MUNIX and incremental stimulation MUNE in ALS patients and control subjects

This study compares the new Motor Unit Number Estimation (MUNE) technique, MUNIX, with the more common incremental stimulation MUNE (IS-MUNE) with respect to reproducibility in healthy subjects and as potential biomarker of disease progression in patients with ALS.

Thirteen ALS patients and 48 control subjects were prospectively investigated - both groups were studied with MUNIX and IS-MUNE applied on the abductor digiti minimi (ADM) muscle. Additional retest was performed on 14 control subjects. Follow-up tests were carried out on 6 patients. The analysis included measures of reproducibility (Intraclass Correlation Coefficient (ICC)) and diagnostic performance (Receiver Operating Characteristic (ROC) analysis).

Test-retest reproducibility was low to moderate for MUNIX and IS-MUNE (ICC=0.38 and 0.56, respectively). Repeated MUNIX and IS-MUNE measurements on the same subject had a mean percentage difference (MPD) of 20% and 46%, respectively (p=0.039). In the control group, the coefficient of variation was markedly lower for MUNIX than for IS-MUNE (26% and 44%, respectively, p<0.0005). In ALS patients MUNIX had a notably better responsiveness in follow-up than IS-MUNE (percent change per month, 9.4 versus 5.6, p=0.046). ROC analysis suggested similar diagnostic accuracy of both tests.

MUNIX is a useful MUNE indicator when assessing progression of lower motor neuron affection in ALS. Furthermore, MUNIX displayed lower intrasubject variability, but no evident better diagnostic yield compared with IS-MUNE.

This study has established comparative assessment of MUNIX and IS-MUNE performance in test-retest setting and as diagnostic tests on a distal muscle in ALS patients.