Quantitative sensory testing of cold and vibration perception during compression of median nerve at the wrist

We conducted a sequential study of quantitative sensory testing (QST) during compression-induced conduction block of the median nerve to determine relative vulnerability of the small and large myelinated nerve fibers. We tested cold (CPT) and vibratory perception thresholds (VPT) of the third digit in 15 healthy subjects during constant, localized compression for 30 min of the median nerve at the wrist. The orthodromic sensory nerve action potentials (SNAPs) recorded at wrist and elbow served to monitor the degree of associated conduction block. After the onset of nerve compression, it took 16 min for CPT to show the first change; VPT remained normal for 26 min. CPT recovered 2 min later than VPT after release of compression. The SNAP amplitude at the wrist diminished immediately at the start of compression and declined progressively, whereas the response at the elbow remained the same initially, showing no latency change for 20 min. A nearly identical time course of SNAP changes in the two experiments justified the comparison of separately tested CPT and VPT as a measure of modality-specific vulnerability. Contrary to the common belief, a focal compression sufficient to produce rapidly reversible conduction abnormalities affects the slow-conducting small myelinated fibers mediating cold perception before the fast-conducting large myelinated fibers transmitting vibration perception. The data document the order of modality-specific vulnerability of sensory nerve fibers to mild compression. The finding suggests that testing CPT, rather than VPT, provides a better QST to delineate rapidly reversible symptoms induced by compression.

Persistent multifocal conduction block in vasculitic neuropathy with IgM anti-gangliosides

A 30-year-old man with essential cryoglobulinemia presented with an axonal neuropathy and was found to have vasculitis at nerve biopsy. After 44 months, in accord with clinical deterioration, motor conduction studies showed excessive temporal dispersion multifocally, with partial conduction block persisting for 3 years. Antibody testing showed the presence of IgM anti-GM1, anti-GD1a, and anti-GM2 antibodies. Transitory conduction block has been reported occasionally in patients with vasculitis. The persistent multifocal conduction abnormalities found in this patient were more likely due to a superimposed immunomediated demyelination rather than to chronic nerve ischemia secondary to vasculitis.

Intraoperative muscle measurements reveal a relationship between contracture formation and muscle remodeling

Children with cerebral palsy often have spasticity of both the extensors and flexors, but how and why a flexion contracture of the wrist will develop during growth is not thoroughly understood. In order to understand the muscle adaptations that occur during contracture formation, the relationship between intraoperative sarcomere length and the extent of contracture was measured in 23 children (average age, 14.3 +/- 2.9 years) undergoing tendon transfers involving the flexor carpi ulnaris (FCU) or extensor carpi radialis brevis (ECRB) muscles. For both ECRB and FCU, sarcomere lengths measured intraoperatively were longer compared to sarcomere lengths predicted from a regression relationship obtained from "control" patients with radial nerve injury (P < 0.001). The most interesting aspect of the long FCU sarcomere lengths measured was that there was a highly significant correlation between the degree of contracture formation and intraoperative sarcomere length (r2 = 0.5, P < 0.005). These data clearly show that greater contracture severity is associated with longer intraoperative FCU sarcomere length. No such correlation was observed for the ECRB. The data suggest that the clinical presentation of the contracture is related to degree of FCU, but not ECRB adaptation, to the central nervous system lesion.

Saccade-related potentials recorded from human subthalamic nucleus

OBJECTIVE

To investigate an ocular motor role for the STN in Parkinson's disease (PD) patients.

METHODS

Potentials were recorded from deep brain stimulation (DBS) electrodes implanted in the vicinity of STN in five PD patients, while patients simultaneously performed visually cued saccades, self-paced saccades and in two patients self-paced wrist extensions.

RESULTS

Premovement potentials related to visually cued saccades were found in 4/5 patients and 56% (5/9) of potentials showed phase reversal indicating a local generator. Onsets of these potentials began closer to saccade initiation from STN contacts (0.88+/-0.30s) than thalamic ones (1.39+/-0.28 s). Self-paced saccade-related potentials were found in 4/4 patients. Self-paced saccade potential onsets (1.82+/-0.88 s) were not different from self-paced wrist extension onsets (1.27+/-0.98 s), suggesting a non-specific mechanism could be responsible for both potentials. 50% (3/6) of potentials to self-paced saccades and 66% (2/3) of potentials to self-paced wrist extensions showed phase reversal. Potentials could be found either ipsilaterally or contralaterally with respect to saccade direction.

