Electrodiagnostic testing is a useful tool in the evaluation of suspected myopathy and helps to confirm the presence of a myopathy and exclude disease mimickers. The electrodiagnostic pattern of findings during testing guides subsequent laboratory evaluation, genetic testing, and in identifying potential muscle biopsy targets. It also provides a baseline for subsequent assessment of disease progression or response to therapy. This article summarizes the approach to electrodiagnostic assessment in various myopathic disorders.

Purpose/Aim: Acquired neuromyotonia or Isaacs syndrome is a type of peripheral nerve hyperexcitability of autoimmune origin. It may occur as an isolated, paraneoplastic or accompanied with some autoimmune diseases. This report describes acquired neuromyotonia in a child with a new reported association with vitamin D deficiency. Case report: A 9-year-old child, in whom the diagnosis of acquired neuromyotonia was made by clinical and typical electromyographic findings. All paraneoplastic and autoimmune workup was normal, except for a vitamin D deficiency state. A dramatic improvement was recorded on both clinical and electrophysiological base after vitamin D replacement. Conclusion: An in-depth future analysis of vitamin D status in patients with neuromyotonia will help to establish whether the association of neuromyotonia with vitamin D deficiency is casual or whether these two conditions may be causally related.

Assessment of spontaneous waveforms recorded in a resting muscle during needle electromyography is important to determine the type of underlying neuromuscular disorder, temporal course of a disease, and severity and prognosis. A variety of different spontaneous waveforms may be recorded. Some waveforms may be recording in patients without neuromuscular disorders, such as end-plate activity or fasciculation potentials, while others occur only in abnormal muscles. Fibrillation potentials are the most common abnormal spontaneous waveform and are encountered in a wide variety of neuromuscular disorders causing denervation or damage to muscle fibers. Myotonic discharges, when diffuse, are seen in a small number of myopathies or muscle channelopathies. Complex repetitive discharges are nonspecific spontaneous waveforms that may be encountered in chronic or longstanding neurogenic or myopathic disorders. Myokymic and neuromyotonic discharges are rare spontaneous waveforms that suggest either focal or diffuse peripheral nerve hyperexcitability. When interpreted in conjunction with voluntary motor unit potentials as well as nerve conduction study findings, spontaneous waveforms are useful to fully assess the types of disorders of patients with neuromuscular complaints.

Needle electromyography (EMG) findings help confirm myopathy and may indicate specific pathologic changes on muscle biopsy.
We conducted a retrospective chart review of 218 consecutive patients referred for muscle biopsy. Presence of specific needle EMG findings was correlated with pathologic findings of inflammation, necrosis, splitting, and vacuolar changes. Sensitivity, specificity, and positive and negative predictive values of specific EMG findings for pathologic changes were calculated.
Short-duration motor unit potentials (MUP) were sensitive (83%-94%) but not specific (34%-49%) for pathologic changes. Fibrillation potentials were 65%-74% sensitive and 58%-81% specific for inflammation, necrosis, splitting, or vacuolar changes. The absence of fibrillation potentials had high negative predictive value (82%-93%) for inflammation, splitting, or vacuolar changes.
Fibrillation potentials and short-duration MUPs predict pathologic changes of muscle fiber necrosis, splitting, and/or vacuolar changes (as seen with inflammatory myopathies and muscular dystrophies). Absence of fibrillation potentials suggests other myopathologic changes (e.g., congenital myopathy). Muscle Nerve 59:315-320, 2019.

Schwartz-Jampel syndrome is a rare autosomal recessive disorder with joint contractures, generalized myotonia, skeletal anomalies, and facial dysmorphism. The patients with Schwartz-Jampel syndrome have muscle stiffness and electromyography reveals complex, repetitive discharges as myotonic discharges. It is unusual for a Schwartz-Jampel syndrome case to have recurrent gastrointestinal bleeding episodes. The stable endothelial barrier is provided by perlecan which is an important component of vascular structures. Thus, perlecan deficiency may cause recurrent gastroduodenal bleeding. Our report is unique with being the first reported Schwartz-Jampel syndrome case with gastrointestinal bleeding.

Multiminicore disease is a congenital myopathy characterized pathologically by the presence of multiple minicore structures in the sarcoplasm. Mutations in the selenoprotein N1-encoding gene (SEPN1) and ryanodine receptor 1-encoding gene (RYR1) are responsible for half of the reported cases. Mutations in multiple epidermal growth factor-like domains 10-encoding gene (MEGF10) have been identified only recently in a few patients with antenatal to infantile-onset myopathy, with and without minicore pathology.
We report 2 sisters with adult-onset respiratory insufficiency followed by development of limb weakness. Both had scoliosis, distal joint hyperlaxity, and high-arched feet.
A biopsy of the right triceps muscle in 1 sister showed multiple minicore structures. She had electromyographic changes of myopathy with fibrillation potentials and myotonic discharges. Next generation sequencing identified novel compound heterozygous missense variants in MEGF10 c.230G>A (p.Arg77Gln) and c.1833T>G (p.Cys611Trp) in both sisters.
MEGF10 mutations can cause myopathy with adult-onset respiratory insufficiency. Muscle Nerve, 2016 Muscle Nerve 53: 984-988, 2016.

