AbstractBackgroundMeningoencephalitis of unknown origin (MUO) comprises a group of debilitating inflammatory diseases affecting the central nervous system of dogs. Currently, no validated clinical scale is available for the objective assessment of MUO severity.ObjectivesDesign a neurodisability scale (NDS) to grade clinical severity and determine its reliability and whether or not the score at presentation correlates with outcome.AnimalsOne hundred dogs with MUO were included for retrospective review and 31 dogs were subsequently enrolled for prospective evaluation.MethodsMedical records were retrospectively reviewed for 100 dogs diagnosed with MUO to identify the most frequent neurological examination findings. The NDS was designed based on these results and evaluated for prospective and retrospective use in a new population of MUO patients (n = 31) by different groups of independent blinded assessors, including calculation of interobserver agreement and association with outcome.ResultsThe most common clinical signs in MUO patients were used to inform categories for scoring in the NDS: seizure activity, ambulatory status, posture and cerebral, cerebellar, brainstem, and visual functions. The intraclass correlation coefficient (ICC) for prospective use of the NDS was 0.83 (95% confidence interval [CI], 0.68‐0.91) indicating good agreement, and moderate agreement was found between prospective and retrospective assessors (ICC, 0.71; 95% CI, 0.56‐0.83). No association was found between NDS score and long‐term outcome.Conclusions and Clinical ImportanceThe NDS is a novel clinical measure for objective assessment of neurological dysfunction and showed good reliability when used prospectively in MUO patients but, in this small population, no association with outcome could be identified.