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Testing the neuromuscular junction: what neurophysiology can offer the neurologist
  1. R G Whittaker
  1. Correspondence to Dr R G Whittaker, Department of Clinical Neurophysiology, Royal Victoria Infirmary, Queen Victoria Road, Newcastle upon Tyne NE1 4LP, UK; roger.whittaker{at}nuth.nhs.uk

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Introduction

The range of diseases affecting the neuromuscular junction (NMJ) is many and varied. Congenital forms due to single gene defects affecting various components of the NMJ include slow channel myasthenic syndrome, choline acetyltransferase deficiency and the recently described DOK-7 mutation.1 Of the acquired disorders, the archetypal example is autoimmune myasthenia gravis caused by antibodies to the acetylcholine receptor on the muscle membrane.2 Drugs (eg, magnesium and penicillamine) and toxins (eg, botulinum toxin) can also cause NMJ dysfunction.

Patients often complain of non-specific symptoms such as tiredness and fatigue, and there may be little to find on clinical examination. Many of the available treatments have significant side effects, and it would be a brave physician indeed who instigated potentially lifelong therapy without confirmatory tests.

For many NMJ diseases, there are tests of very high specificity. Examples include antiacetylcholine receptor antibody testing in myasthenia gravis, botulinum toxin assays for botulism and genetic testing for several forms of congenital myasthenic syndrome. These are the respective ‘gold standard’ tests. However, it often takes a long time for these results to become available, they can be expensive and although specific they may not be particularly sensitive. Furthermore, for some forms of congenital myasthenic syndrome, the responsible genetic defect has not yet been identified. Hence, there is a need for rapidly available functional tests to screen for the presence of NMJ dysfunction, to aid in the patient's immediate management and to guide further investigation.

Bedside tests of the NMJ

Although there are many bedside tests of NMJ function, these are not without problems. The rest test3 and the ice test4 (see Reddy and Backhouse5) each show reasonable sensitivity and specificity in ocular myasthenia, although sometimes with false negative responses.6 The Tensilon test also shows good sensitivity and specificity,7 8 but is associated with a …

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