Differential effects of in vitro heating on rat sciatic nerve branches and spinal nerve roots

Selective neural inhibition via photobiomodulation alleviates behavioral hypersensitivity associated with small sensory fiber activation

OBJECTIVE

Photobiomodulation at higher irradiances has great potential as a pain-alleviating method that selectively inhibits small diameter nerve fibers and corresponding sensory experiences, such as nociception and heat sensation. The longevity and magnitude of these effects as a function of laser irradiation parameters at the nerve was explored.

METHODS

In a rodent chronic pain model (spared nerve injury-SNI), light was applied directly at the sural nerve with four delivery schemes: two irradiance levels (7.64 and 2.55 W/cm2 ) for two durations each, corresponding to either 4.8 or 14.4 J total energy, and the effect on sensory hypersensitivities was evaluated.

RESULTS

At emitter irradiances of 7.64 W/cm2 (for 240 s), 2.55 W/cm2 (for 720 s), and 7.64 W/cm2 (for 80 s) the heat hypersensitivity was relieved the day following photobiomodulation (PBM) treatment by 37 ± 8.1% (statistically significant, p < 0.001), 26% ± 6% (p = 0.072), and 28 ± 6.1% (statistically significant, p = 0.032), respectively, and all three treatments reduced the hypersensitivity over the course of the experiment (13 days) at a statistically significant level (mixed-design analysis of variance, p < 0.05). The increases in tissue temperature (5.3 ± 1.0 and 1.3 ± 0.4°C from 33.3°C for the higher and lower power densities, respectively) at the neural target were well below those typically associated with permanent action potential disruption.

CONCLUSIONS

The data from this study support the use of direct PBM on nerves of interest to reduce sensitivities associated with small-diameter fiber activity.

Massive Lumbar Disk Herniation Following "Therapeutic" Water Boiling of the Lower Extremities: Case Report and Literature Review

BACKGROUND

Legs burning for treating lumbar radicular pain are still in use nowadays in low socioeconomical environments. They are dangerous as the case we report shows clearly.

CLINICAL CASE

A 49-year-old man came to our attention with severe flaccid paraparesis occurred 10 days before, almost immediately after he had immersed his legs in boiling water to treat his severe left lumbocrural pain. This was known to be due to a right L3/4 herniated disk diagnosed by magnetic resonance imaging. At the examination he showed severe motor paresis and absent reflexes of his lower limbs, while crural pain was mild and sensation and urinary function were unaffected. The results of his neurologic examination led us to suspect an acute motor axon degeneration related to thermal shock. Lumbar magnetic resonance imaging, performed before the planned electromyogram as an exception to the established routine, showed instead a giant 5- × 5.5-cm, herniated disk compressing the dural sac at L3.

CONCLUSIONS

Prompt surgical decompression led to rapid improvement. We discuss here the pathophysiology of this unusual case and point out how medieval practices for treating sciatica-like pain are not only unjustified from a medical viewpoint but also potentially dangerous.

Bone temperature elevation by drilling friction and neurological outcome in the cervical spino-laminoplasty

BACKGROUND

Drill-induced heat has been suspected as a cause of tissue injury, and there are ample experimental data to substantiate the implication. However, no clinical results have been presented with measurement of temperature in the vicinity of neural structures during the actual spinal procedures.

METHODS

Using a thermocouple, temperature in the gutters drilled in the midline and the lateral margins of the lamina was monitored closely in 61 patients, who underwent the French-door style of cervical spino-laminoplasty. The drilling was performed intermittently for a duration of 5 or 10 seconds, using 3- or 5-mm diamond burrs with sufficient continuous cooling irrigation. The correlations between bone temperature elevation and postoperative sensorimotor symptoms were then analyzed.

RESULTS

In the lateral gutters at the most cephalad level (typically C3), where the drilling was performed underneath an overlying bundle of muscle attached to C2's spinous process, the temperature rose significantly. This occurred even with 5-second drilling sessions. The average peak temperature was 44 °C at this level. At all other sites, the temperature was maintained below 40 °C. In three patients, transient neurological deficit developed postoperatively, which did not correlate with the incidence of bone temperature elevation.

