Recovery of C-fiber-induced extravasation following peripheral nerve injury in the rat

Pillar Pain After Minimally Invasive and Standard Open Carpal Tunnel Release: A Systematic Review and Meta-analysis

Purpose

Pillar pain is a recognized postoperative complication of carpal tunnel release (CTR). Minimally invasive and alternative surgical techniques can theoretically prevent pillar pain, and the aim of this review was to compare the incidence of pillar pain after standard open CTR and alternative surgical techniques.

Methods

MEDLINE, Embase, and Scopus databases were thoroughly searched. Randomized controlled trials comparing minimally invasive surgical techniques to standard open CTR were identified. Data, including surgical technique, number of hands, incidence of pillar pain, and follow-up intervals, were extracted. Odds ratios (OR) were expressed as pillar pain incidence in the intervention group relative to standard open CTR.

Results

There were 12 studies included. No statistically significant differences were noted among endoscopic (OR = 0.53, P = .20), flexor retinaculum lengthening (OR = 1.00, P = 1.00), short incision (OR = 0.41, P = .07) or illuminated knife techniques (OR = 0.18, P = .16). There was a statistically significant decrease in pillar pain after minimally invasive CTR (OR = 0.41, 95% confidence interval 0.20-0.86, I2 = 0%, P = .02) between 3- and 6-months follow-up; however, analyses at all other follow-up periods failed to reach statistical significance.

Conclusions

Although our findings suggest that standard open CTR may be associated with an increased duration of pillar pain between 3 and 6 months postoperatively, our results suggest that minimally invasive CTR techniques do not affect either the initial development or persistence of pillar pain.

Clinical relevance

Our results illustrate the natural history of pillar pain with the majority of cases resolving after 6 months, highlighting the utility of symptomatic and conservative treatments and patient education in the management of pillar pain.

Transection of the pelvic or vagus nerve forestalls ripening of the cervix and delays birth in rats

Innervation of the cervix is important for normal timing of birth because transection of the pelvic nerve forestalls birth and causes dystocia. To discover whether transection of the parasympathetic innervation of the cervix affects cervical ripening in the process of parturition was the objective of the present study. Rats on Day 16 of pregnancy had the pelvic nerve (PnX) or the vagus nerve (VnX) or both pathways (PnX+VnX) transected, sham-operated (Sham) or nonpregnant rats served as controls. Sections of fixed peripartum cervix were stained for collagen or processed by immunohistochemistry to identify macrophages and nerve fibers. All Sham controls delivered by the morning of Day 22 postbreeding, while births were delayed in more than 75% of neurectomized rats by more than 12 h. Dystocia was evident in more than 25% of the PnX and PnX+VnX rats. Moreover, on prepartum Day 21, serum progesterone was increased severalfold in neurectomized versus Sham rats. Assessments of cell nuclei counts indicated that the cervix of neurectomized rats and Sham controls had become equally hypertrophied compared to the unripe cervix in nonpregnant rats. Collagen content and structure were reduced in the cervix of all pregnant rats, whether neurectomized or Shams, versus that in nonpregnant rats. Stereological analysis of cervix sections found reduced numbers of resident macrophages in prepartum PnX and PnX+VnX rats on Day 21 postbreeding, as well as in VnX rats on Day 22 postbreeding compared to that in Sham controls. Finally, nerve transections blocked the prepartum increase in innervation that occurred in Sham rats on Day 21 postbreeding. These findings indicate that parasympathetic innervation of the cervix mediates local inflammatory processes, withdrawal of progesterone in circulation, and the normal timing of birth. Therefore, pelvic and vagal nerves regulate macrophage immigration and nerve fiber density but may not be involved in final remodeling of the extracellular matrix in the prepartum cervix. These findings support the contention that immigration of immune cells and enhanced innervation are involved in processes that remodel the cervix and time parturition.

Neuropathic changes in equine laminitis pain

Laminitis is a common debilitating disease in horses that involves painful disruption of the lamellar dermo-epidermal junction within the hoof. This condition is often refractory to conventional anti-inflammatory analgesia and results in unremitting pain, which in severe cases requires euthanasia. The mechanisms underlying pain in laminitis were investigated using quantification of behavioural pain indicators in conjunction with histological studies of peripheral nerves innervating the hoof. Laminitic horses displayed consistently altered or abnormal behaviours such as increased forelimb lifting and an increased proportion of time spent at the back of the box compared to normal horses. Electron micrographic analysis of the digital nerve of laminitic horses showed peripheral nerve morphology to be abnormal, as well as having reduced numbers of unmyelinated (43.2%) and myelinated fibers (34.6%) compared to normal horses. Sensory nerve cell bodies innervating the hoof, in cervical, C8 dorsal root ganglia (DRG), showed an upregulated expression of the neuronal injury marker, activating transcription factor-3 (ATF3) in both large NF-200-immunopositive neurons and small neurons that were either peripherin- or IB4-positive. A significantly increased expression of neuropeptide Y (NPY) was also observed in myelinated afferent neurons. These changes are similar to those reported in other neuropathic pain states and were not observed in the C4 DRG of laminitic horses, which is not associated with innervation of the forelimb. This study provides novel evidence for a neuropathic component to the chronic pain state associated with equine laminitis, indicating that anti-neuropathic analgesic treatment may well have a role in the management of this condition.

