Compound Muscle Action Potential (CMAP) Duration and Myosin Loss in Patients with Critical Illness Myopathy: Correlation and Prognostication
Shahar Shelly1, Pannathat Soontrapa2, Nicolas Madigan3, Michael Polzin3, Tarun Singh4, Sri Raghav Sista5, Pritikanta Paul6, Bing Liao7, Sherri Braksick3, Andrea Boon3, William Litchy3, Margherita Milone3, Teerin Liewluck3
1Rambam Medical Center, 2Siriraj Hospital, 3Mayo Clinic-Rochester, 4University of Michigan, 5UTHouston, 6UCSF, 7Houston Methodist Hospital
Objective:
To study the correlation between compound muscle action potential (CMAP) duration and myosin loss and their prognostic values in critical illness myopathy (CIM).
Background:
CIM is a common cause of weakness in critically ill patients. Prolonged CMAP duration and myosin loss are the diagnostic hallmarks of CIM but their correlation has not been studied.
Design/Methods:
We searched the Mayo Clinic database (1986-2021) for patients who developed new weakness during ICU stay or within a month after ICU dismissal, had prolonged CMAP duration and underwent muscle biopsy. Clinical, electrophysiological, and myopathological findings were reviewed.
Results:
Twenty patients were identified. Median time of weakness-onset was 16 days (range: 1-84) since ICU admission. Muscle biopsy showed myosin loss in 14 patients, 9 of whom had >50% of myofibers affected (high grade myosin loss). Type 2 fiber atrophy was observed in 13/14 patients with myosin loss and in 6 patients without myosin loss. Patients with myosin loss had significantly (p≤ 0.05) higher frequency of steroid exposure (14 vs 3), higher median number of necrotic fibers per low power field (2.5 vs 1), and longer median CMAP duration (msec) of peroneal (13.4 vs 8.75), tibial (10 vs 7.8) and ulnar (11.1 vs 7.95) nerves compared to those without myosin loss. Eleven patients had concomitant peripheral neuropathy. Ten patients died during median follow-up time of 3 months. The mortality rate was similar in both groups; however, patients with high grade myosin loss had a lower overall survival rate compared to those with low grade or no myosin loss (p=0.05).
Conclusions:
Myosin loss occurred in 70% of CIM patients with prolonged CMAP duration. Longer CMAP duration predicts myosin-loss pathology. The extent of myosin loss directly correlates with the mortality rate. Our findings highlight the potential prognostic values of CMAP duration and myosin-loss severity helping predict disease outcome.