Development and longitudinal neurocognitive functioning in mucopolysaccharidosis type IIIC: a case study.

This case study presents a comprehensive analysis of the neurocognitive, medical, and developmental functioning of a 9-year-old girl diagnosed with mucopolysaccharidosis type IIIC (MPS IIIC). Genetic testing revealed a homozygous pathogenic variant of the HGSNAT gene (c.1872C > A), typically associated with severe neurodegeneration. However, her clinical presentation has been milder compared to the expected progression based on her genetic profile and residual enzyme levels. The child's current overall intellectual functioning was at the level of moderate intellectual disability; however, her developmental age has remained at the level of 5;3 for the last 3 years. The neuropsychological assessment showed some moderate difficulties in the patient's functioning, and brain magnetic resonance imaging showed no abnormalities. The results revealed that the child maintains the majority of her cognitive skills at a stable level, except for a marked decline in working memory. The study highlights the complexity and variability in the progression of MPS IIIC, emphasizing the need for early diagnosis, regular monitoring, and a multidisciplinary approach. This case highlights the need to consider individual variability in MPS IIIC progression, even when genetic and biochemical markers suggest a more severe course.