Paramyotonia Congenita is a rare genetic disease that affects the muscles. It is also known as Myotonia Congenita Thomsen, Thomsen's disease, and hereditary myotonia. Symptoms include muscle stiffness and difficulty in relaxing after contraction, especially in cold temperatures. Other synonyms for Paramyotonia Congenita include "muscular inertness," "muscles that are slow in contracting," "congenital muscle stiffness," and "myotonia." The condition is caused by mutations in the SCN4A gene, which affects the production of sodium channels that regulate muscle contractions. Though it is an inherited disorder, Paramyotonia Congenita can be managed through several interventions such as medication and avoiding triggers that can worsen muscle stiffness.