Because of the lack of an unambiguous answer as for MS etiopathogenesis and, consequently, the limited options of causal treatment, no effective therapy has been developed to combat the disease . The currently available interventions allow to reduce the disease activity, inhibit its progression, modify the autoimmune process, influence the intensity of relapses, and alleviate or treat the concomitant clinical symptoms. MS therapy is a difficult process, which is related, inter alia, to the multitude of symptoms and their overlapping [27, 28]. Despite the increasingly detailed knowledge about the disease pathomechanism, the pharmacological treatment options that might be widely applied are quite limited. Therefore, symptomatic treatment, including physical activity and physiotherapy, plays an important role besides pharmacotherapy [29, 30].
Rheology investigates blood flow through blood vessels. It concerns whole blood, as well as plasma and morphotic elements, especially red blood cells . The basic factors that influence blood flow involve RBC, red blood cell deformability and aggregation, plasma viscosity, and HCT [31–33].
The phenomenon of erythrocyte deformability plays an important role in the flow of blood cells through capillaries with a diameter even two times smaller than the cells . Normal erythrocytes deform under stress mainly because they do not have cell nuclei, their cytoplasm has a relatively low viscosity, the cell membrane presents favorable viscoelastic properties, and the appropriate shape ensures a high ratio of free surface to volume . Changes in the blood cell shape depend on the quality of the spectrin-actin network in connection with calcium and ATP ions, and the reasons for this capacity decrease mainly involve the cell age, mechanical damage, and disease factors [10, 35].
According to Maeda , appropriate erythrocyte deformability plays a key role in the blood flow in the vascular system. In turn, the appropriate erythrocyte shape, intracellular viscosity, and the cell membrane wall stiffness depend on the HGB level, which significantly influences the deformation capacity. Red blood cell deformability is crucial as it allows the cells to pass through capillaries. The lower the deformability, the higher the blood viscosity and the worse the blood flow in the microcirculation .
Spontaneous erythrocyte aggregation in whole blood, i.e., the formation of three-dimensional erythrocyte structures, is a reversible physiological phenomenon that plays a key role in blood flow at low shear rates and significantly increases blood viscosity . Erythrocyte aggregation is a complex dynamic phenomenon, activated by various factors, especially plasma-related ones, and pathological processes. Physiological shear stresses are to prevent aggregate formation, and the absence of shear stresses may be among the many causes of cardiovascular disease .
The degree of red blood cell aggregation depends on the flow conditions, properties of cell membranes, and physicochemical properties of cells . Small blood vessels, where the shear rates are usually low, are particularly susceptible to the formation of erythrocyte aggregates. This ultimately causes a decrease in blood flow velocity, or even its inhibition, and, as a consequence, under-oxygenation of cells and tissues .
The hematological symptoms in MS patients are unspecific. A homogeneous mechanism of their occurrence is difficult to determine and very likely not to exist at all. It has been suggested that hematological changes in MS may be related to such factors as complex humoral and cellular response, biochemical disorders, changes related to the attachment and transport of vitamin B12, the applied pharmacological treatment, and the phase of the disease (relapse or remission) [39–41].
Erythrocyte elasticity is significantly influenced, inter alia, by intracellular viscosity, affected by HGB. Appropriate MCH and MCHC values determine the potential for erythrocyte deformability even in cases of hyper- or hypotonic changes in the blood environment . Najim al-Din et al.  did not find significant HGB level differences between MS patients and those with other neurological diseases; they also confirmed the thesis by Reynolds and Linnell  about the tendency for macrocytosis without concomitant anemia in MS patients. In the presented study, a statistically significantly higher mean baseline level of HGB was observed in the group of healthy women compared with the mean level in MS patients before whole-body cryotherapy (P = 0.045). After the intervention, a statistically significantly lower mean HGB level was noted in healthy women (P = 0.014). The analysis showed no statistically significant differences in MCH after whole-body cryotherapy application or at baseline. However, there was a statistically significant minimal increase in MCHC in healthy women after whole-body cryotherapy application (P = 0.013).
Grasso et al.  found no pathological HGB or RBC values in MS patients. They performed blood tests in MS patients (EDSS score: 1–9) and observed no difference in MCV as compared with the control group. At the same time, as the comparative group consisted of individuals who were not completely healthy, the results were also referred to the assumed physiological norms. Notably, the MCV results exceeded the assumed norms in one person from the study group and one from the control group only. No correlation was established between the patient’s functional status expressed in the EDSS score and the MCV value.
