Longitudinal study of EEG patterns in a child with a KCNH1 mutation showing non-epileptic myoclonus

The potassium voltage-gated channel subfamily H member 1 is encoded by the KCNH1 gene. Mutations in KCNH1 result in Temple-Baraitser (TBS) and Zimmermann-Laband (ZLS) syndromes, which are characterized by developmental delay, epilepsy, and nail dysplasia. These syndromes have clinical overlap and are found in patients with atypical TBS/ZLS phenotypic features. Although KCNH1 -related diseases are associated with a high incidence of epilepsy, electroencephalogram (EEG) changes over time have not been studied. In this study, we investigated the longitudinal evolution of EEG findings and types of seizures from the neonatal period to 6 years of age in a patient with a KCNH1 variant. A 6-year-old girl showed features of KCNH1 -related disease. Exome analysis revealed a heterozygous de novo missense variant (NM_172362.3; c.1481T>C; p.Ile494Thr) in KCNH1 . She presented generalized tonic-clonic seizures (GTCS) at the age of 3 years and 1 month. She also presented with myoclonus in the neonatal period, determined to be involuntary movement because of the absence of corresponding EEG discharges and persistence. The EEG findings evolved as follows: normal at 1 month of age, multifocal spikes started at 3 months of age, and high-voltage slow waves started at 3 months of age, which initially appeared during awake periods. Patients with a KCNH1 variant may have GTCS and involuntary movements including myoclonus. Furthermore, high-voltage slow waves in EEG, which are characteristic of patients with KCNH1 variants, were not observed immediately after birth but appeared at several months of age.