|Disease||Types||Age of onset||Life expectancy||Primary symptoms||Key similarities/differences within disease types||Similarities between MLD, MPS II and MPS IIIA|
(onset before 3 years of age) 
|Median 1.5 years ||Mean age at death 4.2 years ||Motor related (e.g. weakness, gait abnormalities, quadriparesis, dysarthria, hearing difficulties, vision impairment, incontinence) .||Motor decline is typical for both late-infantile and juvenile MLD (more rapid in late-infantile); in the juvenile form, it may be preceded by cognitive and behavioral problems .||
All three diseases tend to have a pediatric onset and are associated with significantly reduced life expectancy.|
Juvenile MLD, severe MPS II and MPS IIIA are associated with behavioral problems and eventual motor decline. In late-infantile MLD, patients develop motor deficits very young, making manifestations of behavioral problems difficult to detect.
|Juvenile (onset before 16 years of age) ||Median 6 years ||Mean age at death 17.4 years ||Neuropsychiatric or cognitive prodrome (i.e. frontal lobe dysregulation, followed by gradual neurologic decline) .|
|MPS II||Severe (neuronopathic) – two-thirds of patients, with signs and symptoms appearing by 3 years of age ||Median 1.5 years ||Median age at death 11.7 years ||
Affects multiple organs and physiologic systems (e.g. facial dysmorphism, organomegaly, joint stiffness and contractures, pulmonary dysfunction, myocardial enlargement and valvular dysfunction, and neurologic involvement).|
In patients with neurologic involvement, intelligence is impaired .
|Patients with the severe form of MPS II have cognitive impairment; patients with the less severe form do not experience significant cognitive involvement .|
|Mild (non-neuronopathic)||Median age at death 14.1 years ||Individuals with non-neuronopathic MPS II are of normal intelligence .|
|MPS IIIA||NA||Mean 3 years ||Median age at death 15.0 years ||
Primarily characterized by degeneration of the central nervous system, resulting in severe cognitive impairment (e.g. speech delay) as well as hyperactivity and aggressive behavioral problems .|
Behavioral difficulties tend to become increasingly severe for 5 or 10 years, after which there is a regression in behavioral disturbances, which is associated with a progressive and severe loss of intellectual and motor functioning .
Somatic symptoms include coarse facial features and skeletal pathology that affects growth and causes degenerative joint disease, hepatosplenomegaly, macrocrania and hearing loss .
|The clinical spectrum in MPS IIIA is broad (e.g. patients typically survive until early teens in the most severe cases or as late as the sixth decade in attenuated forms) .|