Exome sequencing detection of two untranslated GFPT1 mutations in a family with limb-girdle myasthenia.

The term 'limb-girdle myasthenia' (LGM) was first used to describe three siblings with proximal limb weakness without oculobulbar involvement, but with EMG decrement and responsiveness to anticholinesterase medication. We report here that exome sequencing in the proband of this family revealed several sequence variations in genes linked to proximal limb weakness. However, the only mutations that cosegregated with disease were an intronic IVS7-8A>G mutation and the previously reported 3'-UTR c.*22C>A mutation in GFPT1, a gene linked to LGM. A minigene assay showed that IVS7-8A>G activates an alternative splice acceptor that results in retention of the last seven nucleotides of intron 7 and a frameshift leading to a termination codon 13 nucleotides downstream from the new splice site. An anconeus muscle biopsy revealed mild reduction of the axon terminal size and postsynaptic fold simplification. The amplitudes of miniature endplate potentials and quantal release were also diminished. The DNA of the mildly affected father of the proband showed only the intronic mutation along with sequence variations in other genes potentially relevant to LGM. Thus, this study performed in the family originally described with LGM showed two GFPT1 untranslated mutations, which may cause disease by reducing GFPT1 expression and ultimately impairing protein glycosylation.

Congenital myasthenic syndrome associated with epidermolysis bullosa caused by homozygous mutations in PLEC1 and CHRNE.

Mutations in the plectin gene (PLEC1) cause epidermolysis bullosa simplex (EBS), which may associate with muscular dystrophy (EBS-MD) or pyloric atresia (EBS-PA). The association of EBS with congenital myasthenic syndrome (CMS) is also suspected to result from PLEC1 mutations. We report here a consanguineous patient with EBS and CMS for whom mutational analysis of PLEC1 revealed a homozygous 36 nucleotide insertion (1506_1507ins36) that results in a reduced expression of PLEC1 mRNA and plectin in the patient muscle. In addition, mutational analysis of CHRNE revealed a homozygous 1293insG, which is a well-known low-expressor receptor mutation. A skin biopsy revealed signs of EBS, and an anconeus muscle biopsy showed signs of a mild myopathy. Endplate studies showed fragmentation of endplates, postsynaptic simplification, and large collections of thread-like mitochondria. Amplitudes of miniature endplate potentials were diminished, but the endplate quantal content was actually increased. The complex phenotype presented here results from mutations in two separate genes. While the skin manifestations are because of the PLEC1 mutation, footprints of mutations in PLEC1 and CHRNE are present at the neuromuscular junction of the patient indicating that abnormalities in both genes contribute to the CMS phenotype.

Mutations in LAMB2 causing a severe form of synaptic congenital myasthenic syndrome.

BACKGROUND: We describe a severe form of congenital myasthenic syndrome (CMS) associated with congenital nephrosis and ocular malformations caused by two truncating mutations in the gene encoding the laminin beta2 subunit (LAMB2). METHODS AND RESULTS: Mutational analysis in the affected patient, who has a history of a serious untoward reaction to treatment with acetylcholinesterase inhibition, revealed two frame-shifting heteroallelic mutations, a maternally inherited 1478delG and a paternally inherited 4804delC. An anconeus muscle biopsy demonstrated a profound distortion of the architecture and function of the neuromuscular junction, which was strikingly similar to that seen in mice lacking laminin beta2 subunit. The findings included: pronounced reduction of the axon terminal size with encasement of the nerve endings by Schwann cells, severe widening of the primary synaptic cleft and invasion of the synaptic space by the processes of Schwann cells, and moderate simplification of postsynaptic folds and intact expression of the endplate acetylcholinesterase. The endplate potential quantal content was notably reduced, while the frequencies and amplitudes of miniature endplate potentials were only moderately diminished and the decay phases of miniature endplate potentials were normal. Western blot analysis of muscle and kidney tissue and immunohistochemistry of kidney tissue showed no laminin beta2 expression. CONCLUSION: This case, which represents a new type of synaptic CMS, exemplifies the wide variability of phenotypes associated with LAMB2 mutations and underscores the fundamental role that laminin beta2 plays in the development of the human neuromuscular junction.

Presynaptic failure of neuromuscular transmission and synaptic remodeling in EA2.

