Risk factors of peripheral neuropathy in patients with type 2 diabetes in Isfahan: Results of a cohort study in Iran

Introduction: Peripheral neuropathy (PN) is among themost prevalent complications of diabetes that can lead to impairment of mobility of diabetic patients. The purpose of the current study was to predict relative factors influencing the occurrence of peripheral neuropathy (PN) in patients with type 2 diabetes.Methods: This was a cohort study on diabetic patients in the Isfahan Province of Iran. The studied population consisted of patients with type 2 diabetes, of ages 18 or older, who were diagnosed as new cases of diabetes from 2007 to 2014, and whose follow-up was completed by the end of 2016. In this study, with regards to the presence of time-varying co-variates, timedependent Cox regression model was employed in order to estimate the Hazard Ratio (HR) of PN in the diabetic patients. Results: Overall, 1874 patients with diabetes participated in the study, of which 839 (44.77%) were men and 1035 (55.23%) were women. During the study period, PN occurred in 17.98% of the patients; the ratio was 17% in women and 19.18% inmen. In comparison to the reference group, the adjusted HR of PN in males was equal to 3.66 (95% CI: 1.15-11.67), in housewives was equal to 4.09 (95% CI: 1.02-16.38), and divorced or wife died patients was equal to 3.02 (95% CI: 1.61-5.65). In addition, for each 6 month follow-up of the patients, the adjusted HR of PN increased to 1.19 (95% CI: 1.17-1.22). Conclusions: The adjusted HR of PN in men, in housewives, and elderly people, divorced or wife died patients, with elementary education level were greater than the reference group. Thus, training, screening and diagnostic programs should be carried out with greater sensitivity in patients who are at greater risk for PN.


INTRODUCTION
Diabetes includes a group of metabolic disorders that is associated with high blood glucose levels and impaired carbohydrate, lipid and protein metabolism. It is a disease that results from a disorder in the function or secretion of insulin. The two common groups of diabetes are known as type 1 (diabetes dependent on insulin) and type 2 (diabetes not dependent on insulin) 1 . According to the existing estimates, in 2011 more than 366 million people all over the world had diabetes and this number will increase up to 522 million people by 2030 2 . Peripheral neuropathy (PN) is among the most prevalent complications of diabetes that can lead to impaired mobility of patients with diabetes 2,3 . The prevalence of PN is different in numerous studies and also differs from country to country; the prevalence has been reported to be between 1.5 to 80% [4][5][6][7][8] . PN involves about 37% of patients with type 1 diabetes, and at least 20-40% of patients with type 2 diabetes 9 . The prevalence of PN was equivalent to 34% in the study carried out by Nitiyanant et al. that evaluated the control of diabetes across 230 diabetes care centers in 12 Asian countries 10 . In a review study, which used the meta-analysis method to assess the results of 21 studies conducted in Iran from 1991 to 2013, the prevalence of diabetic-associated PN was estimated to be about 53%. Therefore, based on the results of that study, it can be stated that the prevalence of PN in Iranian patients is high; as such, more than half of the patients with diabetes have PN 11 . In the European Diabetes (EURODIAB) Prospective Complications Study, in which 1172 patients were followed for 7 years, the cumulative incidence rate of PN was equivalent to 23.5% 12 . According to the results of that study, the risk of having PN was related to the level of glycated hemoglobin (HBA1c) and the duration of diabetes 13 . Moreover, in the Pittsburgh Epidemiology of Diabetes Complications (EDC) Study, it was observed that the cumulative incidence of PN after 4-year follow-up of the diabetic patients was about 13% 13 and after 10 years of follow up, it increased up to 34.2% 14 . Therefore, in light of the high prevalence of type 2 diabetes and importance of PN in diabetic patients and that, thus far, limited studies have been conducted to determine the factors affecting the occurrence of PN in patients with diabetes in Iran, this current study was conducted with the aim of examining the incidence rate and factors of PN occurrence in patients with type 2 diabetes in Isfahan, Iran. The results of this study could help to identify the people most at risk for PN in order to accelerate initiation of effective diagnostic and therapeutic interventions. This should decrease the incidence of PN in diabetic patients, as well as improve the quality of life and survival of these patients.

Research design and data source
This is a cohort study on diabetic patients living in the Isfahan province of Iran. After the diagnosis of diabetes, a file called "diabetic's file" was created for each diabetic in the Diabetic Patient Care Program in the healthcare system of Iran. In this study, data of the assessed variables were extracted by the researcher from the diabetic patient's records. For time-varying variables, collection of data was conducted through repeated measurements at a time interval of 6 months until the end of the follow-up period. Furthermore, the information about occurrence of PN was extracted from the patient's files and referral forms to specialists.

Study population
The study population consisted of patients with type 2 diabetes mellitus (ages 18 or older) who were diagnosed as new cases of diabetes from 2007-2014 and who were registered in diabetes files of health centers, health houses, and diabetes clinics. These patients received regular health care (at least once every 180 days) for a minimum of 2 years. Patient follow-up was done by the end of 2016.
Inclusion criteria of the study 1 Censored patients in the research were as follows 1. Patients who missed the follow-up period.
2. Patients with no regular treatment.
3. Cases with no study outcomes until the end of the follow-up period.

