Cardiac syndrome X is a historical term used to describe patients with angina, tests suggestive of ischemia, and epicardial coronary arteries without obstructive stenoses on angiography. The definition was important because it recognized that a ‘normal’ angiogram does not exclude an ischemic mechanism; today, however, it is considered too imprecise and is being replaced by pathophysiological classification within ANOCA and INOCA.
ANOCA indicates angina with non-obstructive coronary arteries, whereas INOCA indicates documented ischemia with non-obstructive arteries; these terms describe a presentation, not a single disease. The main endotypes include microvascular angina, epicardial vasospasm, microvascular spasm, mixed forms, and patients whose symptoms ultimately prove nonischemic.
Microvascular angina results from dysfunction of the coronary microcirculation, vessels too small to be assessed directly by conventional coronary angiography. The problem may be structural, with increased minimal resistance and arteriolar remodeling; functional, with impaired vasodilation; or vasomotor, with microvascular spasm.
The syndrome frequently affects women, but is not exclusively a female disease. The greater representation of women in studies also reflects differences in the prevalence of obstructive coronary artery disease, vascular physiology, and diagnostic pathways. Men with ANOCA/INOCA must be assessed according to the same pathophysiological principles.
The most important concept is that epicardial coronary angiography alone does not complete the diagnostic pathway. In persistently symptomatic patients with suspected ANOCA/INOCA and impaired quality of life, the 2024 ESC guidelines recommend invasive coronary functional testing to identify treatable endotypes, taking preferences and the clinical context into account.
The microcirculation comprises prearterioles and arterioles that regulate most coronary resistance. Unlike the large epicardial arteries, their tone changes rapidly in response to myocardial metabolism, pressure, endothelial signals, and neurohumoral control; this regulation keeps flow relatively constant at rest and allows it to increase during exertion.
Coronary flow reserve is the ratio of maximal flow during vasodilation to resting flow. Reduced CFR indicates an inability to increase flow adequately, but may result from epicardial stenoses, diffuse disease, high resting flow, or microvascular dysfunction. To attribute it to the microcirculation, epicardial components must be excluded or measured.
In the structural endotype, arteriolar medial hypertrophy, perivascular fibrosis, and capillary rarefaction increase minimal resistance. Hypertension, diabetes, and ventricular remodeling may promote this phenotype. The result is reduced vasodilatory capacity even when the epicardial endothelium appears relatively normal.
In the functional endotype, minimal resistance may be normal but the relationship between resting and hyperemic flow is abnormal. High resting flow caused by increased demand or dysregulation may mathematically reduce CFR, which is why it must be interpreted alongside resistance measurements and the hemodynamic context.
IMR is calculated during hyperemia using distal pressure and mean transit time and provides a relatively specific estimate of microvascular resistance. An elevated value supports structural dysfunction; the historically used threshold is approximately 25, although technology and measurement method may affect it.
Hyperemic microvascular resistance can be measured with Doppler guidewires. Continuous thermodilution and newer techniques allow absolute flow and resistance to be estimated with greater reproducibility; these tools are expanding phenotyping but require dedicated expertise.
The endothelium modulates tone through nitric oxide, prostacyclins, endothelin, and other mediators. Smoking, LDL, inflammation, diabetes, and oxidative stress reduce NO bioavailability and may produce a paradoxical response to cholinergic stimulation.
The acetylcholine test assesses vasomotion. In the presence of endothelial dysfunction or smooth-muscle hyperreactivity, acetylcholine may induce spasm. Epicardial spasm requires marked visible constriction associated with symptoms and ischemic changes; microvascular spasm causes symptoms and ECG ischemia without diagnostic epicardial constriction.
Microvascular angina and vasospastic angina may coexist; this mixed phenotype is clinically important because a drug effective for one mechanism may be insufficient or unsuitable for the other. This is precisely the rationale for an endotype-guided strategy.
Microvascular dysfunction is not synonymous with absence of atherosclerosis. CCTA or IVUS may demonstrate non-obstructive plaque in patients with INOCA. Diffuse atherosclerosis and vasomotor dysfunction share risk factors and may potentiate each other.
Microvascular ischemia tends to be more heterogeneous and subendocardial than ischemia caused by a single epicardial stenosis; this may reduce the sensitivity of some methods that seek clear regional defects, whereas PET with quantification of global flow may detect a diffuse reduction in reserve.
