Paramyotonia Congenita is caused by mutations in the SCN4A gene. This gene provides instructions for making a protein that is critical for the normal function of skeletal muscle cells. Muscle contractions are triggered by the flow of positively charged ions as detailed in the diagram below.
The SCN4A protein forms channels that control the flow of sodium ions into these cells. Mutations in these genes alter the sodium channels. These altered channels cannot regulate the flow of sodium ions into skeletal muscle cells. The increase in sodium results in muscle contraction and relaxing not functioning properly. Hence, the episodes of muscle stiffness and weakness characteristic of PMC.
This is a genetic condition. It is inherited in an autosomal dominant pattern. In other words, only one copy of the altered gene in each cell is sufficient to cause the disorder. Most of the time, someone affected has one parent also affected by this condition.
For more in-depth information, check out this site: https://ghr.nlm.nih.gov/condition/paramyotonia-congenita#inheritance