The effect of ischemia on nerve conduction

Provocative changes in nerve conductions: Fact or fiction?

At times electrodiagnostic medical consultants (EMCs) are asked to perform studies in both a neutral position, and then again after the patient is in a provocative position that exacerbates symptoms, to assess for measurable electrophysiologic changes. While this approach might seem initially appealing, particularly when standard studies are not effective at diagnosis, empiric studies in several conditions have been unimpressive. Studies in median neuropathy at the wrist, thoracic outlet syndrome, piriformis syndrome, and radial tunnel syndrome have failed to demonstrate reproducible changes in nerve conduction studies in positions that exacerbate symptoms. Furthermore, there is lack of a plausible pathophysiologic mechanism for producing both measurable and rapidly reversible electrophysiologic changes after just a few minutes, or less, of compression. Axon loss and demyelination would not be rapidly reversible, and positional changes of 2 min or less (the durations generally studied) would be insufficient to produce measurable nerve ischemia. Last, we have gained a greater appreciation for how much nerves move within limbs with changes in joint position; this movement can lead to misleading changes in nerve conduction studies. It is thus appropriate to conclude that testing nerve conduction in provocative or symptomatic positions adds no value to electrodiagnostic testing.

An in Vivo Mouse Model to Investigate the Effect of Local Anesthetic Nanomedicines on Axonal Conduction and Excitability

Peripheral nerve blocks (PNBs) using local anesthetic (LA) are superior to systemic analgesia for management of post-operative pain. An insufficiently short PNB duration following single-shot LA can be optimized by development of extended release formulations among which liposomes have been shown to be the least toxic. In vivo rodent models for PNB have focused primarily on assessing behavioral responses following LA. In a previous study in human volunteers, we found that it is feasible to monitor the effect of LA in vivo by combining conventional conduction studies with nerve excitability studies. Here, we aimed to develop a mouse model where the same neurophysiological techniques can be used to investigate liposomal formulations of LA in vivo. To challenge the validity of the model, we tested the motor PNB following an unilamellar liposomal formulation, filled with the intermediate-duration LA lidocaine. Experiments were carried out in adult transgenic mice with fluorescent axons and with fluorescent tagged liposomes to allow in vivo imaging by probe-based confocal laser endomicroscopy. Recovery of conduction following LA injection at the ankle was monitored by stimulation of the tibial nerve fibers at the sciatic notch and recording of the plantar compound motor action potential (CMAP). We detected a delayed recovery in CMAP amplitude following liposomal lidocaine, without detrimental systemic effects. Furthermore, CMAP threshold-tracking studies of the distal tibial nerve showed that the increased rheobase was associated with a sequence of excitability changes similar to those found following non-encapsulated lidocaine PNB in humans, further supporting the translational value of the model.

Arterial Anatomy of the Posterior Tibial Nerve in the Tarsal Tunnel

BACKGROUND

Both vascular and compression etiologies have been proposed as the source of neurologic symptoms in tarsal tunnel syndrome. Advancing the understanding of the arterial anatomy supplying the posterior tibial nerve (PTN) and its branches may provide insight into the cause of tarsal tunnel symptoms. The purpose of this study was to describe the arterial anatomy of the PTN and its branches.

METHODS

Sixty adult cadaveric lower extremities (thirty previously frozen and thirty fresh specimens) were amputated distal to the knee. The vascular supply to the PTN and its branches was identified, measured, and described macroscopically (the thirty previously frozen specimens, prepared using a formerly described debridement technique) and microscopically (the thirty fresh specimens, processed using the Spälteholz technique).

RESULTS

On both macroscopic and microscopic evaluation, the PTN and the medial and lateral plantar nerves were observed to have multiple entering vessels within the tarsal tunnel. On microscopic evaluation, a vessel was observed to enter the nerve at the bifurcation of the PTN into the medial and lateral plantar nerves in twenty-two (73%) of the thirty specimens. There was a significant difference (p < 0.05) in vascular density between the PTN and each of its branches.

CONCLUSIONS

The abundant blood supply to the PTN and its branches identified in this study is consistent with observations of other peripheral nerves. This rich vascular network may render the PTN and its branches susceptible to nerve compression related to vascular congestion. The combination of vascular and structural compression may also elicit neurologic symptoms.

