SHORT REPORT ABSTRACT: The purpose of this study was to compare the diagnosticyield of repetitive radial nerve stimulation (RNS) while recording from exten-sor indicis proprius (EIP) to other commonly used muscle–nerve combina-tions in patients with myasthenia gravis (MG). Radial RNS with recordingfrom EIP was performed in 20 controls and 20 patients with MG. It revealedan abnormal decrement at rest in 35% of patients compared to 11% withulnar nerve stimulation, 64% with spinal accessory nerve stimulation, and74% with facial nerve stimulation. Radial-EIP RNS is a reliable technique inthe evaluation of MG and appears more sensitive than ulnar nerve RNS.

Muscle Nerve 33: 817–819, 2006

RADIAL NERVE REPETITIVE STIMULATIONIN MYASTHENIA GRAVIS

ADRIANA PETRETSKA, MD, RANDA JARRAR, MD, and DEVON I. RUBIN, MD

Department of Neurology, Mayo Clinic, 4500 San Pablo Rd., Jacksonville, Florida 32224, USA

Accepted 23 November 2005

Repetitive nerve stimulation (RNS) is a commonlyused technique to evaluate patients with suspectedmyasthenia gravis (MG). The diagnostic sensitivity ofRNS depends on the distribution of weakness anddisease severity, with a higher yield occurring fromproximal than distal nerves.1,5,7,8 Technical factorssuch as limb movement and patient discomfort limitthe reliability of proximal RNS, however, and distalnerves are therefore often studied initially.4 Onestudy has shown that finger and wrist extensor weak-ness is more common than interossei weakness inpatients with generalized MG.9 Therefore, evalua-tion of a distal finger extensor muscle by RNS mayprovide a higher yield in identifying abnormalitieson RNS. The aim of this study was to assess thereliability of radial RNS, with recording from theextensor indicis proprius (EIP) muscle, in MG pa-tients and to compare the frequency of abnormaldecrement of radial–EIP RNS to other nerve–musclecombinations.

MATERIALS AND METHODS

Patient Recruitment. The study was approved by ourinstitutional review board and all subjects provided

informed consent. All patients seen in the Neuromus-cular Clinic in 2003–2004 with a new or establisheddiagnosis of generalized MG were offered participa-tion. Patients who had another neuromuscular disor-der identified by routine EMG and MG patients inpharmacologic remission were excluded. Disease sever-ity was defined by the criteria from the MyastheniaGravis Foundation of America (MGFA).3 The study wasnot performed in Class I or Class V patients.

Technique. RNS was performed on the ulnar nerve/abductor digiti minimi, spinal accessory nerve/trapezius,or facial nerve/nasalis with 2-Hz stimulation before andafter exercise according to standard laboratory tech-nique.2 Anticholinesterase medications were held for atleast 6 h prior to the study. For radial–EIP RNS, 5-mm discelectrodes were taped over the EIP (G1) approximatelyfour fingerbreadths proximal to the ulna styloid process,where the reference electrode (G2) was placed. Theground electrode was taped on the dorsal forearm. Stim-ulation was performed at the spiral groove at 2 Hz at restand up to 3 minutes following 1 minute of isometricexercise. The fingers were held in extension by the tech-nician. In two patients a reliable response could not beobtained with stimulation at the spiral groove due tostimulator movement off the nerve, and the nerve wasstimulated at the elbow. One patient had no reliableresponse at the spiral groove or the elbow and wasexcluded from the study.

RESULTS

Normal Controls. Twenty subjects were studied (8men, 12 women), with a mean age of 38 years

Abbreviations: AChR, acetylcholine receptor; ADM, abductor digiti minimi;CMAP, compound muscle action potential; EIP, extensor indicis proprius;MG, myasthenia gravis; MGFA, Myasthenia Gravis Foundation of America;MRC, Medical Research Council; RNS, repetitive nerve stimulationKey words: abductor digiti minimi; extensor indicis proprius; myastheniagravis; radial nerve; repetitive stimulationCorrespondence to: A. Petretska; e-mail: petretska.adriana@mayo.edu

© 2006 Wiley Periodicals, Inc.Published online 22 March 2006 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mus.20508

Radial Repetitive Stimulation MUSCLE & NERVE June 2006 817

(range, 24–58 years). The mean radial–EIP CMAPamplitude to a single supramaximal stimulus was 4.7mV (range, 2.0–7.1 mV). The mean percentageCMAP amplitude decrement at rest was 1.6% (max-imum 6%) and after exercise was 0.8% (maximum5%).