CONCLUSIONS

These subcortical premovement potentials to saccades are similar to Bereitschaftspotentials and contingent negative variations to limb movements recorded in cortical and subcortical regions.

SIGNIFICANCE

These studies further support a role of STN in ocular motor control and suggest a common mechanism of motor preparation for both eye and limb movements in the basal ganglia.

Sensory communication of the median and ulnar nerves in the palm

A communicating branch between the median and superficial ulnar nerve in the palm of the hand has been described, but its relationship to the cutaneous anatomy of the hand has had little emphasis. Fifty preserved cadaveric hands were dissected. A communicating branch was found in 37 of 50 specimens. In 34 specimens, the connecting branch proceeded from the ulnar nerve to enter the median nerve distally; in three specimens it proceeded from the median nerve to reach the ulnar nerve distally. This study describes the communicating branch in relation to the distal crease of the wrist with the axis of the third webspace and fifth ray as the radial and ulnar borders, respectively. This study may aid surgeons in determining the likelihood of injury in trauma or during various surgical procedures.

The effects of circumferential air-splint pressure on flexor carpi radialis H-reflex in subjects without neurological deficits

The purposes of this study were to investigate the effects of circumferential pressure on flexor carpi radialis (FCR) H-reflex in subjects without neuromuscular deficits and to evaluate the skin's contribution to this effect. FCR H-reflex was assessed in 43 subjects by measuring the peak-to-peak amplitude change before, during, and after circumferential pressure was applied to the forearm. Twelve H-reflexes (H/M ratio: M = 25%, SD = 14) were recorded before pressure application to obtain a baseline value (H(baseline)) to which all data were compared. A pneumatic 15 to 20-cm air splint inflated to 51-60 mmHg provided the pressure around the forearm. H-reflex recordings were taken at 1, 3, and 5 min. during (H(pressure)) and after pressure application. A second smaller study (placebo), in which the air splint was inflated to 0 mmHg, was conducted in 5 subjects to ensure that changes in reflex amplitudes were not a result of cutaneous effects. Two types of responses were observed in the FCR H-reflex following pressure application. One group of subjects significantly increased in H-reflex amplitude while another group decreased in H-reflex amplitude when compared to H(baseline). Regression analysis found that H(max) explained 37.2% of the variance when controlling for H(baseline). Subjects with larger H(max) showed an increase in H(pressure) while subjects with lower H(max) showed decreases in H(pressure) The placebo study revealed no differences in H-reflex amplitude from baseline values, implying that skin stimulation from the air splint has no role in the effects observed. The dichotomous result indicates that pressure influences the upper extremity differently than it does the lower extremity in certain individuals. Clinicians, using circumferential pressure as a therapeutic modality to lower muscle activity of the upper extremity, need to be cognizant that pressure may have contrasting effects on their patients.

Cross-talk between language processes and overt motor behavior in the first 200 msec of processing

A recently emerging view sees language understanding as closely linked to sensory and motor processes. The present study investigates this issue by examining the influence of processing action verbs and concrete nouns on the execution of a reaching movement. Fine-grained analyses of movement kinematics revealed that relative to nouns, processing action verbs significantly affects overt motor performance. Within 200 msec after onset, processing action verbs interferes with a concurrent reaching movement. By contrast, the same words assist reaching movement when processed before movement onset. The cross-talk between language processes and overt motor behavior provides unambiguous evidence that action words and motor action share common cortical representations and could thus suggest that cortical motor regions are indeed involved in action word retrieval.

