Mucopolysaccharidosis type IIIB (MPS IIIB) masquerading as a behavioural disorder.

Inborn errors of metabolism (IEMs) that manifest primarily as psychiatric and behavioural symptoms in childhood are often mistaken for idiopathic primary psychiatric disorders. The pathophysiological basis of these symptoms may be overlooked until later in the disease course when neurological deficits become dominant; this results in a significant delay in establishing a proper diagnosis. To illustrate this, we describe two siblings who presented with behavioural issues and mild learning disabilities in childhood, and were consequently given multiple psychiatric diagnoses. In early adulthood, however, they manifested a rapid cognitive decline. Subsequent cranial MRI imaging revealed progressive brain iron accumulation in deep brain nuclei. Whole exome sequencing and biochemical investigation confirmed the diagnosis of mucopolysaccharidosis type IIIB. Their long diagnostic odyssey illustrates the importance of considering IEMs when assessing individuals with behavioural abnormalities and cognitive impairment.