Paroxysmal tonic upgaze in a child with SCN8A-related encephalopathy.

Pathogenic variants in the SCN8A gene have been associated with a broad phenotypic spectrum, ranging from benign familial infantile seizures to severe, early-onset developmental and epileptic encephalopathy. This spectrum also includes an "intermediate phenotype" characterized by different degrees of cognitive disability, mild neurological impairment, and therapeutically manageable epilepsy. We report on a child harbouring a de novo, novel SCN8A deletion, whose clinical picture is consistent with an SCN8A-related "intermediate phenotype". This patient's peculiar feature is the occurrence of paroxysmal tonic upgaze (PTU), a non-epileptic disorder consisting of sustained conjugate upward deviation of the eyes, with neck flexion, and downbeat saccades. PTU has been described in otherwise healthy children, as well as in a few genetic syndromes, but has never been observed in SCN8A-related phenotypes. This report, therefore, adds a new symptom to the spectrum of movement disorders associated with SCN8A-related developmental and epileptic encephalopathy. In this short communication, we provide video-EEG documentation of PTU and seizures, and discuss the challenging differential diagnosis between the two symptoms.