CPT® Code 81190

The CPT® Code 81190 pertains to the molecular genetic testing of the CSTB (cystatin B) gene, specifically focusing on known familial variants associated with Unverricht-Lundborg disease. This condition is a progressive myoclonic epilepsy characterized by involuntary muscle jerking or twitching, which typically manifests between the ages of 6 and 15. The CSTB gene, located on the long arm of chromosome 21 at position 22.3 (21q22.3), encodes the cystatin B protein, which plays a crucial role in regulating cathepsin enzyme activity within cell lysosomes. The gene features a distinctive DNA allele with a dodecamer repeat pattern consisting of 12 amino acids, specifically CCCCG-CCCCG-CG, which usually repeats 2 to 3 times. In individuals diagnosed with Unverricht-Lundborg disease, it is common to find more than 30 dodecamer repeats in both copies of the CSTB gene. However, some patients may present with one gene exhibiting an expanded allele while the other carries one of nine identified mutations, such as amino acid substitutions or a truncated protein that is dysfunctional. The symptoms of this disease can progress to include ataxia, tremors, dysarthria, and cognitive decline, with myoclonus potentially evolving into tonic-clonic seizures, which may be managed with medication. The analysis performed under this code is critical for identifying at-risk family members by examining specific sites on the CSTB gene for mutations that have already been documented in other family members.