Thesis: Neuropsychological dysfunction in Obstructive Sleep Apnoea
Disease severity, as quantified by the Apnoea Hypopnoea Index (AHI; the average number of times someone stops breathing over the time they are asleep) , does not relate consistently to neurocognitive dysfunction and, as such, a defined profile of neurocognitive dysfunction is yet to be established (Aloia, Arnedt & Davis, 2004; Beebe, Groesz & Wells, 2003; Fulda & Schulz, 2003; Tsai, 2010). Several papers have suggested that the disparity within research is due to a lack of clear cut characterisation of disease severity, a multitude of inter-individual health differences that can mediate disease severity, the diverse range and questionable suitability of tests, and small sample sizes (Yahoui et al., 2009; Sanchez et al., 2009; Wong et al., 2006; Tsai, 2010).
Tsai (2010) and Bolig (2007) critique the use of the AHI as a measure of disease severity as it does not reflect patient differences in levels of hypoxia, sleep fragmentation, type of respiratory event, or inter-individual health differences. Research demonstrates that sleep fragmentation and levels of hypoxia are intimately related to neurocognitive disturbance and are measured in overnight polysomnography, however these metrics are, as yet, peripheral to disease severity (Bolig, 2007; Butkov, 2007). Whilst the AHI may be a useful and convenient “catch-all” measure of breathing disturbance, its utility as a measure of neurocognitive disease severity is questionable. The AHI does not account for hypoxia or sleep fragmentation and does not relate consistently to neurocognitive dysfunction, as such the AHI may not be the most suitable metric (Hunt, 2004; Tsai, 2010). A stronger relationship between nocturnal physiological disturbance and diurnal neurocognitive dysfunction may be found by investigating levels of hypoxia, sleep fragmentation, and inter-individual health differences.
The present thesis investigates the profile of neuropsychological damage in OSA with specific reference to measures of disease severity, other than the AHI.
Why my research is important
Obstructive sleep apnoea (OSA) is a frequent and often under-diagnosed condition that is associated with upper airway collapse, oxygen desaturation, sleep fragmentation, and neurocognitive disturbance (Al-Lawati, Patel & Ayas, 2009; Bolig, 2007). The estimated prevalence of sleep apnoea in the general population is 9% of middle aged women and 27% of middle aged men (Young et al., 1993). However, the clinical prevalence is between 1-5%, leaving a large proportion of people with sleep apnoea undiagnosed and untreated (Butkov & Lee-Chiong, 2007).
Untreated OSA is associated with increased healthcare utilization, occupational injuries, and motor-vehicle accidents (Al-Ghanim et al., 2008). Hillman et al (2006) completed a comprehensive study evaluating the financial cost of sleep disorders in Australia. They estimated that the total economic burden of sleep disorders in Australia was $US7.494 billion in the year 2004, representing 1.4% of the total burden of disease for Australia. Sassani et al. (2004) estimate that for the United States in the year 2000 there were more than 800,000 motor-vehicle collisions, costing more than $15.9 billion, that claimed more than 14,000 lives that were attributable to sleep apnoea. These same authors estimated that these figures could be more than halved if sleep apnoea were appropriately treated (Sassani et al., 2004).
Hence research is needed to define what nocturnal events are associated with the neuropsychological damage of OSA and the profile of damage in OSA, in order to successfully treat and prevent further damage and understand what support is needed for people with OSA.