CLINICAL RELEVANCE

Advancing the understanding of the arterial anatomy supplying the PTN and its branches may provide insight into the cause and treatment of tarsal tunnel syndrome.

Activity attenuates skeletal muscle fiber damage after ischemia and reperfusion

INTRODUCTION

In this investigation we aimed to determine whether: (1) physical activity protects rat skeletal muscle from ischemia/reperfusion (I/R) injury; and (2) continued activity after I/R improves the rate of healing.

METHODS

Rats were divided into sedentary or active (voluntary wheel running) groups. Active rats ran for 4 weeks before I/R or 4 weeks before plus 4 weeks after I/R.

RESULTS

Activity before I/R resulted in 73.2% less muscle damage (Evans blue dye inclusion). Sedentary and active rats had a similar decline in neural-evoked (∼ 99%) and directly stimulated (∼ 70%) in vivo muscle torque, and a similar reduction in junctophilin 1. Active rats produced 19% and 15% greater neural-evoked torque compared with sedentary rats at 14 and 28 days postinjury, respectively, although the rate of recovery appeared similar.

CONCLUSIONS

Activity protects against long-term muscle damage, but not short-term neural injury or excitation-contraction uncoupling. Continued activity neither accelerates nor hinders the rate of functional recovery.

Acute nerve compression and the compound muscle action potential

Detecting acute nerve compression using neurophysiologic studies is an important part of the practice of clinical intra-operative neurophysiology. The goal of this paper was to study the changes in the compound muscle action potential (CMAP) during acute mechanical compression. This is the type of injury most likely to occur during surgery. Thus, understanding the changes in the CMAP during this type of injury will be useful in the detection and prevention using intra-operative neurophysiologic monitoring. The model involved compression of the hamster sciatic nerve over a region of 1.3 mm with pressures up to 2000 mmHg for times on the order of 3 minutes. In this model CMAP amplitude dropped to 50% of its baseline value when a pressure of roughly 1000 mmHg is applied while, at the same time, nerve conduction velocities decline by only 5%. The ability to detect statistically significant changes in the CMAP at low force levels using other descriptors of the CMAP including duration, latency variation, etc alone or in conjunction with amplitude and velocity measures was investigated. However, these other parameters did not allow for earlier detection of significant changes. This study focused on a model in which nerve injury on a short time scale is purely mechanical in origin. It demonstrated that a pure compression injury produced large changes in CMAP amplitude prior to large changes in conduction velocity. On the other hand, ischemic and stretch injuries are associated with larger changes in conduction velocity for a given value of CMAP amplitude reduction.

Generalized peripheral nerve failure during thoracic spine surgery: a case report

OBJECTIVE

Intra-operative systemic changes impairing peripheral nerve function are not commonly detected with electrophysiology. This case presentation illustrates how somatosensory evoked potential (SSEP) monitoring can detect global changes in peripheral nerve excitability during spine surgery.

METHODS

A posterior thoracic spine fixation was performed on a young male with multiple traumatic injuries. Bilateral tibial nerve SSEPs were intraoperatively recorded, along with the right median nerve SSEP for control.

RESULTS

A rapid, progressive loss of tibial and median nerve potentials (followed by cortical SSEP loss) occurred 90 min after anaesthetic induction. Oxygenation and fluid volume were adequate throughout the case, despite mean airway resistance being elevated (33 cmH(2)0) and blood pressure being low (80/45 mmHg). Corresponding to the decrease in peripheral nerve responses was a drop in end-tidal CO(2) partial pressure (PaCO(2)) from 37 to 25 mmHg. Approximately, 100 min later, the peripheral and cortically generated SSEPs recovered in 2 of 3 limbs monitored. On emergence from anesthesia it was clear that the patient had bitten and kinked the endotracheal tube thus increasing the airway resistance. Ventilation difficulties were magnified with the patient's prone position. Post-operatively there were no sensorimotor deficits.

CONCLUSIONS

Somatosensory evoked potential monitoring during spine surgery can detect uncommon generalized nerve conduction block, and further alert surgical teams to a systemic impairment. This was discovered to result from a compromised endotracheal tube. This can apply in various monitoring situations, as the changes affecting the SSEPs were not related to surgical manipulation.

