Development and reliability of a classification system for gross motor function in children with metachromatic leucodystrophy

Aim Motor deterioration is a key feature of late infantile and juvenile metachromatic leucodystrophy (MLD). Assessment of the disease course implies the need for a standardized description of motor decline. The aim of this study was to establish a classification system for gross motor function in MLD and to assess its interrater reliability. Method The Gross Motor Function Classification in MLD (GMFC-MLD) was modelled analogous to the Gross Motor Function Classification System in cerebral palsy. Motor data from 59 individuals (27 male; 32 female) with MLD (21 late infantile; 38 juvenile) born between 1970 and 2007 were gathered from a nationwide survey and classified by six independent raters. Median age at onset was 17 months (range 9-27mo) for the late infantile group and 74 months (35-168mo) for the juvenile group. Results The GMFC-MLD consists of seven levels and is applicable from the age of 18 months. It represents all clinically relevant stages from normal (level 0) to loss of all gross motor function (level 6). The kappa coefficient was 0.90 for overall rater agreement. There were no significant differences between level-specific kappa coefficients. Interpretation The GMFC-MLD is a highly reliable, feasible tool for standardized assessment of gross motor function in MLD which can be used for the description of the natural course of the disease and for evaluation of therapeutic options such as stem cell transplantation and enzyme replacement, both of which are topics of current research.