Numerous studies have documented a significant association between symptom severity and cognitive functioning in bipolar disorder (BD). These findings advanced speculations about a potential link between the physiological stress associated with illness severity and cognitive dysfunction. To explore this hypothesis, the current study employed heart rate variability (HRV) as a physiological measure that is sensitive to the effects of chronic stress, and a scale of trait anxiety for assessing a psychological condition that is correlated with hyper sympathetic arousal. Analyses indicated that BD patients with High Illness Severity reported more symptoms of trait-anxiety (i.e., State Trait Anxiety Inventory), performed more poorly on a computerized neuropsychological battery (i.e., CNS Vital Signs), and exhibited a more constricted HRV profile (i.e., lower SDNN with elevated LF/HF ratio) than patients with Low Illness Severity. Illness severity was determined by a history of psychosis, illness duration, and number of mood episodes. A third group of healthy controls (n=22) performed better on the neuropsychological battery and exhibited a healthier HRV profile than the BD groups. This study provides preliminary evidence that illness severity and cognitive impairment in BD may be associated with state anxiety and neuro-cardiac alterations that are sensitive to physiological stress.
Neuropsychological research in bipolar disorder (BD) revealed the presence of cognitive deficits that linger beyond the resolution of mood disturbance (Martinez-Aran et al., 2004b and Goldberg and Burdick, 2008). Studies suggest that chronic cognitive impairment tends to emerge in patients who suffer from a more severe course of illness (Martinez-Aran et al., 2004a, Robinson et al., 2006 and Torres et al., 2007), as indicated by the presence of psychosis (Bora et al., 2007), and increased number of mood episodes and psychiatric hospitalizations (Denicoff et al., 1999 and Robinson and Ferrier, 2006). These findings led to speculations about the role of physiological stress associated with illness severity in the development of cognitive dysfunction (Goodwin et al., 2008, Kapczinski et al., 2008, Berk et al., 2011 and Vieta et al., 2013).
Previous findings provide preliminary evidence for the ill effects of physiological stress in BD. Physiological stress was implicated by research that revealed disproportionally high stress-related pathology in BD. Studies point to increased medical burden in BD, as indicated by higher rates of cardiovascular disease, diabetes mellitus, and obesity (Kupfer, 2005 and McIntyre et al., 2007). On the psychological level, emotional correlates of excessive activation of physiological responses to stress, specifically in the form of anxiety, are also evident in BD.
A large volume of studies has documented extensive comorbidity between BD and anxiety disorders (Freeman et al., 2002 and Simon et al., 2004). Co-morbid anxiety disorders predict more frequent mood episodes (McElroy et al., 2001, Baldassano, 2006 and Pini et al., 2006) and poor prognosis (Henry et al., 2003, El-Mallakh and Hollifield, 2008, Lee and Dunner, 2008, Azorin et al., 2009 and Coryell et al., 2009). In an integrative view, studies have separately linked chronic anxiety and cognitive impairment to greater illness severity in BD. However, these variables have not been examined jointly within a single sample. In addition, there is little data linking signs of chronic physiological stress to cognitive impairment or illness severity in BD.
The current study thus aimed to explore whether the cognitive impairment that develops in more symptomatic BD patients is accompanied by physiological and emotional correlates of chronic physiological stress. For this purpose, the study administered a measure of trait anxiety. This measure assesses an emotional state of chronic worry, which has been previously linked to an over activation of the hypothalamic pituitary adrenal (HPA) axis mediated stress responses (O׳Connor et al., 2009; Walker et al., 2011). On the physiological side, the study employed heart rate variability (HRV) as a quantitative measure of neuro-cardiac activity that is sensitive to the effects of chronic physiological stress.
HRV refers to the temporal variance in heartbeats, and it is often measured by the standard deviation of the beat-to-beat interval (SDNN). Constriction in HRV is associated with chronic stress and poor cardiac health (Dishman et al., 2000). In psychiatric research, studies have documented a wide range of conditions, such as generalized anxiety disorder (Yeragani et al., 1998) and depression (Yeragani et al., 2002) that are susceptible for both chronic physiological stress and abnormal reduction in HRV. In addition, there is evidence that cardiac data predict physical (Lombardi, 2002 and Rugulies, 2002) and mental health (Rottenberg et al., 2002) outcomes in psychiatric patients. In BD research, several studies detected constricted HRV in euthymic patients relative to controls (Cohen et al., 2003 and Latalova et al., 2010). The current study explored whether constriction in HRV, elevation in trait anxiety and cognitive impairment co-occur in BD patients with a more severe course of illness.