Effects produced in human arm and forearm motoneurones after electrical stimulation of ulnar and median nerves at wrist level

Effects of electrical stimulation of ulnar and median nerves at wrist level were investigated in post-stimulus time histograms (PSTHs) of single motor units from both flexors and extensors in human arm and forearm. Stimulation of ulnar nerve produced late (mean extra time-after monosynaptic group Ia excitation-10.7 +/- 0.1 ms) high-threshold (>1.2 x motor threshold, MT) excitation, which was not reproduced by purely cutaneous stimulation, in all the investigated motor nuclei except in Extensor Carpi Radialis. Stimulation of median nerve, and of the skin of fingers II and III (at palmar side level), produced short latency inhibition (mean extra time 3.8 +/- 0.3 ms), which was most often truncated or followed by late excitation (mean extra time 11.8 +/- 0.3 ms); both effects were of low threshold (0.8 x MT). Short latency inhibition was very strong, and late excitation was rare and weak in almost all the investigated motor units except in those supplying flexors in forearm, in which the main effect was the late facilitation (stronger than in other motoneurones). Since extra time was not more than 13 ms, it is suggested that the late effects may be mediated through spinal pathways, at least during their 3-5 first ms. Based on the electrophysiological results and on the anatomical characteristics of ulnar and median nerves, it is assumed that ulnar-induced late high-threshold peak in PSTHs might reflect group II excitation in spinal motoneurones, and median-induced modifications in motor unit discharge, mainly cutaneous control of motoneurone discharge. Since the central delay of median-induced inhibition is longer the more caudal the motoneurone, inhibitory propriospinal-like interneurones are supposed to mediate cutaneous inhibitory control from hand upon muscles in arm and forearm. Potential roles of proprioceptive and cutaneous control from hand to more proximal musculature, provided by ulnar and median nerve, respectively, during precise hand movements are discussed.

Variant location of the musculocutaneous nerve during axillary nerve block

We present the case of a 56-year-old man who underwent axillary nerve block for a wrist arthroscopy procedure, with real-time ultrasound and peripheral nerve stimulator guidance. The ulnar nerve and radial nerve were located medial and posterior to the brachial artery, respectively. A large complex structure was noted in the position typically occupied by the median nerve. Contact of this structure with the stimulating needle produced strong biceps contraction, and slight adjustment of the needle resulted in forearm pronation. After injection of 10 mL of local anesthetic near this structure, it appeared to consist of two separate components on ultrasound. We believe that these components represented the median and musculocutaneous nerves lying together, lateral to the artery. Radial, median, ulnar, and musculocutaneous nerve block ensued, and wrist arthroscopy was carried out uneventfully. Knowledge of this anatomical variation may improve anesthesiologists' ability to provide effective axillary block.

Partial reconstruction of muscle activity from a pruned network of diverse motor cortex neurons

Primary motor cortex (M1) neurons traditionally have been viewed as "upper motor neurons" that directly drive spinal motoneuron pools, particularly during finger movements. We used spike-triggered averages (SpikeTAs) of electromyographic (EMG) activity to select M1 neurons whose spikes signaled the arrival of input in motoneuron pools, and examined the degree of similarity between the activity patterns of these M1 neurons and their target muscles during 12 individuated finger and wrist movements. Neuron-EMG similarity generally was low. Similarity was unrelated to the strength of the SpikeTA effect, to whether the effect was pure versus synchrony, or to the number of muscles influenced by the neuron. Nevertheless, the sum of M1 neuron activity patterns, each weighted by the sign and strength of its SpikeTA effect, could be more similar to the EMG than the average similarity of individual neurons. Significant correlations between the weighted sum of M1 neuron activity patterns and EMG were obtained in six of 17 muscles, but showed R(2) values ranging from only 0.26 to 0.42. These observations suggest that additional factors-including inputs from sources other than M1 and nonlinear summation of inputs to motoneuron pools-also contributed substantially to EMG activity patterns. Furthermore, although each of these M1 neurons produced SpikeTA effects with a significant peak or trough 6-16 ms after the triggering spike, shifting the weighted sum of neuron activity to lead the EMG by 40-60 ms increased their similarity, suggesting that the influence of M1 neurons that produce SpikeTA effects includes substantial synaptic integration that in part may reach the motoneuron pools over less-direct pathways.

Amplitude, area and duration of the compound muscle action potential change in different ways over the length of the ulnar nerve

OBJECTIVE

To study physiological changes of the compound muscle action potential (CMAP) obtained from stimulation at different sites over the full length of a motor nerve and to study possible effects of anthropometrical factors.

METHODS

Multicentre study of ulnar motor nerve conduction in five segments to Erb's point performed bilaterally on 100 healthy subjects aged 17-83 years.

