## 4 Discussion  
Our study showed a positive effect of transcutaneous gaseous CO\({}_{2}\) application on feet sensations tested with monofilament and vibration. The vibration sensation and monofilament test were performed before treatment and after four weeks of CO\({}_{2}\) therapy or placebo treatment. Before the therapy, patients in both groups had no vibration sensation in five of the ten standard points. The monofilament test showed a loss of sensation in 11 (study group) or ten (placebo group) out of 16 standard points (Table 2). The pretreatment temperature of the big toe was comparable in both groups. After four weeks of therapy, sensation was significantly improved in the study group (\(p<0.001\) for vibration and monofilament test), while no improvement was noted in the control group. Similarly, the temperature of the big toe was significantly higher in the study group, while there was no difference in the control group. These results are of great clinical importance because neuropathy is a strong cofactor in the etiology of chronic wounds in patients with diabetes and no effective therapy yet exists for this condition [6]. The only remaining approach is enhanced glycemic control, which seems to be effective predominantly in type 1 diabetes [6]. The pathogenesis of DSPN is not completely understood, but it seems that disturbances in blood flow play an important role [26]. The two theories explaining pathogenesis (metabolic and vascular) are obviously both involved in the pathogenesis of DSPN [27]. Metabolic changes are being intensively investigated to understand and treat the condition that affects millions of patients worldwide [6]. The immediate effects of transcutaneous gaseous CO\({}_{2}\) application are vasodilation and elevation of oxygen release from Hb via the Bohr effect. As the therapy is repeated, neoangiogenesis is induced [16; 20; 21]. There is no simple explanation for the observed positive effect of transcutaneous application of gaseous CO\({}_{2}\) on DSPN in patients with diabetes. The improved microcirculation could be the underlying mechanism. The vasodilatory effect in transcutaneous CO\({}_{2}\) therapy is mediated via the NO-dependent pathway [28; 29; 30]. The role of the NO-dependent pathway in DSPN is not clear. Several studies to date have evaluated the role of endothelium-dependent and endothelium-independent microvascular dysfunction in peripheral neuropathy with no clear explanation of pathogenic pathway so far [6; 22; 31; 32; 33; 34; 35]. The improved microcirculation after transcutaneous CO\({}_{2}\) therapy was confirmed in a previous study [36]. In this study, we measured the temperature of the great toe as an additional parameter depending on microcirculation. Figure 3: Temperature of the big toe before and after the treatment. Legend: wl1—leg with a wound before treatment, wl2—leg with a wound after treatment, hl1—contralateral leg without a wound before treatment, hl2—contralateral leg without a wound after treatment, * \(p<0.001\). The role of natural CO\({}_{2}\)-rich water is known for its positive effects on wound healing. Therefore, its medical application has been modified over the past decades. CO\({}_{2}\)-enriched water is used more often than gaseous CO\({}_{2}\). The primary goal in our first published research was chronic wound healing, and we decided to use gaseous CO\({}_{2}\) to avoid the moisturizing of the wound and inhaling of evaporated CO\({}_{2}\) from water [16]. Since one of the positive observations was also recovery of sensory loss, we decided to adhere to the same mode of CO\({}_{2}\) application. According to our knowledge, this is the first double-blind, randomized research investigating the influence of the transcutaneous application of gaseous CO\({}_{2}\) on DSPN. However, a positive effect was previously observed in two publications with patients treated for chronic wounds on feet [37, 38]. [PERSON] et al.