AIM To investigate the interobserver agreement on phenotypic early-onset ataxia (EOA) assessment and to explore whether the Scale for Assessment and Rating of Ataxia (SARA) could provide a supportive marker.METHOD Seven movement disorder specialists provided independent phenotypic assessments of potentially ataxic motor behaviour in 40 patients (mean age 15y [range 5-34]; data derived from University Medical Center Groningen medical records 1998-2012). We determined interobserver agreement by Fleiss' kappa. Furthermore, we compared percentage SARA subscores ([subscore/total score]9100%) between 'indisputable' (primary ataxia recognition by at least six observers) and 'mixed' (ataxia recognition, unfulfilling 'indisputable' criteria) EOA phenotypes.RESULTS Agreement on phenotypic EOA assessment was statistically significant (p<0.001), but of moderate strength (Fleiss' kappa=0.45; 95% CI 0.38-0.51). During mild disease progression, percentage SARA gait subscores discriminated between 'indisputable' and 'mixed' EOA phenotypes. In patients with percentage SARA gait subscores >30%, primary ataxia was more frequently present than in those with subscores <30% (p=0.001).INTERPRETATION Among movement-disorder professionals from different disciplines, interobserver agreement on phenotypic EOA recognition is of limited strength. SARA gait subscores can provide a supportive discriminative marker between EOA phenotypes. Hopefully, future phenotypic insight will contribute to the inclusion of uniform, high-quality data in international EOA databases.Ataxia is described by an impairment of the smooth performance of goal-directed movements, 1 resulting in impaired 'unconscious' decision making about balance, speed, force, and direction of intended movements. [2][3][4] Intentional motor behaviour may thus be affected by ataxic limb movements (intention and action tremor, dysdiadochokinesis, rebound, hypermetria), trunk movements (with staggering, swaying, and titubation), eye movements (nystagmus, saccades, overand undershoot), and speech (dysarthria, dysrhythmia). The underlying neuropathology involves abnormal spinal afferent input and/or cerebellar dysfunction, hampering multisensory fine-tuning and timing of motor output. In the literature, the concept of 'early-onset ataxia' (EOA) is used to define the initiation of ataxia before the 25th year of life. 5,6 The estimated EOA prevalence is about 14.6 per 100 000.7 As implicated by the large range in the age at onset, there is an enormous variety in underlying (genetic and metabolic) disorders. In this perspective, international EOA databases aim to (1) provide insight in the longitudinal disease course, (2) identify new genes, (3) design new treatment strategies, and (4) characterize uniform and transparent markers for disease monitoring. In the absence of a criterion standard, EOA patient inclusion will depend on subjective phenotypic recognition of ataxia. This process is complex for several reasons. In young children, it is wellknown that the physiological maturation of th...

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