John C. King

Prevalence of denervation in paraspinal and foot intrinsic musculature

Dumitru D, Diaz CA, King JC: Prevalence of denervation in paraspinal and foot intrinsic musculature. Am J Phys Med Rehabil 2001;80:482–490. ObjectiveThe primary purpose of this investigation was to determine the prevalence of abnormal spontaneous activity (positive sharp waves (PSWs) and fibrillation potentials (FPs)) in selected lumbosacral paraspinal and foot intrinsic muscles in an asymptomatic healthy population. DesignThis was a prospective assessment of 50 individuals without history or physical findings suggestive of peripheral neuromuscular disease whereby a monopolar needle electrode was located in the unilateral L4 and L5 paraspinal as well as abductor hallucis and extensor digitorum brevis muscles. These muscles were extensively evaluated for the presence of PSWs, FPs, and fasciculation potentials. ResultsTen subjects per decade from 20-59 yr and ten subjects from 60-80 yr comprised the 50 participants (28 women), resulting in a mean age of 45 ± 15.9 (range, 20-76) yr. A single individual (prevalence, 2%) demonstrated fibrillation potentials in the extensor digitorum brevis, and FPs and PSWs were detected in two subjects’ (4% prevalence) L4/L5 paraspinal muscles. Ninety-four percent of the subjects had fasciculation potentials in the abductor hallucis, whereas 60% had these waveforms in the extensor digitorum brevis. Only 6% of subjects had fasciculation potentials in the L4 but not L5 paraspinal muscles. All subjects demonstrated both prototypical and “atypical” appearing endplate spikes in all of the muscles examined. ConclusionsWe failed to confirm the previously reported prevalence of FPs and PSWs in both the paraspinal and foot intrinsic musculature. Atypical appearing endplate spikes, however, display configurations similar to FPs and PSWs and were present in all subjects. Failure to pay close attention to the discharge rate and rhythm of endplate spikes can lead to misinterpreting these waveforms as FPs and PSWs. It is likely that the previously reported high prevalence of spontaneous activity in healthy persons resulted from not fully appreciating the similarity between innervated and denervated spontaneous single muscle fiber discharge configurations.

Bowel training in spina bifida: importance of education, patient compliance, age, and anal reflexes.

Bowel incontinence is a major social impairment for 90% of patients with spina bifida. This study assess the bowel continence of children and young adults with spina bifida before and after a toileting intervention that emphasized patient/family education and a regular, consistently timed, reflex-triggered bowel evacuation. Bowel continence defined as one or fewer incontinent stools per month, rose from 13% (5/40) to 60% (24/40) following intervention. Twenty-four of the 35 initially incontinent patients were compliant. Seventy-nine percent (19/24) of the compliant subjects achieved continence whereas 0/11 of the noncompliant subjects achieved continence p < 0.0001). Presence of the bulbocavernosus (BC) and anocutaneous (AC) reflexes correlated significantly with achieving continence (either vs none p < .02, AC vs no AC p < .01). Instituting bowel training before age 7 correlated with improved outcomes by means of better compliance. Excluding noncompliant subjects, 83% (24/29) of the original sample of 40 patients satisfied our strict definition of bowel continence after this simple low technology intervention.

Anodal block V anodal stimulation. Fact or fiction.

Anodal block and stimulation are poorly documented electrophysiologic phenomenon. Median and superficial radial nerves are examined in a prospective study to explore the significance of anodal block in routine nerve conduction studies. In addition, the anodes ability to stimulate the peripheral nervous system is evaluated. A monopolar stimulation technique is employed to achieve pure anode-generated responses. Additionally, a similar monopolar cathode stimulation technique is utilized and found to be equivalent to the traditional bipolar cathode stimulation. Based on the findings in this investigation, anodal block does not appear to occur during routine nerve conduction studies; however, transposition of the anode and cathode is clinically significant because the increased distance between the cathode and recording electrode results in predictably prolonged latencies. With higher levels of stimulus intensity, sensory, motor and F wave responses are generated by anodal stimulation in all cases. The actual mechanism of anodal stimulation remains uncertain and requires further study. Predicated on the results of this investigation, it appears that anodal block is an unlikely occurrence during routine electrodiagnostic medicine evaluations.

Motor unit action potential components and physiologic duration

Motor unit action potentials (MUAPs) recorded from the same motor unit at two distances along the biceps brachii muscle with monopolar needle electrodes at high amplifier gains (20 μV/division) and averaged 2000–3000 times reveal total potential durations of 39.6 ± 4.6 ms. In addition, the terminal segment for each of these two MUAPs contained a late far‐field potential with a mean duration of 23.8 ± 4.1 ms. Computer simulations of MUAPs suggest that this long‐duration positive far‐field mirrors the true morphology of the intracellular action potential (IAP), which is monophasic positive, possessing a terminal repolarization phase approaching 30 ms. This investigation suggests that the MUAPs physiologic duration is directly proportional to the muscle fiber length and the IAPs duration, which becomes manifest as a positive far‐field potential when the IAP encounters the musculotendinous junction and slowly dissipates. The leading/trailing dipole model is used to explain qualitatively this studys quantitative clinical and computer simulation findings.

