Frequency and severity of myocardial perfusion abnormalities using Tc-99m MIBI SPECT in cardiac syndrome X

Syndrome X is defined by a typical angina pectoris with normal or near normal (<40% stenosis) coronary angiogram with or without ECG change or atypical angina pectoris with normal or near normal coronary angiogram plus a positive none-invasive test (exercise tolerance test or myocardial perfusion scan) with or without ECG changes [1, 2]. Patients with coronary artery spasm (Prinzmetal's or variant angina), left ventricular hypertrophy, systemic hypertension, and valvular heart disease are not included in this syndrome [3]. The term "Microvascular Angina" (MVA) includes all such patients with coronary microcirculatory derangements but with normal coronary angiograms irrespective of the presence or absence of exercise-induced ST segment depression [4].

The exact pathophysiological mechanisms underlying this condition are not well understood, and many mechanisms for the chest pain have been suggested. In some studies, microvascular dysfunction has been proposed as the cause [4‐7] whereas in others, metabolic abnormalities, such as net myocardial lactate production have been demonstrated [8‐12]. Noninvasive imaging has been used to determine whether ischemia is present or not. Controversial findings have been reported regarding left ventricular function in MVA. Though regional wall motion abnormalities have been reported using stress nuclear techniques, two dimensional echocardiography has not disclosed any segmental contractile dysfunction [13]. Some studies in which positron-emission tomography (PET) was used have shown abnormal heterogeneity in perfusion [14, 15] whereas others have shown no abnormality [16, 17]. However, the possibility of the development of exercise induced ischemia in these patients is supported by the fact that approximately two-thirds of the patients develop regional abnormalities of left ventricular contraction or show an abnormal ejection fraction response (i.e., less than 5% increment) following exercise [18].

Tl-201 scintigraphy has been employed in the investigation of patients with syndrome X, and often shows regional defects after stress [19‐21]. However, the physical limitations due to lower energy emission of Tl-201 as a myocardial perfusion agent have long been recognized [22‐25] yielding inconsistent results [19, 26]. The development of new and higher energy emission Tc-99m labeled agents, such as Tc-99m MIBI, significantly improved image quality and eliminated false-positives results in Tl-201 SPECT [25, 27]. Although, in these patients the diagnosis of microvascular involvement as a "yes-no" binary outcome may be adequate for further management, however, information on both the presence and extent of coronary artery disease (CAD) are desirable, as both have implications in assessment of prognosis or selection of therapy [28‐30]. In this study we evaluated myocardial perfusion in syndrome X, using Tc-99m MIBI SPECT.

Thirteen patients out of 36 (36.1%) showed abnormal myocardial perfusion scan. Of the 23 females patients, 9 (39.1%) had abnormal Tc-99m MIBI SPECT and 14 (60.9%) showed normal scan. Of the 13 males 4 (30.8%) showed abnormal and 9(69.2%) cases revealed normal studies. There was no significant difference between two genders (p value > 0.05). Of the 9 positive female patients, 3 (33.33%) cases showed multiple perfusion defects and 6(66.66%) cases revealed single localized perfusion abnormality. Of the 4 males with positive scan 2 (50%) cases revealed multiple and 2 (50%) single perfusion defects. The defects were localized in the apex in 3, the apical segments in 4, the midventricular segments in 12 and in the basal segments in 6 cases. All patients had reversible perfusion abnormality and none of them had transient left ventricular dilation (TLV). The involvement of three vessel territories approximately were the same(LAD;8, RCA;9 and LCX;8). In the cases with multiple perfusion defects, the ischemic pattern was distributed throughout the different segment of myocardium. The detailed data of uptake reduction are shown in table 1 and a sample picture is presented [Fig. 2]. Exercise treadmill test (ETT) was performed in this study as a part of the protocol. The results of ETT were positive in 25 (69%) patients and negative in 11(30%). 65% of the results of the exercise ECG were not concordant with perfusion defects found on myocardial perfusion imaging.

