Discussion
Many studies have been conducted to evaluate the prevalence of silent myocardial ischemia in asymptomatic diabetic patients, reporting a prevalence of 9% to 57% (14). Silent ischemia is characterized by confirmed myocardial ischemia objectively without symptoms of angina or angina equivalents(15). The autonomic neuropathy in diabetes has been proposed to be the reason for blunted perception of chest pains, justifying a higher proportion of silent ischemia in these patients (16). The incidence of cardiovascular events and death are significantly higher in type 2 diabetic patients with MPI SPECT abnormalities (17).
Various diagnostic tests are implemented to diagnose CAD, each having their own limitations. For example, rest ECG could yield normal findings in more than a half of patients with chronic stable angina (18). Also, no agreement has been achieved on which asymptomatic diabetic patients should be screened (19, 20). Scholte et al. reported a 13% rate of cardiac deaths and cardiovascular events in patients with type 2 diabetes during a 3-year follow-up. None of the known cardiovascular risk factors have been shown to have enough sensitivity and specificity to predict silent myocardial ischemia in diabetic patients (21).
However, our study revealed a 41.3% prevalence of silent ischemia in asymptomatic type 2 diabetic patients, higher than 37% reported by Al-Humaidi et al (22). They reported that perfusion defects were related to disease duration, insulin use, nephropathy, and neuropathy. Their results regarding the duration of diabetes or insulin use was in accordance with our findings which should be examined in further studies as well.
CAD was diagnosed in our patients despite having no compromise in LVEF in echocardiography or electrocardiographic (ECG), highlighting the superior sensitivity of MPI SPECT in detecting silent ischemia/CAD. This is in accordance with the findings of Mohagheghie et al. who reported that CAD was detected by MPI in one third of asymptomatic diabetic patients with normal ECG and no evidence of peripheral arterial disease (23). Moreover, CAD was significantly more prevalent in patients with diabetes mellitus of more than 15 years, consistent with other studies which indicate an increased risk of cardiac events in prolonged diabetes mellitus (24-26).
Wu et al. reported that asymptomatic postmenopausal women with DM had a higher SSS and SDS than age-matched men (27). While our study found that male gender, regardless of smoking and requiring insulin treatment, is a strong predictor of myocardial perfusion abnormalities in asymptomatic diabetic patients. This might be due to poor control of blood sugar in men and intentional non-adherence to medications compared to women. However, women in our study were mainly pre-menopausal.
Moreover, Salehi et al, concluded that DM was independently associated with abnormal MPI in asymptomatic patients with a normal LVEF, also claiming that smoking and longer duration of DM were correlated with myocardial perfusion abnormalities, which is consistent with our findings. However, they found no significant difference between males and females in terms of abnormal MPI findings (28). This might be due to different study population or inclusion criteria.
MPI SPECT is a readily accessible, easily performed modality compared to other evolving modalities such as CT-angiography, MRI or newly discovered biomarkers, as evidence is scarce in supporting their incremental values in predicting further cardiac events. Moreover, MPI SPECT offers an automated evaluation and is less operator-dependent (29).
Our study had some limitations. Our sample size was quite small. Besides, the effect of some of possible confounding factors such as underlying liver or kidney disease (abnormal liver function tests or serum creatinine levels) was not assessed in our study, as we excluded such patients from our study. Further, prospective studies, with larger population and longer follow-ups are necessary to determine the prognostic value of an abnormal MPI in asymptomatic diabetic patients.