Diagnosis of congenital myotonia is based upon characteristic clinical symptoms, but since this disease is very rare lot of physicians are not familiar with myotonia phenomenon they might attribute myotonic symptoms to the other, more common conditions.

Clinical testing can be performed by using reflex hammer, which when kicking skeletal muscles produces clearly discernible dimpling (percussion myotonia) in tested muscle. Kick can as well produce fasciculation (muscle jerks) upon impact that lingers for few seconds.

Electromyograph is characterized by high frequency discharges after inserting needle or voluntary muscle movement.

Patohystologically, muscle biopsy shows changes in muscle cells (myocites), fiber hypertrophy and absence of type 2b fibers, but none of these changes is specific for myotonia and can be found in other non-dystrophic myotonias.