AAEM minimonograph #25: Single-fiber electromyography

Single-fiber electromyography (SFEMG) is a selective recording technique in which a needle electrode with a small recording surface in the side is used to identify action potentials from individual muscle fibers. The SFEMG parameters of greatest clinical use are fiber density (FD) and neuromuscular jitter. FD reflects the local organization of muscle fibers within the motor unit; jitter reflects the safety factor of neuromuscular transmission at individual neuromuscular junctions. SFEMG can be of great value in demonstrating or excluding abnormalities in mild or questionable disease of nerve, muscle, or the neuromuscular junction. The neuromuscular jitter may be measured during nerve stimulation, which is particularly useful in uncooperative patients or when it is desirable to control the firing rate precisely, or during voluntary muscle activation, which is less subject to technical artifact. The SFEMG findings may not be specific to a particular diseases, but they frequently increase understanding of the disease process by demonstrating abnormal neuromuscular transmission or rearrangement of muscle fibers within the motor unit, which complements information from more conventional EMG examinations. © 1996 American Association of Electrodiagnostic Medicine. Published by John Wiley & Sons, Inc.     

AAEE minimonograph #29: automatic quantitative electromyography

The present status of different computerized methods of automatic quantitative electromyography are reviewed. Interference pattern methods-turns analysis, spectral analysis-are efficient, but the results usually cannot be directly related to the physiological properties of the motor units. Integration analysis does not currently have a major role in diagnostic electromyography. Traditional measurement of single motor unit action potentials during weak contraction can be facilitated and made more objective with computer assistance, but only the lowest-threshold motor units in the muscle are amenable to study. A new class of methodologies under development permit the decomposition of interference patterns into their constituent motor unit action potentials for measurement of configurational and behavioral properties. Patient data from these various methods can be statistically compared with normative data bases available on-line in computerized electromyographs. Both quantitative and quantitative electromyography have applications in the neuromuscular electrodiagnostic examination.     

Single-fiber electromyography in myasthenia gravis.

One-hundred-sixty single-fiber EMG studies of the extensor digitorum communis muscle were performed on 127 patients with myasthenia gravis; 131 demonstrated defective neuromuscular transmission. Jitter determinations in the biceps, deltoid or frontalis muscles increased the diagnostic yield significantly. Evoked-potential EMG studies were abnormal in less than 50 percent of patients in whom they were performed. The most sensitive criterion of abnormality was the percentage of fibers with increased jitter; the sensitivity of the test was enhanced, however, if the mean jitter of the tested muscle was also used as a criterion of abnormality. Since increased jitter may also be seen in primary muscle and nerve disease, these disorders must be excluded by other means before diagnosing myasthenia gravis on the basis of the single-fiber studies.     

Single-fiber electromyography in experimental autoimmune myasthenia gravis

The sensitivity of stimulated single-fiber electromyography in the detection of early abnormalities in neuromuscular transmission in experimental autoimmune myasthenia gravis (EAMG) was tested. Increased jitter and blocking were seen up to 3 weeks before clinical illness or decrement developed. Stimulation at 10 Hz appeared more sensitive in detection of abnormalities than stimulation at 3 or 5 Hz. Jitter values did not correlate with anti-Torpedo acetylcholine receptor (AChR), nor with anti-rat AChR antibody titer. No correlation was found between jitter and AChR loss or AChR-antibody complexes in muscle. It is concluded that, in addition to AChR loss and the presence of AChR-antibody complexes, other factors must determine the neuromuscular dysfunction in EAMG and possibly myasthenia gravis.     

Single-fiber electromyography of masseter muscle in myasthenia gravis

Jitter after axonal microstimulation in the masseter muscle was studied in 30 consecutive patients (12 women) with myasthenia gravis (MG). Patients' mean age was 42.3 (12-75), median disease duration was 3 months (1-72), and onset was ocular (15 cases), oculobulbar (7), bulbar (6), or generalized (2). There were 23 newly-diagnosed patients. Nine cases developed purely ocular MG and 21 cases developed generalized MG. In the latter group, five subjects had a rapidly progressive course and 16 subjects had stable or well-controlled disease (MGFA grade 2-3). Six patients did not have circulating anti-acetylcholine receptor antibodies. Masseter single-fiber electromyography (SFEMG) was abnormal in 6 of 9 ocular MG patients and in all generalized cases (overall sensitivity 27 of 30 cases or 90%; confidence interval 79.3%-100.0% at P = 0.95). Masseter should be considered for SFEMG in diagnosis of MG, especially in cases with bulbar onset.     

