Science News: Measurement of Excitation-Contraction Coupling Time in Critical Illness Myopathy

Submitted by: Eman Tawfik, MD
Edited by: Marcus Pai, MD, PhD

Hirose B, Ikeda K, Yamamoto D, et al. Measurement of excitation-contraction coupling time in critical illness myopathy. Clin Neurophysiol. 2022;135:30-36. doi:10.1016/j.clinph.2021.10.022

Summary: This study describes a novel electrophysiological method using an accelerometer to measure the excitation-contraction coupling of the soleus muscle for early diagnosis of critical illness myopathy (CIM). The authors hypothesized that the new tool can be useful for assessing impairment of excitation contraction coupling in unconscious patients and thus is applicable in CIM. The study included 41 consecutive patients with critical illness admitted to the intensive care unit (ICU).
 
Routine tibial motor, and sural sensory nerve conduction studies were performed, and the excitation contraction coupling (ECC) was measured, as soon as possible after ICU admission and repeated every week until discharge. To measure the ECC, the tibial nerve compound motor action potential (CMAP) was stimulated at the popliteal fossa with recording from the soleus muscle and ‘movement related potential’ (MRP) was simultaneously recorded using an accelerator sensor taped to the base of the big toe. The excitation contraction coupling time (ECCT) was determined by calculating the difference between CMAP latency and MRP latency and the MRP amplitude, was measured from the baseline to the maximal deflection and reflects the maximum acceleration of plantar flexion.
 
Of the 41 ICU patients evaluated, 23 patients were diagnosed as having CIM. Normal value of the ECCT obtained from 20 healthy subjects was 5.1 ± 0.8 ms with the upper limit of normal set at 6.7 ms. The normal values of MRP amplitude was 4.6 ± 1.3 m/s² and the lower limit of normal was set at 2.0 m/s².
 
ECCT was prolonged in 65.2% of the CIM patients and occurred within 1 week of admission in most of the patients. MRP amplitude was abnormal in 7 patients. Of note, the abnormality in ECCT became apparent earlier than abnormalities in the other electrophysiological parameters.

Comments: This is an interesting article describing an easy and applicable test that can detect ECC impairment, which was observed to occur early in the course of CIM in most of the patients. Another advantage of the new method is that it does not need patient’s cooperation in contrast to EMG, thus is suitable for unconscious or uncooperative ICU patients. 

Articles of similar interest 
There are articles that address excitation-contraction coupling in healthy and myasthenia gravis patients:
1. Asada Y, Imai T. Measurement of excitation-contraction coupling time in lower extremities. Phys Ther Res. 2020;24(1):29-34
2. Tsuda E, Imai T, Hozuki T, et al. Correlation of bite force with excitation-contraction coupling time of the masseter in myasthenia gravis. Clin Neurophysiol. 2010;121(7):1051-1058