Neuromuscular Features in XL-MTM Carriers

Background and Objective X-linked myotubular myopathy (XL-MTM) is an early-onset congenital myopathy characterized by mild to severe muscle weakness in male individuals. The objective was to characterize the clinical spectrum of neuromuscular features in X-linked myotubular myopathy (XL-MTM) carriers.

Methods We performed a nationwide cross-sectional study focusing on neuromuscular features in an unselected cohort of Dutch XL-MTM carriers. Participants were recruited from neuromuscular centers in the Netherlands and through the Dutch and European patient associations. Genetic results were collected. Carriers were classified based on ambulatory status and muscle weakness. We used a questionnaire focusing on medical and family history and neuromuscular symptoms. In addition, we performed a neurologic examination including manual muscle testing (MMT), timed up and go (TUG) test, and 6-minute walking test (6MWT).

Results We included 21 carriers (20 genetically confirmed and 1 obligate), of whom 11 (52%) carriers were classified as manifesting, with severe (nonambulatory; n = 2), moderate (minimal independent ambulation/assisted ambulation; n = 2), mild (independent ambulation but with limb or axial muscle weakness; n = 3), and minimal (only facial muscle weakness, n = 4) phenotypes. Three of the manifesting carriers (2 severe and 1 moderate) were from families without genetically confirmed male XL-MTM patients. Furthermore, 7 manifesting carriers (1 moderate; 2 mild; and 4 minimal) were not classified as manifesting carriers before participation in our study. Three carriers reported a history of pneumothorax. The obstetric history revealed frequent polyhydramnios (50%) and reduced fetal movements (36%) in pregnancies of affected sons. Muscle weakness was most pronounced in proximal and limb girdle muscles. Other frequently reported signs included (asymmetric) facial weakness (73%), reduced or absent deep tendon reflexes (45%), scoliosis (40%), and ptosis (45%). Ten participants (48%) were classified as nonmanifesting. Manifesting carriers had lower functional testing scores on 6MWT and TUG compared with nonmanifesting carriers.

Discussion This study showed that 52% of an unselected group of XL-MTM carriers has muscle weakness (3 of whom were previously unclassified as manifesting). This corresponds to findings of our recent questionnaire study on self-reported symptoms in XL-MTM carriers. These observations should raise awareness of the neuromuscular manifestations of the XL-MTM carrier state and provide important epidemiologic information required for future clinical trials.