Abstract

Myotonic disorders represent rare, genetically heterogeneous neuromuscular diseases, whose clinical hallmark is the presence of myotonia. Myotonic disorders are generally classified as either dystrophic (DM) or non-dystrophic myotonia (NDM). Non-dystrophic myotonias (NDM) are skeletal muscle channelopathies characterized by a disruption of normal sarcolemmal ion conductance that results in abnormal muscle fiber excitability. Depending on the affected ion channels, non-dystrophic myotonias include myotonia congenita (MC), paramyotonia congenita (PMC) and sodium channel myotonia (SCM).

Myotonia congenita (MC) is the most common type of non-dystrophic myotonia, occurring due to mutations in CLCN1, the gene encoding the primary skeletal muscle voltage-gated chloride channel (ClC-1). Diagnosis is suspected in the setting of myotonia, muscular stiffness, the warm-up phenomenon, family history and electromyographic studies suggestive of myotonia congenita and is ultimately established through genetic testing.

In this case report, we present a 9-year-old male patient with complaints of muscular stiffness, difficulty in climbing stairs, with calf and buttock hypertrophy and percussion myotonia in both hands and calves evident in physical examination. A diagnosis Myotonia Congenita was made based on the presence of a positive Lid lag sign, a positive warm up test and the results of electromyography (EMG) studies. It was then confirmed by genetic testing.

Along with showcasing a rare pathology, this case report emphasises the importance of a thorough clinical evaluation and electrophysiological studies as the best initial assessments of myotonia, when considering the differential diagnosis and in guiding appropriate genetic testing.

Keywords: Non-Dystrophic Myotonia; Channelopathies; Myotonia Congenita; Electrophysiology

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