The Use of Stimulated EMG in the Diagnosis of Neuromuscular Junction Abnormality

Abstract

Despite the increasing availability of genetic panels and their diminishing cost genetic screening for myasthenic conditions in children remains difficult. Additionally some children with undoubted myasthenic conditions have yet to be genetically characterised. For this reason a neurophysiological test for the diagnosis of disorders of the neuromuscular junction (NMJ) is valuable. Of the options available repetitive nerve stimulation up (RNS) is relatively insensitive and uncomfortable and reliance on a form of single fibre EMG (SFEMG) is essential. Volitional SFEMG presents formidable challenges in children and is unlikely to be successful in any child under the age of eight years. Stimulation techniques increase the feasibility of studies in children. A modification of stimulation SFEMG using concentric needle electrodes has been developed. Because multiple motor units are stimulated true single-fibre potentials are not seen with this technique and we therefore describe the technique as Stimulated Potential Analysis using Concentric needle Electrodes under the acronym SPACE. Examinations are conducted on orbicularis oculi being the most sensitive muscle in most myasthenic conditions. With the use of local anaesthetic it is possible to effect successful examination in most children. Monopolar needles are placed near the facial nerve as it crosses the zygomatic arch with the potentials recorded from orbicularis oculi. The use of peak detection algorithms on most EMG machines allows some quantification of the results producing a value for the Mean Consecutive Difference (MCD) and these are unaffected by the high-frequency filter used. Quantification of the MCD becomes increasingly difficult with increasing abnormality of the underlying NMJ. Normative data have been derived from the extrapolated norms (e-norms) technique. When applied to a large cohort of patients the technique demonstrates sensitivity 84%, specificity 71%, negative predictive value 96%, and positive predictive value 36% for the diagnosis of myasthenia. Specificity can be further enhanced if those conditions caused by a neurogenic abnormality are excluded. The technique is within the competence of any trained clinical neurophysiologist and is well tolerated by patients and parents alike. It is an important neurophysiological technique for the diagnosis of neuromuscular junction abnormality.