Conductivity of dorsal column fibers during experimental spinal cord compression and after decompression at various stimulus frequencies

Use of enhanced stimulation voltage to determine the severity of compressive peripheral nerve injury

BACKGROUND

The aim of this study was to investigate whether enhanced stimulation voltage could be a predictor of the extent of injury in acute compressive peripheral nerve trauma.

METHODS

The femoral nerves were exposed on both sides, in 11 anesthetized rabbits. Supramaximal stimulation voltage was used to produce a maximal-amplitude compound muscle action potential (CMAP) from the quadriceps femoris muscle. Afterward, the left femoral nerve was clipped for 1 minute, and the right femoral nerve for 5 minutes to produce an acute compressive injury. Immediately after removal of the clip, the proximal and distal sides of the clippage site were stimulated by gradually increased voltage until CMAPs were obtained. The same procedure was repeated at the 30th and 60th minutes. The ratio of the CMAP amplitudes obtained from proximal and distal stimulation was measured to establish a classification.

RESULTS

The stimulation voltages and amplitudes of the CMAPs before clippage were similar with the after-clippage values obtained from distal stimulation (p > 0.05), but the after-clippage values obtained from proximal stimulation were different in both sides (p < 0.05). Doubled stimulation voltage was enough to obtain CMAPs on the left side, but eightfold the initial level was required on the right side. The amplitude ratios recovered to preinjury levels in all of the subjects on the left side, but only two showed recovery on the right side (p < 0.001). Histopathologically, there was axonal compression without discontinuity on the left side, whereas the fibers were dispersed on the right side.

CONCLUSION

Stimulation voltage was found to discriminate the severity of the lesion in experimental peripheral nerve injury. Proximal to distal amplitude ratio seems to be a prognostic factor when the injury is less severe.

Changes in axonal physiology and morphology after chronic compressive injury of the rat thoracic spinal cord

The spinal cord is rarely transected after spinal cord injury. Dysfunction of surviving axons, which traverse the site of spinal cord injury, appears to contribute to post-traumatic neurological deficits, although the underlying mechanisms remain unclear. The subpial rim frequently contains thinly myelinated axons which appear to conduct signals abnormally, although it is uncertain whether this truly reflects maladaptive alterations in conduction properties of injured axons during the chronic phase of spinal cord injury or whether this is merely the result of the selective survival of a subpopulation of axons. In the present study, we examined the changes in axonal conduction properties after chronic clip compression injury of the rat thoracic spinal cord, using the sucrose gap technique and quantitatively examined changes in the morphological and ultrastructural features of injured axonal fibers in order to clarify these issues. Chronically injured dorsal columns had a markedly reduced compound action potential amplitude (8.3% of control) and exhibited significantly reduced excitability. Other dysfunctional conduction properties of injured axons included a slower population conduction velocity, a longer refractory period and a greater degree of high-frequency conduction block at 200 Hz. Light microscopic and ultrastructural analysis showed numerous axons with abnormally thin myelin sheaths as well as unmyelinated axons in the injured spinal cord. The ventral column showed a reduced median axonal diameter and the lateral and dorsal columns showed increased median diameters, with evidence of abnormally large swollen axons. Plots of axonal diameter versus myelination ratio showed that post-injury, dorsal column axons of all diameters had thinner myelin sheaths. Noninjured dorsal column axons had a median myelination ratio (1.56) which was within the optimal range (1.43-1.67) for axonal conduction, whereas injured dorsal column axons had a median myelination ratio (1.33) below the optimal value. These data suggest that maladaptive alterations occur postinjury to myelin sheath thickness which reduce the efficiency of axonal signal transmission.In conclusion, chronically injured dorsal column axons show physiological evidence of dysfunction and morphological changes in axonal diameter and reduced myelination ratio. These maladaptive alterations to injured axons, including decrease in myelin thickness and the appearance of axonal swellings, contribute to the decreased excitability of chronically injured axons. These results further clarify the mechanisms underlying neurological dysfunction after chronic neurotrauma and have significant implications regarding approaches to augment neural repair and regeneration.

4-Aminopyridine enhances motor evoked potentials following graded spinal cord compression injury in rats

Although several experimental and clinical studies have demonstrated the ability of 4-aminopyridine (4-AP) to restore electrophysiological and/or behavioral function following chronic spinal cord injury, the mechanism by which this occurs remains unclear. Demonstration of efficacy in rat spinal cord injury has not been reported, evidently because even relatively mild spinal cord contusions that produce only minor permanent locomotor disturbances abolish hind limb myoelectric motor evoked potentials (mMEPs). In this study, mMEPs were recorded acutely 25 days following graded thoracic spinal cord compression in rats. mMEP amplitudes were significantly enhanced by a single, 2 mg/kg i.v. dose of 4-AP. mMEPs were increased in all rats showing some evoked responses initially, and also in some animals which had no responses prior to treatment. 4-AP was further found to increase the maximum following frequency of mMEPs in both normal and injured rats from about 0.1 Hz to between 1 and 10 Hz. These data suggest that 4-AP might act by enhancing synaptic efficacy, as well as enhancing conduction in spinal axons whose myelination has been rendered dysfunctional by trauma.

