Repetitive nerve stimulation in myasthenia gravis—relative sensitivity of different muscles Joa ˜o Costa a , Teresinha Evangelista a , Isabel Conceic ¸a ˜o a,b , Mamede de Carvalho a,b, * a Department of Neurology, Hospital de Santa Maria, Av. Prof. Egas Moniz, 1649 Lisbon, Portugal b Laboratory of Electromyography, Centro de Estudos Egas Moniz, Faculty of Medicine, Institute of Molecular Medicine, Lisbon, Portugal Accepted 24 May 2004 Available online 18 September 2004 Abstract Objective: To correlate repetitive nerve stimulation (RNS) decrement in different muscles with the predominant clinical presentation in myasthenia gravis (MG), and to study single fibre EMG (SFEMG) sensitivity in ocular MG. Methods: Sixty-nine, untreated, consecutive patients suspected for MG were observed prospectively for a minimum of 6 months. Those who improved on medical treatment were diagnosed as MG. The others, in whom the neurophysiological studies were normal and that did not improve on medical treatment served as a control group, from which normative data for RNS and SFEMG was obtained. The MG patients were further classified in 3 subgroups according to the predominant clinical presentation: group I (ocular); group b (bulbar); and group a (axial). We performed RNS in nasalis, trapezius, anconeus, and abductor digiti minimi. All patients with ocular MG underwent jitter determination of the orbicularis oculi muscle. Results: Thirty-seven patients were diagnosed as MG (group I, 15; group b, 13; group a, 9). In group I, RNS was abnormal in 33% of the patients. RNS studies disclosed at least one abnormal muscle response in every patient in groups a and b. Trapezius was significantly more sensitive in group a, and anconeus and nasalis in group b ðP , 0:01Þ: Jitter was abnormal in all patients in group I, and the most sensitive parameter was an increased number of unstable pairs, 100%. Conclusions: Based on these observations, we recommend that a shoulder muscle, as the trapezius, should be studied first in the limb-axial presentation of MG, and the anconeus –nasalis muscles in predominant bulbar MG. In ocular MG, RNS is not sensitive and jitter should be performed in facial muscles. Significance: This paper shows the unequal sensitivity of several muscles to RNS in different forms of MG. q 2004 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. Keywords: Myasthenia gravis; Repetitive nerve stimulation; Single-fibre EMG; Jitter 1. Introduction Repetitive nerve stimulation (RNS), introduced by Harvey and Masland (1941), has become the most routinely used test for the diagnosis of myasthenia gravis (MG) (Keesey, 1989; Stalberg, 1980). RNS is commonly available in neurophysiology laboratories and shows a good correlation with the clinical assessment of disease severity (Keesey, 1989). Most often recordings are from the small hand muscles, although proximal muscles have been shown to be more sensitive than distal ones (Claussen et al., 1995; Kennett and Fawcett, 1993; Krarup, 1977; Ozdemir and Young, 1976; Stalberg, 1980), and facial muscles are more sensitive in ocular MG (Oey et al., 1993). Overall RNS is abnormal in more than 70% of patients with MG when proximal muscles are investigated (Oh et al., 1992). This value is significantly lower in patients with mild generalized MG (Claussen et al., 1995; Stalberg and Sanders, 1981), and in patients with ocular MG the abnormality rate can be less than 30% of the tested subjects (Claussen et al., 1995; Stalberg, 1980). RNS studies in proximal muscles are more painful and more prone to artefactual changes (Claussen et al., 1995; Stalberg, 1980), but stimulation of the upper trapezius or anconeus muscles are better tolerated than stimulation of other proximal muscles at Erb’s point, in the axilla or in the leg (Claussen et al., 1995; Keesey, 1989; Kennett and Fawcett, 1993; Schumm and Stohr, 1984). Clinical Neurophysiology 115 (2004) 2776–2782 www.elsevier.com/locate/clinph 1388-2457/$30.00 q 2004 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.clinph.2004.05.024 * Corresponding author. Tel.: þ351-21-780-5219; fax: þ 351-21-752- 0801. E-mail address: mamedemg@mail.telepac.pt (M. de Carvalho).