AAEE minimonograph #30: electrophysiologic studies of myoclonus

Definition as well as classification of myoclonus and electrophysiologic methods for investigating myoclonus were reviewed. Among the electrophysiologic techniques currently available in most laboratories, the EEG-EMG polygraph is the most essential one and can provide us with the most important information. Jerk-locked averaging and evoked potential studies are useful for further investigating the pathophysiology of myoclonus and can be performed by using the same recording electrodes as those used for the polygraph. Jerk-locked evoked potentials and double-stimulation evoked potentials can be employed only for further investigating how cerebral cortex is involved in the generation of certain myoclonia. All these techniques can be used in proper combinations depending on the clinical features of the myoclonus in question, the purpose of the study, and the facilities available in each laboratory. These techniques also will be useful for following the clinical course during the treatment with antimyoclonus agents.     

Single-fiber electromyography during menstrual exacerbation and ovulatory suppression in MuSK antibody-positive myasthenia gravis

We report a patient with muscle-specific tyrosine kinase (MuSK) antibody-positive myasthenia gravis who experienced worsening of myasthenic weakness associated with alterations in estrogen and progesterone levels during the administration of oral contraceptive therapy. Single-fiber electromyography was performed to document changes in neuromuscular transmission associated with the clinical exacerbation and subsequent resolution of the menstrual exacerbation and clinical improvement experienced with continuous monophasic oral contraceptive therapy. The potential long-term benefit of ovulatory suppression in MuSK antibody-positive myasthenia gravis is discussed.     

AAEE minimonograph #28: injury to peripheral motor nerves

Nerve injury triggers a sequence of electrophysiological events dependent upon changes occurring both at the neuromuscular junction and the muscle membrane. The time course of these physiologic and morphologic alterations is reviewed in light of recent information provided by studies of extrajunctional acetylcholine receptors, axoplasmic transport, and freeze-fracture electron microscopy. A thorough knowledge of these changes is essential to understanding the clinical impact of nerve injury.     

AAEE minimonograph #27: differential diagnosis of myotonic syndromes

Recent advances in neuromuscular diseases have also widened the diagnostic spectrum of myotonic disorders. Treatment, prognosis, and genetic aspects are different in the various syndromes and mandate a correct diagnosis. The combination of neurologic examination, standard EMG, exercise test, cold exposure, potassium loading, eye examination, and pedigree analysis allows correct classification of nearly all patients with myotonic disorders. In this review emphasis is placed on clinical features and electrophysiologic evaluation.     

AAEE minimonograph #26: The electrodiagnosis of carpal tunnel syndrome

The electrodiagnosis of carpal tunnel syndrome is reviewed, including discussions of old and new techniques of motor and sensory nerve conduction, anomalous innervation, needle electrode examination, and one method of examining a patient with suspected carpal tunnel syndrome. The results of electromyographic testing of 505 patients with carpal tunnel syndrome in Rochester, Minnesota, from 1961 to 1980 are compared with results from previous studies. In the appendixes, a method of performing median motor and sensory nerve conduction studies and Mayo Clinic normal values are provided.     

AAEE minimonograph #32: the electrophysiologic examination in patients with radiculopathies

A brief history of the evolution of radiculopathy as a clinical entity, and the use of electrodiagnostic studies to diagnose it, are provided. Root anatomy and the concept of myotomes and dermatomes are reviewed, as is the pathophysiology of radiculopathy. The value and limitations of the various electrophysiologic procedures used in the diagnosis of radiculopathies are discussed, including motor and sensory nerve conduction studies, late responses, somatosensory evoked potentials, nerve root stimulation, and the needle electrode examination. The specific muscles are enumerated which most often appear abnormal on needle electromyography with lesions of the various roots. The electrodiagnostic differentiation of root lesions from plexus lesions is described, and the various electrodiagnostic findings with lumbar canal stenosis are discussed. Finally, the value and limitations of the electrodiagnostic assessment in the evaluation of patients with suspected radiculopathies are reviewed.     

AAEM minimonograph #11: Needle examination in clinical electromyography

The physiologic and histologic principles underlying clinical electromyographic studies are briefly reviewed as an introduction to the normal and abnormal findings in human subjects. Technical aspects of recordings as well as the specific types of discharges and their significance are discussed.     

AAEM minimonograph #45: The early development of electromyography

The use of electricity for therapeutic purposes began in the first century and became more refined as the properties of electricity became more understood. The works of Franklin, Galvani, Volta, and others contributed to this body of knowledge. Development of the string galvanometer, the advent of the vacuum tube, the introduction of concentric needle electrodes, and the development of the cathode-ray oscilloscope occurred during the first half of the 20th century. The science of electromyography and electrodiagnosis grew in its sophistication, leading to the formation of the American Association of Electromyography and Electrodiagnosis (now the American Association of Electrodiagnostic Medicine) with James Golseth, MD, as its first president in 1953. © 1995 John Wiley & Sons, Inc.     