Transient conduction block following acute peripheral nerve ischemia

We studied the effects of transient focal ischemia on the electrophysiologic function of rat sciatic nerves. Focal and generalized impairment of impulse conduction, measured by falling evoked muscle and nerve compound action potential amplitudes, occurred within 10 minutes of femoral artery occlusion. Conduction failure reached a nadir at 45-60 minutes and then improved to normal within 24 hours. Fastest motor and mixed nerve conduction velocities were reduced less than 15% of baseline values during the period of acute conduction block. There were no detectable morphological abnormalities at the site of conduction failure. Transient ischemia produces reversible conduction block without evident structural changes. The fall in amplitude without significant conduction slowing implies that slower conducting myelinated fibers are relatively more sensitive to the effect of acute ischemia.

The use of pulsed electromagnetic fields with complex modulation in the treatment of patients with diabetic polyneuropathy

Clinical and electroneuromyographic studies were performed in 121 patients with diabetic polyneuropathy (DPN) before and after courses of treatment with pulsed electromagnetic fields with complex modulation (PEMF-CM) at different frequencies (100 and 10 Hz). Testing of patients using the TSS and NIS LL scales demonstrated a correlation between the severity and frequency of the main subjective and objective effects of disease and the stage of DPN. The severity of changes in the segmental-peripheral neuromotor apparatus--decreases in muscle bioelectrical activity, the impulse conduction rate along efferent fibers of peripheral nerves, and the amplitude of the maximum M response--depended on the stage of DPN and the duration of diabetes mellitus. The earliest and most significant electroneuromyographic signs of DPN were found to be decreases in the amplitude of the H reflex and the Hmax/Mmax ratio in the muscles of the lower leg. Application of PEMF-CM facilitated regression of the main clinical symptoms of DPN, improved the conductive function of peripheral nerves, improved the state of la afferents, and improved the reflex excitability of functionally diverse motoneurons in the spinal cord. PEMF-CM at 10 Hz was found to have therapeutic efficacy, especially in the initial stages of DPN and in patients with diabetes mellitus for up to 10 years.

Ultrasound therapy facilitates the recovery of acute pressure-induced conduction block of the median nerve in rabbits

Though the use of ultrasound for the treatment of carpal tunnel syndrome (CTS) or compression neuropathy has been described, its effect remains controversial. A test model of acute CTS was developed using rabbits. Acute median nerve compression was induced by the infusion of saline into the carpal tunnel under general anesthesia to elevate the intracarpal pressure. A reduction in the compound muscle action potential (CMAP) amplitude of the abductor pollicis was noted after intracarpal pressure increased. To investigate the efficacy of ultrasound in acute CTS, rabbits with acute median nerve compression were divided into 3 groups (10 each) and ultrasound was applied at different intensities to each group as follows: 1.5 W/cm(2) to group 1; 0.2 W/cm(2) to group 2; 0.0 W/cm(2) (sham) to group 3. A total of 10 treatment sessions were given over a period of 2 weeks. Following ultrasound application, the CMAP amplitudes showed significant improvement in group 1 compared to the other two groups (P < 0.05), indicating facilitated recovery from acute CTS in this pressure-induced median nerve compression rabbit model. The benefits of ultrasound application in a clinical setting must be verified by further clinical trials.

Effects of Tourniquet Ischemia on Current Perception Thresholds in Healthy Volunteers

The neuroselective effects of tourniquet ischemia/compression in healthy volunteers were evaluated using the automated electrodiagnostic sensory Nerve Conduction Threshold (sNCT) test. The sNCT evaluation generates reliable, painless Current Perception Threshold (CPT) measures. Standardized CPT measures using constant alternating current sinusoid waveform stimulus at 3 different frequencies 5 Hz, 250 Hz, and 2 kHz (NeurometerEG CPT/C Neurotron, Inc. Baltimore, MD) were obtained from 10 individuals at baseline and after 5, 10, 15, and 20 minutes of tourniquet ischemia and 30 minutes post-tourniquet release. The data were analyzed to determine the significance of any changes in CPTs. Increases in CPTs after 15 and 20 minutes of tourniquet ischemia at 2000 Hz and 250 Hz reached statistical significance. There were no significant changes in 5 Hz CPT measures. The results of this study demonstrate the ability of the sNCT test to quantify previously described differential neuroselective effects of tourniquet ischemia on sensory nerve function. Demonstration of statistically significant increases in CPT values at 2000 Hz and 250 Hz secondary to tourniquet ischemia, with no change in 5 Hz CPT values, is consistent with the understanding that 2000 Hz sine wave stimuli activate the large myelinated sensory fibers, 250 Hz sine wave stimuli activate small myelinated sensory fibers, and 5 Hz sine wave stimuli activate small unmyelinated sensory fibers.

