Delayed Diagnosis of Mild GLUT1 Deficiency Syndrome Caused by an Apparently De Novo SLC2A1 p.(Phe445del) Variant in a Child with a History of Severe Neonatal Hyperkalemia

Background/Objectives: Glucose transporter type 1 deficiency syndrome (GLUT1DS) is a rare neurometabolic disorder with an expanding clinical spectrum, including mild and non-classical presentations. We report a boy with severe transient neonatal hyperkalemia, bilateral congenital cataracts, and later subtle neurological and neurocognitive symptoms, in whom genomic testing supported the diagnosis of mild GLUT1DS. Methods: This single-patient case report describes clinical follow-up from birth to nine years of age, including neurological, metabolic, neuropsychological, imaging, and genetic investigations. Whole-exome sequencing using next-generation sequencing technology was performed. Results: The patient required intensive care immediately after birth because of severe transient hyperkalemia of unclear etiology. Bilateral congenital cataracts were surgically corrected during infancy. Later, he developed two brief seizure episodes, reduced exercise tolerance, episodic fatigue, attentional difficulties, motor restlessness, and mild graphomotor impairment. Neuropsychological assessment showed overall average intellectual functioning, below-average verbal abilities, low-average non-verbal abilities, and attention-deficit/hyperactivity disorder. Repeated metabolic investigations, electroencephalography, and brain magnetic resonance imaging were unrevealing. Whole-exome sequencing identified an apparently de novo heterozygous SLC2A1 variant, NM_006516.4.1333_1335del, p.(Phe445del), supporting the diagnosis of mild GLUT1DS. Because of the mild phenotype and preserved everyday functioning, ketogenic diet therapy was not initiated. Conclusions: This case highlights the diagnostic challenges of mild GLUT1DS and the value of genomic testing in children with unexplained neurological or neurocognitive symptoms despite normal routine investigations. Although neonatal hyperkalemia and GLUT1DS coexisted in this patient, current evidence is insufficient to establish a causal relationship.