An autosomal recessive disorder clinically almost indistinguishable from sialidosis, type II, but due to deficiency of both sialidase and β-galactosidase. As in sialidosis, multiple variants occur, of increasing severity with decreasing age of onset; congenital, infantile, and juvenile forms have been identified. The defect appears to be in a protein necessary for activation or protection of the two enzymes.