In this 1-year prospective observational research on patients with normotensive type 2 DM, we investigated the frequency of CAN, CAN risk factors, and their association with the progression of CAN. We found that advanced age at baseline and high systolic blood pressure at 1-year follow-up were associated with CAN.
Due to a lack of standardization in the tests to diagnose CAN, we were not precisely able to determine the prevalence of CAN 2. This rate was reported to range between 60-66.5% 9-11. Here, CAN prevalence was 78.3% at baseline and 70% at the end of the first year. The design of the current study was methodologically more detailed than many others for selecting normotensive patients, using all Ewing Battery tests, and considering age-dependent reference ranges for heart rate response to respiration. This may explain the differences in prevalence rates.
Advanced age at baseline and high systolic blood pressure at 1-year follow-up were associated with CAN. We found no correlation between sex and CAN. The literature includes some studies that found no significant correlation between CAN and sex 12, while there are some that found a significant correlation 13,14. Regarding the conflicting results in these studies, the effect of sex on CON seems to remain unclear.
Again, some previous studies showed no correlation between BMI and CAN 12, while in others, obesity was a risk factor for CAN 15. In the current study, BMI and CAN were not correlated. Because of the inconsistent results in clinical studies, the effect of obesity on CAN remains unclear, as is the case with sex.
In almost all studies, age was defined as a risk factor for CAN 10. Here, the risk of CAN increased with increasing age. Also, age nearly significantly increased Ewing scores. In other words, age was associated with the severity of neuropathy. Numerous studies strongly support that CAN is an early complication that can occur even within two years of the onset of diabetes 16 and that chronic hyperglycemia has a key role in the pathophysiology of CAN 17,18. Increasing age possibly leads to higher exposure to chronic hyperglycemia, accelerating the formation of CAN. Besides, as age progresses, loss of function develops in both sympathetic and parasympathetic ganglia and nerve conduction decreases in different pathways of the autonomic nervous system for various reasons (oxidative damage, decrease in neuroprotective substances, synaptic deterioration of organelles, changes in the extracellular matrix, etc.) 17,19. This is why increased age leads to decreased HRV in non-diabetic healthy individuals at 5-year follow-up (20). Remarkably, the correlation between age and CAN disappeared at the end of the first year. We think the fact that the numerical distributions of the groups changed at the end of 1 year (from 13-47 to 18-42) caused the disappearance of the age difference, which was significant at baseline, although not statistically strong.
Another risk factor that is strongly associated with CON is the duration of diabetes. Previous research indicates an association between diabetes duration and CAN 21,22 Long-term hyperglycemia is considered the main culprit in the development of diabetic neuropathy 23. Experimental studies have reported changes in polyol-myoinositol metabolism and Na-K ATPase system due to hyperglycemia. Non-enzymatic glycosylation leads to decreased axon diameter and transport, decreased nerve conduction, axoglial junction disorders, microangiopathy, endoneurial hypoxia, and demyelination. These mechanisms are involved in the pathophysiology of neuropathy. The increase in diabetes duration raises exposure to chronic hyperglycemia, as with age. In the present research, we could not find a correlation between diabetes duration and CAN. The difference in diabetes duration between patients with and without CAN was 2 years at baseline and 1.5 years at the end of 1 year. The increased number of patients may possibly reveal the effect of diabetes duration on CAN. On the other hand, here, diabetes duration nearly significantly increased Ewing scores. The severity of neuropathy increases as the Ewing score rises 11. According to long-term data from the Rochester Diabetic Neuropathy Cohort Study, duration and severity of exposure to hyperglycemia were correlated with the severity of neuropathy alone 24.
Some studies suggest that chronic smoking is a risk factor for cardiac autonomic neuropathy in diabetic individuals 25 while others fail to find a correlation between smoking and diabetic neuropathy 26. The rate of smoking was very low in our sample, preventing a definite assessment in this regard.
So far, no correlation has been demonstrated between antidiabetic treatment regimen and CAN 22. In line with previous research, we found that antidiabetic treatment was not associated with CAN.
Another finding obtained here was the correlation between SBP and CAN. Neil HA et al. showed an association between impaired CAN tests and increased SBP 27.Sympathetic baroreflex decreases with an increase in blood pressure and HRV decreases with an increase in norepinephrine levels 28. Endothelial dysfunction develops with decreased NO and prostacyclin levels and increased endothelin-1 level 29. As superoxide levels increase, NO synthesis decreases and direct nerve damage occurs 30. The frequency of CAN increases with all these pathophysiological processes occurring with increased blood pressure. In the current study, Ewing scores were found to be positively correlated with SBP. Thus, SBP was associated with both neuropathy and the severity of neuropathy.