Transient forearm conduction block in the median nerve

We present two cases referred for electrophysiological confirmation of carpal tunnel syndrome (CTS). Initial nerve conduction studies were normal. Approximately 20 min into the examination, both patients developed sensory symptoms and weakness in the distal median nerve territory while the elbow was extended and forearm supinated. Further studies demonstrated complete conduction block across the forearm in the median motor and sensory nerve fibers. When measurable, conduction velocities remained normal or were modestly slow. Complete clinical and electrophysiological recovery occurred within 2 min following forearm pronation, suggesting that dysfunction was probably due to focal transient ischemia. Patients describing increased sensory symptoms during routine electrophysiological assessments for CTS should be investigated to rule out the possibility of a more proximal abnormality.

Functional degradation of the rabbit sciatic nerve during noncompressive segmental ischemia

The purpose of this study was to evaluate functional degradation in a nerve with a local ischemic segment created without a direct compression effect. Ischemia of one segment of a rabbit sciatic nerve was induced by stripping the nerve's extrinsic blood supply along 15 cm. Blood flow of both in situ and ischemic nerves was quantitatively measured with radioactive microspheres in six serial segments in seven animals. The flow in one middle segment of the stripped nerve was significantly reduced to 0.1 ml/min per 100 g (p = 0.006). In another eight animals, both in situ and stripped nerves were metabolically challenged with repetitive stimuli (200 Hz). Conduction velocity and peak amplitude were measured before stimuli, after 30 and 60 minutes of stimuli, and after a 30-minute recovery period. Conduction velocity was reduced in both nonischemic and stripped nerves during prolonged repetitive stimulation. Peak amplitude was reduced slightly in the nonischemic group and markedly in the stripped group. Normal or higher values were seen again in both groups during the recovery period. It was demonstrated, therefore, that conduction properties of the nerve, especially amplitude, can be affected by localized ischemia in one segment.

Effects of a selective A beta afferent block on the pain-related SEP scalp topography

This study examined the effects of a selective ischemic block of the A beta peripheral afferents on the pain-related somatosensory evoked potential (SEP). Pain descriptions given during the A beta block suggest that the SEP elicited by noxious sural nerve stimulation arises from activity in the A delta and not the C peripheral afferents. The SEP recorded during the A beta block was characterized by a potential whose latency and topographic pattern was very similar to a late pain-related positive potential (SP6) that we have described in previous work. These results provide further evidence for SP6 being generated by brain areas (sources) that receive noxious inputs and make it very unlikely that sources involved exclusively in innocuous somatosensory processes contribute to SP6.

Role of adenosine in postischemic dysfunction of coronary innervation

We sought to determine the role of adenosine in the sustained but reversible decrease in cardiac neurotransmission that occurs after brief ischemia. Adult mongrel dogs were anesthetized and instrumented for measurements of heart rate, arterial pressure, and left anterior descending coronary artery (LAD) and left circumflex coronary artery (LCX) flow velocities. Changes in coronary vascular resistance were measured during bilateral stimulation of the stellate ganglia. After beta-adrenergic blockade and bilateral vagotomy, stellate stimulation increased coronary vascular resistance in the LAD and LCX beds 28 +/- 2% and 30 +/- 3%, respectively. After a 15-minute infusion of adenosine into the LAD, the peak increase in LAD resistance was significantly reduced (18 +/- 2%) compared with LCX (34 +/- 5%) and control (P < .05, n = 6) resistance. The LAD response after infusion of the vasodilator papaverine was unchanged (n = 6). Intracoronary infusion of adenosine deaminase (n = 10) but not vehicle (n = 5) into the LAD during a 15-minute LAD occlusion prevented the attenuation in constriction to stellate stimulation. We conclude that adenosine, exogenously infused or endogenously produced, is capable of reducing cardiac neurotransmission.

