One of the greatest losses in life comes while watching a previously witty and thoughtful loved one lose the things that make them, them. I suspect most people can relate to that sentiment – and some folks have probably experienced it in a much more intimate way than others. Professionally, those of us that look after outpatient hemodialysis patients have also seen that decline happen within the context of dialysis.
Indeed, the end-stage renal disease (ESRD) population has long been known to carry a higher burden of cognitive dysfunction than the general population; estimates of the prevalence of cognitive impairment among hemodialysis patients who are over age 55 ranges between 30- 70%. What is poorly understood, however, is how hemodialysis impacts those cognitive deficits. Given the personal/ familial burden of cognitive decline, as well as the impact of this disease on their medical care, understanding cognitive impairment in the context of hemodialysis should be of great interest to us all.
Previous work has sought to ascertain the prevalence of cognitive decline among patients with chronic kidney disease (CKD). Some of those have found a correlation between declining cognition and progressive CKD; other publications, such as the work done by Tamura et al as discussed in a previous blog post, have not shown that progressive CKD tracks with declines in cognition when captured by the mini-mental status exam (MMSE).
In a recent AJKD article, Drew et al prospectively assessed cognitive function over time using a robust battery of tests in order to ascertain how memory and executive function – representing two distinct cognitive domains – were affected by dialysis. They collected data at least annually from 314 outpatients and had median follow-up of 2.1 years. This design also allowed them to assess for risk factors for decline.
Executive function refers to a collection of cognitive processes, but it is probably best thought of as an individual’s ability to reason through an issue and to problem-solve. That contrasts with memory, representing an individual’s ability to recall information previously learned. Drew et al found that memory was preserved or slightly improved when evaluated year-over-year, but that executive function had a statistically significant decline over time. The decline in executive function was noted over a variety of different tests, making the result fairly robust. Further covariate analysis identified older age as the most significant risk factor. It should be noted that the neurocognitive studies were performed during the first hour of dialysis rather than during some interval following treatment. While the authors have shown that there is no change between tests performed one hour prior to and those performed during dialysis, it is possible that this is not the optimal time to assess a patient’s problem-solving abilities.
The logical question that emerges is: Why?
I think all of us could come up with several reasonable explanations:
- Could this decline be unrelated to dialysis, instead related to underlying comorbid diseases?
- Do we negatively impact cognition through the metabolic flux induced by dialysis?
- Or, as hypothesized by the authors, is this pattern of decline a reflection of the high prevalence of cerebrovascular disease in this population, with consequences akin to multi-infarct dementia?
Obviously, this remains an open question, though by beginning to understand the baseline rate of decline and developing a toolset for cognitive assessment, Drew et al have allowed us to meander a bit further down the road to answering it.