Myotonia congenita

This code describes a rare, inherited neuromuscular disorder characterized by delayed relaxation of skeletal muscles after voluntary contraction. Patients experience muscle stiffness, which often improves with repeated movement (warm-up phenomenon). It is typically present from birth or early childhood and is non-progressive.

Assign this code when documentation specifies a diagnosis of myotonia congenita, such as Thomsen's disease or Becker's disease. This diagnosis is usually established through clinical presentation, electromyography (EMG) findings, and genetic testing confirming mutations in the CLCN1 gene.

• Differentiate from other myotonic disorders, such as myotonic dystrophy (G71.11), which has distinct clinical features and genetic causes.
• Ensure documentation clearly specifies "congenita" to avoid confusion with other forms of myotonia.
• Code any associated symptoms or complications, such as muscle hypertrophy or weakness, as secondary diagnoses.