In subsequent studies, Kocer et al.  also failed to confirm macrocytosis in MS patients examined during a relapse. In their research, MCV remained within the normal range in over 77% of the participants, while microcytosis was observed in the others. De Freitas et al.  also found no differences in MCV between MS patients undergoing steroid therapy and those who received no pharmacological treatment.
In the present study, there were also no statistically significant WBC changes in MS patients compared with the control group. However, a statistically significant difference was observed in the baseline RBC levels, which were lower in patients with MS (CRYO-MS and CONTROL-MS groups) than in healthy women (CONTROL-CRYO group). Baseline MCV did not differ between groups, with a slight MCV reduction observed in healthy women after whole-body cryotherapy application (P = 0.028).
The red blood cell distribution width (RDW), a measure of erythrocyte size and volume, affects deformability. A study found an increased RDW in patients with relapsing-remitting MS compared with healthy controls, which may underlie the altered red blood cell deformability in MS. Furthermore, increased RDW was observed to be positively associated with EDSS scores. A study that involved 109 MS patients and 130 healthy individuals failed to control other potential confounding factors affecting RDW and lacked adequate pathological controls. More research is needed on the use of RDW as a biomarker . Morphological changes in erythrocytes (macrocytes and echinocytes) are positively correlated with MS severity and may impair red blood cell deformability .
Platelets also play an important role in the coagulation cascade; they are abundant in MS . Platelets themselves do not directly affect erythrocyte aggregation, but influence thrombotic processes [49, 50], which supports the idea of ischemic tissue damage . In the present study, no statistically significant changes in PLT were established after whole-body cryotherapy application or at baseline.
Brunetti et al.  observed increased values of whole blood viscosity indices, with coexistent normal levels of plasma viscosity and HCT. They suggested that the raised blood viscosity was due to a decrease in erythrocyte deformability. Later studies by Pollock et al.  did not confirm this hypothesis, showing no visible differences in erythrocyte deformability between MS patients and healthy controls. Research related to the morphology of erythrocytes was also carried out by Simpson et al. , who reported that MS patients presented a lower level of sphingomyelin in the erythrocyte cell membrane as compared with the control group. They demonstrated impaired erythrocyte deformability, which can increase blood viscosity in MS patients. The authors observed an increase in MS patients’ blood viscosity that varied depending on sex. In women, the values of blood viscosity were significantly higher than in the control group at the shear rates of 1 ∙ s− 1, 10 ∙ s− 1, and 100 ∙ s− 1, while in men, blood viscosity was statistically significantly higher only at one shear rate (1 ∙ s− 1). Ernst  noted, however, that changes in viscosity indices did not always correlate with the level of erythrocyte deformability, as they were also influenced by the administered steroid therapy. This was confirmed by de Freitas et al. , who investigated the effect of steroid therapy on red blood cell membrane stability. They found that erythrocyte membranes in MS patients were less resistant to damaging factors than erythrocyte membranes in healthy individuals, while steroid therapy improved erythrocyte membrane resistance and brought it closer to that observed in healthy people.
When analyzing the level of HCT in the conducted study, a statistically significant difference was observed in the baseline values: these were lower in MS patients receiving the intervention (35.97 ± 5.65%) and in the MS control group (35.90 ± 5.88%) compared with healthy women (40.54 ± 2.50%). Moreover, healthy women were characterized by a statistically significantly lower HCT level after whole-body cryotherapy application (P = 0.003). In turn, with reference to EI, a statistically significant difference was demonstrated in baseline values at the shear stress of 4.24–60.30 Pa: the result was unexpectedly lower in healthy women compared with those with MS who received or who did not receive cryotherapy (P ≤ 0.016). In healthy women, a favorable increase in EI was noted at the shear stress of 2.19–60.30 Pa after the intervention. The average level of EI variability equaled 28.07%. The application of whole-body cryotherapy significantly increased erythrocyte deformability and decreased HCT values in healthy women, which positively influences their rheological blood properties.
Kowal and Marcinkowska-Gapińska  published the preliminary results of their research on the comparison of hemorheological properties in MS patients and individuals with acute cerebral ischemia. They observed a statistically significantly higher HCT value and a lower measure of red blood cell aggregation capacity in patients with MS compared with those after an acute cerebral ischemia incident. The differences in erythrocyte stiffness and the tendency to aggregation were not statistically significant. In the study, however, MS patients constituted a very small group, and the research was not continued among a wider MS cohort. The mean AI values in the study group and in the control group were not significantly different before and after whole-body cryotherapy. There were also no statistically significant changes in baseline AI or T1/2 values.
