Motor and Sensory Conduction in the Musculocutaneous Nerve
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J Neurol Neurosurg Psychiatry: first published as 10.1136/jnnp.39.9.890 on 1 September 1976. Downloaded from Journal ofNeurology, Neurosurgery, and Psychiatry, 1976, 39, 890-899 Motor and sensory conduction in the musculocutaneous nerve W. TROJABORG From the Laboratory of Clinical Neurophysiology, Rigshospitalet, University Hospital, Copenhagen, Denmark SYNOPSIS Motor and sensory conductionvelocity in the musculocutaneous nerve were determined in 51 normal subjects. The maximal velocity from the anterior cervical triangle to the axilla was the same in motor and sensory fibres. The conduction velocity decreased 2 m/s per 10 years increase of age. It was 70 m/s at 15-24 years and 58 m/s at 65-74 years. The velocity ofthe slowest components in sensory fibres was 17 m/s. Three selected case reports illustrate the diagnostic value of the method. The musculocutaneous nerve originates from the old, and 13 were 55 to 74 years old. The procedures Protected by copyright. lateral cord of the brachial plexus at or about the used for recording motor and sensory conduction lower border of the pectoralis minor muscle and were the same as those described for other limb nerves its fibres derive chiefly from the fifth and sixth (Buchthal and Rosenfalck, 1966, 1971; Payan, 1969; Trojaborg and Sindrup, 1969; Behse and Buchthal, cervical nerve roots (Sunderland, 1968). Con- 1971; Buchthal et al., 1974). In short, motor con- duction along motor fibres from the axilla to the duction was determined by stimulating the musculo- brachial biceps muscle has been described by cutaneous nerve at two sites: (1) at the anterior Redford (1958) and from Erb's point to the cervical triangle just behind the sternocleidomastoid brachial biceps by Gassel (1964) and Kraft (1972). muscle and approximately 6 cm above the clavicle, Nelson and Currier (1969) stimulated the nerve at and (2) at the axilla between the axillary artery and two sites in seven normal subjects and determined the median nerve medially, and the coracobrachialis the motor conduction velocity along the fibres to muscle laterally, just above the level of the tendon of the brachial biceps in the nerve segment between the latissimus dorsi muscle. The electric stimulus, the anterior cervical triangle and the axilla. 0.2 ms in duration, was applied through insulated The study presented in this report deals with stainless steel needle electrodes, 0.7 mm in diameter. conduction velocity along the musculocutaneous The near-nerve electrode, with a bared tip of 3 mm, was adjusted close to thenerveas indicated bya thresh- http://jnnp.bmj.com/ nerve in normal subjects in proximal (fossa old of the muscle action potential of0.5-1 mA (Troja- supraclavicularis-axilla) and distal segments of borg and Sindrup, 1969). The remote electrode, with a motor fibres and in proximal (fossa supra- bared tip of 5 mm, was placed at the same level as clavicularis-axilla) and intermediate (elbow- the near-nerve electrode at a transverse distance of axilla) segments of sensory fibres. Three selected 3 cm. The stimulus was delivered from a constant- case reports are presented to illustrate the current stimulator isolated from ground. The usefulness of the method. maximal stimulus current was 5-7 mA-that is, 10 times the threshold current. The action potentials on October 2, 2021 by guest. were recorded with concentric needle electrodes METHOD (0.07 mm2 leading-off area, DISA 13KO51 1) placed Fifty-onesubjectswithout history, signs, or symptoms in the endplate zone and adjusted to the region of of neuromuscular disorders were examined; 20 were maximal response in the brachial biceps muscle. The between 15 and 34 years old, 18 were 35 to 54 years motor responses and the stimulus current were recorded on two channels of an electromyograph (DISA 13K051 1). Theconduction time was measured (Accepted 27 April 1976.) from the onset of the stimulu, to the onset of the 890 J Neurol Neurosurg Psychiatry: first published as 10.1136/jnnp.39.9.890 on 1 September 1976. Downloaded from Motor and sensory conduction in the musculocutaneous nerve 891 muscle action potential; the amplitude was measured increased neither the amplitude nor the number of peak-to-peak. The distance between the near nerve slow components of the response. At the axilla and stimulating electrodes was measured with an anterior cervical triangle, electrodes positioned by obstetric caliper (Gassel and Diamantopoulos, means of