Single fiber electromyography in the differential diagnosis of myopathic limb girdle syndromes and chronic spinal muscular atrophy

Guidelines for single fiber EMG

This document is the consensus of international experts on the current status of Single Fiber EMG (SFEMG) and the measurement of neuromuscular jitter with concentric needle electrodes (CNE - CN-jitter). The panel of authors was chosen based on their particular interests and previous publications within a specific area of SFEMG or CN-jitter. Each member of the panel was asked to submit a section on their particular area of interest and these submissions were circulated among the panel members for edits and comments. This process continued until a consensus was reached. Donald Sanders and Erik Stålberg then edited the final document.

Spinal muscular atrophy: diagnosis and management in a new therapeutic era

Spinal muscular atrophy (SMA) describes a group of disorders associated with spinal motor neuron loss. In this review we provide an update regarding the most common form of SMA, proximal or 5q-SMA, and discuss the contemporary approach to diagnosis and treatment. Electromyography and muscle biopsy features of denervation were once the basis for diagnosis, but molecular testing for homozygous deletion or mutation of the SMN1 gene allows efficient and specific diagnosis. In combination with loss of SMN1, patients retain variable numbers of copies of a second similar gene, SMN2, which produces reduced levels of the survival motor neuron (SMN) protein that are insufficient for normal motor neuron function. Despite the fact that understanding of how ubiquitous reduction of SMN protein leads to motor neuron loss remains incomplete, several promising therapeutics are now being tested in early-phase clinical trials.

Single-fiber electromyography in hyperCKemia: the value of fiber density

Although persistently raised serum creatine kinase (sCK), or hyperCKemia, is considered the biological hallmark of neuromuscular diseases, pauci- or asymptomatic- or isolated-hyperCKemia can often be found. Single-fiber electromyography (SFEMG) is an electrophysiological technique of great value in the assessment of neuromuscular, neuropathic and myopathic disorders. We hypothesize that SFEMG fiber density (FD) evaluation is able to detect subclinical electrophysiological abnormalities indicating a myopathic process in subjects with hyperCKemia. Nineteen subjects with hyperCKemia without evident clinical signs of muscle involvement and 15 healthy controls were studied. Electrophysiological investigations including nerve conduction studies (NCS), quantitative EMG (QEMG), SFEMG with focus on FD measurements, and muscle biopsy were performed. NCS, QEMG, SFEMG were normal in all controls. In subjects with hyperCKemia, NCS were normal; QEMG was abnormal in 5, while both SFEMG and muscle biopsy disclosed abnormalities in 12 subjects. The mean FD value was 2.6 ± 0.5 in the control and 4 ± 1.4 (p = 0.003) in the hyperCKemia group. SFEMG revealed subclinical changes in the majority of subjects with hyperCKemia. To the best of our knowledge, this is the first study demonstrating that SFEMG FD evaluation is able to detect the presence of muscle diseases, which are in a subclinical phase and would remain unidentified otherwise. SFEMG may be used to distinguish hyperCKemia associated to asymptomatic muscle disorders from idiopathic hyperCKemia. We believe that SFEMG FD evaluation should be added to the routine examinations in the screening of idiopathic hyperCKemia.

Single-fiber electromyography

Single-fiber EMG is a technique introduced in 1963 by Stålberg and Ekstedt for recording single muscle fiber action potentials by means of a specially constructed needle with a 25-microm recording surface. The needle is positioned in the muscle to record from two or more time-locked potentials belonging to the same motor unit. Jitter is the variability in the arrival time of action potentials to the recording electrode between consecutive discharges. This variability reflects end-plate conduction and is measured along with fiber density, which is the average number of fibers belonging to the same motor unit that is in the recording area. An abnormal test is one in which more than 10%, or the mean, of 20 fiber pairs has increased jitter when compared with normal reference values. Increased fiber density is seen with reinnervation. Single-fiber EMG is more sensitive than conventional EMG and is the most sensitive, but not specific, test for myasthenia gravis. Lambert-Eaton myasthenic syndrome, and other neuromusculasr junction pathology. It has been useful in the evaluation of some neuropathies and myopathies and has provided valuable information on the motor unit spatial arrangement, territory, microphysiology, and pathophysiology.

Limb-girdle muscular dystrophy: a follow-up study of 79 patients

The limb-girdle muscular dystrophies (LGMD) are autosomally inherited neuromuscular diseases. Recently six different loci for LGMD have been reported: 5q (LGMD1A), 15q (LGMD2A), 2p (LGMD2B), 13q (LGMD2C), 17q (LGMD2D) and 4p-14-q21.2 (LGMD2E) respectively. We have studied 79 patients affected by LGMD during the period 1976 to 1995. All patients were examined clinically, and various investigations, including genetics were performed. According to their data we divided them as follow: 1) Cases with autosomal recessive inheritance (34.19%) of these two families are linked to chromosome 2p and the others were subdivided according to the age at onset into childhood LGMD and juvenile-adult LGMD; 2) Cases with dominant inheritance (13.92%); 3) Sporadic cases (51.89%). Onset of symptoms occurs from the first to the third decade. The clinical course varies considerably, as does the degree of disability. Our study allowed to identify two different groups of patients who relatively homogeneous with respect to their clinical and laboratory characteristics.

Limb girdle muscular dystrophy: reappraisal of a rejected entity

The term limb girdle muscular dystrophy (LGMD) has been introduced to delineate a distinct form of muscular dystrophy with predominantly proximal upper and lower extremity weakness. Families with evidence of both autosomal recessive and autosomal dominant modes of inheritance have been described. The recognition of other disorders presenting with weakness in a limb girdle distribution, such as the spinal muscular atrophies, dystrophinopathies, inflammatory and metabolic myopathies, casted doubt on the existence of LGMD as a separate entity. Recent linkage studies showing association between various forms of LGMD and loci on chromosome 15, 13 and 5 respectively, and the demonstration of 50K dystrophin associated glycoprotein deficiency in some cases of LGMD, strongly support the notion that limb girdle muscular dystrophy constitutes a separate group of phenotypically and genotypically distinct disorders. Further investigations are necessary to recognize the different subtypes of this disease and to identify the underlying mutations.