RESULTS

CMAP amplitude decreased linearly with conduction distance (0.31%/cm) from wrist to Erb's point. CMAP area decreased with the square of conduction distance. Decrease in area was smaller than decrease in amplitude especially distally. CMAP duration increased linearly (0.17%/cm). Amplitude decay correlated with age, height and BMI and dispersion correlated with age and height. There were no correlations between area decay and anthropometrical factors. There was no significant inter-examiner variation.

CONCLUSIONS

Area decay may be preferred to amplitude decay in the evaluation of conduction block over short segments due to smaller physiological changes and independence of anthropometrical factors. The absence of inter-examiner variation indicates that the results are robust and may be used by other laboratories.

SIGNIFICANCE

This study provides knowledge of physiological changes of CMAP parameters that may be of importance in the evaluation of nerve pathology, in particular conduction block.

Effects of sex and age on strength–duration properties

OBJECTIVE

To evaluate the possible effects of sex and age on strength-duration time constant (SDTC).

METHODS

The SDTC of 126 healthy volunteers was measured following stimulation of right median nerve at the wrist. Variations in values were evaluated according to sex and age.

RESULTS

The SDTC was 438.6+/-114.5 micros in women and 396.2+/-90.3 in men (P=.023). In men, as age increased, so did SDTC. However, this was not true in women. Comparing the values of women and men, aged below 40, demonstrated a difference in excitability, confined to younger patients.

CONCLUSIONS

As SDTC depends on the biophysical properties of the axonal membrane and can provide some information about Na(+) channel function, these data raise the possibility of a difference in Na(+) channel function between men and women and a difference in the conductance with age.

SIGNIFICANCE

The age- and sex-related differences shown in this study suggest a possible biochemical or hormonal influence on axonal excitability.

Lewis-Sumner syndrome in hepatitis C virus infection: a possible pathogenetic association with therapeutic problems

A patient with chronic hepatitis from hepatitis C virus (HCV) infection developed Lewis-Sumner syndrome (LSS). The neuropathy worsened after intravenous immunoglobulins, remitted after intravenous methylprednisolone, relapsed during interferon-alpha, but responded again to steroids continued for 68 weeks with clinical remission and without worsening of hepatitis. We are not aware of other reports of HCV infection and LSS. This association may be coincidental or related to a virus-triggered immune-mediated process. Although the coexistence of a dysimmune neuropathy with hepatitis makes problematic the choice of treatment, we emphasize that the patient's condition during treatment with steroids and the 46 following weeks without therapy has been excellent.

Langzeitergebnisse nach Handgelenkdenervation bei fortgeschrittenem karpalem Kollaps (SLAC-/SNAC-Wrist Stadium II und III)

INTRODUCTION

Mid-carpal arthrodesis, proximal row carpectomy and wrist arthrodesis are commonly cited in the literature as successful methods for pain reduction in patients with advanced carpal collapse (SLAC-/SNAC-wrist stages II and III). However, studies of isolated wrist denervation in patients who still possess good wrist mobility appear to be an underrepresented topic, especially in the German literature.

PURPOSE

Based on follow-up results, it was possible to establish to what extent patients benefit long-term from isolated wrist denervation, whilst retaining wrist mobility.

PATIENTS AND METHOD

46 patients (6 female, 40 male) with a diagnosis of SLAC-wrist (n = 10) or SNAC-wrist (n = 36) stages II and III underwent wrist denervation of points 1 - 4, 6, 9 and 10 according to Wilhelm, between 1990 and 2001, following a positive denervation test. No previous reconstructive wrist surgery had been performed on any of the patients, whose average age at the time of denervation was 47 years. In 29 cases the dominant hand was affected, and in 22 cases the symptoms could be classified as post-traumatic. Post-operative follow-up was possible in 32 out of 46 cases (70 %), and occurred on average 6.3 years post-denervation (range 2.3 to 14 years).

