Accurate diagnosis is important as a first step for managing MD. This is contingent on a targeted history and examination, biochemical, and genetic testing; sometimes with additional testing such as muscle biopsy, neurophysiological assessment, and muscle imaging. Muscle biopsy used to be the gold standard; however, it is increasingly replaced by genetic testing. The detection rate with genetic testing for DMD and BMD is ~95% using deletion/duplication study and reflex to sequencing analysis if deletion/duplication study is negative [1]. The genetic basis for FSHD has been elucidated in recent decades, and the genetic testing detection rate is ~95% for patients with FSHD where a contraction mutation of the D4Z4 macrosatellite array in the subtelomeric region of chromosome 4q35 can be identified [2]. Next generation sequencing technology such as whole exome sequencing (WES) has significantly improved our ability to diagnose subtypes of LGMD, as the traditional sequencing method limits testing to one gene at a time. While electromyography and nerve conduction studies have been a traditional part of the assessment of a patient with MD, these tests are not believed to be indicated or necessary for diagnosis unless other means are inconclusive. Muscle imaging is becoming more widely accepted as it is noninvasive and various forms of MD often result in unique patterns. This approach is also very sensitive, enabling inflammatory myopathies (also called myositis) and metabolic myopathies—which may mimic MD but require different management—to be ruled out. For these conditions, specific treatments exist and accurate early diagnosis improves outcomes.

It is also worth mentioning that some subtypes of MD, such as myotonic dystrophy, are often missed in the presentation. Hilbert et al. found that patients with myotonic dystrophy type 1 (DM1) experienced an average of seven years delay to diagnosis, and members with myotonic dystrophy type 2 (DM2) had an even more stunning delay of 14 years before receiving a correct diagnosis [3]. Thus, the importance of clinical suspicion from clinicians and families cannot be overestimated.