Polymyositis and dermatomyositis belong to the group of inflammatory muscle diseases. This chapter deals only with the “idiopathic” polymyositis and dermatomyositis syndromes; parasitic, bacterial, spirochetal, and viral infectious myopathies will not be discussed. For recent reviews of the general subject, the reader may wish to refer to Banker and Engel (1986), Dalakas (1991, 1992), Engel (1992)

https://www.thelancet.com/pdfs/journals/lancet/PIIS0140-6736(03)14368-1.pdf

Polymyositis and Dermatomyositis

THE polymyositis and dermatomyositis complex encompasses a heterogeneous group of acquired muscle diseases called inflammatory myopathies because muscle weakness and inflammatory infiltrates within the skeletal muscles are the principal clinical and histologic findings.1 2 3 4 5 6 7 8 9 10 11 12 Traditionally, polymyositis and dermatomyositis have been viewed as pathogenetically similar and part of the spectrum of "idiopathic" inflammatory myopathies,9 , 10 , 13 , 14 despite the variability in their clinical and laboratory characteristics, prognosis, and response to therapy. With the evolution over the past 10 years of rather well defined clinical, demographic, histologic, and immunopathological criteria and the identification of inclusion-body myositis as a distinct type of polymyositis,15 the inflammatory myopathies now . . .

Polymyositis, Dermatomyositis, and Inclusion-Body Myositis

AGA technical review on management of oropharyngeal dysphagia

The purpose of the study was to quantify the influence of selected motor unit properties and patterns of activity on amplitude cancellation in the simulated surface electromyogram (EMG). The study ...

https://www.physiology.org/doi/pdf/10.1152/japplphysiol.00894.2004

Influence of amplitude cancellation on the simulated surface electromyogram.

A model of the motor unit action potential was developed to investigate the amplitude and frequency spectrum contributions of motor units, located at various depths within muscle, to the surface detected electromyographic (EMG) signal. A dipole representation of the transmembrane current in a three-dimensional muscle volume was used to estimate detected individual muscle fiber action potentials. The effects of anisotropic muscle conductance, innervation zone location, propagation velocity, fiber length, electrode area, and electrode configuration were included in the fiber action potential model. A motor unit action potential was assumed to be the sum of the individual muscle fiber action potentials. A computational procedure, based on the notion of isopotential layers, was developed which substantially reduced the calculation time required to estimate motor unit action potentials. The simulations indicated that: 1) only those motor units with muscle fibers located within 10---12 mm of the electrodes would contribute significant signal energy to the surface EMG, 2) variation in surface area of electrodes has little effect on the detection depth of motor unit action potentials, 3) increased interelectrode spacing moderately increases detection depth, and 4) the frequency content of action potentials decreases steeply with increased electrode-motor unit territory distance.

Detection of motor unit action potentials with surface electrodes: influence of electrode size and spacing

Automatic analysis of the EMG interference pattern

Analysis of amplitude and area of concentric needle EMG motor unit action potentials

Computer simulations of motor unit action potentials (MUAPs) as measured by a concentric needle (CN) electromyography (EMG) electrode in normal motor units (MUs) indicated that the MUAP amplitude is determined mainly by the proximity of the electrode to the closest muscle fiber. The area and duration of the simulated MUAPs were affected by all muscle fibers in front of the active recording surface but mainly by those that were less than 2 and 2.5 mm, respectively, from the active recording surface. The MUAP area was also affected by the proximity of the electrode to the closest muscle fiber. The number of phases of the simulated MUAPs increased when the dispersion of the arrival times of individual muscle fiber APs at the electrode was increased. Increased temporal dispersion of APs decreased the MUAP amplitude and area slightly but did not affect the MUAP duration. It is inferred that different features of the CN MUAP are determined by the distribution of muscle fibers within different portions of the MU territory and thus provide complementary information about the MU architecture.

Simulation of concentric needle EMG motor unit action potentials

This study was performed to evaluate an automatic method of motor unit action potential (MUAP) analysis developed in our laboratory. MUAPs were recorded from the biceps brachii muscle of 68 normal subjects and 122 patients with nerve or muscle disease. The values of mean MUAP durations from normal subjects obtained by automatic analysis were similar to those reported in the literature. However, the normal range of MUAP amplitude and the incidence of polyphasic MUAPs were much higher. Normal ranges of mean MUAP area, area/amplitude ratio, and the number of turns were also defined. Automatic analysis demonstrated an abnormality of at least one MUAP feature in 70% of patients. There was concordance between automated analysis and visual assessment of MUAPs in 76% of patients with neuropathy but in only 50% of patients with myopathy. The relationships between different MUAP features seen in neuropathy and myopathy are explained in physiologic terms.

Evaluation of an automatic method of measuring features of motor unit action potentials.

The electromyographic (EMG) interference pattern (IP) was measured in the biceps muscle of 16 normal male and 17 normal female subjects. The activity, upper centile amplitude (UCA), and the number of small segments (NSS) (defined in a companion paper) were measured from 500-msec epochs of the IP. The normal values of these features were defined separately for men and women by plotting the UCA and NSS values against activity for each epoch and defining an area on these plots, called a "cloud," that contained more than 90% of the datum points from each study. The mean deviation of the individual datum points from the overall mean values was also calculated for each study. A study in one muscle is considered to be normal if more than 90% of the datum points from that muscle are within the normal clouds and the deviation values are within their normal range. In patients with neuropathy, the characteristic pattern was increased UCA with normal or decreased NSS. In patients with myopathy, NSS was increased and the UCA was normal or decreased. In all studies, the interpretations of the IP from the plots agreed with qualitative assessments of the IP made independently by an electromyographer. The use of these features to understand and quantitate the changes in the motor units produced by disease is demonstrated by serial studies performed in a patient with motor neuron disease.

Sanjeev D. Nandedkar

Papers

Automatic analysis of the EMG interference pattern

Analysis of amplitude and area of concentric needle EMG motor unit action potentials

Simulation of concentric needle EMG motor unit action potentials

Evaluation of an automatic method of measuring features of motor unit action potentials.

Automatic analysis of the electromyographic interference pattern. Part II: Findings in control subjects and in some neuromuscular diseases.