Daniel Dumitru

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.

Dermatomal/segmental somatosensory evoked potential evaluation of L5/S1 unilateral/unilevel radiculopathies.

Dermatomal and segmental somatosensory evoked potentials (SEPs) have been reported to be of diagnostic utility in unilateral/unilevel L5 and S1 radiculopathies. This investigation employs history, physical examination, imaging studies, and electrodiagnostic medicine evaluations to clearly define unilateral/unilevel L5 or S1 nerve root compromise. Inclusion criteria require all of the preceding diagnostic methods to corroborate a specific nerve root lesion. Regression equation analysis for cortical P1 latencies evaluating age and height based on comparable patient and control reference populations reveals segmental and dermatomal sensitivities for L5 radiculopathies to be 70% and 50%, respectively, at 90% confidence intervals. Similar sensitivities are obtained for 2 standard deviation mean cortical P1 latencies. Side‐to‐side cortical P1 latency difference data reveal segmental and dermatomal sensitivities for L5 radiculopathies to be 40% at 2 standard deviations. Regression equation analysis for age and height regarding segmental and dermatomal studies for S1 radiculopathies reveal sensitivities of 30% and 20%, respectively, at 90% confidence intervals. Similar data are delineated for 2 standard deviation mean cortical P1 latencies. Side‐to‐side cortical P1 latency difference data reveal segmental and dermatomal sensitivities for S1 radiculopathies to be 50% and 10%, respectively, at two standard deviations. The clinical utility of both segmental and dermatomal SEPs are questionable in patients with known unilateral/unilevel L5 and S1 nerve root compromise.

Electrophysiologic evaluation of the facial nerve in Bell's palsy. A review.

Facial nerve paralysis is the most common mononeuropathy and idiopathic facial paralysis (Bells palsy) the most common seventh nerve disease electromyographers may be asked to evaluate. The electrophysiologic method of choice to assess the facial nerve is side-to-side evoked amplitude comparison with the affected side expressed as a percentage of the nonaffected side. This examination should be performed on days 3, 5, 7, 9,11 and 13 after onset of paralysis. If the percentage of surviving axons falls below 10% within the first 14 days, an incomplete recovery is suggested. Electromyography may assist in prognosticating a functional return, determining neural conduction across the site of injury and following reinervation in the recovery period. The persistence or early return of an absent R1 component of the blink reflex may qualitatively suggest a satisfactory functional outcome in facial paralysis. Supramaximally exciting the facial nerve at the stylomastoid foramen and comparing the clinical response on the affected and nonaffected side, maximum stimulation test, can also predict eventual seventh nerve return. Observing a minimal twitch, utilizing the nerve excitability test or measuring the facial nerve latency have yielded poor correlations with functional return and are of limited usefulness in the prognostication of acute facial palsies. Trigeminal somatosensory evoked potentials can be employed to evaluate the status of the trigeminal nerve as approximately 50% of patients with Bells palsy also have lesions involving the fifth nerve. Side-to-side amplitude comparison and electromyography are the two most valuable electrophysiologic methods of assessing facial nerve functioning.

Physiologic basis of potentials recorded in electromyography

The extracellularly recorded configuration of a single muscle fiber discharge is generally appreciated to be triphasic with an initially positive deflection. However, careful attention to waveform appearance during the electrodiagnostic medicine examination reveals that both innervated and denervated muscle waveforms may display a pantheon of configurations. Further, despite the fact that innervated and denervated single muscle fiber discharges arise from distinctly different intracellular action potential (IAP) configurations, their extracellularly recorded waveforms can appear quite similar, leading to potential misidentification and, hence, the possibility of an erroneous diagnostic conclusion. The least appreciated, but nevertheless critical, aspect of explanations for muscle waveform configurations is the relationship between the muscle fiber and recording electrode. Additionally, it is important to appreciate both the near‐field and far‐field aspects of single fiber and compound muscle action potentials. In this review, the leading/trailing dipole model is used to explain muscle waveform configurations in both innervated and denervated tissues.

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.

Issues & opinions: Single muscle fiber discharges (insertional activity, end‐plate potentials, positive sharp waves, and fibrillation potentials): A unifying proposal

The exact origin and precise morphologic explanation of positive sharp waves (PSWs) are presently lacking. Observing normal needle electromyographic insertional activity reveals two types of waveforms: (1) biphasic negative/positive spikes, and (2) positive spikes followed by a small negative phase. In the end‐plate region, it is possible to occasionally observe a biphasic end‐plate spike transform into a monophasic positive end‐plate waveform. It is postulated that this waveform is simply a form of intracellular recording for the biphasic end‐plate spike or a form of extracellularly recorded but blocked single muscle fiber discharge. Similarly, the observed monophasic positive insertional activity may be an intracellularly recorded single muscle fiber discharge or a blocked extracellular discharge originating about the needle electrode. Applying this reasoning to PSWs suggests that they may also be an intracellular recording of a fibrillation potential, or needle‐induced extracellular blocked local single muscle fiber discharge. This unifying concept is applied to various clinical situations purported to demonstrate “different” types of PSWs.

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.