Nemaline Myopathy Type 6 Caused by Variants in the KBTBD13 Gene

Background and Objectives: Nemaline myopathy type 6 (NEM6) is the most prevalent type of nemaline myopathy in the Netherlands. Because a detailed clinical characterization is not available yet, we here provide a detailed assessment of 24 patients. Methods: gene, creatine kinase levels, and the results of previous muscle biopsies were collected. We evaluated experienced health-related quality of life, fatigue severity, prevalence of falls, balance control (Mini-Balance Evaluation Systems Test [Mini-BESTest]), functional motor score (Motor Function Measure [MFM]), and 6-minute walk distance. We used transcranial magnetic stimulation to assess muscle relaxation kinetics. Discussion: This cross-sectional study in patients with NEM6 shows a relatively mild clinical phenotype and mildly abnormal functional tests. However, patients report an important impact on the daily activities, which is illustrated by functional difficulties, reduced quality of life, increased fatigue severity, and increased prevalence of falls. This might be related to delayed muscle relaxation. This study provides a comprehensive overview of the clinical presentation and functional limitations in patients with NEM6.