Consequences of Peripheral Nerve Demyelination: Basic and Clinical Aspects

Motor Nerve Conduction Block Estimation in Demyelinating Neuropathies by Deconvolution

A deconvolution method is proposed for conduction block (CB) estimation based on two compound muscle action potentials (CMAPs) elicited by stimulating a nerve proximal and distal to the region in which the block is suspected. It estimates the time delay distributions by CMAPs deconvolution, from which CB is computed. The slow afterwave (SAW) is included to describe the motor unit potential, as it gives an important contribution in case of the large temporal dispersion (TD) often found in patients. The method is tested on experimental signals obtained from both healthy subjects and pathological patients, with either Chronic Inflammatory Demyelinating Polyneuropathy (CIDP) or Multifocal Motor Neuropathy (MMN). The new technique outperforms the clinical methods (based on amplitude and area of CMAPs) and a previous state-of-the-art deconvolution approach. It compensates phase cancellations, allowing to discriminate among CB and TD: estimated by the methods of amplitude, area and deconvolution, CB showed a correlation with TD equal to 39.3%, 29.5% and 8.2%, respectively. Moreover, a significant decrease of percentage reconstruction errors of the CMAPs with respect to the previous deconvolution approach is obtained (from a mean/median of 19.1%/16.7% to 11.7%/11.2%). Therefore, the new method is able to discriminate between CB and TD (overcoming the important limitation of clinical approaches) and can approximate patients’ CMAPs better than the previous deconvolution algorithm. Then, it appears to be promising for the diagnosis of demyelinating polyneuropathies, to be further tested in the future in a prospective clinical trial.

Molecular regulators of nerve conduction - Lessons from inherited neuropathies and rodent genetic models

Myelinated nerve fibers are highly compartmentalized. Helically wrapped lipoprotein membranes of myelin are integrated with subsets of proteins specifically in each compartment to shape the physiological behavior of these nerve fibers. With the advance of molecular biology and genetics, many functions of these proteins have been revealed over the past decade. In this review, we will first discuss how action potential propagation has been understood by classical electrophysiological studies. In particular, the discussion will be concentrated on how the geometric dimensions of myelinated nerve fibers (such as internodal length and myelin thickness) may affect nerve conduction velocity. This discussion will then extend into how specific myelin proteins may shape these geometric parameters, thereby regulating action potential propagation. For instance, periaxin may specifically affect the internodal length, but not other parameters. In contrast, neuregulin-1 may affect myelin thickness, but not axon diameter or internodal length. Finally, we will discuss how these basic neurobiological observations can be applied to inherited peripheral nerve diseases.

The role of axonopathy in the mechanisms of development of demyelination processes in the central and peripheral nervous system

The role of axonopathy in the development of demyelinating processes in the CNS and peripheral nervous system was addressed in studies of 43 patients with multiple sclerosis (MS) and 144 patients with chronic inflammatory demyelinating polyneuropathy (CIDPN). Patients with MS were found to have foci of reduced MRI intensity in the T1 regime ("black holes," present in 28%) and regional atrophy of the cerebral cortex (in 46%), which showed a significant association with the degree of invalidity on the EDSS (Kendall tau = 0.38 and 0.43; p = 0.038 and 0.021, respectively). The mean fatigue score on the FSS was 4.9 (3.6; 5.4). A significant increase in the central conduction time on the background of fatigue (p = 0.016), along with an absence of signs of impaired reliability of neuromuscular transmission and an absence of past-activation phenomena, suggested that central mechanisms were predominant in the formation of fatigue phenomena in MS. In addition, 34.9% of patients with MS showed signs of peripheral nervous system involvement, while the clinical-electrophysiological pattern in 12.5% of patients with CIDPN showed signs of CNS involvement. These data widen existing concepts of the mechanisms of formation of axonopathy in the CNS, based on evidence for the development of axon-demyelinating processes in CIDPN, which is the most accessible model of demyelination for study using contemporary neurophysiological methods.

A new method for the estimation of motor nerve conduction block

OBJECTIVE

A method for conduction block (CB) estimation, based on compound muscle action potentials (CMAP) elicited by stimulation at sites proximal and distal to the region in which a block is suspected, which is less sensitive to temporal dispersion than methods based on area and amplitude estimation, routinely used in clinical practice.

METHODS

The method is based on deconvolution of CMAPs. It provides the delay distribution that convolved with a kernel (estimated by an optimisation method) gives a reconstruction of the CMAPs. The integral of the delay distribution was used to estimate CB. The method was tested on phenomenological signals (sum of delayed and amplitude scaled versions of the same signal), structure based simulated signals (from a plane layer generation model of surface EMG), and experimental signals (eight healthy subjects; CMAPs recorded over abductor digiti minimi; different temporal dispersions obtained comparing above-elbow stimulation of ulnar nerve with below-elbow stimulation or with wrist stimulation; conduction distances about 10 and 35 cm, respectively).