OBJECTIVE: To further investigate the basis of abnormal neuromuscular transmission in two patients with congenital myasthenic syndrome associated with episodic ataxia type 2 (EA2) using stimulated single fiber EMG (SFEMG) and in vitro microelectrode studies. METHODS: Two patients with genetically characterized EA2 previously shown to have abnormal neuromuscular transmission by voluntary SFEMG were studied with stimulated SFEMG and anconeus muscle biopsy with microelectrode studies and electron microscopy of the neuromuscular junction. RESULTS: In vivo stimulated SFEMG showed signs of presynaptic failure, with jitter and blocking that improved with increased stimulation frequency. Additional evidence of presynaptic failure was provided by the in vitro microelectrode studies, which showed marked reduction of the end plate potential quantal content in both patients. Of note, the end plate potentials showed high sensitivity to N-type blockade with omega-conotoxin not seen in controls. The ultrastructural studies revealed some evidence of small nerve terminals apposed to normal or mildly overdeveloped postsynaptic membranes, suggesting an ongoing degenerative process. CONCLUSIONS: The authors demonstrated presynaptic failure of neurotransmission in patients with heterozygous nonsense mutations in CACNA1A. The contribution of non-P-type calcium channels to the process of neurotransmitter release in these patients likely represents a compensatory mechanism, which is insufficient to restore normal neuromuscular transmission.

Presynaptic congenital myasthenic syndrome due to quantal release deficiency.

OBJECTIVE: To provide clinical, electrophysiologic, and ultrastructural findings in three patients with a presynaptic congenital myasthenic syndrome (CMS). BACKGROUND: Familial infantile myasthenia and paucity of synaptic vesicles are the only two fully characterized CMS. We are describing here three patients with another form of presynaptic CMS characterized by deficiency of the action potential-dependent release without reduction of the spontaneous release of neurotransmitter from the nerve terminal. METHODS: The authors performed electromyography and anconeus muscle biopsies that included intracellular recordings and electron microscopy of the neuromuscular junction in three patients with presynaptic CMS. They also sequenced part of the P/Q-calcium alpha(1)-subunit gene (CACNA1A) and the acetylcholine receptor subunit (AChR) genes in these patients. RESULTS: In these patients there were additional neurologic findings including nystagmus and ataxia. In all three patients the end-plate potential quantal content (m) was markedly reduced but neither the amplitudes nor the frequencies of miniature end-plate potentials were diminished. Ultrastructurally, postsynaptic end-plate folds, nerve terminal size, and synaptic vesicle number were normal but double-membrane-bound sacs containing synaptic vesicles were present in the nerve terminal of all three patients. The screening of reported pathogenic mutations in the CACNA1A and a mutational analysis of AChR subunit genes were negative. CONCLUSION: This form of CMS appears to result only from a deficiency of the quantal release of neurotransmitter that may be due to an abnormal calcium mechanism or impaired endocytosis and recycling of synaptic vesicles.

AAEM case report 16. Botulism. American Association of Electrodiagnostic Medicine.

Early diagnosis of botulism is essential for effective treatment. Electrophysiologic testing can be of major help to establish a prompt diagnosis, but the classic electrodiagnostic features of botulism are often elusive. Decrement or increment of compound muscle action potential (CMAP) amplitudes to slow or fast rates of nerve stimulation are often unimpressive or totally absent. Reduction of CMAP amplitudes, denervation activity, or myopathic-like motor unit potentials in affected muscles are found more frequently but they are less specific. In general, the electrophysiologic findings taken together suggest involvement of the motor nerve terminal, which should raise the possibility of botulism. The case reported here illustrates a common clinical presentation of botulism. This study emphasizes realistic expectations of the electrodiagnostic testing, the differential diagnosis, and the potential pitfalls often encountered in the interpretation of the electrophysiologic data.

Diagnostic difficulties in patients with adul botulism type a.

Although the ekcuodiagnostic abnormalities in botulism have bean well characterized, the expected abnormalities, such as facilitation of compound muscle anion potential amplitudes after sustained activation or with repetitive stimulation at fast tales, may not always be present especially early m the disease. This may lead to etectrodtagnosttc difficulties and sometimes a delay in establishing a diagnosis. We describe four serologically proven cases of type A botulism in adults that illustrate the variability in ekctrodiagnostic findings in this disease.

Morvan's fibrillary chorea: a paraneoplastic manifestation of thymoma.

