To describe an unusual presentation of a rare congenital myopathy.

MTM1 myopathy is an X-linked myopathy, characterized by muscle weakness, ranging from mild to severe, and can involve facial and extra-ocular muscles.  Diagnosis is by genetic testing.

The patient is a 15 year old African American boy who was first seen in Neuromuscular clinic in 2016. He had difficulty walking since 2 years of age, described as toe walking. Gross motor and language milestones were delayed. He had difficulty walking up and down stairs. He fatigued easily. He had recurrent falls. He also complained of intermittent thigh pain and tingling. He had no muscle cramps, chest pain, shortness of breath or difficulty swallowing. Clinical examination was pertinent for mid-face flattening, high arched palate, mild scoliosis, bilateral ptosis, impaired upgaze, lower face weakness, decreased strength in all muscle groups and neck flexor weakness, decreased muscle tone, and toe walking on gait exam. Reflexes were absent. 

CK, TSH, lactate, and pyruvate were unremarkable. EMG/NCS showed evidence of myotonic discharges in at least 2 muscle groups in lower extremities. Motor unit morphology was otherwise normal. Muscle biopsy of the left quadriceps muscle showed diffuse type I myofiber hypotrophy, frequent internalized nuclei, and numerous necklace fibers. Genetic analysis was done and revealed hemizygous VUS (variation of uncertain significance) in MTM1 gene.

MTM1 myopathy is a congenital myopathy that is caused by mutations of the MTM1 gene that encodes for the ubiquitous myotubularin protein. There is extensive heterogeneity in mutations and both truncating (nonsense, frameshift, large deletion) and non-truncating mutations (missense, in-frame insertion/deletion) have been identified. There is marked phenotypic variability, ranging from mild to severe muscle weakness, requiring ventilatory support. Typically, non-truncating mutations are associated with a milder phenotype. Phenotypic variations are seen within families as well as with recurrent mutations in unrelated patients.