Novel agonistic action of mustard oil on recombinant and endogenous porcine transient receptor potential V1 (pTRPV1) channels

Neurogenic components play a crucial role in inflammation and nociception. Mustard oil (MO) is a pungent plant extract from mustard seed, horseradish and wasabi, the main constituent of which is allylisothiocyanate. We have characterized the action of MO on transient receptor potential V1 (TRPV1), a key receptor of signal transduction pathways in the nociceptive system, using fura-2-based [Ca(2+)](i) imaging and the patch-clamp technique in a heterologous expression system and sensory neurons. In human embryonic kidney (HEK) 293 cells expressing porcine TRPV1 (pTRPV1), MO evoked increases of [Ca(2+)](i) in a concentration-dependent manner. A high concentration of MO elicited irreversible cell swelling. Capsazepine, ruthenium red and iodoresiniferatoxin dose-dependently suppressed the MO-induced [Ca(2+)](i) increase. MO elicited outward rectified currents in pTRPV1-expressing HEK 293 cells with a reversal potential similar to that of capsaicin. [Ca(2+)](i) responses to MO were completely abolished by the removal of external Ca(2+). MO simultaneously elicited an inward current and increase of [Ca(2+)](i) in the same cells, indicating that MO promoted Ca(2+) influx through TRPV1 channels. In cultured porcine dorsal root ganglion (DRG) neurons, MO elicited a [Ca(2+)](i) increase and inward current. Among DRG neurons responding to MO, 85% were also sensitive to capsaicin. The present data indicate that MO is a novel agonist of TRPV1 channels, and suggest that the action of MO in vivo may be partly mediated via TRPV1. These results provide an insight into the TRPV1-mediated effects of MO on inflammation and hyperalgesia.

Peripheral mGluR5 antagonist attenuated craniofacial muscle pain and inflammation but not mGluR1 antagonist in lightly anesthetized rats

The present study investigated the role of peripheral group I metabotropic glutamate receptors (mGluRs) in MO-induced nociceptive behaviour and inflammation in the masseter muscles of lightly anesthetized rats. Experiments were carried out on male Sprague-Dawley rats weighing 300-400 g. After initial anesthesia with sodium pentobarbital (40 mg/kg, i.p.), one femoral vein was cannulated and connected to an infusion pump for intravenous infusion of sodium pentobarbital. The rate of infusion was adjusted to provide a constant level of anesthesia. Mustard oil (MO, 30 microl) was injected into the mid-region of the left masseter muscle via a 30-gauge needle over 10s. After 30 microl injection of 5, 10, 15, or 20% MO into the masseter muscle, the total number of hindpaw shaking behaviour and extravasated Evans' blue dye concentration in the masseter muscle were significantly higher in the MO-treated group in a dose-dependent manner compared with the vehicle (mineral oil)-treated group. Intramuscular pretreatment with 3 or 5% lidocaine reduced MO-induced hindpaw shaking behaviour and increases in extravasated Evans' blue dye concentration. Intramuscular pretreatment with 5 mM MCPG, non-selective group I/II mGluR antagonist, or MPEP, a selective group I mGluR5 antagonist, produced a significant attenuation of MO-induced hindpaw shaking behaviour and increases in extravasated Evans' blue dye concentration in the masseter muscle while LY367385, a selective group I mGluR1 antagonist, did not affect MO-induced nociceptive behaviour and inflammation in the masseter muscle. These results indicate that peripheral mGluR5 plays important role in mediating MO-induced nociceptive behaviour and inflammation in the craniofacial muscle.