Further observations on ischaemic suppression of motor units in human soleus muscle: A single case study

Changes in action potentials (APs) of human soleus motor units (MUs), that occur during ischaemia produced by thigh compression, were studied in five experiments on one subject. In each experiment, a well-isolated AP of a low threshold MU was recorded by a needle electrode beginning with ischaemia onset up to the final MU block. All registered low-threshold MUs responded to Achilles tendon jerks throughout their life cycle. The block of low-threshold MUAps either coincided with or preceded complete tendon reflex loss (min 25 of ischaemia). Low-threshold MUAPs were suppressed in two phases. During the first phase of MUAP slowing, beginning with ischaemia onset up to min 17-22, MUAP amplitude decreased, while MUAP area remained approximately unchanged. During the second phase, MUAP area as well as MUAP amplitude decreased, and in 2-5 min MUAP failed. More ischaemia-resistant high-threshold MUAPs were recorded after the population of low-threshold Mus was blocked. Some high-threshold MUAPs deteriorated similarly to low-threshold MUAPs. Others were blocked abruptly, without any marked changes in shape. It is likely that early extinction of low-threshold MUs contributes to the effect of the stretch reflex suppression during ischaemia. It is suggested that the pattern of MUAP changes during ischaemia in different MUs is related to their degree of aerobic metabolism.

Peripheral nerve function in chronic venous insufficiency

Abnormalities of vasomotion, impairment of the venoarteriolar reflex and increased skin blood flow reported in the liposclerotic skin of patients with chronic venous insufficiency (CVI) suggest altered nervous control of the skin microcirculation. The aim of this study was to determine whether patients with CVI have a peripheral neuropathy. Forty patients with CVI and lipodermatosclerosis (LDS) and 35 age and sex-matched controls were examined for neuropathy using three modalities of testing. Threshold to warming was used to assess unmyelinated fibres, and threshold to cooling and vibration to assess myelinated fibres. Warming and cooling thresholds were measured on the sole of the foot by a purpose built, computer controlled instrument. The threshold to vibration was measured on the big toe using the Ohio Bio-thesiometer. A significantly raised threshold to warming and vibration was found in the CVI group compared to the normal controls [median threshold to warming (interquartile range) in CVI group = 5.3 (0.1-9.1) median threshold to warming (interquartile range) in controls = 1.21 (0.17-3.5), p = 0.005 and median threshold to vibration (interquartile range) in CVI group = 22 (13-31) median threshold to vibration (interquartile range) in controls = 12 (8.5-27.5), p = 0.024]. The thresholds to cooling was not statistically different in the two groups. This study demonstrates the presence of a peripheral neuropathy in patients with chronic venous insufficiency, and this may be important in the pathogenesis of venous ulceration.

Adaptations in gait resulting from unilateral ischaemic block of the leg

The purpose of this study was to develop a major temporary perturbation to the peripheral neuromuscular system and to quantify the motor adaptations on the first gait strides taken after the perturbation becomes effective. A unilateral ischaemic block to the leg was induced by a cuff inflated to 200 mmHg; sensory block was evident after 15-20 min and total motor block after about 30 min. Electromyographs from seven muscles on each limb were recorded for the first 16s after walking commenced and were compared with electromyographs from cadence-matched strides recorded 30 min after the cuff was removed. Ensemble-averaged profiles of the ischaemic and control trials from six subjects showed that almost half of the non-ischaemic muscles (4 on the ischaemic limb and 7 on the intact limb) had significantly different profiles. Only two muscles showed consistent changes for all six subjects (gastrocnemius and rectus femoris of the intact limb).