CONCLUSIONS

Intermittent drilling with sufficient irrigation can prevent thermal neuronal damage generated by high-speed drills. Drilling with small diamond burrs in deep and narrow spaces covered by overlying muscles predisposes to inadequate irrigation and thermal elevation. Continuous, protracted drilling without frequent irrigation may result in excessive heat generation and nerve injury.

Influence of intraoperative neurophysiologic monitoring on the development of surgical dissection techniques

Intraoperative neurophysiologic monitoring (IONM) is essential in the preservation of function of nervous system. IONM is thus becoming the gold-standard method in nerve-sparing surgical procedures. Apart from spine and brain surgery, IONM is essential in significantly reducing morbidity in colorectal surgery, prostate and thyroid surgery, as well as in hip replacement, to name a few. IONM measures weak electric nerve potentials and, therefore, it is easily disturbed by other electromagnetic sources. Surgical dissection techniques and devices interfering with IONM make this technique useless because this is dissection that mainly endangers nerve structures. Therefore, there is a need to take into consideration the influence of various dissection techniques on IONM, and to develop or modify inert techniques that are currently not widely used.

Effects of hyperthermia on the peripheral nervous system: a review

The present paper overviews the current knowledge about effects of hyperthermia at temperatures used in clinical oncology on the peripheral nervous system. From the experimental studies it may be concluded that the heat sensitivity of the nerve is determined by the sensitivity of the nerve vasculature. These studies show that in order to avoid induction of severe neuropathy, application of heat to the peripheral nerves should not be in excess of doses of 30 min at 44 degrees C or equivalent. Using modern equipment for application of loco-regional hyperthermia the incidence of even mild neurological complications is very low. In hyperthermic isolated limb perfusion (HILP) neurotoxicity is an often-mentioned side effect, this is in spite of the fact that in all studies a relatively mild hyperthermic temperature is used that, based on the experimental studies, should be well tolerated by the nerves and other normal tissues in the limbs. It seems that the neurotoxicity observed after HILP results from thermal enhancement of drug toxicity, very probably combined with effects of a high tourniquet pressure that is used to isolate the blood flow in the leg. Whole body hyperthermia (WBH), using anesthesia and appropriate monitoring to avoid cardiovascular stress is at present considered a safe procedure. Still in the recent past cases of neuropathy after treatment have been described. When chemotherapy, and notably cisplatin, is administered before or during hyperthermia there are several clinical and experimental observations that indicate a limited tolerance of the peripheral nervous tissue in such case. Also previous radiotherapy may limit the tolerance of nerves to hyperthermia, notably when radiation is applied with a large field size. Experimental studies show that combined treatment with radiation and heat leads to enhancement of effects of radiation (enhancement ratio approximately 1.5 at 60 min at 44 degrees C). A clear contraindication for the application of hyperthermia in patients is the presence of a neurodegenerative disease, such as multiple sclerosis. Vigilance is also required in the treatment of diabetic patients with hyperthermia, this based on experimental animal studies, but so far no clear clinical data are available.

Effects of temperature on neuromuscular electrophysiology

Like nearly all biologic structures, the peripheral nervous system is remarkably temperature sensitive. Clinical neurophysiologists are most aware of the untoward effects of cooling on nerve conduction studies, including reduced conduction velocity, prolonged distal latency, and increased response amplitude and duration. However, familiarity with the effects of temperature variation on the peripheral nervous system can also provide a deeper understanding of the physiological mechanisms underlying the function of nerve, muscle, and neuromuscular junction in health and disease. Intentional temperature alteration can also improve the diagnostic accuracy of certain electrophysiologic tests, such as the use of heat when performing repetitive nerve stimulation in myasthenia gravis or the use of cold during needle electromyography in some of the myotonic disorders. Finally, extremes of temperature have long been known to produce permanent neuronal dysfunction; recent investigations are beginning to elucidate the mechanisms of such injury.

Noncontusive segmental spinal cord injury using radiofrequency current

OBJECTIVE

To develop a nondisruptive model for the study of spinal cord injury.

METHODS

A 2-MHz radiofrequency heating chamber was mounted over the rat T13-L1 vertebral column via a short dorsal incision. Epidural temperature at chamber level was monitored via a small proximal laminotomy. Forty-three rats were studied using time-temperature heating regimens from 45 to 48.5 degrees C and 4 to 15 minutes. A blinded numerical hind limb impairment score (Neurologic Impairment Score) was determined at intervals up to 2 weeks after injury. Segmental spinal cord blood flow was measured using [14C]butanol tissue uptake in injured and control rats.