Substance P does not play a critical role in neurogenic inflammation in the rat masseter muscle

In this study, we performed a series of experiments to investigate whether substance P (SP) contributes to neurogenic inflammation in the skeletal muscle tissue. Intramuscular injection of an inflammatory irritant, mustard oil (MO), induces significant edema formation in the rat masseter muscle. In order to study the contribution of endogenous SP in the MO-induced edema, groups of rats were pretreated with two different doses (100 nmol; 1 microl) of either peptidergic (Sendide) or non-peptidergic (L703, 606) neurokinin 1 (NK1) receptor antagonist in one masseter muscle 15 min prior to the MO injection in the same muscle. The extent of edema was assessed as the percent weight difference of the injected muscle compared to the non-injected muscle. Neither Sendide nor L703,606 pretreatment resulted in a significant inhibition of the MO-induced edema in the masseter muscle. Exogenous application of SP also produced a significant swelling of the muscle, which was blocked by L703,606 (1 microl) pretreatment, suggesting that evoked release of SP following MO injection is not sufficient to induce significant edema formation. Capsaicin (1% in 25 microl), which is known to cause neurogenic inflammation, failed to produce edema formation in the masseter muscle. The same concentration of capsaicin injected into the hindpaw produced significant swelling of the injected paw. Taken together, these results provide compelling evidence that, unlike cutaneous or joint tissue, SP does not play a critical role in inducing neurogenic inflammation in the skeletal muscle tissue.

Regeneration of unmyelinated and myelinated sensory nerve fibres studied by a retrograde tracer method

Regeneration of myelinated and unmyelinated sensory nerve fibres after a crush lesion of the rat sciatic nerve was investigated by means of retrograde labelling. The advantage of this method is that the degree of regeneration is estimated on the basis of sensory somata rather than the number of axons. Axonal counts do not reflect the number of regenerated neurons because of axonal branching and because myelinated axons form unmyelinated sprouts. Two days to 10 weeks after crushing, the distal sural or peroneal nerves were cut and exposed to fluoro-dextran. Large and small dorsal root ganglion cells that had been labelled, i.e., that had regenerated axons towards or beyond the injection site, were counted in serial sections. Large and small neurons with presumably myelinated and unmyelinated axons, respectively, were classified by immunostaining for neurofilaments. The axonal growth rate was 3.7 mm/day with no obvious differences between myelinated and unmyelinated axons. This contrasted with previous claims of two to three times faster regeneration rates of unmyelinated as compared to myelinated fibres. The initial delay was 0.55 days. Fewer small neurons were labelled relative to large neurons after crush and regeneration than in controls, indicating that regeneration of small neurons was less complete than that of large ones. This contrasted with the fact that unmyelinated axons in the regenerated sural nerve after 74 days were only slightly reduced.

C-Fiber Structure Varies with Location in Peripheral Nerve

Recent advances in regeneration and pain research have revealed gaps in the understanding of normal C-fiber anatomy. In the rat PNS, C-fiber axons assemble into Remak bundles, but beyond this, features of C-fiber organization are not defined. Systematic sampling and quantitation reveals that Remak bundles exiting from the L5 dorsal root ganglion (DRG) contain large numbers of axons, for example, 56% of unmyelinated axons were in bundles of >20 axons. This is different from distal nerve segments such as the hindpaw plantar nerve where the median number of axons per bundle is 3. The cross-sectional area of unmyelinated axons in dorsal root was homogeneous near the DRG but variability in axonal area increased near the spinal cord (p = 0.00001) and the mean axonal area was unchanged. Unmyelinated axons in peripheral nerve were almost always isolated from one another by Schwann cell processes; however, in dorsal root 7% to 9% of unmyelinated axons were immediately adjacent within pockets containing 2 or more axons. Remak bundles in the distal peripheral nerve clustered with other Remak bundles. We observe that multiple unmyelinated axons are juxtaposed within the C-fiber/Remak bundle and that the close association of afferent axons may have important functional implications.

Mustard oil induces a transient receptor potential vanilloid 1 receptor-independent neurogenic inflammation and a non-neurogenic cellular inflammatory component in mice

