Table 1 What is mucopolysaccharidosis type II (MPS II)?

• First described in two brothers by Dr Charles A. Hunter in 1917.

• Caused by deficient activity of the lysosomal enzyme iduronate-2-sulfatase (EC 3.1.6.13), which catalyses a step in the catabolism of the glycosaminoglycans (GAG) dermatan sulfate and heparan sulfate. The accumulation of these in tissues and organs throughout the body contributes to the chronic, progressive, multisystemic manifestations of MPS II [14, 15].

• The initial clinical signs and symptoms typically emerge within the first few years of life and include recurrent respiratory infections, coarse facial features, joint stiffness, otitis media, hearing loss, umbilical/inguinal hernias, and hepatosplenomegaly [45].

• The severity of the disease spans a broad range. For clinical purposes, patients are generally considered to fall into one of two categories according to the presence or absence of cognitive impairment. All patients will experience somatic disease manifestations, although progression may be slower in individuals without cognitive impairment [14, 15]. About two-thirds of patients will display progressive central nervous system involvement, initially resulting in learning impairment and abnormal behaviour, followed by the development of profound cognitive impairment [12].