Peripheral neuropathy diagnosis
In the current study, the diagnosis of PN was carried out by a trained doctor, the Michigan questionnaire, and the monofilament test. Some information about the signs of PN in the diabetic patients, as well as the nail and skin conditions, were investigated via the Michigan questionnaire. In the Michigan questionnaire, 4 factors were addressed, namely the outer appearance of the foot skin (in terms of the skin's dryness or cracked skin, callus, infection, and/or deformity of body), existence of ulcer, reflex of Achilles Tendon, and situation of vibration feeling. The factors were assessed by utilizing 128 hertz diapason in the toe, and a score bigger than 2 was considered as PN existence 15 . In terms of PN, the ten-point monofilament test was performed in ten parts of the sole and back of the leg; the lack of monofilament at one or more points indicated PN 16 .

Data analysis
In this study, use was made of the statistical chisquare test in order to assess the relations of qualitative variables. An independent t-test was employed to compare the means of quantitative variables in patients with PN versus patients without PN, as well among men and women. In this study, with regards to the presence of time-varying co-variates, the timedependent Cox regression method was employed in order to estimate the Hazard Ratio (HR) of PN in diabetic patients. The adjusted HR for any variable was adjusted for all other variables. The HR was calculated by considering the 95% confidence interval (CI). The significance level in all tests was considered as 5%. The statistical software SPSS 18 and Stata 15 were used for analysis of data.    In the current study, it was observed that the average age in the diabetic patients who developed PN was 6 years greater than those patients without PN development. Moreover, for each year of increase in age of patients, the HR occurrence of PN will increase by 2% 18 . In other studies carried out around the world, the same results were obtained. In a study by Barbosa et al. on the patients with PN in Portugal, it was observed that on average, the mean age of patients with PN was 5.7 years greater than those patients without PN 19 . Therefore, by raising the age, the risk of PN is increased in diabetic patients.

Overall
Moreover, in the current study, the adjusted HR of the PN in men was equal to 3.66 (95% CI: 1.15-11.67). In a study by Booya et al. with the purpose of examining the factors affecting the incidence of diabeticassociated PN, the adjusted odds ratio of PN in men was equal to 2.9 18 . These results are consistent with the findings of the DCCT study (diabetes control and complications trial study) 20 .However, in some studies, the female gender was introduced as the predictive factor for incidence of PN in diabetic patients 21,22 . Nonetheless, in some studies, no significant statistical relation was observed between gender and risk of developing PN 23,24 .
In the present study, it was also observed that the type of diabetes treatment affects the occurrence of PN. The incidence rate of PN in patients who simultaneously received oral medicines and insulin therapeutic regimen was greater than those in other groups; therefore, in comparison to the oral treatment group, the un-adjusted HR of PN was equal to 1.29 (95% CI: 1.02-1.66). In a study by Marvasti et al., it was observed that compared to the oral treatment group, the HR of PN in the insulin-treated group was equal to 1.42 and in the group of insulin and oral treatment, it was equal to 1.41 25 .
In the present study, it was observed that increasing the follow-up duration of the Type 2 diabetes patients led to an increase in the HR of PN occurrence. Such results were observed in other studies 23,24,26 . In a study by Booya et al, it was observed that for each 1-year increase of patient follow-up period, the HR of PN occurrence will increase by 10% 18 . Nonetheless, in some studies, no significant statistical relations were observed between the duration of diabetes and incidence of PN 27,28 . Furthermore, in our study, it was observed that the average of HBA1c in patients with PN was equal to 10.55% and in patients without the symptoms of PN, it was 9.17%; this difference was statistically significant (p ≤ 0.001). Moreover, the un-adjusted HR of PN increased 10% per one percent increase in HBA1c. The relation between HBA1c and complications of Macrovascular (Heart disease)-related diabetes, and Microvascular diabetes (including peripheral neuropathy, retinopathy, and nephropathy) have been demonstrated in numerous studies 29,30 . In the prospective study in Britain in which the diabetes patients were followed up for an average of 10 years, it was observed that per 1% increase in level of HBA1c, the complications of diabetes increased by 37% 31 . In this study, no significant statistical relation was observed between smoking, hypertension and hyperlipidemia, and development of peripheral neuropathy; similar results were observed in the study by Booya et al 18 .
There were some limitations, including the time when there was no record or the information was incomplete regarding some medical cases, in which case   the information was then completed through contacting the patient to provide information on the missing variables. Furthermore, this research study only investigated patients with active medical files in health centers, houses, or diabetes units.

CONCLUSIONS
The maximum rates of PN incidence were observed in men, urban patients, and patients with oral medicines and insulin therapeutic regimens, as well as patients who were overweight or obese and those with low physical activity. In addition, the adjusted HR of PN in men, housewives, divorced or wife died patients, and people with high school education level were higher than the others. Thus, training, screening and diagnostic programs should be conducted with more sensitivity and accuracy in patients who are at greater risk of PN. Furthermore, in order to identify the complications in the initial steps of the disease as well, based on the data in this study, it is advisable to initiate effective and proper therapeutic actions in order to prevent greater complications, and to improve the quality of life in diabetic patients.

ETHICAL CONSIDERATIONS
This article was extracted from the Ph.D. thesis of Abdollah Mohammadian-Hafshejani from Tehran University of Medical Sciences with the following code: 9221128003. In this study, there was no intervention, nor was any personal information exclusively studied. Furthermore, in order to respect individual privacy, no full names or other private details were included in the checklist of the data collection. All patient information were included in the checklist as codes (including the codes of health centers and patients).