Patients with Myasthenia Gravis. Twenty subjectswere studied with a mean age of 65 years (range,26–82 years). The median duration of clinical symp-toms was 41 months (range, 1 month to 15 years).Fifteen (75%) patients were MGFA Class II and five(25%) were Class III. The mean radial–EIP CMAPamplitude was 4.4 mV (range, 2.0–8.3 mV).

The percentage of patients exhibiting abnormaldecrement among different nerves is shown in Table1. Baseline abnormal decrement with radial RNS wasseen in 35% of patients, compared to 11% with ulnarRNS. Abnormal decrement occurred most com-monly with stimulation of the facial (73%) and spi-nal accessory (64%) nerves. Abnormal decrementwas identified more frequently with radial than ulnarRNS in Class II and III patients. None of the Class IIpatients demonstrated abnormal decrement on ul-nar RNS compared to 27% on radial RNS.

The mean percent decrement in the radial–EIPat rest (11%) was higher than for the ulnar (5%) andless than for the spinal accessory (12%) and facial(18%) nerves. In Class II patients, it was higher forthe radial (9%) than ulnar (2%) nerves, but lessthan for the spinal accessory (13%) and facial (17%)nerves. Similar results were seen in Class III patients(radial 18%, ulnar 11%, spinal accessory 12%, andfacial 22%).

Among the patients, 35% demonstrated mild ormoderate weakness [grade 3 or 4 on the MedicalResearch Council (MRC) scale] in the EIP, com-pared to 20% in the abductor digiti minimi (ADM).Six patients had more pronounced weakness in theEIP than ADM. Patients with more severe EIP muscleweakness demonstrated a higher percent of abnor-mal radial decrement (50% of MRC 4; 67% of MRC

3) compared to ulnar (0% of MRC 4; 33% of MRC3). Five patients demonstrated abnormal decrementin the radial but not ulnar nerve, and only one ofthem had more pronounced weakness in the EIPmuscle than ADM.

DISCUSSION

The sensitivity of RNS in distal muscles in patientswith MG has been reported to be as low as 31%, incontrast to 67%–85% of proximal muscles.5,6,10 Fin-ger extensor weakness, commonly present in MGpatients, occurs more frequently than weakness ofother distal muscles, such as the interossei.9 Thisstudy confirmed that more weakness occurs in theEIP than ADM in 30% of patients, and that RNSrecorded from a distal radial-innervated muscledemonstrated a higher frequency and degree of dec-rement than ulnar RNS.

Although RNS of the radial nerve with recordingfrom the anconeus muscle has been reported previ-ously to be more sensitive than ulnar RNS, thattechnique may be more difficult due to movementartifact with extension of the elbow during stimula-tion.1,4 We studied the EIP muscle since it is a smalldistal muscle that is often clinically weak and onethat can be compared to more commonly testeddistal nerve–muscle combinations. Abnormal decre-ment was present more frequently in the radial–EIPRNS than the ulnar–ADM, especially in mild gener-alized MG. The degree of EIP weakness correlatedwith the frequency of abnormal decrement, al-though some patients with normal EIP strengthdemonstrated a decrement.

Radial RNS recording from EIP can be used tosupplement RNS with recording from a proximal orcranial muscle. Technical difficulties with supra-maximal stimulation of the radial nerve exist, mak-ing this technique more difficult than ulnar RNS,but it was possible to obtain reliable responsesin most patients. In terms of practical implications,our study suggests that rather than beginning an

Table 1. Percentage of patients with abnormal (�10%) decrement to repetitive nerve stimulation according to MGFA classification ofdisease severity

Nerve

All patients (n � 20) Class II (n � 15) Class III (n � 5)

At rest(%)

Postexercise(%)

At rest(%)

Postexercise(%)

At rest(%)

Postexercise(%)

Radial 35 45 27 40 60 60Ulnar 11 22 0 23 40 20Accessory 64 61 60 50 75 75Facial 73 73 67 67 100 100

818 Radial Repetitive Stimulation MUSCLE & NERVE June 2006

electrodiagnostic evaluation of MG patients with ul-nar RNS, radial–EIP RNS may be a more appropriateinitial nerve–muscle combination.

This material was presented in part at the annual meeting of theAmerican Academy of Neurology in Miami Beach, April 2005.

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