the motor threshold were used for recording 1964). of sensory potentials. The maximum sensory The sensory fibres of the musculocutaneous nerve conduction velocity was determined from the latency were stimulated with needle electrodes placed along to the initial positive peak of the potential; the the nerve where it emerges as the lateral cutaneous minimum velocity along sensory fibres was deter- nerve of the forearm at the level of the elbow crease. mined when the resolution of the recording was high Here it can be palpated just lateral to the tendon of enough to distinguish potentials 0.05 ,uV in amplitude the biceps brachii and medial to the brachioradialis by means of electronic averaging of 250, 500, or 1000 muscles (Fig. 1). The near-nerve electrode was traces (Buchthal et al., 1974). adjusted with the amplitude of the sensory potential recorded at the axilla as indicator. The current pulse TEMPERATURE NEAR NERVE To maintain a tempera- was 5-10 mA. Further increase in stimulus current ture of 36-380C along the surface of the limb Sensory m. biceps brachii 68 70 Protected by copyright. / N R I 0 5 10 71 S \\/_0lOmV s + L 0 2.5-. 5 0 5 lOms http://jnnp.bmj.com/ on October 2, 2021 by guest. FIG. 1 Motor and sensory potentials evoked by supramaximal stimulation of the musculocutaneous nerve. The nerve was stimulated at the axilla and at the anterior cervical triangle ('Erb's point') and the responses were recorded from the brachial biceps muscle (right). Sensory potentials were evoked by stimulating the lateral cutaneous nerve oftheforearm at the elbow (S) and recorded over the nerve at the axilla (lower left) and at 'Erb's point' (upper left, photographic superimposition of 15 traces). The figures above the traces denote the maximal conduction velocity (m/s) in the segment distal to thepoint ofrecording. Time scale in milliseconds (N= near-nerve electrode, R = remote electrode). Normal subject, 22 years old. F J Neurol Neurosurg Psychiatry: first published as 10.1136/jnnp.39.9.890 on 1 September 1976. Downloaded from 892 W. Trojaborg corresponding to 35-37°C near the nerve, an infra-red cutaneous nerve at the axilla, and at the anterior heating element (45 cm long, 500 W, controlled by a cervical triangle ('Erb's point', Table 1). thermocouple on the limb) was placed 20-30 cm The motor above the upper extremity. latency increased with the distance between sites of stimulation and recording The RESULTS (Fig. 2). increase in latency was 0.16 ms per cm increase in a NORMAL SUBJECTS distance, corresponding to velocity of 64 m/s. However, the motor velocity MOTOR CONDUCTION The latencies were mea- and the amplitude of the evoked response were sured from 51 nerves by stimulating the musculo- related to age (Table 2, Fig. 3). With increasing BLE 1 MOTOR LATENCY TO M. BICEPS BRACHII (51 SUBJECTS AGED 15-74 YEARS) 95% limit Stimulus Distance* (cm) n Mean (ms) % Upper (ms) Lower (ms) At axilla 7 9 2.0 2.6 1.3 9 21 2.3 3.0 1.6 1 1 31 2.6 3.3 2.0 1 3 9 2.9 3.6 2.3 At 'Erb's point' 25 5 4.8 5.5 4.1 Protected by copyright. 27 17 5. 5.8 4.4 29 1 3 5.4 6.1 4.8 31 12 5.7 6.4 5.1 33 4 6.0 6.7 5.4 *MeasLlred wit'l O6st:tric c-dipzrs. 7r7 MOTOR ms 6 800 00A3 1 4 FIG. 2 Latency to the brachial http://jnnp.bmj.com/ biceps as a function of conduction distance. The thick line represents the regression 3 line, the thin and dashed lines the 95 and 99 % confidence limits, respectively. 2 on October 2, 2021 by guest. 7z7 AXILLA OR ER3'S P. TO M.BICEPS BRACHII 7 n I "0 5 10 15 20 25 30 35 40 distance from stim.electrode,cm J Neurol Neurosurg Psychiatry: first published as 10.1136/jnnp.39.9.890 on 1 September 1976. Downloaded from Motor and sensory conduction in the musculocutaneous nerve 893 TABLE 2 MOTOR CONDUCTION VELOCITY ALONG MUSCULOCUTANEOUS NERVE FROM ANTERIOR CERVICAL TRIANGLE ('ERB'S POINT') TO AXILLA. AND AMPLITUDE OF EVOKED RESPONSES IN M. BICEPS BRACHII Velocity Amplitude Stimulus at avilla Stimulus at 'Erb's point' 95°, limit 95 limitl 95 °' limit Age n Mean Lower Upper Mean Lower Upper Mean Lower Upper (yr) (m/s) (m/s) (m/s) (m V) (m V) (m V) (m V) (m V) (m V) 15-24 14 70 63 78 17 9 32 14 7 27 25-34 6 68 60 75 16 8 30 13 6 26 35-44 8 65 58 73 15 8 28 12 6 24 45-54 10 63 55 71 13 7 26 11 6 22 55-64 9 60 53 68 13 7 24 10 5 21 65-74 4 58 50 66 1 2 6 22 9 5 1 9 age of the subjects the velocity decreased 2 rn/s (axilla-'Erb's point') and in the intermediate per 10 years of age: the velocity at 15-24 years nerve segment (elbow-axilla) and the same as in being 12 mr/s faster than at 65-74 years (P < 0.001).