RESULTS

Twelve out of 32 patients (32 %) reported to be pain-free at follow-up, with a post-operative period ranging from 2.3 to 11.4 years (average 6.2 years). Six patients (18.75 %) continued to experience pain when load-bearing, which remained unchanged over a period of several years. Three patients (9 %) reported pain after stress, five patients (15.6 %) the recurrence of severe pain, which developed on average 3.8 years post-operatively. Two patients (6.25 %) experienced no analgesic benefits following wrist denervation, and four patients underwent wrist arthrodesis on average 13.5 months post-denervation, due to remaining or additional symptoms. Nineteen out of 32 patients subjectively reported a significant improvement following wrist denervation. Reassessment of range of movement post-denervation showed a reduction of 8.1 % in wrist extension/flexion, and 20 % in radio/ulna-abduction. Compared to the unaffected side, this represented an average loss of 28 % wrist extension/flexion, and 33.5 % radio/ulna-abduction. An average 51 % increase in hand strength could be shown at follow-up, leaving an average 30 % deficit compared to the unaffected hand. Evaluation of these results using the Krimmer score showed a good - very good outcome in 20 of the 32 patients (62.5 %); six patients were found to have a satisfactory outcome, and six patients a poor outcome. Subjective measures of pain (obtained using visual analogue scales) showed a reduction from 68.13 pre-operatively to 25.63 post-operatively. The DASH assessment (parts A and B) attained an average post-operative value of 17.1.

CONCLUSION

Denervation of the wrist in patients with SLAC-/SNAC-wrist stages II and III can achieve a long-term elimination or reduction of pain, whilst improving hand strength, and having only a minimal impact on wrist range of movement. Wrist denervation should therefore be given preference over wrist arthrodesis, midcarpal arthrodesis or proximal row carpectomy in patients with good wrist mobility.

Microsurgical dissection of the carpal tunnel with respect to neurovascular structures at risk during endoscopic carpal tunnel release

PURPOSE

The goal of this descriptive study was to investigate the anatomy of the carpal tunnel with respect to the related neurovascular structures, because a detailed knowledge of the complex anatomy of this region is essential to perform endoscopic carpal tunnel release.

METHODS

Sixteen fresh-frozen cadaveric hands were used for the study. Dissection of the palmar aspect of the hand was performed by use of magnifying loupes, an operative microscope, and microsurgical instruments. All anatomic components were photographed, and dimensions were recorded by use of a micrometer. The distance from the radial aspect of the os pisiformis and the proximal and distal portals to the main structures that may be injured was measured. Topography of the transverse ligament and possible adhesions to the tendons and median nerve were also recorded.

RESULTS

The mean distance from the radial aspect of the os pisiformis to the radial border of Guyon's canal and the ulnar edge of the palmaris longus tendon was 10.3 mm (range, 9 to 12 mm) and 16.1 mm (range, 12 to 22 mm), respectively. The mean distance from the distal portal to the superficial palmar arch and the ulnar artery was 10.4 mm (range, 5 to 15 mm) and 7.6 mm (range, 4.5 to 9 mm), respectively. The mean distance from the distal edge of the transverse ligament to the thenar branch of the median nerve was 2.7 mm (range, 0 to 4.1 mm). The mean length of the transverse ligament was 31 mm (range, 25 to 34.5 mm). In 14 hands we also identified the palmaris longus tendon. In 11 hands we found adhesions between the transverse ligament and the sheath of the flexor tendons.

CONCLUSIONS

The palmaris longus can be used as a guide for the placement of the proximal portal. Staying at the ulnar side of the palmaris longus keeps the superficial palmar branch of the median nerve at a safe distance from the instruments. The "fat drop sign" is also a useful guide for the placement of the distal margin of the transverse carpal ligament, keeping the distal portal away from the superficial palmar arch. Synovial adhesions can usually cover the inferior surface of the transverse ligament, and they need to be removed for clear endoscopic identification of the transverse fibers before the ligament is cut.

CLINICAL RELEVANCE

Detailed knowledge of the complex anatomy of the carpal tunnel is essential to perform endoscopic carpal tunnel release.