Pain may persist after exertion has ended or be less reproducible than classic epicardial angina. Nociceptive sensitization and altered visceral perception may contribute in some patients, but must not be used to psychologize symptoms before genuine coronary dysfunction has been excluded.
Symptoms include exertional chest pressure or tightness, pain at rest, dyspnea, and reduced exercise tolerance. Duration may be longer and the response to nitrates less predictable than in angina caused by epicardial stenosis; the clinical presentation nevertheless cannot reliably distinguish endotypes.
Historically, symptoms in women may have been inappropriately attributed to anxiety or noncardiac causes after angiography showed no stenosis. Modern coronary physiology has demonstrated that a substantial proportion have documentable microvascular dysfunction or vasospasm.
Physical examination is often normal. Hypertension, obesity, or signs of systemic disease may provide clues to the determinants. Transient ECG changes and arrhythmias may occur during a vasospastic episode.
The clinical burden is substantial: persistent symptoms, repeated emergency department visits, repeated coronary angiograms, and impaired quality of life. The problem is therefore not resolved by simply stating that ‘the coronary arteries are normal.’
Diagnosis begins by excluding obstructive epicardial coronary artery disease with CCTA or coronary angiography according to clinical probability. Intermediate stenoses must be assessed physiologically with FFR or iFR before symptoms are attributed to the microcirculation.
The COVADIS criteria for microvascular angina include symptoms of ischemia, absence of obstructive coronary artery disease, objective evidence of ischemia, and documentation of microvascular dysfunction by reduced reserve or microvascular spasm. Combining the criteria distinguishes definitive from suspected forms.
Stress PET can measure myocardial blood flow and myocardial flow reserve. Reduced reserve in the absence of obstructive stenoses supports CMD and has prognostic value. Correction for resting flow and hemodynamic conditions improves interpretation.
Stress CMR with quantitative or semiquantitative perfusion may document diffuse subendocardial ischemia and simultaneously assess structure and scar. It is particularly useful when myocarditis or cardiomyopathies must be excluded.
Doppler echocardiography of the left anterior descending artery may estimate CFR at experienced centers, whereas conventional stress echocardiography may be normal if ischemia is diffuse and does not produce clear regional abnormalities. A negative test therefore does not always exclude CMD.
Comprehensive invasive testing assesses epicardial significance, microvascular function with CFR and IMR or equivalent methods, and vasoreactivity with acetylcholine; the order of testing may vary with the protocol. This allows classification as microvascular angina, vasospastic angina, a mixed phenotype, or absence of vasomotor dysfunction.
The 2024 ESC guidelines strengthened the role of invasive testing in patients with persistent symptoms despite treatment and impaired quality of life, recognizing that endotype diagnosis may guide more effective symptom therapy.
The CorMicA trial randomized patients with angina and non-obstructive coronary arteries to stratified therapy based on functional testing or usual care. The endotype-guided strategy improved Seattle Angina Questionnaire scores and quality of life at 6 and 12 months, without demonstrating a difference in MACE in the relatively small sample.
The differential diagnosis includes pericarditis, reflux, musculoskeletal pain, pulmonary embolism, anemia, cardiomyopathies, valvular heart disease, and tachyarrhythmias. The presence of CMD does not exclude a concomitant noncardiac cause, and vice versa.
Treatment begins with lifestyle and risk factors. Smoking, blood pressure, diabetes, lipids, and obesity influence the endothelium and microcirculation. If non-obstructive atherosclerosis is present, prevention must be proportionate to plaque burden and overall risk.
In the endotype with reduced CFR and elevated microvascular resistance without spasm, beta-blockers may reduce demand and prolong diastole. ACE inhibitors or ARBs may improve blood-pressure control and endothelial function; statins are indicated when warranted by the atherosclerotic profile.
For epicardial or microvascular vasospasm, calcium channel blockers are the principal therapy. Long-acting nitrates may be added, particularly for epicardial vasospasm. Nonselective beta-blockers may worsen vasospasm in some patients and are not the first choice for this endotype.
Ranolazine may be used in patients with persistent symptoms, with a variable response and some evidence of greater benefit when CFR is reduced. Its effect on major outcomes has not been demonstrated specifically in MVA.
