congenital disease definitionadld leukodystrophyhypomyelinating leukodystrophymetachromatic leukodystrophy symptomsdemyelinating leukodystrophymitochondrial leukodystrophygloboid leukodystrophyleukodystrophy diagnosisinfantile leukodystrophyleukodystrophy prognosisalexander leukodystrophymetabolic leukodystrophy