To our knowledge, this is the first large scaled, observational, longitudinal cohort study to investigate risk factors for DPN in a Taiwanese adult population. Using MNSIE for the diagnosis of DPN, we found that participants without DPN at baseline had a 13% cumulative incidence of DPN over the 6 years of follow-up (corresponding with an annual incidence of 2.204%) in a population where the duration of DM was as long as 15.2±6.9 years. The incidence of DPN in our study is comparable with that of a previous longitudinal, large-scale, nationwide, population-based study in Taiwan (n = 37375, annual incidence of 3.2%) [18] However, it was lower than that reported Western populations [19, 20]. This discrepancy might be due to differences in the sample size, ethnicity of the study population (the prevalence of DPN is about 32.1% in the UK [21] and about 23.5% in Taiwan [22]), diagnostic criteria, and measurement instruments.
Apart from these, one of the crucial factors is the baseline duration of DM. One study well established that the prevalence of diabetic neuropathy increased from 8–42% in patients with T2DM when patients were monitored for 10 years [23]. Compared with the previous cohort study, patients had newly diagnosed DM with a cumulative incidence of 10% over the 13-year follow-up period and an annual incidence of 0.7% [9]. The relatively high cumulative incidence over our 6-year follow-up period might be attributable to the longer baseline duration of DM.
The association between renal function and incident DPN
In our study, baseline renal function was found to be an independent risk factor for DPN, including baseline eGFR (Table 3) and baseline serum creatinine concentration (Table 4), particularly in people under the age of 65. This finding was inconsistent in people aged 65 and above, which might be due to the decline of eGFR in the aging process. This is consistent with the Rochester cohort longitudinal assessment [12], in which Dyck et al. reported that the presence of DPN is associated with the severity of nephropathy and might be implicated in its cause. Our previous studies also indicate that the prevalence of DPN increases significantly in patients with impaired renal function [6]. On the other hand, the baseline UACR did not show the same result, which may be attributed to the large standard deviations (94.9±196.8 mg/g vs. 72.8±259.1 mg/g, p=0.44).
To date, the mechanisms of neurotoxicity in T2DM patients with renal impairment remains unclear, but they have been demonstrated in some studies [24, 25]. Experimental evidence indicates that alterations in membrane excitability is induced by inhibition of the axonal Na+/K+ pump, which abolishes the direct contribution of the hyperpolarizing pump current to the membrane potential, leading to an accumulation of extracellular K+ that causes depolarization [26]. Disruption of these various ionic gradients may affect the Na+/Ca2+ exchanger, leading to increased levels of intracellular Ca2+ and axonal loss [27].
In addition, it is clear from previous research that impaired renal function results in microvascular endothelial dysfunction, even in the early stages of chronic kidney disease. Endothelial injury is caused by various factors, including inflammation, hypertension, diabetes-associated factors, and a uremic milieu [25, 28]. Eventually, it leads to neuropathy due to impaired nerve blood flow, epineurial arterio-venous shunting, and reduced nerve oxygen tension [29].
Other studies examining nephropathy as a risk factor for DPN have been inconclusive [13]. However, it is suggested that the selection of disease markers for renal impairment may be important (for example, eGFR or creatinine), and further investigation is needed. Based on the current study, we recommend that increased serum creatinine concentration or lower baseline eGFR be used as an indicator to enhance the awareness of incident DPN.
Other risk factors of future DPN
After adjustment for potential confounding factors, we also found that a higher risk of DPN was linked with increased age, body weight, duration of DM, and male gender. Our findings are consistent with most previous reports from cross-sectional studies and a meta-analysis of patients with T2DM in Western, Korean, and Taiwanese populations [5, 6, 30]. Concerning sugar control, previous studies indicated hyperglycemia as a risk factor for the development of DPN [5, 8], but we found no association between baseline HbA1c levels and incident DPN. This is likely explained by low levels of HbA1c at baseline (7.3±1.2% in the no-DPN group and 7.6±1.7% in the incident-DPN group) compared with the levels usually found in previous studies. These data possibly reflect better medication adherence among Taiwanese DM patients [31] compared with worldwide [32]. Our study also showed equally high numbers of hypoglycemic medication prescriptions in both groups.
In the current study, increased weight was independent risk factor of incident DPN, but no statistically significant associations with incident DPN were found for BMI and waist circumference. This is inconsistent with previous studies [5, 9, 10] but previous studies have not identified a consistent list of risk factors related to markers of obesity [10, 12]. A possible explanation is that previous investigators did not adequately correct the reference cut-off values and the units for tests. This is not to say that markers of obesity may not be risk factors for DPN, but corrections must first be made for these characteristics in the cut-off values and the units [12].
In terms of dyslipidemia, we found that serum lipid components had no statistically significant associations with the risk of DPN in T2DM. As stated above, these findings were consisted with some previous studies [34, 35]. In fact, accumulated evidence has shown a correlation between DPN and serum lipid profiles but has shown inconsistent results [33]. The possible underlying mechanisms of dyslipidemia leading to DPN are complex which may include insulin resistance, chronic inflammatory status, oxidative stress induced by elevated LDL, and demyelination [33]. Nevertheless, these mechanisms are mainly reported in preclinical studies [36–38]. It is well established that DPN is a multifactorial disease and our findings indicate that lipid metabolism may play a minor role in its pathogenesis.
The major strengths of the current study are its large sample size in a cohort design, the unselected nature of participants, standardized data collection procedures, and inclusion of several potential risk factors at baseline. But despite these strengths, there are still plenty of limitations. First, our results might not apply to treatment-naïve cohorts of early-stage T2DM. A high proportion of medication prescription might have affected the cardiovascular risk factors. Furthermore, we did not use confirmatory tests such as nerve conduction studies or skin biopsy for DPN diagnosis. However, the diagnosis of DPN is principally a clinical one according to ADA recommendations, and the MNSIE is a sensitive, specific, validated clinical screening tool. Lastly, we included participants from a single hospital, which might limit the generalizability of the results.