Fibrinogen is a 340-kDa glycoprotein synthesized in the liver, with a plasma concentration of approximately 150–400 mg/dl. The protein is involved in blood clotting and hemostasis, as well as in inflammation and tissue repair. Fibrinogen facilitates platelet aggregation by glycoprotein IIb/IIIa receptor binding and forming a fibrin monomer that rapidly polymerizes to create a clot [55, 56]. With inflammatory response, plasma fibrinogen levels rise 2–3-fold, leading to cell aggregation and an increase in blood viscosity . Studies have shown that fibrinogen can modulate inflammatory response by leukocyte activation and synthesis of pro-inflammatory mediators (cytokines and chemokines) [55, 58]. Fibrinogen is not a clear indicator of the blood-brain barrier disruption, but it does activate glial cells, which results in blood-brain barrier dysfunction in MS patients .
Acuña et al.  demonstrated that high plasma fibrinogen levels (> 417 mg/dl) were associated with active changes in magnetic resonance imaging in MS patients during relapses. In an earlier study on fibrinogen in MS patients, Ehling et al.  showed no elevation of fibrinogen in the cerebrospinal fluid or plasma, probably because they compared fibrinogen levels between MS patients and those with central nervous system infections; in addition, less than a third of MS patients experienced an acute relapse at the time of fibrinogen analysis, with the rest presenting an inactive disease or a chronic progressive course.
Despite the inflammatory events, it seems that patients with relapsing-remitting MS do not have elevated levels of fibrinogen in remission . However, D-dimer levels are elevated  and low fibrinogen levels during remission (MS patients vs. controls) do not rule out fibrinogen increase during relapse, especially if one considers the role of fibrinogen in MS pathology, where fibrin is involved in cytokine release and microglia activation in the central nervous system [64, 65]. When analyzing the mean levels of fibrinogen in the present study, no increased values were observed in women with MS, while after the application of a series of 20 whole-body cryotherapy sessions, there was a statistically significant rise in healthy women (by 34.13%) and a statistically insignificant rise (owing to the large discrepancy) in MS patients (by as much as 44.24%).
In clinical practice, plasma fibrinogen may be a valuable and easy biomarker of activity during relapses in MS patients, but prospective studies in larger groups are needed to confirm these results.
It has been found that although all MS patients show an increase in oxidative damage biomarkers, there seems to be no correlation between the degree of the increase and the disease severity. It is now suggested that oxidative changes are not accompanied by inflammatory activity in these patients as measured by changes in WBC and C-reactive protein levels. This would mean that oxidative stress precedes the inflammatory response, which would indirectly support the hypothesis that oxidative stress alters blood-brain barrier permeability and stimulates monocyte adhesion to vascular endothelium . Erythrocytes may contribute to the pathophysiological mechanisms of MS through impaired antioxidant capacity and altered hemorheology, leading to an increase in oxidative stress and to potential ischemic tissue damage, respectively .
Studies on the effect of whole-body cryotherapy in MS patients were also conducted by Bryczkowska et al. . In healthy individuals, the first changes in the lipid profile were observed after 20 daily whole-body cryotherapy sessions , while in MS patients, after a series of 30 whole-body cryotherapy sessions, researchers did not report significant changes in total protein, albumin, uric acid, and glucose concentrations or in the lipid profile . In the present study, lower baseline total protein levels were demonstrated in the groups of MS patients (70.17 ± 3.98 g/l and 70.29 ± 2.79 g/l) compared with healthy women (73.56 ± 3.68 g/l); there was also a statistically significant reduction of the parameter among healthy women after whole-body cryotherapy application (2.49%). No significant differences were indicated in the other proteinogram indices, i.e., albumin, alpha-1 globulins, alpha-2 globulins, beta-1 globulins, beta-2 globulins, except for lower baseline levels of gamma globulins in the groups of MS patients compared with healthy women (by 18.13% on average).
Studying the role of erythrocytes in MS may reveal further specific differences that could be used as the disease biomarkers, as well as broaden our understanding of the pathological mechanisms of this complex and heterogeneous disease. This, in turn, could lead to the discovery of new and innovative therapeutic targets, thus significantly improving patients’ quality of life.
Summing up, in the light of the available literature and the results of own research, it can be concluded that whole-body cryotherapy is an effective method to combat or inhibit the progress of many diseases and their negative consequences, thus contributing to maintaining the best possible body fitness. The presented study is probably the first one to assess the influence of whole-body cryotherapy on rheological blood properties, including EI and AI, in MS women. No pathological or harmful changes were observed after whole-body cryotherapy application. In healthy participants, the intervention resulted in improved erythrocyte flexibility, which adds to better cell oxygenation.