Nicorandil and trimetazidine are available in some healthcare systems and may be considered in selected cases according to local guidelines. Ivabradine may reduce heart rate in patients in sinus rhythm when tachycardia contributes to symptoms, but does not directly correct structural microvascular resistance.
Sublingual nitrates may be tried for episodes, but the response in MVA is less predictable than in epicardial stenosis. A favorable response does not identify the mechanism, and a lack of response does not exclude ischemia.
Endotype-guided therapy is preferable to a random sequence of drugs. A patient with spasm requires vasodilation; one with elevated resistance and tachycardia may benefit more from reducing demand; a mixed phenotype may require a combination.
The WARRIOR trial, published in 2026 after presentation in 2025, did not demonstrate a significant reduction in the composite of events with a standardized intensive strategy of a high-intensity statin, ACE inhibitor/ARB, and aspirin versus usual care in women with suspected ANOCA/INOCA. Interpretation is limited by lower-than-expected statistical power and extensive use of these therapies in the control group as well.
The WARRIOR result should not be interpreted as evidence that statins or RAAS inhibitors are ineffective when they have an independent indication. Rather, it demonstrates that a uniform strategy does not replace phenotyping and that the event benefit specifically attributable to intensive therapy in this population remains uncertain.
Aspirin must not be prescribed automatically to every patient with INOCA and no documented atherosclerosis, because bleeding risk must be balanced against an undefined preventive benefit. The presence of plaque and global risk guide the decision.
Rehabilitation, progressive exercise, and stress management may improve capacity and symptom perception. Psychological support is useful when anxiety and fear of activity amplify disability, but must be integrated with a rigorous cardiac diagnosis and not used as a substitute.
The prognosis of ANOCA/INOCA is not uniformly benign. Patients with markedly reduced CFR, diffuse atherosclerosis, diabetes, or other risk factors may have a higher incidence of events. The risk of death generally remains lower than with complex obstructive coronary artery disease, but the burden of symptoms and hospitalizations is high.
The most frequent complication is persistent angina with impaired quality of life. Failure to diagnose the endotype often produces cycles of inconclusive tests, arbitrary treatment changes, and patient mistrust.
Vasospasm may be associated with ventricular arrhythmias and, rarely, myocardial infarction or sudden death, especially when it causes prolonged occlusion. Recognition is therefore important beyond symptom control.
CMD may coexist with HFpEF and other cardiometabolic phenotypes. Reduced coronary reserve, inflammation, and increased filling pressures may influence one another, but not every patient with HFpEF and reduced CFR has ‘syndrome X.’
Follow-up should monitor symptoms, functional capacity, blood pressure, lipids, and new evidence of epicardial disease. The emergence of a new clinical pattern requires reassessment because a patient with previous INOCA may subsequently develop obstructive atherosclerosis or a true ACS.
Contemporary terminology therefore has practical value: replacing ‘syndrome X’ with microvascular angina, vasospasm, or another defined endotype connects diagnosis, treatment, and prognosis to a verifiable mechanism. The historical term may be retained for recognizability, but should not conclude the clinical reasoning.
The term ‘syndrome X’ originated in an era when coronary angiography was the principal anatomical tool and the microcirculation could not be measured routinely. Patients with pain, a positive exercise test, and apparently normal coronary arteries were grouped together despite having different mechanisms. Modern physiology has transformed a diagnosis of exclusion into a series of positively diagnosable endotypes.
Structural microvascular angina is characterized by increased minimal resistance and reduced vasodilatory capacity. The patient may have low CFR and elevated IMR. Associated factors include hypertension, diabetes, hypertrophy, and diffuse atherosclerosis.
Functional microvascular angina may show low CFR without markedly increased minimal resistance, often because of high resting flow or dysregulation; this distinction is attracting increasing interest because it may require different interventions targeting heart rate, demand, or vascular tone.
Microvascular spasm is diagnosed when acetylcholine reproduces symptoms and ischemic ECG changes without the ≥90% epicardial constriction that would define epicardial spasm. The mechanism involves arteriolar hyperreactivity and endothelial dysfunction.
Epicardial vasospasm may coexist with CMD. In these patients, calcium channel blockers are essential, whereas nonselective beta-blockers may be unfavorable. A generic diagnosis of MVA would therefore lose essential therapeutic information.