Startle produces early response latencies that are distinct from stimulus intensity effects

Recent experiments pairing a startling stimulus with a simple reaction time (RT) task have shown that when participants are startled, a prepared movement was initiated earlier in comparison to voluntary initiation. It has been argued that the startle acts to trigger the response involuntarily. However, an alternative explanation is that the decrease in RT may be due to stimulus intensity effects, not involuntary triggering. Thus the aim of the current investigation was to determine if RT simply declined in a linear fashion with increasing stimulus intensity, or if there was a point at which RT dramatically decreased. In the present experiment participants completed 50 active wrist extension trials to a target in response to an auditory stimulus of varying stimulus intensity (83-123 dB). The presented data show that RTs associated with a startle response are separate from stimulus intensity facilitated responses. Furthermore, this startle facilitation is more highly associated with sternocleidomastoid electromyographic (EMG) activity, rather than the EMG from the widely used startle response indicator muscle orbicularis oculi.

Surface recording of orthodromic sensory nerve action potentials in median and ulnar nerves in normal subjects

Reference values of sensory nerve compound action potentials (CAP) were established in 50 healthy subjects by examination with surface electrodes placed in a transverse and a longitudinal position at the wrist. Longitudinal electrodes record potentials of larger amplitude and shorter duration than transversally recorded CAPs. Both amplitude and duration were increased by an increase of interelectrode distance. The use of amplitude and duration as variables is encouraged by the well-defined biologic variation obtained when fixed interelectrode distance is used and when values are corrected for age and temperature. The strong correlation between amplitude and duration might indicate that the decrease of amplitude with age is caused by the reduction of conduction velocity rather than axonal loss. The ulnar nerve CAPs differ from those of the median nerve mostly with regard to influence of age and temperature. The reasons for this are discussed.

Wide-pulse-width, high-frequency neuromuscular stimulation: implications for functional electrical stimulation

Electrical stimulation (1-ms pulses, 100 Hz) produces more torque than expected from motor axon activation (extra contractions). This experiment investigates the most effective method of delivering this stimulation for neuromuscular electrical stimulation. Surface stimulation (1-ms pulses; 20 Hz for 2 s, 100 Hz for 2 s, 20 Hz for 3 s) was delivered to triceps surae and wrist flexors (muscle stimulation) and to median and tibial nerves (nerve stimulation) at two intensities. Contractions were evaluated for amplitude, consistency, and stability. Surface electromyograph was collected to assess how H-reflexes and M-waves contribute. In the triceps surae, muscle stimulation produced the largest absolute contractions (23% maximal voluntary contraction), evoked the largest extra contractions as torque increased by 412% after the 100-Hz stimulation, and was more consistent and stable compared with tibial nerve stimulation. Absolute and extra contraction amplitude, consistency, and stability of evoked wrist flexor torques were similar between stimulation types: torques reached 11% maximal voluntary contraction, and extra contractions increased torque by 161%. Extra contractions were 10 times larger in plantar flexors compared with wrist flexors with muscle stimulation but were similar with nerve stimulation. For triceps surae, H reflexes were 3.4 times larger than M waves during nerve stimulation, yet M waves were 15 times larger than H reflexes during muscle stimulation. M waves in the wrist flexors were larger than H reflexes during nerve (8.5 times) and muscle (18.5 times) stimulation. This is an initial step toward utilizing extra contractions for neuromuscular electrical stimulation and the first to demonstrate their presence in the wrist flexors.

Anatomy of the anterior interosseous innervation of the pronator quadratus: evaluation of structures at risk in the single dorsal incision wrist denervation technique

PURPOSE

Patients with chronic wrist pain often are treated with wrist denervation, which typically involves transecting both the anterior interosseous nerve (AIN) and the posterior interosseous nerve. A single dorsal incision approach is an improvement over the more traditional multiple-incision technique. The purpose of our study was to describe the branches of the AIN to the pronator quadratus and evaluate the risk of denervation with the single dorsal incision technique.

METHODS

Twelve fresh-frozen cadaver forearms were dissected. The branches of the AIN to the pronator quadratus were identified and the individual branch points were measured from the articular edge of the distal radius. Wrist denervation was then performed on each specimen through the single dorsal incision (as suggested by Berger).

RESULTS

There were an average of 3 branches from the AIN to the pronator quadratus. All forearms had at least 1 branch to the pronator quadratus more proximal to the distal end of the dorsal skin incision; however, in only 2 of the forearms was the most proximal branch more than 2 cm proximal to the distal end of the dorsal skin incision.

CONCLUSIONS

Wrist denervation through the recommended single dorsal incision poses a serious risk for completely denervating the pronator quadratus. Therefore the resection of the AIN must be performed close to the distal margin of the pronator quadratus.

