Adult Neuropsychiatric Manifestation of Hartnup Disease With a Novel SLCA6A19 Variant: A Case Report.

Objectives: In adults, inborn metabolic diseases are often missed in routine diagnostic settings due to a low level of suspicion.

Methods: A patient in their twenties was admitted for an apparent acute exacerbation of anxiety disorder. Medical treatment was unsuccessful, and presumed catatonic psychosis was treated by electroconvulsive treatment. The patient was referred to neurology with reduced level of consciousness, mutism with no targeted movements, obvious anxiety and tetraspasticity, eczema, and reduced body weight.

Results: EEG was normal; repeat brain MRI showed progressive atrophy and leukoencephalopathy. Autoimmune encephalitis was assumed and treated with plasma exchange, high-dose glucocorticoids, and intravenous immunoglobulin. Repeated CSF analyses remained normal. Metabolic workup showed hyperaminociduria, low neutral amino acids, and undetectable tryptophane. Whole-exome sequencing and segregation analysis revealed compound heterozygous, pathogenic and a novel, likely pathogenic variant in the SLC6A19 gene: c.718C>T, p.(Arg240*) and c.170G>A, p.(Arg57His). Diagnosing Hartnup disease, high-protein diet, and niacin supplementation led to rapid considerable improvement. At 4 months, plasma amino acids were normal; communication and behavior were age-adequate; and spasticity had almost resolved, but polyneuropathy was unchanged.

Discussion: Metabolic workup and whole-exome sequencing are recommended in rapidly progressive neuropsychiatric disease, especially with additional neurologic signs and when standard treatment fails.