-tolerated.

Small fiber neuropathy: a common and important clinical disorder

Small fiber neuropathy (SFN) is a neuropathy selectively involving small diameter myelinated and unmyelinated nerve fibers. Interest in this disorder has considerably increased during the past few years. It is often idiopathic and typically presents with peripheral pain and/or symptoms of autonomic dysfunction. Diagnosis is made on the basis of the clinical features, normal nerve conduction studies (NCS) and abnormal specialized tests of small nerve fibers. Among others, these tests include assessment of epidermal nerve fiber density, temperature sensation tests for sensory fibers and sudomotor and cardiovagal testing (QSART) for autonomic fibers. Unless an underlying disease is identified, treatment is usually symptomatic and directed towards alleviation of neuropathic pain.

Laser evoked potentials for assessing sensory neuropathy in human patients

Sensory neuropathy usually impairs tactile sensations related to large myelinated afferents (Abeta) as well as thermal-pain sense related to small myelinated (Adelta) and unmyelinated (C) afferents. By selectively affecting large or small fibres, some sensory neuropathies may also provoke a dissociated sensory loss. Standard nerve conduction studies and somatosensory evoked potentials assess Abeta-fibre function only. Laser pulses selectively excite free nerve endings in the superficial skin layers and evoke Adelta-related brain potentials (LEPs). From earlier studies and new cases we collected data on 270 patients with sensory neuropathy. LEPs often disclosed subclinical dysfunction of Adelta fibres and proved a sensitive and reliable diagnostic tool for assessing small-fibre function in sensory neuropathy.

Functional assessment of A? and C fibers in patients with Fabry's disease

The pathophysiology of neuropathic pain in Fabry's disease (FD) is still largely unknown. Seven FD patients were studied by laser evoked potentials (LEPs) to assess the function of the A delta and C fibers. Laser pulses were delivered on the skin of the hand and perioral region at painful intensity to record LEPs related to A delta-fiber inputs and at nonpainful intensity to obtain LEPs related to C-fiber inputs. When the perioral region was stimulated, a vertex positive component was recorded with a mean latency of 260.3 ms and 376 ms after A delta- and C-fiber stimulation, respectively. The mean A delta-LEP amplitude was significantly lower in FD patients (N1/P1 mean values were 2.8 microV and 4.5 microV after hand and face stimulation, respectively, compared to 4 microV and 8.9 microV for controls; N2/P2 mean values were 8.2 microV and 11.1 microV after hand and face stimulation, respectively, and 16.7 microV and 22.3 microV in controls). Unlike the healthy subjects, 6 FD patients, suffering from neuropathic pain, showed a late positive potential related to C-fiber function (mean latency, 377.1 ms) also after facial stimulation at painful intensity, suggesting a relative overflow of C-fiber input, which may be relevant in the pathophysiology of pain in this disease.