Distal excitability changes in motor axons in amyotrophic lateral sclerosis

OBJECTIVE

Previous axonal excitability studies in amyotrophic lateral sclerosis (ALS) have suggested that impaired potassium channel function could be responsible for the generation of fasciculations, but the ectopic activity arises predominantly from the motor nerve terminals. This study tested the hypothesis that dysfunction of potassium channels is more pronounced in the more distal parts of axons.

METHODS

Threshold electrotonus was used to compare accommodation at the motor point of abductor pollicis brevis, and at the wrist portion of the median nerve, between 22 patients with ALS and 19 normal subjects. As target responses for motor point stimulation, movement-related potentials were recorded using an accelerometer.

RESULTS

Compared to normal subjects, ALS patients showed greater threshold changes to depolarizing conditioning currents at both the motor point and wrist, suggesting less accommodation by potassium currents. Differences in the threshold electrotonus curves between the normal and ALS groups were much more prominent at the motor point than at the wrist.

CONCLUSIONS

In ALS, axonal potassium channels are impaired more prominently in distal portions of axons than at the nerve trunk, and this is consistent with evidence that fasciculations mostly arise from the nerve terminals.

SIGNIFICANCE

Excitability testing at the motor point provides additional information about the pathophysiology of ALS.

Axonal excitability properties in amyotrophic lateral sclerosis

OBJECTIVE

To investigate axolemmal ion channel function in patients diagnosed with sporadic amyotrophic lateral sclerosis (ALS).

METHODS

A recently described threshold tracking protocol was implemented to measure multiple indices of axonal excitability in 26 ALS patients by stimulating the median motor nerve at the wrist. The excitability indices studied included: stimulus-response curve (SR); strength-duration time constant (tauSD); current/threshold relationship; threshold electrotonus to a 100 ms polarizing current; and recovery curves to a supramaximal stimulus.

RESULTS

Compound muscle action potential (CMAP) amplitudes were significantly reduced in ALS patients (ALS, 2.84+/-1.17 mV; controls, 8.27+/-1.09 mV, P<0.0005) and the SR curves for both 0.2 and 1 ms pulse widths were shifted in a hyperpolarized direction. Threshold electrotonus revealed a greater threshold change to both depolarizing and hyperpolarizing conditioning stimuli, similar to the 'fanned out' appearance that occurs with membrane hyperpolarization. The tauSD was significantly increased in ALS patients (ALS, 0.50+/-0.03 ms; controls, 0.42+/-0.02 ms, P<0.05). The recovery cycle of excitability following a conditioning supramaximal stimulus revealed increased superexcitability in ALS patients (ALS, 29.63+/-1.25%; controls, 25.11+/-1.01%, P<0.01).

CONCLUSIONS

Threshold tracking studies revealed changes indicative of widespread dysfunction in axonal ion channel conduction, including increased persistent Na+ channel conduction, and abnormalities of fast paranodal K+ and internodal slow K+ channel function, in ALS patients.

SIGNIFICANCE

An increase in persistent Na+ conductances coupled with reduction in K+ currents would predispose axons of ALS patients to generation of fasciculations and cramps. Axonal excitability studies may provide insight into mechanisms responsible for motor neuron loss in ALS.

Anatomy of neurovascular structures around the carpal tunnel during dynamic wrist motion for endoscopic carpal tunnel release

OBJECTIVE

The purpose of this study is to investigate the anatomic relationship between neurovascular structures and the transverse carpal ligament (TCL) so as to avoid complications during an endoscopic carpal tunnel release procedure.

METHODS

Fresh cadaver hands from seven men and 12 women (age range, 48-74 yr) were used. The neurovascular structures just over and under the TCL were meticulously dissected under loupe magnification. Several anatomic landmarks were calculated (average length of the TCL; average distance between the TCL distal margin and the neurovascular structures; and average lengths of the superficial palmar arch, ramus communicantes, recurrent motor branch, and palmar cutaneous branch of the median nerve). The ulnar neurovascular structure was studied with the wrist positioned in neutral, ulnar flexion, and radial flexion.