Morvan's fibrillary chorea is a rare disease characterised by symptoms which include neuromyotonia, cramping, weakness, pruritus, hyperhidrosis, insomnia, and delirium. The first case of Morvan's fibrillary chorea to be associated with clinical manifestations of myasthenia gravis with thymoma, psoriasis, and atopic dermatitis is reported. Muscle histopathology disclosed chronic denervation and myopathic changes and in vitro electrophysiology demonstrated both presynaptic and postsynaptic defects in neuromuscular transmission. Serum antibodies to acetylcholine receptors, titin, N-type calcium channels, and voltage gated potassium channels were detected. Plasmapheresis, thymectomy, and long term immunosuppression induced a dramatic resolution of symptoms. The association of thymoma with other autoimmune disorders and autoantibodies, and prolonged and sustained remission with chronic immunosuppression, place Morvan's fibrillary chorea on the range of neurological diseases arising as a paraneoplastic complication of cortical thymomas.

Fulminant demyelinating neuropathy mimicking cerebral death.

Guillain-Barré syndrome can very rarely present with acute quadripares and cranial nerve involvement resembling a locked-in state. We describe a very unusual case of fulminant neuropathy in a child who was previously exposed to vincristine. The clinical picture resembled brain death; however, electrodiagnostic studies led to the diagnosis of a peripheral neuropathy. Serial electrodiagnostic studies and pathologic findings confirmed demyelination.

Cluster of wound botulism in California: clinical, electrophysiologic, and pathologic study.

Over a period of 15 months we have seen 6 patients with long-standing history of subcutaneous heroin injections who experienced acute blurred vision, dysphagia, dysarthria, and generalized weakness. Decreased or absent deep tendon reflexes, pupillary abnormalities, incremental responses to fast repetitive nerve stimulation, and positive serology for Clostridia botulinum toxin A were found, but not in all cases. Muscle biopsies showed variable signs of neurogenic atrophy. In vitro electrophysiology studies revealed decreased end-plate potentials quantal content, confirming the presynaptic nature of the disorder. Mechanical ventilation was required in 5 patients. Half of the patients were treated with polyvalent antitoxiin. Prognosis was favorable, though recovery was slow. In conclusion, acute bulbar weakness with visual symptoms in patients with subcutaneous heroin abuse strongly suggets the possibility of wound botulism. High diagnostic suspicion combined with histology and in vitro electrophysiology confirmation of presynaptic failure, especially in seronegative cases, may significantly improve morbidity.

End-plate electromyography: use of spectral analysis of end-plate noise.

An automated technique for estimating the channel open time of human end-plate acetylcholine receptors (AChR) is described. The method, which is based on the power spectral analysis of electromyographic end-plate noise, is simple and permits a rapid estimation of the dominant AChR ion channel kinetics. Values of AChR channel open time determined by spectral analysis of end-plate noise in adults (1.23 +/- 0.01 ms at 33 degrees C) were consistently shorter than those estimated from the decay phase of extracellularly recorded miniature end-plate potentials. The channel open time extracted from the end-plate noise compared well with that derived from the analysis of in vitro recorded acetylcholine-induced current noise. Therefore, spectral analysis of end-plate noise may provide a good estimation of the kinetics of the human AChR ion channel. The technique is particularly useful in identifying congenital myasthenic syndromes with altered AChR ion channel function.

Function and ultrastructure of the neuromuscular junction in post-polio syndrome.

We performed a detailed morphological and electrophysiological analysis of the neuromuscular junction in muscle biopsies from 10 patients with post-polio syndrome (PPS). This was done to clarify the basis for the apparent neuromuscular transmission impairment in PPS. In six patients, intracellular microelectrode recordings demonstrated either reduction of amplitudes of miniature end-plate potentials (MEPPs) or decreased quantal content or both. In one patient, reduction of quantal content was only present with prolonged or high-frequency nerve stimulation. In three patients no significant abnormalities were found by the intracellular microelectrode studies. Histologically, atrophy of individual muscle fibers were present in 6 out of the 10 biopsies, but grouped atrophy was not seen. Fiber type grouping suggesting reinnervation was seen in 8 out of the 10 muscle biopsies. Fragmentation and dispersion of the end plate was present in three patients. In two of these patients dispersion of the end plate was associated with an apparent increase of the quantal content. Electron microscopy revealed either normal neuromuscular junctions or small axon termini apposed to normal postsynaptic folds. In summary, variable degrees and different types of failure of neuromuscular transmission were seen in association with histological signs of reinnervation in the muscle biopsies of affected patients. Functional and structural abnormalities of the neuromuscular junction, although very common, were not invariably present and, therefore, they do not appear to be a necessary condition to define the post-poliomyelitic syndrome.