Determinants of motor unit action potential duration

OBJECTIVE Motor unit action potential (MUAP) recordings are modeled by means of a single muscle fiber simulation program, to define two key subcomponents comprising the complete physiologic MUAP duration. A number of defining properties of these subcomponents are further developed. METHODS A single muscle fiber simulation program is utilized with various muscle fiber lengths and conduction velocities to generate near-field and far-field waveforms. RESULTS Two key subcomponents to the total physiologic single muscle fiber and hence MUAP duration are identified. One, defined as the near-field component, is directly dependent upon muscle fiber hemi-length. The other, defined as the far-field component, is independent of fiber length, but matches the internal action potential in duration. Both the near-field and far-field components are inversely dependent upon intracellular action potential conduction velocity. Additionally, temporal dispersion among the individual fibers contributing to a MUAP must be included in the overall MUAP duration calculation. CONCLUSIONS It is hoped that this approach to MUAP duration may allow a more complete appreciation of the components contributing to the MUAP, than permitted by the empirically derived values for MUAP duration presently under clinical use.

Concentric needle electrode duration measurement and uptake area

Motor unit action potentials (MUAPs) were recorded with a standard concentric needle electrode inserted into the right biceps brachii muscle with different angular orientations of the beveled recording surface to the muscle fibers. Contrary to the predictions from computer simulations, the MUAP duration remained constant during needle rotation. This finding is used to reexamine the previous assumptions regarding the concentric needles spatial uptake recording territory and the implications with respect to MUAP duration measurements.

Effect of reference electrode position on the compound muscle action potential (CMAP) onset latency

Compound muscle action potential (CMAP) onset latency is interpreted to reflect the arrival time at the muscle of impulses in the fastest‐conducting motor nerve fiber. However, we have observed that the position of the reference or indifferent electrode (E2) affects CMAP onset latency. Motor nerve conduction studies (NCS) of the median, ulnar, and deep ulnar motor (DUM) nerves on 20 normal hands were performed using both traditional bipolar and experimental monopolar (referenced to the contralateral hand) montages. As the position of E2 was altered, the CMAP onset latency varied 0.1–0.5 ms for the median NCS, 0.1–0.3 ms for the ulnar NCS, and 0.1–1.5 ms for the DUM NCS. This study demonstrates that E2 recorded potentials are significant and vary with positioning, affecting motor onset latency. This has implications both for reference values and the physiologic interpretation of the CMAP waveform.

Concentric and single fiber electrode spatial recording characteristics

A better appreciation of the specific spatial recording characteristics of the single fiber and concentric needle electrode can result in more accurate physiologic and theoretical interpretations of single fiber and quantitative motor unit action potential analysis. We demonstrate by physical modeling that the 90% and 99% amplitude sensitivity envelopes are not simple hemispherical shapes. The 90% sensitivity concentric electrode volume does not extend beyond the insulated portion of the 15° beveled surface between the core and cannula and extends only 280 μm perpendicularly from the center of the cores surface. The 99% envelope extends approximately 830 μm perpendicularly from the cores center. This is a much smaller volume of sensitivity than exists for a similarly modeled monopolar electrode. The 90% and 99% envelopes extend to 110 and 320 μm perpendicularly from the exposed single fiber core. Both the single fiber and concentric needle volumes of sensitivity have specific asymmetries described.

Motor unit action potentials recorded with concentric electrodes: physiologic implications

Computer simulations of concentric needle electrode recording characteristics assume a hemisphere spatial recording territory for the electrodes core with the cannula shielding electrical activity arising from those muscle fibers located behind the cannula with respect to the electrodes core. It is also believed that the motor unit action potentials (MUAP) duration is generated by the number of muscle fibers within the electrodes hemispherical recording territory. This presumption suggests that rotating the needle will necessarily alter the number of muscle fibers within the hemispherical recording territory and hence lead to an alteration in MUAP duration. Comparisons were performed for different needle orientations with documentation of no statistically significant alteration in MUAP duration. Additionally, referential recording montages with the concentric needle electrode revealed that the electrodes core records MUAPs with durations comparable to those detected by the cannula. These findings strongly suggest that the recording territory of the concentric needle electrode, with respect to MUAP duration, is not a hemisphere but a sphere encompassing most if not all of the MUAPs muscle fibers in a manner similar to that of a monopolar needle. These findings have significant implications regarding presently used MUAP simulation techniques and require a reconceptualization of how the concentric needle electrode records electrical activity within a volume conductor.

Isolated sural neuropathy presenting as lateral ankle pain

A case of job-related, unilateral traumatic sural neuropathy causing severe lateral ankle pain and impaired work performance for a 26-yr-old female grocery clerk is reported. This diagnosis is made both clinically and electrophysiologically. We review the pertinent electrophysiologic features, anatomy, and clinical findings in our patient with an isolated sural neuropathy. A review of the literature demonstrates that trauma is the most common cause of this unusual isolated neuropathy. Despite its rare occurrence, it should be considered in patients who present with lateral ankle pain and concomitant loss of sensation in the sural nerve distribution. The establishment of a neuropathic origin assists with management strategies that will differ from the more common musculoskeletal causes of lateral ankle pain. After an appropriate diagnosis and treatment, an excellent outcome resulted for our patient.