The role of ischaemia in the analgesia which follows Bier's block technique

The effect of 30-min tourniquet ischaemia (Bier's block) on the antidromic homolateral left median nerve sensory potential (SP) and on the bilateral sympathetic skin response (SSR) was studied in 6 healthy volunteers. The SSR was provoked both acoustically and by electrical stimulation of the median nerve; the latter stimulus was also used to provoke the SP. After 28 min of tourniquet ischaemia, the electrical stimulus failed to provoke the SP and bilateral SSR, indicating blockade of the afferent limb of the reflex. The acoustic SSR was unaffected by ischaemia, and thus the efferent limb of the SSR was not blocked, indicating that ischaemia does not affect the post-ganglionic efferent C fibres. These findings confirm that 30 min of ischaemia blocks A beta afferent fibres but does not block efferent C fibres. Thus the analgesia following Bier's block alone, in some patients with sympathetically maintained pain, most likely results from the ischaemic blockade of sensory A beta fibres, confirmed both acoustically and by electrical stimulation of the median nerve.

Involvement of thin afferents in carpal tunnel syndrome: evaluated quantitatively by argon laser stimulation

The thin afferent nerves were tested quantitatively by determining the thresholds of warmth and pricking pain to argon laser stimulation and by measuring the brain potentials related to pricking pain. In 27 patients with electrophysiologically verified carpal tunnel syndrome these parameters were measured from fingers 3 and 5 on both hands. All patients had had sensory symptoms ranging from 3 months to 25 years. Both the thresholds were elevated (P less than 0.05) at finger 3 compared to measurements from finger 5, and compared to finger 3 in a group of 39 controls. Four patients with symptoms for more than 7 years had thresholds below the control values. The power of the pain-evoked brain potentials elicited from finger 3 was lower (P less than 0.05) compared to finger 5, and compared to the control group (P less than 0.01). No correlations were found between the measured parameters and the clinical electrophysiological investigation. The findings support previous assumptions that chronic low-force compressions cause impairment of intraneural microcirculation, and hence can affect the function of the thin afferents.

Effects of nerve compression or ischaemia on conduction properties of myelinated and non-myelinated nerve fibres. An experimental study in the rabbit common peroneal nerve

Compound action potentials of both myelinated (A) and non-myelinated (C) fibres in the common peroneal nerve of rabbits were studied during and after acute, graded compression of the nerve at 200 or 400 mmHg applied for 2 h or during ischaemia created by nitrogen inhalation or aortic occlusion. Compression of the nerve at 200 mmHg blocked the AI component (large myelinated fibres) after about 23 min, while compression at 400 mmHg shortened this time to 11 min. The A2 component (thinner myelinated fibres) had a lower conduction velocity and a higher resistance to compression. There was just a slight decrease in conduction velocity of the non-myelinated fibres when the nerves were compressed at 200 mmHg for 2 h. However, compression at 400 mmHg for 2 h induced a marked deterioration of amplitude and conduction velocity of the C-fibres. There was an incomplete restitution of function of A- and C-fibres during 2 h of recovery. The thinner myelinated fibres were more susceptible to deprivation of oxygen than the thicker ones, while non-myelinated fibres differed in response according to method of ischaemia induction. It is concluded that non-myelinated fibres are very resistant to compression and a very high pressure (greater than 400 mmHg) is needed to affect these fibres.

The sympathetic skin response: normal values, elucidation of afferent components and application limits

The sympathetic skin response (SSR), recorded at the hand and foot, was elicited using different classes of stimuli in 20 normal controls and 10 patients with peripheral neuropathy. We found that SSR latencies changed significantly with different recording sites, but not with different stimulation sites. Additionally, after ischemic conduction block of the arm in 3 normal controls, the previously obtainable SSR recorded at the hand became unobtainable with median nerve stimulation. Also, in one patient with subacute ganglionitis and 3 patients with demyelinating neuropathies, the SSR could not be elicited by electrical stimulation, but it could with deep inspiration. These results suggest that large diameter myelinated fibers may serve as afferents for the SSR. Furthermore, these findings imply that an unobtainable SSR by electrical stimulation may be due not only to dysfunction of the autonomic efferent nerve fibers, but also to abnormalities of the sensory afferents of the reflex. Therefore, investigations of autonomic dysfunction utilizing the SSR must be interpreted with caution in patients with peripheral neuropathies.