RESULTS

Deconvolution method gives more precise estimates of the simulated CB with respect to area and amplitude methods (phenomenological signals: bias in CB estimation in the worst case about 10% for deconvolution, 30% for area, 60% for amplitude). Experimental data: by increasing temporal dispersion, in the average CB estimation increases 4% for area and 10% for amplitude, no increase for deconvolution.

CONCLUSIONS

The new method for CB estimation is less sensitive to temporal dispersion than area and amplitude methods.

SIGNIFICANCE

The proposed method provides a precise CB estimation. Being stable to temporal dispersion, it allows to diagnose CB with a lower confidence threshold than in the case of area and amplitude.

Pure motor mononeuropathy with distal conduction block: an unusual presentation of multifocal motor neuropathy with conduction blocks

OBJECTIVE

To report an hitherto undescribed presentation of Motor Neuropathy with Multifocal Conduction Block (MMNCB).

METHODS

Description of two patients presenting with complete paralysis of the 3 heads of the trapezius muscle (case one) and progressive weakness of finger extension (case 2).

RESULTS

Nerve conduction studies (NCS) established that the corresponding nerves were distally inexcitable. In the affected muscles, no voluntary activity was elicited in both patients with spontaneous activity noted in patient 2. Systematic NCSs documented an asymptomatic, partial conduction block (CB) in a median nerve forearm segment in both patients. Neurophysiological follow-up after a dramatic response to intravenous immunoglobulins demonstrated recovery of the initially unobtainable motor responses.

CONCLUSIONS

This indicates that a complete, distal CB of the motor fibers destined to the trapezius muscle in patient 1, and to the extensor indicis proprius in patient 2, had caused the heralding deficits.

SIGNIFICANCE

These findings underscore the possibility of distal CB in this disorder and the need for extensive NCSs, including asymptomatic nerves, for an accurate diagnosis.

Surface electromyographic recording of volitional activity: a technique to detect partial motor conduction block

Partial motor conduction block, an electrophysiological hallmark of demyelination, helps to identify acquired demyelinating neuropathies but its electrophysiological detection can be difficult. We report a technique that may be helpful in this regard. Twenty-five patients with partial motor conduction block secondary to acquired demyelinating polyneuropathy (ADP), 7 with amyotrophic lateral sclerosis (ALS), 7 with stroke, and 11 control subjects, were studied. Amplitude of compound muscle action potentials was recorded after distal electrical (E) stimulation and for volitionally (V) induced responses in 82 muscles. Mean +/- SD V/E ratio was 12.3 +/- 6.6 for ADP patients, 58.1 +/- 17 for ALS patients, 11.4 +/- 9 for stroke patients, and 55.4 +/- 12.3 for controls. The V/E ratios for patients with partial motor conduction block and stroke were significantly reduced compared with ALS patients and healthy controls (P < 0.05). Surface electromyographic (EMG) recording for determination of the V/E ratio may be a useful technique for detection of a proximal conduction block if a central lesion or poor effort can be excluded. Further study of this novel technique is necessary.

Facts, fallacies, and fancies of nerve conduction studies: twenty-first annual Edward H. Lambert Lecture

Optimal application of the nerve conduction study depends on an understanding of the principles and a recognition of the pitfalls of the technique. The conventional methods deal primarily with distal nerve segments in an extremity. Other techniques allow one to assess nerve segments in less accessible anatomical regions, to improve the accuracy in precisely localizing a focal lesion, and to increase sensitivity in detecting subclinical abnormalities. Despite certain limitations, these methods can provide diagnostically pertinent information if they are used judiciously in appropriate clinical contexts. I wish to review the fundamental concepts of nerve stimulation techniques and their proper applications in the clinical domain. The discussion primarily relates to the importance of reproducibility of various measures and pitfalls in the evaluation of conduction block.

Chronic motor neuropathies: diagnosis, therapy, and pathogenesis

Pure motor neuropathy syndromes resemble amyotrophic lateral sclerosis variants with no upper motor neuron signs. Their identification is important, as, in contrast to amyotrophic lateral sclerosis, they are often immune mediated and treatable. Typically the immune-mediated motor neuropathy syndromes are distal and asymmetrical and progress slowly. The clinical features may help alert the clinician to the diagnosis, but other ancillary evidence such as abnormalities on electrophysiological testing and the presence of serum autoantibodies to neural antigens are helpful in making the diagnosis more secure. Electrophysiological abnormalities include not only motor conduction block but also other evidence of a demyelinative process such as prolonged distal latencies or F-wave abnormalities. High-titer anti-GM1 antibodies occur frequently but more specific patterns of reactivity may be especially helpful. Treatment of these motor neuropathy syndromes includes cyclophosphamide, which we use in combination with plasma exchange, and in some patients, human immune globulin. Clinical responses to therapy may occur within the first 2 to 4 months in patients with motor neuropathy syndromes with demyelinative features, but only become obvious 6 months or later after starting treatment in patients with predominantly axonal disorders.