RESULTS

Above the injury threshold, increasing the time-temperature regimens was associated with a progressively worse Neurologic Impairment Score (r = 0.73-0.87 up to 24 hours after injury). Cord blood flow was unchanged at 2 hours but was 44% depressed at the injury level 6 hours after injury (p < 0.01). Histologically, injury extended minimally beyond the injured segment. Vascular thrombosis was not seen.

CONCLUSION

This comparatively noninvasive model does not mechanically disrupt cord components and results in progressive neurologic impairment that correlates with the time-temperature regimen used for injury. It should be useful in identifying secondary phenomena that worsen functional status after cord trauma.

Hyperthermia-induced damage to rat sciatic nerve assessed in vivo with functional methods and with electrophysiology

A 5-mm segment of the rat sciatic nerve was treated in vivo with hyperthermia (43-45 degrees C) for different times using a brass thermode. The effect of this local heat treatment on the nerve was assessed with electrophysiology and using two functional assays. Hyperthermia led to a dose-dependent decrease of motor and sensory function. Electrophysiological examination showed a decrease in amplitude of motor and reflex responses rather than a decrease in conduction velocities. Calculated ED50 values were not significantly different for the two functional and for the electrophysiological methods. Functional recovery from nerve damage took place in all cases. Measured at the same level of damage, i.e., 50% function loss, it took 14 days to recover from complete sensory function loss and 20 days from complete motor function loss. Although both motor and sensory functions were restored, 30 days after hyperthermia no responses could be detected with electrophysiology, this as a result of the thin myelin sheaths that occur upon recovery.

Hyperthermic injury versus crush injury in the rat sciatic nerve: a comparative functional, histopathological and morphometrical study

Functional and morphological changes of the rat sciatic nerve after local hyperthermia (30 min, 45 degrees C) and crush treatment were compared. After hyperthermic injury nerve function loss developed in a time period of about 7 h. Nerve crush led to an immediate loss of nerve function. Nerve function loss was assessed by a motor and a sensory function test. Recovery from function loss took place in both treatment groups and was complete in 4-5 weeks. Early (within 8 h post-treatment) histopathological changes in the nerve after heating included edema, possible blood stasis and changes in the blood vessel wall, like swelling of the media. During this period some axonal changes were observed. Immediate after crushing axons were severely damaged, while many blood vessels remained normal. Within one week after both treatments, degeneration of axons and myelin was observed at the site and distal from the site of the lesion (Wallerian degeneration). Three weeks after treatment a major part of the axons had regenerated and remyelinated. Vascular changes at the site of lesion could still be observed in the heat-treated nerves. Twelve weeks after both treatments, blood vessels appeared to be normal again. Morphometrical analysis of the treated nerves confirmed the histological observations. Three and 12 weeks after treatment average axon diameters were significant smaller and average myelin sheaths were significant thinner compared to untreated nerves. These parameters did not differ significantly when the two treatment groups were compared.

Effects of 4-aminopyridine on protein phosphorylation in heat-blocked peripheral nerve

The topical application of 4-aminopyridine (4-AP) reverses within 1-3 min the partial conduction block that results from heating 5-mm segments of rat posterior tibial, peroneal and sural nerves to 45 degrees C for several minutes. Nerves previously blocked in vitro or in vivo were incubated in vitro in the presence of [gamma-32P]ATP. The rate of phosphorylation of soluble nerve proteins that had entered the media was determined. Labeled proteins were separated electrophoretically and autoradiograms prepared. We found that 5 mM 4-AP increases the phosphorylation rate in heat-blocked nerve by approximately 50-fold. The process is calcium-dependent and is heat-labile. Soluble proteins with a molecular weight in the 53-55 kDa range are preferentially but not exclusively phosphorylated in the presence of 4-AP at levels effective in restoring conduction. The results suggest that the potassium channel blocker 4-AP may restore conduction by inducing changes in channel proteins.

