Background
cognitive functioning, MPS II, OMIM 309900, X-linked lysosomal storage disease
DISEASE, MUCOPOLYSACCHARIDOSIS II, HUNTER SYNDROME, GCA
Mucopolysaccharidosis II (MPS II; Hunter syndrome; OMIM 309900) is a rare, X-linked lysosomal storage disease with an estimated worldwide prevalence of between 1 in 100,000 and 1 in 170,000 male live births [The current standard of care for MPS II is intravenous enzyme replacement therapy with recombinant I2S (idursulfase; Takeda Pharmaceuticals USA, Inc., Lexington, MA, USA), which has been shown to be effective in the treatment of somatic manifestations of the disease and in improving survival [Global ability scores are used to assess broad cognitive ability on major intellectual tests such as the Wechsler scales, DAS, Bayley, and Kaufman ability scales [To address this fundamental limitation of using norm-based scores in isolation, the Projected Retained Ability Score (PRAS) methodology has been developed to characterize the absolute change (i.e., change in absolute performance in the same child from baseline to endpoint) in norm-based scores over time. When applied to DAS-II GCA scores, PRAS allows differentiation between below-average cognitive growth rates versus plateauing and deteriorating (i.e., loss of milestones) cognitive functioning in children with MPS II [