Quality Control of Motor Unit Number Index (MUNIX) Measurements in 6 Muscles in a Single-Subject “Round-Robin” Setup

Abstract

BackgroundMotor Unit Number Index (MUNIX) is a neurophysiological measure that provides an index of the number of lower motor neurons in a muscle. Its performance across centres in healthy subjects and patients with Amyotrophic Lateral Sclerosis (ALS) has been established, but inter-rater variability between multiple raters in one single subject has not been investigated.ObjectiveTo assess reliability in a set of 6 muscles in a single subject among 12 examiners (6 experienced with MUNIX, 6 less experienced) and to determine variables associated with variability of measurements.MethodsTwelve raters applied MUNIX in six different muscles (abductor pollicis brevis (APB), abductor digiti minimi (ADM), biceps brachii (BB), tibialis anterior (TA), extensor dig. brevis (EDB), abductor hallucis (AH)) twice in one single volunteer on consecutive days. All raters visited at least one training course prior to measurements. Intra- and inter-rater variability as determined by the coefficient of variation (COV) between different raters and their levels of experience with MUNIX were compared.ResultsMean intra-rater COV of MUNIX was 14.0% (±6.4) ranging from 5.8 (APB) to 30.3% (EDB). Mean inter-rater COV was 18.1 (±5.4) ranging from 8.0 (BB) to 31.7 (AH). No significant differences of variability between experienced and less experienced raters were detected.ConclusionWe provide evidence that quality control for neurophysiological methods can be performed with similar standards as in laboratory medicine. Intra- and inter-rater variability of MUNIX is muscle-dependent and mainly below 20%. Experienced neurophysiologists can easily adopt MUNIX and adequate teaching ensures reliable utilization of this method.

Improving the repeatability of Motor Unit Number Index (MUNIX) by introducing additional epochs at low contraction levels

“…However, the repeatability of MUNIX under repeatability conditions has rarely received sufficient attention. In one study, a “round-robin” setup was performed (Neuwirth et al, 2016), in which twelve raters made two MUNIX measurements of six muscles on the same subject in two sessions separated by one day. A considerate amount of intra-rater coefficient of variance was reported (mean 14.1 ± 4.3%, range 6.6 – 22.9%).…”

“…The research protocol was approved by the local ethic committee. The biceps brachii muscle was selected as it was considered one most challenging muscle for MUNIX calculation according to Neuwirth et al (2016). The experimental setup was configured following a previous study (Peng et al, 2016), as shown in Figure 1(a), to record the isometric contraction of the biceps brachii during flexion, while spurious force contributions from other muscles were minimized with the application of a fiberglass cast.…”

“…A poor repeatability in selecting the optimal electrode location, often caused by operator’s error, can present direct sources of variations. It could be a challenging task even for a trained neurologist to properly identify the optimal electrode location (Neuwirth et al, 2016). Recent advance in high-density surface EMG measurements can properly capture the optimal electrode location (indicated by the largest CMAP response) and provide a tool to eliminate this source of error.…”

Correlations between MUNIX and adapted multiple point stimulation MUNE methods

“…AMPS has good reproducibility based on the test-retest coefficient of variation (COV) in healthy subjects (COV = 10.4%) (Wang and Delwaide, 1995), and is even higher in amyotrophic lateral sclerosis (ALS) patients where the number of MU is reduced (COV = 4%) (Wang and Delwaide, 1998). A good within raters COV (between 5.8 and 9%) and between raters COV (between 13.5 and 16.4%) is also found when MUNIX is applied to abductor pollicis brevis muscle of healthy subjects (Neuwirth et al, 2016). MUNIX and AMPS were widely used in clinical studies, particularly to document the motor unit loss which characterized ALS (Wang and Delwaide, 1998;Wang et al, 2002Wang et al, , 2009Nandedkar et al, 2010;Boekestein et al, 2012;Furtula et al, 2013).…”

Muscle and Bone Mass Loss in the Elderly Population: Advances in diagnosis and treatment

“…Age-related loss of motor neurons is a major component of sarcopenia and muscle mass loss [9], which results in a decrease in muscle function caused by disuse. The Motor Unit Number Index (MUNIX) is a neurophysiological measure that provides an index of the number of lower motor neurons in a muscle using the compound muscle action and surface electromyographic interference pattern [15]. Weaker individuals exhibit a smaller motor unit number index (MUNIX) value [9, 16].…”