A mixed endotype with reduced CFR/high resistance and positive spasm provocation also exists; these patients may have a greater symptom burden and require combinations of drugs. Response to a single agent must not be used retrospectively as a diagnostic test.
Some patients with angina and non-obstructive coronary arteries show no abnormality on vasomotor testing. Pain may arise from noncoronary cardiac or extracardiac causes. A negative functional test is therefore useful because it helps avoid indefinite antianginal therapy without a target.
The presence of non-obstructive plaque constitutes another, separate diagnostic axis. One patient may have CMD and atherosclerosis; another may have vasospasm without plaque; a third may have non-obstructive plaque without vasomotor dysfunction. Atherosclerotic prevention must be calibrated accordingly.
MINOCA is not synonymous with INOCA. MINOCA is a working diagnosis of myocardial injury with non-obstructive coronary arteries, whereas INOCA concerns ischemia without obstructive coronary artery disease. Confusing the terms leads to errors in the choice of imaging and therapy.
HFpEF and CMD may also overlap. In HFpEF, elevated filling pressure and capillary rarefaction may reduce flow reserve; the presence of low CFR, however, does not demonstrate that the microcirculation is the sole cause of dyspnea.
Before studying the microcirculation, significant epicardial stenosis must be excluded. FFR >0.80 or nonhyperemic indices above validated thresholds support the absence of an epicardial gradient sufficient to explain ischemia in the segment assessed. Diffuse disease may nevertheless reduce pressure along the entire vessel without a single focally severe stenosis.
Invasive CFR with thermodilution uses the ratio between resting and hyperemic mean transit times; with Doppler it uses the ratio of hyperemic to resting flow velocity. Values <2.0-2.5 have been used as abnormal in different studies and techniques. The threshold depends on the method and must not be extrapolated without knowing the protocol.
IMR is calculated as distal pressure multiplied by mean transit time during hyperemia. A threshold ≥25 is commonly used for structural CMD. Because the measurement depends on injection quality and stable hyperemia, technical experience is important.
Continuous thermodilution allows estimation of absolute coronary blood flow and microvascular resistance; this technique may improve reproducibility and characterize phenotypes that conventional measurements represent incompletely, but its clinical adoption is still evolving.
Acetylcholine testing is performed with escalating doses and monitoring of symptoms, ECG, and angiography. Provocation must be performed at experienced centers because it may induce severe spasm, bradycardia, or arrhythmias, which are generally reversible with intracoronary nitrate and appropriate treatment.
The diagnosis of epicardial spasm typically requires symptoms, ischemic changes, and marked angiographic constriction. If symptoms and ECG changes occur without diagnostic epicardial constriction, microvascular spasm is considered. The endothelial response may be assessed further at nonspasmogenic doses.
Adenosine and acetylcholine interrogate different functions. Adenosine primarily assesses endothelium-independent vasodilatory capacity and minimal resistance, whereas acetylcholine assesses endothelium-dependent vasomotion and predisposition to spasm. One test may be normal and the other abnormal.
Coronary functional testing must be interpreted with the blood pressure and heart rate at the time. Tachycardia, hypertension, or anemia may increase resting flow and reduce CFR. The diagnosis is not a simple number but a coherent physiological pattern.
Reproduction of the usual symptom during provocation increases clinical relevance. A mild physiological abnormality unrelated to pain may be less convincing as the sole explanation for symptoms than a response that clearly reproduces the patient's clinical picture.
The procedure may be incorporated into the same session as diagnostic coronary angiography; this avoids declaring the coronary arteries ‘normal’ and sending the patient away without a diagnosis, but requires organization and trained operators.
PET is one of the most validated noninvasive methods for quantifying myocardial blood flow. Tracers such as rubidium-82, nitrogen-13 ammonia, or oxygen-15 water allow calculation of resting and stress flow and myocardial flow reserve. A global reduction without obstructive stenoses suggests CMD or diffuse disease.
Prognosis progressively worsens as myocardial flow reserve decreases, irrespective of sex; this demonstrates that CMD is not merely an explanation for symptoms but may identify a higher-risk vascular phenotype.
Perfusion CMR avoids radiation and simultaneously characterizes function, scar, and edema. Quantitative methods for myocardial perfusion reserve are improving the ability to recognize diffuse ischemia, overcoming the limitation of visual regional assessment alone.
