Sickle Cell Subjects Have a Stronger and Faster Neurally Mediated Vasoconstriction Response to Cold Pain That Correlates with Anxiety Scores

Introduction: Vaso-occlusive pain crisis (VOC) is a significant contributor to the morbidity of Sickle Cell Disease (SCD) and cold exposure has long been associated with increased frequency and intensity of VOC. However, the mechanism by which cold exposure causes the transition from steady state to vaso-occlusion has not been well elucidated. Decreased regional blood flow results in red blood cells spending a longer period of time in the deoxygenated state in the capillaries, increasing the likelihood of hemoglobin S polymerization in the microvasculature and subsequent vaso-occlusion. Regional blood flow is primarily regulated by the autonomic nervous system (ANS) and recent literature shows evidence of cardiovascular autonomic dysfunction in SCD and of its role as a disease modulating factor. We hypothesized that cold exposure triggers a central autonomic response leading to vasoconstriction in the microvasculature and that SCD subjects will have a stronger response when compared to controls.

Methods: 17 SCD and 16 control (healthy or sickle trait) subjects aged 13 to 39 years were exposed to thermal stimuli via a computer-controlled thermode (TSA-II) placed on the thenar eminence of the right hand. Predetermined individual threshold temperatures for heat detection, cold detection, heat pain and cold pain were applied, and changes in the microvascular blood flow (MBF) were measured at a sub-second sampling rate on the contralateral thumb using photo-plethysmography (PPG). Mean MBF was derived from the amplitude of the PPG signal, a greater decrease in mean MBF from baseline signifies stronger vasoconstriction. The vasoconstriction response within the PPG signal, and the time (in sec) to vasoconstriction were determined from the cross-correlation function of the pain stimulus signal and the vascular response.

The R-to-R interval (RRI) derived from electrocardiogram was used to evaluate cardiac autonomic balance. Standard deviation of RRI represents total heart rate variability, Spectral indices of the RRI represent parasympathetic activity (high frequency power; HFP) and sympatho-vagal balance (Low to high Ratio; LHR = low frequency power(LFP)/HFP).

Prior to the study, subjects were administered measures of current anxiety and general level of anxiety (STAI Y1 and Y2, respectively) as well as a measure of pain related anxiety (PASS).

Results: All the thermal stimulation tasks caused a significant decrease in MBF from baseline (p values for cold detection, heat detection, heat pain and cold pain=0.027, <0.001, <0.001, <0.001 respectively), with cold pain task causing the greatest decrease in MBF. Compared to controls, SCD subjects had significantly higher pain anxiety (PASS) score (p=0.013). After controlling for PASS scores, SCD subjects still had significantly stronger vasoconstriction responses compared to controls (p =0.029)(Figure 1). The time to vasoconstriction was significantly shorter in SCD subjects (p=0.032), indicating a quicker vasoconstriction response (Figure 2). Individuals with greater anxiety state (STAI Y1) scores also showed a shorter time to vasoconstriction (p=0.034).

At baseline and across all tasks, SCD subjects showed a significantly lower heart rate variability suggestive of autonomic dysfunction (standard deviation of RRI- 41.2 Vs 50 msec, p=0.007). Both cold and heat pain caused a significant increase in the mean LFP and LHR from baseline with no significant differences in HFP thus suggesting an overall increase in the cardiac sympathetic output with thermal pain.

Conclusion: Using MBF as an objective marker of pain responses, cold pain causes significant decrease in MBF followed by other thermal stimuli. The rapid vasoconstriction responses to the thermal stimuli occurred in the contralateral hand, suggesting a neural mechanism. SCD subjects demonstrated an augmented peripheral vasoconstriction response with evidence of cardiac autonomic imbalance. Our findings imply that global autonomic mediated vasoconstriction and subsequent vaso-occlusion is the likely mechanistic link between cold exposure and VOC in SCD. Thus, vascular autonomic reactivity is a biomarker of basic SCD pathophysiology and serves as a potential target for prevention and treatment of pain in SCD.

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