AAEE minimonograph #31: the electrodiagnosis of ulnar neuropathy at the elbow

Electrodiagnostic testing is useful in evaluating ulnar nerve elbow lesions. A flexed elbow seems preferable for conduction studies, since it eliminates the elbow segment slowing found in normals done in the extended position. Slowing of the motor velocity in the elbow segment was the most frequent abnormality in this study. Sensory conduction studies and needle examination each provided additional helpful data. Latency to ulnar forearm muscles and "inching" stimulations around the elbow are techniques that also deserve to be included in our standard armamentarium.     

Single-fiber electromyography in limb and facial muscles in muscle-specific kinase antibody and acetylcholine receptor antibody myasthenia gravis

We examined the findings from single-fiber electromyography in extensor digitorum communis (EDC) and orbicularis oculi (OOc) in 13 myasthenia gravis (MG) patients with muscle-specific kinase antibodies (MuSK-MG) and 12 MG patients with acetylcholine receptor antibodies (AChR-MG) with similar clinical scores. More than 70% of AChR-MG patients had abnormal jitter in both EDC and OOc, but the majority of MuSK-MG patients had normal jitter in EDC despite abnormal jitter in OOc. These findings demonstrate clear differences between the neurophysiology of MuSK-MG and AChR-MG.     

Single-fiber EMG with a concentric needle electrode: validation in myasthenia gravis

We performed a retrospective study to validate whether a disposable concentric needle electrode (CNE) can be used in place of a single-fiber (SF) electrode for jitter measurements in myasthenia gravis (MG). Normal values for voluntary contraction of orbicularis oculi (OO) and extensor digitorum communis (EDC) were collected from 20 healthy subjects. The method was validated by a retrospective analysis of 56 consecutive MG patients, the "gold standard" being a positive acetylcholine receptor (AChR) antibody titer at the time of the electrophysiological (electromyography) study and the clinical diagnosis. Receiver operating characteristic (ROC) curves were constructed to define maximal sensitivity and specificity of the technique. The sensitivity was 96.4% (95% confidence interval 87.5%-99.6%), with no false-positive results, similar to traditional SF EMG and confirming that the disposable CNE is a justifiable alternative.     

AAEM minimonograph #34: polyneuropathy: classification by nerve conduction studies and electromyography

Electrodiagnostic evaluation of patients with suspected polyneuropathy is useful for detecting and documenting peripheral abnormalities, identifying the predominant pathophysiology, and determining the prognosis for certain disorders. The electrodiagnostic classification of polyneuropathy is associated with morphologic correlates and is based upon determining involvement of sensory and motor fibers and distinguishing between predominantly axon loss and demyelinating lesions. Accurate electrodiagnostic classification leads to a more focused and expedient identification of the etiology of polyneuropathy in clinical situations.     

Surface stimulation single-fiber electromyography in myasthenia gravis.

Surface stimulation for single-fiber electromyography has been proposed previously as a novel technique based on a study performed on healthy subjects. The current study was designed to determine the diagnostic value of surface stimulation single-fiber electromyography in patients with myasthenia gravis. Ten patients with myasthenia gravis were recruited for the study. Surface stimulation of the facial nerve trunk, as well as extramuscular needle stimulation of its zygomatic branch was performed. Twenty single-fiber potentials were recorded from the orbicularis oculi muscle for each stimulation type in each patient. For both stimulation techniques, mean jitter values and the total number of abnormal individual junctions were found to be abnormally high. Jitter values obtained by surface stimulation were comparable with those obtained by needle stimulation.     

Single fiber electromyography in myasthenia gravis during pregnancy

We report the use of single fiber electromyography (SFEMG) to demonstrate changes in the physiologic abnormality of myasthenia gravis (MG) during pregnancy. A 23-year-old became pregnant 15 months after the onset of mild ocular weakness. On initial evaluation, SFEMG jitter measurements demonstrated a slight abnormality of neuromuscular transmission. There was no change in severity of clinical disease or jitter measurements until the third trimester, when she improved. Jitter measurements at that time were normal. Labor was normal and she delivered a normal male. Three days postpartum, myasthenic weakness recurred temporarily and jitter measurements showed worsening. At 16 days and 6 weeks postpartum, she had only minimal medial rectus weakness and jitter studies were normal. Three months postpartum, ocular symptoms recurred and jitter measurements were slightly abnormal. She continued to worsen, developing limb muscle and severe ocular muscle weakness at 4 months postpartum. She was treated with plasma exchange and thymectomy. Prednisone was added 2 months after thymectomy due to continued worsening and development of oropharyngeal weakness. Three years postpartum she was taking prednisone 10 mg every other day and had only slight weakness of neck flexors, and jitter studies were again normal. © 1993 John Wiley & Soncs, Inc.