Traditional SPECT may have reduced sensitivity when ischemia is balanced or diffuse. A normal examination does not exclude CMD, especially if symptoms are typical and clinical probability remains high.
CCTA plays a fundamental role in excluding obstructive atherosclerosis and quantifying plaque. It does not directly measure the microcirculation, although FFR-CT and CT perfusion may provide functional information in specific settings.
Magnetic resonance imaging is particularly useful in the differential diagnosis of myocarditis, hypertrophic cardiomyopathy, and Takotsubo syndrome; these conditions may cause pain, an abnormal ECG, and non-obstructive coronary arteries and must be recognized before everything is attributed to CMD.
Vasospasm may be episodic and not occur during noninvasive testing. Holter monitoring showing episodes of transient ST-segment elevation during symptoms may suggest vasospasm, but sensitivity is limited and definitive diagnosis often requires provocation.
Anemia, hyperthyroidism, and tachyarrhythmias increase demand and may cause symptoms in the presence of reduced microvascular reserve. Treating systemic conditions is part of the pathway and may reduce the burden without an additional antianginal drug.
Noncardiac chest pain syndrome remains common. Gastroesophageal reflux, musculoskeletal pain, and anxiety disorders may coexist with CMD. Precision medicine requires avoiding both cardiac overdiagnosis and premature exclusion of the heart.
In CMD with high resistance and elevated heart rate, beta-blockers reduce demand and prolong diastole. Nebivolol also has NO-mediated vasodilatory properties, but specific evidence on MVA outcomes is limited. The choice is guided by symptoms, blood pressure, and comorbidities.
ACE inhibitors may improve CFR and endothelial function in some studies, particularly in hypertensive patients. However, their event benefit specifically in INOCA has not been conclusively demonstrated. WARRIOR showed how difficult it is to demonstrate an incremental effect of a uniform strategy in a population already receiving good treatment.
Statins are used when there is sufficient atherosclerosis, dyslipidemia, or risk, not because every form of spasm is caused by LDL. They may improve endothelial function in addition to reducing atherosclerotic risk, but the lipid target must follow preventive guidelines.
Dihydropyridine and nondihydropyridine calcium channel blockers reduce vasospasm by relaxing smooth muscle. Severe cases may require high doses or combinations, with monitoring of blood pressure, heart rate, and ventricular function.
Long-acting nitrates are highly effective in epicardial vasospasm but often less effective in structural microvascular angina. Tolerance and headache limit chronic use. A nitrate-free interval may be necessary with some formulations.
Ranolazine inhibits the late sodium current and reduces calcium overload and diastolic tension. Trials in MVA have produced heterogeneous results; analyses suggest possible benefit in patients with more severely reduced CFR. QT prolongation and interactions must be considered.
Ivabradine selectively reduces the If current of the sinus node and may improve symptoms when an elevated heart rate contributes to demand. It must not be used in atrial fibrillation and does not replace treatment for spasm.
Trimetazidine modifies energy-substrate utilization and may be used as an antianginal drug in some countries. It is not available or approved everywhere and has no specific evidence of reducing MACE in MVA.
Nonpharmacological interventions include regular exercise, which improves endothelial function and capacity, and management of weight and sleep. In patients with exertion-induced symptoms, training must be progressive to prevent fear of pain from causing further deconditioning.
Estrogen therapy must not be prescribed with the aim of treating MVA or preventing coronary events. The cardiovascular effects of hormone therapy depend on age, timing, and formulation and follow specific gynecological indications.
Neuromodulation and spinal cord stimulation have been explored in refractory angina, but are not first-line treatments for MVA. Before considering them, it is essential to have a physiological diagnosis and genuinely optimized drug therapy.
Research is assessing new targets involving endothelin, inflammation, and metabolism; these strategies must be considered experimental until randomized trials demonstrate a clinically meaningful benefit.
Studies such as WISE have demonstrated that women with ischemic symptoms and non-obstructive coronary arteries do not have a risk equivalent to that of entirely healthy populations. The burden of risk factors, non-obstructive plaque, and vascular dysfunction determines an intermediate and variable prognosis.
Reduced CFR on PET or invasive testing is associated with a higher risk of events. Prognosis therefore depends on physiological severity, not only on the percentage of epicardial stenosis; this is one of the major changes from the old concept of ‘benign’ syndrome X.