We retrospectively reviewed 2030 childhood electromyograms performed over an 11-year period (2004-2014). Twenty children (1%) with myotonic discharges were identified and placed into 2 groups. Group A (electrical and clinical myotonia) comprised 9 children (8 with myotonia congenita and 1 with paramyotonia congenita); all of them had diffuse myotonic discharges without clinical weakness or elevated creatine kinase. Group B (electrical myotonia without clinical myotonia) comprised 11 children (4 with inflammatory myopathy; 3, congenital myopathy, 3, muscular dystrophy; and 1, congenital muscular dystrophy). Clinical weakness was demonstrated in all of them and elevated creatine kinase in 6; all had a myopathic electromyogram and scattered myotonic discharges. We conclude that myotonic discharges are a rare but characteristic spontaneous discharge identified during electrodiagnostic studies in children. The presence of electrical and clinical myotonia provides helpful clues to differentiate between various muscle disorders in children.

Non-dystrophic myotonic syndromes represent a heterogeneous group of clinically quite similar diseases sharing the feature of myotonia. These syndromes can be separated into chloride and sodium channelopathies, with gene-defects in chloride or sodium channel proteins of the sarcolemmal membrane. Myotonia has its basis in an electrical instability of the sarcolemmal membrane. In the present study we examine the discriminative power of the resulting myotonic discharges for these disorders. Needle electromyography was performed by an electromyographer blinded for genetic diagnosis in 66 non-dystrophic myotonia patients (32 chloride and 34 sodium channelopathy). Five muscles in each patient were examined. Individual trains of myotonic discharges were extracted and analyzed with respect to firing characteristics. Myotonic discharge characteristics in the rectus femoris muscle almost perfectly discriminated chloride from sodium channelopathy patients. The first interdischarge interval as a single variable was longer than 30 ms in all but one of the chloride channelopathy patients and shorter than 30 ms in all of the sodium channelopathy patients. This resulted in a detection rate of over 95%. Myotonic discharges of a single muscle can be used to better guide toward a molecular diagnosis in non-dystrophic myotonic syndromes.

OBJECTIVE: Pathological spontaneous activity (PSA) in electromyography (EMG) has not yet been systematically analysed in various types of myopathies.
METHODS: 136 Patients with well-defined myopathies were retrospectively analysed for the presence of PSA in distal, proximal, and paravertebral muscles. PSA comprised fibrillations (fib)/positive sharp waves (PSW) and high frequency discharges (HFD; i.e., myotonic and complex repetitive discharges).
RESULTS: fib/PSW occurred more frequently than HFD. HFD were rarely myotonic in nature. 50% and more patients presented with HFD in PROMM (80%), Pompe\'s disease (70%), matrin-3 myopathy (60%), sIBM (50%), CNM (50%), while far less than 50% of the patients showed RD in LGMD2I (21%), LGMD2A (17%), LGMD2B (17%), LGMD2L (14%), FSHD (4%), BMD (0%). Four different HFD patterns were proposed.
CONCLUSIONS: The segregation of myopathies relative to the occurrence of PSA and especially HFD in a high prevalence group and in a low prevalence group may be diagnostically valuable.
CONCLUSIONS: The screening for HFD by means of EMG is also valuable in the diagnosis of non-myotonic myopathies.

BACKGROUND: Rhabdomyolysis results from many causes including hypernatremia. Postpartum hypernatremia with osmotic cerebral demyelination is a rare cause of reversible rhabdomyolysis. Electromyographic studies in postpartum hypernatremia have not been reported.
METHODS: Electromyography (EMG) was performed in five women with postpartum hypernatremia and muscle biopsy was performed in one of them.
RESULTS: Among the five women presenting with postpartum hypernatremia associated with marked elevation of serum creatine kinase, four had quadriparesis. All had varying degrees of encephalopathy at admission and recovered without residual deficits after gradual correction of hypernatremia. Needle EMG revealed fibrillations with positive sharp waves in five patients and myotonic discharges in three patients. Serial EMG in one patient revealed the occurrence of transient fibrillations, positive sharp waves and myotonic discharges. Muscle biopsy revealed extensive rhabdomyolysis in one patient.
CONCLUSIONS: EMG in hypernatremic rhabdomyolysis revealed spontaneous activity including fibrillations, positive sharp waves and myotonic discharges along with myopathic potentials. Electromyographic findings depend on the interval from the onset and the degree of rhabdomyolysis.