In this study, clinical variables, neuroimaging features and biomarkers associated to inflammation, atrial dysfunction and endothelial or excitotoxicity were evaluated in order to characterize wake-up IS patients. In our series of patients with acute IS (n = 4,251), who received appropriate treatment according to clinical management guidelines, wake-up episodes occurred in 9.7% of patients. In wake-up IS, 59.1% had a poor functional status at 3 months mainly because they were excluded from acute stroke therapy options. In line with previous studies, we can say that wake-up stroke patients showed an elevated severity at admission [5–7, 36], but their evolution over 3 months was similar to awake IS patients. No difference was found in DWI volume at admission and in clinical DWI mismatch in both group of patients. Findings that support the current trend that MRI or CT scan could be the best methods for extending the time window for wake-up IS patients [17–19].
The results of our study showed that wake-up IS events were independently associated with inflammation markers such as temperature, fibrinogen and C-reactive protein; however, this association was in turn, dependent on low serum vitamin D levels. To our knowledge, low levels of vitamin D have been associated with increased cardiovascular mortality, cancer incidence , autoimmune diseases such as multiple sclerosis , poor prognosis of stroke . Clinical studies have demonstrated that a low serum level of vitamin D is associated with higher risk of stroke and negatively impacts recovery and mortality from stroke . Moreover, a recent study has demonstrated an inverse association with IL-6 and high sensitivity C-reactive protein levels, suggesting a potential anti-inflammatory role for vitamin D in stroke individuals [30, 39]. On the other hand, preclinical data suggest that acute administration of vitamin D can limit infarct progression by modulating post-stroke brain inflammation. Supplementation with vitamin D reduced infarct volume by 50%, reduced of pro-inflammatory mediators IL-6, IL-1β, IL-23a, TGF-β and NADPH oxidase-2 in brains of mice, and increased the expression of the T-regulatory cell marker, Forkhead box-P3 (FoxP3) . The impact of vitamin D deficiency in cerebrovascular diseases may adversely affect endothelial cell function and vascular homeostasis through pleiotropic pro-oxidant (endothelial nitric oxide synthase (eNOS), reactive oxygen species (ROS), upregulation of NADPH oxidases (NOXs)) and pro-inflammatory effects (inflammatory cytokines, nuclear factor kappa-light-chain-enhancer of activated B cells (NF-B), matrix metalloproteinases (MMPs)). In turn, these effects related to vitamin D deficiency can enhance tissue sensitivity to oxidative stress and inflammatory events, with consequent increased susceptibility to the onset and severity of stroke events .
To our knowledge, this study is the first study about the difference of vitamin D level between wake-up stroke and awake stroke. We have found that serum vitamin D levels may be associated with an increased risk of wake-up IS, in particular serum vitamin D levels ≤ 9 ng/ml multiply by 15 the risk of suffering an ischemic stroke during sleep. The cardiovascular system is known to be particularly sensitive to vitamin D levels, which can lead to endothelial dysfunction and vascular abnormalities through a variety of mechanisms, including cytokine release, superoxide migration inhibition, and monocyte adhesion and migration . Taking into account that one of the physiological mechanisms involved to wake-up stroke is the endothelial dysfunction, we hypothesized that the endothelial dysfunction could be the “meeting point”, and the endothelium could be the link between risk factors and vascular lesion due vitamin D deficiency. This, however, will require further studies.
On the other hand, when IL-6 was included in the multivariate regression model, the significance of the markers including vitamin D disappeared. The relationship between inflammation and vitamin D levels is well known [34, 35], so vitamin D could be an excellent therapeutic target to reduce inflammation. We can see that high serum levels of IL-6 are associated with an increased risk of waking-up IS and this risk is, to a large extent, independent of low serum vitamin D levels. Considering the clinical view, the diagnosis and monitoring of vitamin D deficiency could prove a preventive action in routine clinical practice to maintain optimal levels by means of adequate supplementation or healthy habits. However, further research is needed in this area because various factors might influence an individual's response to a particular dose of vitamin D (obesity, genetic factors, etc.) .
This study has the following limitations: First, it is a prospective single center study, although an elevate sample size were included. Second, despite having a very large total patient sample, IS groups were unbalanced. We consider, however, that it would be important to study the two types of stroke independently. Three, only in 61.2% of patients serum glutamate was analyzed, IL-6 in 80.2% of patients and vitamin D in 79.8% of patients. Four, the wake-up group's pathophysiological link between inflammation and stroke could be due to causes other than vitamin D levels. Atrial fibrillation has recently been associated to increased systemic inflammation, and it was shown to be considerably more common in the wake-up stroke group in the current study (p = 0.011). However, clinical and analytical markers of inflammation (temperature, fibrinogen, C-reactive protein, and IL-6) were not found to be higher in wake-up patients with cardioembolic stroke in our data bank. Finally, IL-6, and serum glutamate determinations were not simultaneous. Different researchers performed the determinations, although always blinded to the clinical/neuroimaging data, and supervised by the same senior researcher. In the same lines with the results for clinical and neurological data.