Objective:To compare disease progression between different onset forms of Metachromatic Leukodystrophy (MLD) and to investigate the influence of the type of first symptoms on the natural course and dynamic of disease progression.Methods:Clinical, genetic and biochemical parameters were analyzed within a nationwide study of patients with late-infantile (LI, onset ≤ 2.5 years), early-juvenile (EJ, onset 2.6 - < 6 years), late-juvenile (LJ, onset 6 – < 16 years), and adult (onset ≥ 16 years) forms of MLD. First symptoms were categorized as motor symptoms only, cognitive symptoms only, or both.. Standardized clinical endpoints included loss of motor and language functions, as well as dysphagia/tube feeding.Results:97 Patients with MLD were enrolled. Patients with LI (n=35) and EJ (n=18) MLD exhibited similarly rapid disease progression, all starting with motor symptoms (with or without additional cognitive symptoms). In LJ (n=38) and adult-onset (n=6) patients, the course of the disease was as rapid as in the early-onset forms, when motor symptoms were present at disease onset, while patients with only cognitive symptoms at disease onset exhibited significantly milder disease progression, independent of their age at onset. A certain genotype-phenotype correlation was observed.Conclusions:In addition to age at onset, the type of first symptoms predicts the rate of disease progression in MLD. These findings are important for counselling and therapy.Classification of Evidence:This study provides Class II evidence that in patients with MLD, age at onset and the type of first symptoms predict the rate of disease progression.

Background Metachromatic Leukodystrophy (MLD) is a rare autosomal-recessive lysosomal storage disorder caused by mutations in the ARSA gene. While interventional trials often use untreated siblings as controls, the genotype-phenotype correlation is only partly understood, and the variability of the clinical course between siblings is unclear with some evidence for a discrepant clinical course in juvenile patients. The aim of this study was to systematically investigate the phenotypic variation in MLD siblings in comparison to the variability in a larger MLD cohort and to case reports published in literature. Results Detailed clinical information was available from 12 sibling-pairs (3 late-infantile, 9 juvenile) and 61 single patients (29 late-infantile, 32 juvenile). Variability of age at onset was similar between the siblings and randomly chosen pairs of the remaining cohort (no statistically different Euclidean distances). However, in children with juvenile MLD both the type of first symptoms and the dynamic of the disease were less variable between siblings compared to the general cohort. In late-infantile patients, type of first symptoms and dynamic of disease were similarly homogeneous between siblings and the whole MLD cohort. Thirteen published case reports of families with affected siblings with MLD are presented with similar findings. Conclusions In a systematic analysis of phenotypic variation in families with MLD, siblings with the late-infantile form showed a similar variability as unrelated pairs of children with late-infantile MLD, whereas siblings with juvenile MLD showed a more homogeneous phenotype regarding type of first symptoms and disease evolution in comparison to unrelated children with juvenile MLD, but not regarding their age at onset. These results are highly relevant with respect to the evaluation of treatment effects and for counseling of families with affected siblings.