Mechanisms of Memory Dysfunction during High Altitude Hypoxia Training in Military Aircrew

Abstract

Objectives Cognitive dysfunction from high altitude exposure is a major cause of civilian and military air disasters. Pilot training improves recognition of the early symptoms of altitude exposure so that countermeasures may be taken before loss of consciousness. Little is known regarding the nature of cognitive impairments manifesting within this critical window when life-saving measures may still be taken. Prior studies evaluating cognition during high altitude simulation have predominantly focused on measures of reaction time and other basic attention or motor processes. Memory encoding, retention, and retrieval represent critical cognitive functions that may be vulnerable to acute hypoxic/ischemic events and could play a major role in survival of air emergencies, yet these processes have not been studied in the context of high altitude simulation training. Methods In a series of experiments, military aircrew underwent neuropsychological testing before, during, and after brief (15 min) exposure to high altitude simulation (20,000 ft) in a pressure-controlled chamber. Results Acute exposure to high altitude simulation caused rapid impairment in learning and memory with relative preservation of basic visual and auditory attention. Memory dysfunction was predominantly characterized by deficiencies in memory encoding, as memory for information learned during high altitude exposure did not improve after washout at sea level. Retrieval and retention of memories learned shortly before altitude exposure were also impaired, suggesting further impairment in memory retention. Conclusions Deficits in memory encoding and retention are rapidly induced upon exposure to high altitude, an effect that could impact life-saving situational awareness and response.

Increasing Inaccuracy of Self-Reported Subjective Cognitive Complaints Over 24 Months in Empirically Derived Subtypes of Mild Cognitive Impairment

“…These results add to the accumulating body of evidence that the CDN group – which accounts for a significant proportion of the ADNI MCI cohort – represents false positive diagnostic errors (Bangen et al, 2016; Bondi et al, 2014; Edmonds et al, 2014, 2015, 2016; Eppig et al, 2017; Thomas et al, 2017). Previous studies have suggested that MCI patients have intact awareness of their cognitive abilities (for review, see Piras et al, 2016), which appears contrary to our results.…”

“…In stark contrast, participants who were determined to be cognitively normal on neuropsychological testing had a tendency to over-report cognitive difficulty at all three time points, with remarkable consistency in both self- and informant-report over time. This pattern was seen for both the robust NC group and for the CDN group – a large subtype of ADNI participants who have been intensively studied by our group and appear to be misdiagnosed with MCI based on conventional diagnostic criteria (Bangen et al, 2016; Bondi et al, 2014; Edmonds et al, 2014, 2015, 2016; Eppig et al, 2017; Thomas et al, 2017). …”

“…A final limitation of this study is the lack of assessment of visuospatial functioning, as some patients may show prominent visuospatial impairment and previous research has identified a visuospatial MCI subtype (Clark et al, 2013). However, visuospatial measures in ADNI are limited, and measures that are available have psychometric properties that limit their ability to discriminate between normal and mildly impaired individuals (e.g., ceiling effect; Eppig et al, 2017). …”

“…By applying statistical techniques, such as cluster analysis or latent profile analysis, we have previously identified unique MCI subtypes in several different datasets. These include subtypes with deficits primarily in one cognitive domain (e.g., memory, language), and others with multi-domain impairments (Clark et al, 2013; Bondi et al, 2014; Delano-Wood et al, 2009; Edmonds et al, 2015; Eppig et al, 2017; Libon et al, 2010). Critically, our empirically-derived classifications have also yielded a large subtype of individuals who appear to represent “false positive” diagnostic errors.…”

“…Critically, our empirically-derived classifications have also yielded a large subtype of individuals who appear to represent “false positive” diagnostic errors. These are individuals who were classified as MCI based on the conventional diagnostic criteria, but actually perform within normal limits on a comprehensive battery of neuropsychological tests (Bondi et al, 2014; Clark et al, 2013; Edmonds et al, 2015; Edmonds et al, 2018; Eppig et al, 2017). Within the Alzheimer’s Disease Neuroimaging Initiative (ADNI), this “false positive” subtype, which comprises as much as one-third of the ADNI MCI cohort, demonstrates normal AD cerebrospinal fluid (CSF) and neuroimaging biomarkers (Bangen et al, 2016; Edmonds et al, 2015, 2016).…”