Assessment of axonal dysfunction in an in vitro model of acute compressive injury to adult rat spinal cord axons

An in vitro model of spinal cord injury was developed to study the pathophysiology of posttraumatic axonal dysfunction. A 25 mm length of thoracic spinal cord was removed from the adult male rat (n = 27). A dorsal column segment was isolated and pinned in a recording chamber and superfused with oxygenated (95% O2/5% CO2) Ringer. The cord was stimulated with a bipolar electrode, while two point responses were recorded extracellularly. Injury was accomplished by compression with a modified aneurysm clip which applied a 2 g force for 15 s. With injury the compound action potential (CAP) amplitude decreased to 53.7 +/- 5.4% (P < 0.001), while the latency increased to 115.6 +/- 3.1% (P < 0.0025) of control values. The absolute refractory period increased with injury from 1.7 +/- 0.1 ms to 2.1 +/- 0.1 ms (P < 0.05). The infusion of 5 mM 4-aminopyridine (4-AP), a blocker of voltage-sensitive 'fast' K channels confined to internodal regions, resulted in broadening of the CAP of injured axons to 114.9 +/- 3.1% of control (P < 0.05). Ultrastructural analysis of the injured dorsal column segments revealed marked axonal and myelin pathology, including considerable myelin disruption. In conclusion, we have developed and characterized an in vitro model of mammalian spinal cord injury which simulates many of the features of in vivo trauma. Injured axons display characteristic changes in physiological function including a shift in refractory period and high frequency conduction failure. The ultrastructural data and response of injured axons to 4-AP suggest that myelin disruption with exposure of 'fast' K+ channels contributes to posttraumatic axonal dysfunction.

Effects of induced hypothermia on somatosensory evoked potentials in patients with chronic spinal cord injury

We have investigated the effects of mild whole body hypothermia on the amplitude and latency of somatosensory evoked potentials (SEPs) in control subjects (n = 12) and patients (n = 15) with chronic compressive or contusive spinal cord injury (SCI). Mild hypothermia (-1 degree C) was induced by controlled circulation of propylene glycol through a 'microclimate' head and vest garment while reductions in oral and limb temperatures were monitored. Cooling induced a delayed onset and reduced amplitude of tibial nerve SEPs in control subjects. All SCI patients with recordable SEPs (n = 11) showed similarly delayed onset of the cortical response. In contrast to the controls, nine of the 11 SCI patients showed an increase in amplitude of cortical SEPs. In three of these patients the increase in amplitude exceeded 100% of the precooling values. The cooling-induced changes in SEP amplitude and latency reversed on rewarming for both groups. The cooling-induced increases in cortical SEP amplitude support the a priori hypothesis that cooling would enhance central conduction in some SCI patients with conduction deficits due to focal demyelination.

Randomized double pulse stimulation for assessing stimulus frequency-dependent conduction in injured spinal and peripheral axons

Injury compromises the ability of axons to conduct action potentials at high frequencies. To study stimulus frequency-dependent conduction in injured spinal and peripheral axons, we developed a new stimulation paradigm which applies trains of double pulses at 5 Hz and randomly varied interpulse intervals of 3, 4, 5, 8, 10, 30, 50, and 80 msec. In each double pulse, the first pulse was used to condition the response activated by the second test pulse. Responses elicited by double pulses with 80 msec intervals served as controls. The L5 dorsal root was stimulated to activate dorsal column and dorsal root compound action potentials in pentobarbital anesthetized rats. To injure the spinal cord, we compressed the cord stepwise (0.25 mm every 5 min) until action potential conduction across the compression site was abolished and then decompressed the spinal cord 10 min later. Before injury, conditioning pulses applied 3-80 msec before the test pulses did not alter dorsal column responses except for a slight amplitude augmentation at 20 msec interpulse intervals (mean +/- S. E., + 4.2 +/- 0.8%, P less than 0.02) compared to controls. Injury had 3 effects on the responses. First, it significantly reduced response amplitudes and increased response latencies at 3-5 msec interpulse intervals, i.e., responses activated with 3 msec intervals were 26.0 +/- 7.4% (P less than 0.002, paired t test, n = 6) smaller and had 108 +/- 45 microseconds (P less than 0.04) longer latency than control responses. Second, response amplitude increases at 20 msec interpulse intervals (9.0 +/- 0.7%, P less than 0.0001) significantly exceeded those observed before injury (P less than 0.02, paired t test). Third, injury accentuated response amplitude declines during the stimulus train, most prominently at 80 msec intervals. Spinal cord injury did not affect the dorsal root responses. L5 root compression injury depressed dorsal root action potentials at 3-5 msec interpulse intervals (36.9 +/- 8.4%, n = 4, P less than 0.0001) but had no other effect on the responses. Our data indicate that randomized double pulse evoked potentials are sensitive detectors of acute axonal dysfunction and can be used to quantify stimulus frequency-dependent conduction deficits in injured central and peripheral axons.