Hereditary myopathy with early respiratory failure (HMERF): Still rare, but common enough.

Phenotypic and genetic/allelic heterogeneity is a feature of many neuromuscular disorders, titinopathies being one of them. Hereditary Myopathy with Early Respiratory Failure (HMERF) has been considered an extremely rare disease with definite clinicopathologic hallmarks, and geographically restricted to the Northern European population with one single titin gene defect identified in previous years. The recent availability of massive parallel sequencing techniques, allowing the screening of all coding regions of the genome in undiagnosed patients, together with a growing awareness of the main muscle MRI features of the disease, has led to the discovery of a number of HMERF families and new titin mutations in the last five years. We reviewed the clinical, pathological and muscle imaging findings that are still cornerstones for the diagnosis of this disease, as well as the most recent molecular genetic findings. HMERF is more common and geographically widespread than previously expected, and the knowledge of the whole phenotypic and molecular spectrum of HMERF can increase the number of diagnosed patients considerably.