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.