Investigations over the past two decades have not revealed a specific cause for syndrome X. In the presence of normal coronary arteries, many explanations have been put forth to explain myocardial ischemia, including small vessel abnormalities, coronary artery spasm, cardiomyopathy, metabolic abnormalities, misinterpretation of the coronary angiograms, impaired coronary flow reserve, oxyhemoglobin dissociation defects, psychosomatic factors, altered pain perception, increased sympathetic drive, and endothelial dysfunction [33, 34]. It is now acknowledged that syndrome X most likely encompasses several pathophysiologic diseases. Since the introduction of the term syndrome X, many investigators have used the term with different criteria as to its definition, to create a more homogeneous group of patients. Our results suggest that multiple perfusion defects with different severity and distribution are relatively common in cardiac syndrome X, with the majority of these patients having at least one abnormal perfusion segment. Thus, 36 % of our patients with angina pectoris, who had no evidence of significant organic stenosis on their coronary angiograms, exhibited exercise-induced perfusion defects in their Tc 99m-MIBI scintigrams. In a review of the literature, a number of authors have reported different results when using conventional Tl-201 myocardial perfusion imaging. Berger et al reviewed the exercise Tl-201 scans and clinical data of 41 patients with chest pain and normal coronary arteries (a broad definition of syndrome X). A negative Tl-201 study was the most common finding. Of the 41 patients, 11 (27%) had abnormal exercise Tl-201 scans. Of the 11 patients with abnormal scans, 9 had greater than or equal to 1 cardiac abnormality [19]. Tweddel et al (1992) studied 100 patients with normal arteriograms (a broad definition of syndrome X) undergoing diagnostic angiography for typical angina. Thallium defects were found in 98 patients (a very high incidence) [20]. Legrand et al (1985) studied the results of exercise Tl-201 scintigraphy in 18 patients with chest pain and angiographically normal coronary arteries (a broad definition of syndrome X). Regional exercise Tl-201 perfusion was abnormal in three patients [21]. Chia-Hung et al (1996) reviewed the stress Tc-99m MIBI SPECT results and clinical data of 15 patients with syndrome X. The results of exercise ECG and resting left ventricular ejection fraction (LVEF) were compared with the results of Tc-99m MIBI SPECT. Of these 15 patients, 9 (60%) had a normal Tc-99m MIBI SPECT study and 6 (40%) had an abnormal Tc-99m MIBI SPECT [35]. Recently Cavusoglu et al (2005) have evaluated the regional distribution and extent of perfusion abnormalities, and the lung to heart (L: H) uptake ratios using exercise thallium-201 SPECT in 31 patients. They showed that perfusion abnormalities are relatively common in these patients. Also patients with perfusion abnormalities have significantly higher L: H ratios during exercise than control patients [36]. Jonathan et al (2002) performed myocardial-perfusion cardiovascular magnetic resonance imaging in 20 patients with syndrome X and 10 matched controls, both at rest and during an infusion of adenosine. Quantitative perfusion analysis was performed using the normalized upslope of myocardial signal enhancement to derive the myocardial perfusion index and the myocardial-perfusion reserve index (defined as the ratio of the myocardial perfusion index during stress to that at rest). Following adenosine, control group revealed increased myocardial perfusion in subendocardial and subepicardial layers of myocardium but there was only increased perfusion in subepicardium in patients with syndrome X without any detectable change in the subendocardial layer [37]. In our study, 69 % of exercise tests were abnormal which is similar to the study by Cannon et al (1985) [18]. In this study, we tried to match coronary anatomy with perfusion segments to localize CAD to the level of small vessels, its potential application being in serial studies, following surgical or percutaneous revascularization interventions, and in patients without an established diagnosis of CAD to identify small sized ischemia or infarctions, not detectable by angiography. Our study suggests that SPECT MPI can be used to identify small perfusion defects pointing to the presence of CAD and also suggests that this procedure may have value in the assessment of the physiological significance of small vessel stenosis not identifiable by coronary angiography. We noticed that the ischemic regions in patients with multiple perfusion defects are distributed in the different myocardial segments as previously reported by other investigators [38].

Furthermore, ischemia may not necessarily be localized to an entire myocardial area as in patients with epicardial coronary stenosis.

Indeed myocardial ischemia may be patchily distributed in small areas of myocardium. In these patients, small areas of hypoperfusion were patchily distributed in different anatomic sites, spreading among nonischemic areas, able to compensate for the dysfunction of adjacent myocardium. This pattern is clearly different from that of patients with coronary artery disease, whereas the hypoperfused areas are clustered in the anatomic regions supplied by the stenotic arteries [38].

In addition, in theses patients inappropriate constriction of prearteriolar vessels may be postulated to be sparse in the myocardium and to be non uniform with the possibility that only a minority of these vessels are intensely constricted. Thus, during increased myocardial activity a focal and spotty myocardial ischemia can result although electrocardiographic signs, metabolism and function are found to be within normal limits. Similarly, a focal increase in myocardial adenosine concentration may cause anginal pain even in the absence of significant myocardial ischemia. Depending on the percentage of involvement of prearteriolar abnormality reduced coronary flow reserve and the presence of detectable myocardial ischemia by nuclear scan may vary widely [39].

The patients in our study had well-characterized syndrome X; however, other conditions may lead to microvascular dysfunction, and similar findings might be found in patients with hypertension, hypertrophic conditions, CHD, CHF, LBBB, MVP, valvular diseases, cardiomyopathy or diabetes. Therefore, we excluded patients with these conditions from our study. Although there is predominance of female to male patients (23/13) referred to our hospital, the scan abnormalities are almost similar in both genders (39% Vs 38%). The prognostic implications of the identification of microvessel disease are uncertain. Although one may expect that, if the total myocardium at risk is small, the patient's prognosis will be favorable, the significance of CAD in multiple branches has not been assessed. Our study did not address the prognostic implications of branch vessel disease, as detected by SPECT MPI. Further studies are needed to follow these patients and determine the prognostic significance of these interesting findings.

Finally it should be emphasized that our study is not free of drawbacks. Since the sensitivity of SPECT as compared to PET for the assessment of myocardial ischemia is less than optimal, the absence of documented myocardial ischemia does not allow for the exclusion with certainty of cardiac and/or ischemic genesis of anginal pain in those remaining patients, in whom nuclear scan was normal [40].

Our results support the concept that the chest pain in patients with syndrome X may be related to myocardial ischemia. Myocardial perfusion defects revealed by scintigraphy in 13 out of 36 (36 %) patients with angina and normal coronary arteriograms, raises the possibility of microvascular angina as the cause of chest pain. The term "microvascular angina" is commonly used in these patients; however, the cardiologists should be cautioned when confronting cases with discordant scintigraphic and angiographic findings.