Abstract: Objective To investigate the correlation between the skeletal muscle thickness of the extremities and the British Medical Research Council (MRC) scale scores in elderly patients with ICU-acquired myasthenia (ICU-AW) and to draw the receiver operating characteristic (ROC) curve to evaluateits predictive value for prognosis. 
Methods A total of 60 elderly ICU-AW patients in our hospital were selected as the research group, and 60 elderly ICU patients without ICU-AW who received mechanical ventilation during the same period were selected as the control group. The skeletal muscle thickness of the extremities measured by ultrasound, and MRC score were compared between two groups, and their correlation was analyzed. Clinical data, skeletal muscle thickness of the extremities, and MRC score of patients with different prognosis were compared, and the relationship between skeletal muscle thickness, MRC score and the prognosis of elderly ICU-AW patients as well as their value in predicting prognosis was analyzed.  
Results The thickness and MRC score of the biceps brachii, flexor carpi radialis, quadriceps femoris, and tibialis anterior in the research group were lower than those in the control group (P＜0.05). The thickness ofthe biceps brachii, flexor carpi radialis, quadriceps femoris, and tibialis anterior was positively correlated with the MRC score (P＜0.05). The number of days of mechanical ventilation at 28 d in the deceased was higher than that of surviving patients, while the thickness of the biceps brachii, flexor carpi radialis, quadriceps femoris, and tibialis anterior and MRC score were lower than those of surviving patients (P＜0.05). The thickness of the biceps brachii, flexor carpi radialis, quadriceps femoris, and tibialis anterior and MRC scores were all prognostic protective factors for elderly ICU-AW patients (P＜0.05). The area under the curve (AUC) of MRC scores, the biceps brachii, flexor carpi radialis, quadriceps femoris, and tibialis anterior in predicting the prognosis of elderly ICU-AW patients was 0.805, 0.837, 0.648, 0.785, and 0.799, respectively. The AUC of combined prediction of each index was the largest (0.873). 
Conclusion There is a positive correlation between the skeletal muscle thickness of the extremities and MRC score in elderly ICU-AW patients. The combination of skeletal muscle thickness of the extremities and MRC score has certain value in predicting the prognosis of patients. 

Key words: muscle weakness, ultrasound, skeletal muscle thickness