Selective decrease of small sensory neurons in lumbar dorsal root ganglia labeled with horseradish peroxidase after ND:YAG laser irradiation of the tibial nerve in the rat

Recent electrophysiological evidence indicates that Q-switched Nd:YAG laser irradiation might have selective effects on neural impulse transmission in small slow conducting sensory nerve fibers as compared to large diameter afferents. In an attempt to clarify the ultimate fate of sensory neurons after laser application to their peripheral axons, we have used horseradish peroxidase (HRP) as a cell marker to retrogradely label sensory neurons innervating the distal hindlimb in the rat. Pulsed Nd:YAG laser light was applied to the tibial nerve at pulse energies of 70 or 80 mJ/pulse for 5 min in experimental rats. Seven days later HRP was applied to the left (laser-treated) and to the contralateral (untreated) tibial nerve proximal to the site of laser irradiation. In control animals the numbers of HRP-labeled dorsal root ganglion cells were not significantly different between the right and the left side. In contrast, after previous laser irradiation labeling was always less on the laser-treated side (2183 +/- 513 cells, mean +/- SEM) as compared to the untreated side (3937 +/- 225). Analysis of the dimensions of labeled cells suggested that the reduction of labeled cells on the laser-treated side was mainly due to a deficit in small sensory neurons. Since the conduction velocity of nerve fibers is related to the size of their somata, our histological data imply that laser light selectively affects retrograde transport mechanisms for HRP in slow conducting sensory nerve fibers.

Effects of local hyperthermia on the motor function of the rat sciatic nerve

The effect of local heat treatment of the sciatic nerve was assessed using the toe-spreading test, which mainly assesses the motor function of the sciatic nerve. A 5 mm long segment of the nerve was heated at temperatures from 42.0 to 45.0 degrees C in vivo using a brass thermode. Hyperthermia led to a decrease in spreading of the toes. Recovery from functional loss took place in all cases, and this recovery was completed in 4 weeks. A 50 per cent functional loss in 50 per cent of the treated animals was observed after 58, 32 and 12 min of heating at 43.0, 44.0 and 45.0 degrees C respectively.

Sciatic nerve function following hindlimb thermal injury

Peripheral neuropathy occurs in approximately 20% of patients with major burns and seriously impairs rehabilitation. We describe an experimental model which permits elevation of the tissue temperature in the region of the distal sciatic nerve trunk of rats at a reproducible rate to a predetermined level without inflicting concomitant major cutaneous injury. Radiofrequency current is delivered through parallel copper electrodes mounted in a chamber into which the limb has been inserted. In the present experiments, tissue temperature was arbitrarily elevated to 47 degrees C for 30 sec in 62 rats. There were 43 normal controls. The posterior tibial branch was the most intensively studied, as some of its conduction characteristics can be serially assessed percutaneously. Conduction block, which was apparently irreversible, was present in 67% of posterior tibials by 24 hr postinjury. In branches which were still excitable, prolongation of the absolute refractory period was the most consistent abnormality noted. Slowing of conduction, as evidenced by prolongation of inflection velocity or peak velocity, was never observed. However, this injury resulted in selective conduction failure of sural--but not of peroneal--fibers which conducted at 40 m/sec or greater. Fiber modality is an important determinant of the vulnerability to direct thermal injury of peripheral nerve in vivo.

Potassium ion channel blockade restores conduction in heat-injured nerve and spinal nerve roots

Compound action potentials were recorded in vitro from rat peroneal and sural nerves and from dorsal and ventral roots of the cauda equina before and after radiofrequency heating of 5-mm-length segments of these nerves to 41 to 45 degrees C. The heating was continued for intervals sufficient to reduce response amplitude by 50%. Inflection velocity, potential duration at 1/2 peak height, and the proportion of conducting A alpha fibers were also measured. The topical application of 4-aminopyridine (4-AP) and tetraethylammonium chloride (TEA) to the previously heated segments immediately following the radiofrequency injury completely or near-completely restored amplitude height to the preheat value in all experiments. A alpha sensory fibers were the most susceptible to the conduction block. Conduction in these fibers was also the most readily restored by the application of 4-AP or TEA. The effects of TEA, but not of 4-AP, could be reversed by saline or buffer washing. Topical application of verapamil and of magnesium or calcium ions had no discernible effect on heated nerves. We suggest that the mechanism of heat-induced conduction block may be similar to that from early demyelination or stretch injury. Further, motor and sensory A alpha fibers differ both in their vulnerability to heat and in their subsequent response to the application of potassium channel blockers.