Individuals with the disorder may have trouble releasing their grip on objects or may have difficulty rising from a sitting position and a stiff, awkward gait.

Myotonia may present in the following diseases, with different causes related to the ion channels in the skeletal muscle fiber membrane (sarcolemma).

In myotonic dystrophy a nucleotide expansion of either of two genes, related to type of disease, results in failure of correct expression (splicing of the mRNA) of the ClC-1 ion channel, due to accumulation of RNA in the cytosol of the cell.

[17] Myotonia could be caused by genetic mutations in the SCN4A gene that encodes the skeletal muscle sodium channel subtype 4 (Nav1.4).

[18] This disease results from mutation in the SCN4A gene encoding the voltage-gated sodium channel Nav1.4 in skeletal muscle fiber membrane.

[19] Potassium-aggravated myotonia (PAM) results from in a mututation of the SCN4A gene that causes skeletal muscles to be unable to relax after contracting in bouts, typically following the consumption of potassium rich food.

[20] It is debated if potassium-aggravated myotonia is a distinct disease from Paramyotonia Congenita, and recent academic papers have classified it both ways.

Similar to Paramyotonia Congenita, where potassium exacerbates myotonia in many phenotypes, Hyperkalemic Periodic Paralysis is another disorder of the SCN4A gene where high blood potassium levels result in muscle weakness, muscle paralysis (through weakness or through over excitation preventing movement), and sometimes myotonia.