Aim: In children with Friedreich ataxia (FRDA), ataxia is assessed using the surrogate marker the International Cooperative Ataxia Rating Scale (ICARS). We aimed to determine whether ICARS scores in children with FRDA are confounded by muscle weakness.
Method: In 12 children with FRDA (10 males, two females; mean age 13 y 6 mo, SD 2 y 6 mo) and 12 age-matched children without FRDA (nine males; three females), we determined the association between muscle and ataxia parameters (i.e. muscle ultrasound density (MUD), muscle force, sensory evoked potentials, and ICARS scores). Children with FRDA were included on the basis of FXN gene analysis. Children in the comparison group were included on basis of uneventful pregnancy and normal cognitive and neurological development.
Results: In children with FRDA, muscle ultrasound density was homogeneously increased in the biceps, quadriceps, and tibialis anterior muscles (median 4SD). FRDA muscle weakness was significantly more pronounced in proximal than in distal muscles (-2SD vs -0.5SD respectively; p=0.004), with a stronger impairment of leg muscles than of arm muscles (-2SD vs -0. SD respectively; p=0.001). Comparing MUD between children with FRDA and an age-matched comparison group revealed a relatively strong increase in MUD in the proximal leg muscles in the FRDA group. Under the condition of persistently absent sensory evoked potentials, leg ICARS subscores in the FRDA group appeared to be positively associated with leg muscle force until a maximal plateau level of ICARS subscores was reached.
Interpretation: In children with FRDA, ataxia scales based on ICARS are confounded by muscle weakness. Longitudinal ICARS evaluations in children with FRDA do not necessarily indicate altered ataxia.
© The Authors. Developmental Medicine & Child Neurology © 2011 Mac Keith Press.