To the Editor:
We read with great interest the article by Brown et al.1 that reported an association between delirium diagnosis and cognitive dysfunction at 1 month in patients after cardiac surgery. The study is notable for its rigorous delirium assessment and thorough neuropsychologic battery testing at multiple perioperative time points. The conclusion that there is no difference in cognitive change at 1 yr between delirious and nondelirious patients, however, should be interpreted with caution. In addition to being potentially underpowered, the study measured delirium as a dichotomous yes/no despite previous findings that longer durations of delirium have greater impact on long-term cognitive impairment2,3 and that increased delirium severity is associated with worse outcomes.4,5 Brown et al.’s findings of lower cognitive function scores, starting with baseline assessments, for those who develop postoperative delirium are in agreement with other studies demonstrating the large contribution of preoperative cognitive performance to both acute and long-term brain dysfunction after surgery.6–8 The question of whether this signifies a continuation of a downward trajectory initiated before surgery, is a reflection of greater baseline comorbidities, or has another explanation, has yet to be answered.
The description of resilience, or return to cognitive baseline after episode of postoperative delirium, has previously been described by Inouye et al., who went on to show an accelerated cognitive decline at up to 36 months.9 It would be interesting to see if these patients would have the same biphasic decline within domain-specific cognitive scoring. From a patient perspective, it would also be important to qualify the clinical impact of a lower overall cognitive Z-score and domain-specific cognitive scores. This could help answer whether statistically insignificant changes measured with composite cognitive scoring could potentially miss the clinical impact of delayed executive function, verbal fluency, and other domains that greatly impact patient life or whether statistically significant decline on robust in-depth testing during studies translates into appreciable deficits in the real world.
The study by Brown et al.1 is important in adding to the growing body of literature attempting to define the interrelationship between postoperative delirium and cognitive decline, particularly regarding domain-specific analysis. Trials aimed at reducing delirium and postoperative decline that include similar rigorous assessments but also mechanistic testing are needed. We feel that it is essential to note the potential for recovery of cognition after acute brain dysfunction, but that the extent and quality of that recovery remains to be seen.
The authors declare no competing interests.
Dr. Hughes receives support from the American Geriatrics Society (New York, New York) Jahnigen Career Development Award and the National Institutes of Health (Bethesda, Maryland; grant Nos. HL111111, AG045085, GM120484). Dr. Boncyk receives departmental support under the T32 research training grant from the National Institutes of Health.