Poor glycemic control has a key role in the progression of CAN and its baseline pathophysiology 31. In the DCCT study, intensive glycemic control slowed the deterioration of autonomic functions, decreasing the incidence of CAN by 53% compared to conventional treatment 23. Possibly, early intensive therapy causes this benefit through various pathological pathways associated with autonomicneural functions. These pathways are the formation of AGEs, increased oxidative and nitrosative stress with increased production of free radicals, activation of polyol and protein kinase C pathway, activation of polyADP ribosylation, and activation of genes involving neural damage 2. The DCCT study reported mean HbA1c of 7.4% for the intervention group, compared to 9.1% for the conventional group. Since our research involved no intervention, there was no significant decrease in HbA1c levels after 1 year compared to baseline. This may be the reason for the absence of such benefit. However, total Ewing scores were nearly significantly positively correlated with HbA1c at the end of 12 months. Given the lack of a correlation between antidiabetics and CAN, the prevention or slowing of the development of CAN may not be correlated with the type of drug used (insulin or OAD) but may be correlated with glycemic control.
Regarding the correlation between hyperlipidemia with CAN, some studies found no significant relationship 32, while others reported that hyperlipidemia was a risk factor for CAN 1 and serum lipid control was more important than glucose control to prevent deterioration in CAN 33. Hypercholesterolemia leads to many changes in vascular hemostasis. It decreases NO bioactivity, increases superoxide production, and increases endothelin reactivity 34.
Besides, it increases adhesion molecules and decreases endothelium-dependent vasodilation 35. Common pathophysiological mechanisms could possibly explain the correlation between hyperlipidemia and CAN. In the present study, we found no significant correlation between total cholesterol, LDL, HDL, and triglyceride levels and CAN at baseline or at follow-up, although at the end of 1 year, improvement in Ewing scores was associated with decreased total cholesterol (hence with decreased CAN ratio). Recent research emphasizes the significance of small and dense LDL cholesterol particles in type 2 DM patients, leading to an increase in oxidative stress and a decrease in NO levels, and ultimately to endothelial dysfunction 36. It has been proven that the serum levels of these small and dense LDL particles may differ, even among patients with similar serum LDL levels 37. Although, their effect on CAN remains unknown. The conflicting results regarding the correlation between CAN and serum LDL levels may stem from the different serum levels of small and dense LDL levels. Here, serum LDL levels and CAN were not correlated, but we believe that well-designed research is needed to shed light on this.
After the first year, statin use was not statistically significant but higher in the non-CAN group (71.4% vs 83.3%) and the presence of CAN was found to be affected by statin use with near statistical significance according to regression analysis. Statins inhibit the HMG CoA reductase enzyme, the rate-limiting enzyme in cholesterol biosynthesis, and prevent the synthesis of various intermediates with a key role in the pathogenesis of microvascular and macrovascular complications, apart from cholesterol. Hence, independent of its cholesterol-lowering effect, its pleiotropic effects occur (improved endothelial dysfunction, decreased free radicals such as superoxide and hydrogen peroxide, and increased NO synthesis) 38,39. In this study, the pleiotropic effects of statins may have affected the development of CAN through a positive effect on its pathophysiology. It remains unclear whether the decrease in Ewing scores at the end of 12 months is due to decreased total cholesterol or to statin use, given the pleiotropic effects of statins. Since this is observational research, randomized and placebo-controlled studies can shed more light on this matter.
We found a significant rate of CAN in normotensive type 2 diabetic patients by bedside tests. Age and SBP affected the development of CAN and diabetes duration, HbA1c levels, and age affected the severity of CAN with near significance. Also, there was an independent correlation between decreased total cholesterol levels and improved CAN. Thus, correcting lipid profile, ensuring glycemic control, and preventing blood pressure progression may halt or even regress the progression of CAN in patients with type 2 DM. Moreover, statin therapy may show promise in the treatment of CAN. There is a need for further randomized and placebo-controlled research on this subject.
The main limitations of the present study were the small sample size, the one year follow-up period, and lack of recordings for patients' diet and exercise habits.
Conflict of Interest Statement: The authors declare that they have no conflict of interest.