Quantifying motivational deficits and apathy: a review of the literature.

Varying definitions of apathy in the published literature and a lack of a consensus regarding diagnostic criteria make the identification and quantification of apathy difficult in both clinical trials and clinical practice. The Apathy Evaluation Scale was developed specifically to assess apathy, but variations in the threshold values defined for clinically significant apathy diminish its use as a screening tool in clinical trials, although it has demonstrated sensitivity to changes in treatment in a number of studies. The Neuropsychiatric Inventory contains an Apathy subscale, which has been used to identify clinical trial populations (with a consistent threshold value) and measure changes following treatment. Few of the other assessment tools currently used in patients with neuropsychiatric disorders are specific for apathy or explore it in any depth, most have not been validated in the general population, do not have cut-off points representing clinically significant apathy, and its changes over time and in response to treatment. Further research is required to address these issues in order to facilitate the quantification of apathy and its natural history. Such research should be conducted with the aim of developing new, specific tools for use across neuropsychiatric disorders.