Effect of ischaemia on somatosensory evoked potentials in diabetic patients

The nerve action potential at the elbow and somatosensory evoked potentials (SEPs) at the scalp were recorded over 30 minutes of tourniquet-induced limb ischaemia in 10 diabetic patients and 10 controls. According to the SEP changes, an increased resistance to nerve ischaemia in diabetic patients was observed. The pathways involved in SEP conduction are discussed.

Effects of ischemic nerve block on the early and late components of the stretch reflex in the human forearm

To investigate the peripheral afferent mechanisms which mediate the short latency (M1) and long latency (M2) components of the stretch reflex, we have recorded EMG responses to extension perturbations at the wrist before and following inflation of a blood pressure cuff around the upper arm. After approximately 30 min of ischemia, the reflexes had almost completely disappeared, although subjects were still able to activate the forearm muscles voluntarily. As the ischemic block developed, the M2 component began to decrease in amplitude sooner than M1, and up to the time of complete loss of reflex activity, the suppression of M2 was relatively greater than that of M1. These results suggest that M2 may be mediated by a population of afferent fibers different from the Ia afferents which are believed to generate the M1 response.

Transient focal conduction block following experimental occlusion of the vasa nervorum

Injection of low-dose arachidonic acid into the rat femoral artery occludes the vasa nervorum of the tibial nerve and produces focal and generalized ischemia with transient effects on nerve conduction. Across a severely ischemic segment of the proximal tibial nerve there is a marked fall in amplitude of the compound muscle action potential (CMAP), indicating focal conduction block. There is also significant slowing of maximal motor conduction velocity (MCV) through this nerve segment, but no dispersion of the proximally elicited response. Distally, in a region of less severe ischemia, there is mild slowing of MCV, but no further decrement in the CMAP amplitude. The conduction block begins 5-15 minutes after injection, reaches a nadir at 30 minutes, and persists in more severe cases for at least 2 hours. Despite these prolonged electrophysiologic abnormalities, there is no evidence of axonal degeneration or segmental demyelination.

Median nerve evoked potential changes in an acute carpal tunnel syndrome model in Macaca mulatta

We devised an acute, carpal tunnel syndrome (CTS) model in rhesus monkey, Macaca mulatta. This model was used to observe the relationship between increases in compartmental pressure and nerve function as reflected in the electrophysiologically generated, averaged evoked potential recorded from the skin superficial to the median nerve at the elbow. Two waves, an A beta and A delta were monitored. Increased carpal tunnel pressure produced a decrease in the A beta wave amplitude which was occasionally preceded by a slight elevation, but which eventually resulted in A beta wave abolition with pressures greater than or equal to 31 mm Hg. The A delta wave showed an initial increase in amplitude followed by a decline and abolition. Recovery rate from increased pressure for the A beta wave was directly proportional to the rate of abolition of that wave. The implications of these data are discussed in the context of CTS neuropathology and diagnosis.

Selective fiber vulnerability in acute ischemic neuropathy

To study the relationship between fiber size and vulnerability to ischemia, we produced partial infarction of the proximal posterior tibial nerve of Wistar rats by injecting arachidonic acid into the ipsilateral femoral artery. In the resulting lesion degenerating fibers were concentrated in the center of the nerve. Distal to the infarct, surviving myelinated and unmyelinated fibers were evenly distributed throughout both fascicles. Smaller myelinated fibers (less than 6 micrometers in diameter) were more severely depleted than larger ones ( greater than 6 micrometer). Unmyelinated fibers were reduced in number in proportion to or to a greater extent than myelinated fibers. These findings demonstrate that large myelinated fibers are relatively less vulnerable to peripheral nerve ischemia than smaller fibers, and do not support the contention that unmyelinated fibers are inherently resistant to acute ischemia.

