Discussion
In the current study, the prevalence of neuropathic pain among participants with type 2 DM was 26.8%. This was slightly higher than the prevalence of neuropathic pain among type 2 DM participants (21.6%) that was determined by Ojo et al30 using the PainDETECT questionnaire28; the same assessment method used in our study. Importantly, figure 1 has shown that the neuropathy score in female participants was significantly higher than that for male participants. As well, table 1 has shown that 76.6% of neuropathic participants were females while 23.4% were males. This was almost similar to findings of Ojo et al’s30 study, in which 66.7% of neuropathic participants were females while 33.3% were males. These slight differences could be due to the differences between various populations as Ojo et al’s30 study was conducted in Nigeria and our study was conducted in Jordan. Therefore, these findings suggest that female patients with type 2 DM are more likely to complain of peripheral neuropathic symptoms compared with males.
The current study was also interested in investigating the association between vitamin D deficiency and peripheral neuropathic pain in participants with type 2 DM. The relative similarity in the clinical symptomatology of both conditions especially the feeling of tingling and numbness has driven us to the hypothesis that the two conditions could be associated. Notably, this study did not find any significant difference in vitamin D levels between type 2 DM participants who were nociceptive, with unclear neuropathy score and with neuropathy. Additionally, there was no significant association between vitamin D status (deficient, insufficient and sufficient vitamin D) and neuropathy status (nociceptive, unclear neuropathy and neuropathy) in participants with type 2 DM (table 2). Our findings were inconsistent with the results of the few studies reported in the literature that were interested in finding association between diabetic neuropathy and vitamin D deficiency. For instance, Orabi et al’s study35 has reported that patients with diabetic neuropathy were having significantly lower vitamin D levels compared with controls. Likewise, Shehab et al’s study8 had reported that patients with diabetic neuropathy were having significantly lower vitamin D levels compared with patients with type 2 DM without neuropathy. As well, Shillo et al’s study,12 has reported a significant lower vitamin D level in type 2 DM white Europeans with neuropathy compared with controls. This inconsistency could be due to the small sample size of Orabi et al’s35 and Shillo et al’s12 studies and the different neuropathy assessment methods used in our study. Unfortunately, there was no study in the literature that used the PainDETECT questionnaire28 to assess neuropathic pain in association with vitamin D deficiency although the questionnaire is well validated and was used by other researchers like Ojo et al30 to investigate the prevalence of neuropathic pain in patients with type 2 DM as mentioned above. Even though, we believe that further studies are required to assess the relation between diabetic neuropathy and vitamin D levels using different methods of neuropathy assessment including both neuropathy questionnaires and clinical neuropathy assessment. This will expose the validity of each method in categorizing patients with neuropathy in relation to vitamin D status.
To find predictors of neuropathic pain among participants with Type 2 DM, ordinal logistic regression analysis has shown that female gender was the only significant predictor for neuropathic pain while vitamin D level, age, BMI, FBG, duration of type 2 DM, SBP and DBP were not. The significance level for the HbA1c in the model was on the borderline with a p value of 0.05 and OR (95% CI) of 1.19 (1.00 to 1.43). These results were interesting to us because most of other previous studies had reported different predictors for diabetic neuropathy including the long duration of DM, increased age and elevated HbA1c.36–38 As well, these studies did not find any gender-related differences in neuropathic pain among patients with type 2 DM.36–38 Again, the inconsistency between our results and other previous studies36–38 is almost because these studies used different methods to assess neuropathy. Interestingly, Gryz et al36 tried to assess predictors of diabetic neuropathy in relation to different criteria of its diagnosis and they found that the predictors vary according to the criteria that were used for diagnosis. The only study that used PainDETECT questionnaire to assess neuropathy in patients with type 2 DM was Ojo et al’s study.30 As mentioned above, results of Ojo et al’s study30 were almost similar to our results in regard to the prevalence of diabetic neuropathy, frequency of neuropathy in females compared with males and the lack of any significant difference in FBG, HbA1c and duration of type 2 DM between patients with neuropathy and those without neuropathy. In contrast to Ojo et al’s study,30 age was not a predictor for neuropathic pain in our study. As mentioned above, this could be due to differences in the study populations as both studies were conducted on different populations.
In summary, this study did not find any association between neuropathic pain and vitamin D levels in participants with type 2 DM. So, this finding rejects our hypothesis that both vitamin D deficiency and neuropathic pain could be related. Instead, the current study has found that neuropathic pain in participants with type 2 DM can be predicted from the female gender but not from age, DM duration, FBG or HbA1c. The strengths of the current study comes from its suitable sample size and the method of neuropathy assessment which was well validated and used by other researchers to assess neuropathic pain.30 On the other hand, the current study has also some limitations that may affect its findings. For example, the neuropathy status was not determined clinically but was determined by self-reporting using the PainDETECT questionnaire.28 As well, we only used one assessment tool for neuropathy and we did not compare the validity of this tool compared with other questionnaires that were used previously to assess neuropathy.36–38 So, the comparison of our results with these studies36–38 could not be appropriate because of the differences in the methodology. Another possible limitation to the association between neuropathic pain and female gender is that females may have lower pain threshold and tolerance levels compared with males.39 Unfortunately, the difference in the pain threshold between males and females was not taken in consideration in the method that we used to assess neuropathic pain. Despite of these limitations, we believe that the current study is the first report that did not find any association between vitamin D levels and neuropathic pain in participants with type 2 DM. As well, this is the second study that used the PainDETECT questionnaire28 along with Ojo et al’s study30 to assess neuropathic pain in patients with type 2 DM. Because of the similarity in our findings and results of Ojo et al’s study30 and the inconsistency with other studies36–38 that used different neuropathy assessment methods, we think that it will be wrathful to do further research to compare the various neuropathy assessment tools on same populations and to compare their results with results of the clinical neuropathy assessment methods.