CPT® Code 81188

The CPT® Code 81188 pertains to the molecular genetic testing of the CSTB (cystatin B) gene, specifically aimed at evaluating the presence of abnormal alleles, such as expanded alleles. This testing is particularly relevant for diagnosing Unverricht-Lundborg disease, a progressive myoclonic epilepsy characterized by involuntary muscle jerking or twitching known as myoclonus. The CSTB gene is located on the long arm of chromosome 21 at position 22.3 (21q22.3) and encodes the cystatin B protein, which plays a crucial role in regulating cathepsin enzyme activity within cell lysosomes. The CSTB gene features a distinctive DNA allele with a dodecamer repeat pattern consisting of 12 amino acids, specifically CCCCG-CCCCG-CG, which typically repeats 2 to 3 times in healthy individuals. However, individuals diagnosed with Unverricht-Lundborg disease often exhibit more than 30 dodecamer repeats in both copies of the CSTB gene. In some cases, a patient may have one copy of the CSTB gene with an expanded allele while the other copy contains one of nine other identified mutations, which may include amino acid substitutions or a truncated protein that is dysfunctional. Symptoms of Unverricht-Lundborg disease generally manifest between the ages of 6 and 15 and can include myoclonus that worsens with physical exertion, stress, light, or other stimuli. As the disease progresses, patients may experience additional complications such as ataxia, tremors, dysarthria, and cognitive decline. While myoclonus is typically resistant to anti-epileptic medications, it can evolve into tonic-clonic seizures, for which treatment may be more effective. The analysis performed under CPT® Code 81188 is essential for identifying these genetic mutations, thereby aiding in the diagnosis and management of Unverricht-Lundborg disease.