Analysis of the electrical muscle activity during maximal contraction and the influence of ischaemia

(1) The mechanism underlying muscle fatigue has been studied in maintained isometric maximal contraction of the wrist flexor muscles under normal and ischaemic conditions. Automatic EMG analysis has been used to show the level of motor unit firing rates in fatiguing contractions. (2) Under non-ischaemic conditions the decay of force, turns and amplitude is about the same, whereas during ischaemia force and to a lesser extent amplitude pulses, decline steeply towards zero, while turns, representing the number of impulses, remain in the non-ischaemic range. (3) Depending on the duration of the ischaemia applied before contraction, force and amplitude are initially reduced but turns are nearly unchanged compared with the non-ischaemic values. It is suggested, that this is due to nerve blocking of high threshold motor units. (4) The results show that transmission failure at the neuromuscular junction is a minor factor in muscle fatigue and that this structure is not greatly affected by ischaemia. (5) It is believed that in the first phase of muscle fatigue the force decline is connnected with a slowing of discharge rates. This change of firing frequencies with time must be considered optimal in respect to the force produced because higher as well as lower discharge rates would reduce the force development. In the later phase it is possible that contractile element fatigue, connected with a reduction of action potential amplitudes of single muscle fibres, predominates, especially when the blood supply is obstructed.

Effect of differential nerve block on inhibition of the monosynaptic reflex by vibration in man

The differential nerve block produced by ischaemia has been used in an attempt to identify the afferent nerve fibres responsible for vibratory inhibition of the monosynaptic reflex in man. It is concluded that the inhibition arises mainly from receptors in the lower leg and is carried by myelinated afferent fibres larger than A-delta.

Peripheral nerve injury and recovery after temporary ischemia

Nerve (NCV) and motor (MNCV) conduction velocities of the rat sciatic nerve were examined between 1 and 90 days after ischemia for 1, 2, 3, 4 or 6 h. The results were compared to light and electron microscopy of the nerve. Slight diminution in the MNCV was observed 1 day after 1-2 h ischemia, whereas 3-6 h ischemia resulted in a complete conduction block. Diminution in the NCV occurred first after ischemia for 2 h and a complete block was seen after 4 and 6 h ischemia. Reduced NCV and MNCV were seen between 4 and 18 days only in the animals subjected to ischemia of longer duration of 3-6 h. Both the NCV and MNCV were nearly normalized at the 90th day. Ischemia of 4 and 6 h resulted in denervation of some of the muscle fibers, seen as spontaneous fibrillation at the 4th and 18th day. Electron microscopy and histometric studies showed degeneration of myelinated fibers increasingly after longer durations of ischemia; ischemia for 2 h caused a degeneration of about 5%, 3 h of about 35%, 4 h about 45%, and 6 h about 75% of the fibers. Myelinated fibers of different sizes were equally damaged. In the teased fiber preparations normal and myelin sheaths undergoing Wallerian-like degeneration was seen. Regeneration occurred, but even at the 90th day there was a tendency of the myelin/axon ratio towards values less than control values.

Effect of ischaemia on sensory potentials of normal subjects of different ages

In 73 normal subjects, from 10 to 82 years of age, maximum orthodromic sensory nerve conduction velocity was measured in the median nerve before and during a 30 minute period of vascular occlusion. During ischaemia progressive slowing in conduction velocity, decrease in amplitude, and increase in duration of the sensory action potential evoked at wrist and elbow by supramaximal stimulation of digit III were observed. However, a statistically significant difference (P<0·05 to P<0·01) between subjects was noted by grouping them by age: the older the subject, the longer the persistence of sensory response and the less marked the slowing in conduction velocity. The mechanism of the phenomenon has been discussed in relation to a similar longer resistance to ischaemia found in peripheral nerves of diabetic and uraemic patients.

Padrões de condução de impulsos nervosos, determinados eletronicamente, para análise de comportamento de nervos "in vivo"

Padrões de condução de impulsos nervosos foram determinados eletrônicamente, in vivo, nos nervos ulnar (13), hipoglosso (16) e cutâneo medial do braço (7) de cães anestesiados. Os traçados obtidos confirmaram, em linhas gerais, os descritos na literatura. Todavia, demonstraram um fato novo, adicional, isto é, que fibras motoras e sensitivas, integrantes de nervos periféricos, apresentam uma condução específica, unidirecional (centrífuga nas fibras motoras e centrípeta nas sensitivas), desde que mantidas praticamente intactas em seus respectivos leitos conectivos naturais. Embora este tipo de condução específica corresponda ao que se admite e aceita como comportamento normal, biológico, dessas fibras nervosas, e constitua ainda, base do exame neurológico, esta evidência nunca foi demonstrada anteriormente em experiências e preparações neurofisiológicas.