Reply to Cognitive Reserve: Predictor of Onset of Postoperative Delirium in Older Adults?

To the Editor: I appreciate Zhu and colleagues’ comments on our report that higher levels of cognitive reserve, measured according to participation in cognitive leisure activities, reduced risk and severity of postoperative delirium in older adults undergoing surgery.1, 2 They suggest that muscle function not only could be a target for delirium preventive strategies, but also could serve as a cognitive reserve measure. The concept of cognitive reserve postulates that some individual characteristics help maintains cognitive function in the face of accumulating brain pathology.3 Classical definitions of cognitive reserve incorporate passive and active mechanisms.3 Early work proposed that greater cognitive reserve provides an anatomic buffer (also termed brain reserve) by which greater neuronal or synaptic loss secondary to brain injury or pathology can be sustained before clinical symptoms manifest.3, 4 In active models, cognitive reserve is thought to compensate for brain damage by actively using various mechanisms such as recruiting preexisting cognitive networks, improving efficiency of brain processing, and enlisting compensatory brain networks.3 Activities that enhance muscle function may have modest effects on improving or maintaining cognitive function in aging,5 but this association does not equate to being a good measure of cognitive reserve. Under passive and active cognitive reserve models,3 muscle structure and biology are secondary to the underlying cognitive processes and brain substrates that may more directly contribute to cognitive reserve. Hence, although I agree that leisure activities that increase cognitive reserve should be explored as a delirium-preventive strategy, I do not agree that muscle function is a good proxy for characterizing cognitive reserve.2 Conflict of Interest: JV has received funding support from National Institute on Aging Grants R01 AG039330, R01AG044007, AG044829, and R01AG036921. Author Contributions: Verghese V.: was responsible for study concept and design, analysis and interpretation of data, and preparation of manuscript. Sponsor's Role: NA.