A neurogenic component has been suggested to play a pivotal role in a range of inflammatory/immune diseases. Mustard oil (allyl-isothiocyanate) has been used in studies of inflammation to mediate neurogenic vasodilatation and oedema in rodent skin. The aim of the present study was to analyse mustard oil-induced oedema and neutrophil accumulation in the mouse ear focussing on the roles of neurokinin 1 (NK(1)) and vanilloid (TRPV1) receptors using normal (BALB/c, C57BL/6) as well as NK(1) and TRPV1 receptor knockout mice. A single or double treatment of 1% mustard oil on the BALB/c mouse ear induced ear oedema with responses diminished by 6 h. However a 25-30% increase in ear thickness was maintained by the hourly reapplication of mustard oil. Desensitisation of sensory nerves with capsaicin, or the NK(1) receptor antagonist SR140333, inhibited oedema but only in the first 3 h. Neutrophil accumulation in response to mustard oil was inhibited neither by SR140333 nor capsaicin pre-treatment. An activating dose of capsaicin (2.5%) induced a large oedema in C57BL/6 wild-type mice that was minimal in TRPV1 receptor knockout mice. By comparison, mustard oil generated ear swelling was inhibited by SR140333 in wild-type and TRPV1 knockout mice. Repeated administration of mustard oil maintained 35% oedema in TRPV1 knockout animals and the lack of TRPV1 receptors did not alter the leukocyte accumulation. In contrast repeated treatment caused about 20% ear oedema in Sv129+C57BL/6 wild-type mice but the absence of NK(1) receptors significantly decreased the response. Neutrophil accumulation showed similar values in both groups. This study has revealed that mustard oil can act via both neurogenic and non-neurogenic mechanisms to mediate inflammation in the mouse ear. Importantly, the activation of the sensory nerves was still observed in TRPV1 knockout mice indicating that the neurogenic inflammatory component occurs via a TRPV1 receptor independent process.

Delayed onset of altered sensation following dental implant placement and mental block local anesthesia: a case report

A case of a delayed-onset post-operative altered sensation of the mental nerve is reported with speculations as to etiology. There is a discussion concerning intraoperative radiographs and osteotomy positioning. When postoperative altered sensation occurs, etiologic considerations should always include local anesthetic administration technique. The importance of pulp testing from the first molar to the contralateral lateral incisor is key to determining whether the deficit is in the mandibular nerve or only the mental nerve, which may be a result of mental block local anesthetic administration and not implant placement. This differentiation may be important in treatment and/or legal exposure. The altered sensation was probably caused by the mental block anesthetic technique. The partial anesthetic area in this case was probably a result of crossover innervation from the contralateral mental nerve.

Cutaneous vascular responses evoked by noxious stimulation in rats with the spinal nerve ligation-induced model of neuropathy

Antidromic activation of nociceptive nerve fibres innervating the skin produces an axon reflex that involves extravasation and vasodilation of cutaneous blood vessels. We determined whether the axon reflex of the hindlimb skin is influenced by an experimental model of neuropathy induced by unilateral ligation of spinal nerves L(5) and L(6) in the rat. Ligation of spinal nerves induced symptoms mimicking tactile allodynia, as indicated by a marked decrease of the hindlimb withdrawal threshold to mechanical stimulation. The axon reflex induced by antidromic electrical stimulation of nociceptive fibres innervating the plantar skin ipsilateral to the ligation was attenuated according to determination of extravasation response and blood flow response. Lidocaine block or transection of the sciatic nerve of the neuropathic limb did not induce any change in basal blood flow of the plantar skin. The results indicate that ligation of spinal nerves induces an attenuation of the axon reflex. This attenuation reflects a decrease in the efferent function of primary afferent nociceptors innervating the hypersensitive skin of the hindpaw. The attenuation of antidromically-induced vascular responses was not caused by overriding sympathetic activity, as indicated by lack of blood flow effects by lidocaine blocks or a transection of the sciatic nerve.

Effects of neutralizing antibodies to TNF-alpha on pain-related behavior and nerve regeneration in mice with chronic constriction injury

Inhibition of proinflammatory cytokines reduces hyperalgesia in animal models of painful neuropathy. We set out to investigate the consequences of this treatment for nerve regeneration. Here we examined the sequels of epineurial application of neutralizing antibodies to tumor necrosis factor-alpha (TNF) in chronic constriction injury (CCI) of the sciatic nerve in C57/BL 6 mice. The mice were tested behaviorally for manifestations of thermal hyperalgesia and mechanical allodynia. Nerve regeneration was assessed by morphometry of myelinated nerve fibers in the sciatic nerve and of the epidermal innervation density in the glabrous skin of the hindpaws. Antibodies to TNF reduced thermal hyperalgesia and mechanical allodynia after CCI. Myelinated fiber density in the sciatic nerve was reduced to 30% of normal on day 7 after surgery, and reached 60% on day 45, with no difference between antibody-treated and untreated animals. Epidermal innervation density as shown by PGP 9.5 and CGRP immunohistochemistry was reduced to 25-47% at both time points after CCI, again without differences between antibody treated and untreated mice. Myelinated fiber density but not epidermal innervation density was correlated to thermal and mechanical withdrawal thresholds. We conclude that neutralization of endoneurial TNF attenuates pain related behavior but has no effect on nerve regeneration. Furthermore, the number of epidermal nerve fibers is not relevant to the magnitude of behavioral hyperalgesia in CCI.