Infantile-Onset Unverricht-Lundborg Disease: A Unique Case With Severe Neurological Impairment and Positive Family History.

Progressive myoclonus epilepsies (PMEs) are a group of genetic disorders marked by myoclonus, epilepsy, and progressive neurological decline. Unverricht-Lundborg disease (ULD) is among the more common forms, though prevalence varies. We present the case of an 8-year-old male with infantile-onset ULD, notable for significant developmental delay and recurrent seizures. The patient developed early myoclonic jerks and tonic seizures, accompanied by cognitive impairment and global developmental delay. Family history revealed similar neurological challenges in siblings. Genetic testing confirmed a homozygous missense pathogenic variant in the cystatin B (CSTB) gene. With antiepileptic medications, seizure control improved, highlighting the role of early therapeutic intervention. ULD results from loss-of-function mutations in CSTB, leading to CSTB deficiency and impaired protease regulation. Although progressive in nature, management with appropriate therapy can reduce seizure burden and improve quality of life. This case emphasizes that PME may present early in childhood with tonic seizures and cognitive dysfunction, and early recognition is essential for optimal care.