RESULTS

The anatomic relationships between the TCL and vascular and neural structures were measured. The ulnar neurovascular structures usually passed just over ulnar to the superior portion of the hook of the hamate. However, in 11 hands, a looped ulnar artery coursed 1 to 4 mm radial to the hook of the hamate and continued to the superficial palmar arch. The looped ulnar artery migrates on the ulnar side of Guyon's canal (-2-2 mm radial to the hook of the hamate) with the wrist in radial flexion (of the wrist). During ulnar flexion of the wrist, the ulnar artery shifts more radially beyond the hook of the hamate (2-7 mm).

CONCLUSION

It is appropriate to transect the ligament over 4 mm apart from the lateral margin of the hook of the hamate without placing the edge of the scalpel toward the ulnar side. We would also recommend not transecting the TCL in the ulnar flexed wrist position to protect the ulnar neurovascular structure. The proximal portal could be made just ulnar to the palmaris longus tendon to spare the neurovascular structures in the proximal portion of the TCL.

Superficial surgical landmarks for identifying the posterior interosseous nerve

OBJECT

There is a paucity of information in the neurosurgical literature regarding the surgical anatomy surrounding the posterior interosseous nerve (PIN). The goal of the current study was to provide easily recognizable superficial bone landmarks for identification of the PIN.

METHODS

Thirty-four cadaveric upper extremities obtained from adults were subjected to dissection of the PINs, and measurements were made between this nerve and surrounding superficial bone landmarks. In all specimens the main radial trunk was found to branch into its superficial branch and PIN at the level of the lateral epicondyle of the humerus. Proximally, the PIN was best identified following dissection between the brachioradialis and extensor carpi radialis longus and brevis muscles. At its exit site from the supinator muscle, the PIN was best identified after retraction between the extensor carpi radialis longus and brevis and extensor digitorum communis muscles. This site was a mean distance of 6 cm distal to the lateral epicondyle of the humerus. No compression of the PIN by the tendon of origin of the extensor carpi radialis brevis muscle was seen. One specimen was found to have a proximally split PIN that provided a previously undefined articular branch to the elbow joint. The mean diameter of the PIN proximal to the supinator muscle was 4.5 mm. The leash of Henry crossed the PIN in all but one specimen and was found at a mean distance of 5 cm inferior to the lateral epicondyle. The PIN exited the distal edge of the supinator muscle at a mean distance of 12 cm distal to the lateral epicondyle of the humerus. Here the mean diameter of the PIN was 4 mm. The exit site from the distal edge of the supinator was found to be at a mean distance of 18 cm proximal to the styloid process of the ulna. This exit site for the PIN was best identified following dissection between the extensor carpi radialis longus and brevis and extensor digitorum communis muscles. The distal articular branch of the PIN was found to have a mean length of 13 cm and the proximal portion of this terminal segment was located at a mean distance of 7.5 cm proximal to the Lister tubercle.

CONCLUSIONS

The addition of more anatomical landmarks can help the neurosurgeon to be more precise in identifying the PIN and in avoiding complications during surgery in this region.

Short- and long-term modulation of upper limb motor-evoked potentials induced by acupuncture

The aim of this study was to investigate in humans the effects of acupuncture upon upper-limb motor-evoked potentials (MEPs), elicited by transcranial magnetic stimulation of the primary motor cortex. It is known that peripheral sensory stimulation can be used to induce short- and long-term changes in motor cortex excitability. Data show that the simple insertion of the needle is an adequate somatosensory stimulus to induce a significant modulation of MEP amplitude, the sign of which (facilitation or inhibition) is specific to the investigated muscle and to the point of needle insertion. Moreover, MEP changes in upper-limb muscles are also observed following needling of lower-limb sites, revealing the presence of long-distance effects of acupuncture. Finally, the modulation in muscle excitability considerably outlasts the time period of needle application, demonstrating the induction of long-term plastic changes in the central nervous system. In addition, results have shown that the effects on muscle excitability are not restricted to the stimulation of well-coded acupoints, as described in traditional Chinese medicine, but they can also be induced by needling of nonacupoints, normally not used for therapeutic purposes. The possible neuronal mechanisms underlying the observed effects of acupuncture are discussed in relation to the available neurophysiological data regarding the interlimb reflexes and the changes in the representational cortical maps induced in humans by a prolonged somatosensory stimulation.