The WARRIOR trial showed that events in women with suspected INOCA were strongly dominated by hospitalizations for angina. Even when mortality and myocardial infarction rates are not very high, morbidity and resource use may be substantial.
Improved symptoms with the CorMicA strategy demonstrates that a physiological diagnosis has therapeutic value even without evidence of reduced MACE. Quality of life and reduced angina are important outcomes for a chronic syndrome.
Women with INOCA may have a higher prevalence of CMD, but mechanisms may differ between men and women. Research must avoid generalizing results from female cohorts to all patients without assessing sex as a biological and social modifier.
Menopause modifies endothelial function and the distribution of risk, but no hormonal test diagnoses CMD. Traditional factors and physiological measurements remain central.
Prognosis must be communicated while avoiding two extremes: saying that ‘nothing is wrong’ because the coronary arteries are not obstructed, or presenting every case of CMD as equivalent to severe multivessel coronary artery disease. Accurate communication reduces anxiety and supports adherence.
Follow-up is guided by symptoms and risk. New prolonged angina, ECG changes, or troponin elevation must be assessed as a possible ACS even in someone with a previous diagnosis of MVA. A chronic diagnosis does not protect against future atherothrombosis.
Future research aims to integrate physiology, imaging, omics, and clinical phenotypes to identify subgroups that truly respond to specific therapies. Moving beyond syndrome X is therefore an example of the transition from descriptive to mechanistic medicine.
The coronary endothelium transduces shear stress into vasodilatory signals through eNOS and nitric oxide production. Oxidative stress and inflammation reduce NO bioavailability by both consuming it and altering enzymatic function. The result is less capacity for dilation and greater sensitivity to vasoconstrictor stimuli.
Endothelin-1 is a potent vasoconstrictor, and its levels or receptor response may be increased in some CMD phenotypes. Endothelin antagonists have been studied as potential therapies but are not yet standard treatment for MVA.
Oxidative stress arises from NADPH oxidases, mitochondria, and other sources and may alter ion channels and smooth-muscle function. Smoking, diabetes, and oxidized LDL amplify these pathways, explaining part of the overlap between atherosclerotic and vasomotor factors.
Systemic inflammation may alter the microcirculation through cytokines, endothelial activation, and rarefaction. Conditions such as rheumatoid arthritis or lupus have a higher prevalence of CMD, but immunomodulatory therapy specifically aimed at correcting the microcirculation is not standardized.
Insulin resistance reduces the PI3K-Akt-eNOS pathway and may favor MAPK- and endothelin-mediated vasoconstriction; this mechanism links metabolic syndrome and microvascular function even before overt diabetes develops.
Elevated pressure induces arteriolar remodeling and medial hypertrophy, increasing minimal resistance. Blood-pressure control is therefore a causal treatment of the structural substrate, not merely a general preventive measure.
Capillary rarefaction reduces the surface available for exchange and increases the diffusion distance for oxygen. It may be particularly important in ventricular hypertrophy and HFpEF, where the ratio of myocardial mass to capillary density becomes unfavorable.
The autonomic nervous system modulates coronary tone. Mental stress may provoke vasoconstriction and ischemia in susceptible patients, and studies of mental stress ischemia have shown that physiology does not depend exclusively on physical exertion.
Pain perception involves cardiac afferent and central pathways. Some patients may develop sensitization that amplifies symptoms relative to the degree of ischemia; this does not invalidate the vascular diagnosis, but may require a multimodal approach to achieve good clinical control.
The microbiome and other systemic mechanisms are being studied, but no validated microbiome intervention currently treats CMD. It is important to distinguish biologically interesting hypotheses from clinically proven therapies.
The microcirculation is also influenced by anemia, blood viscosity, and perfusion pressure. Before primary dysfunction is diagnosed, conditions that globally reduce oxygen delivery or increase demand must be corrected.
The biology explains why ANOCA/INOCA is often multifactorial. No single biomarker can replace functional measurement of reserve, resistance, and vasoreactivity.
The first step is to confirm that symptoms are compatible with ischemia and assess the probability of obstructive coronary artery disease. If CCTA shows an intermediate stenosis, physiological significance must be clarified before the patient is labeled as having INOCA.
If no obstructive stenoses are present and symptoms persist, the presence of ischemia is assessed with PET, CMR, or other appropriate methods. In patients with important symptoms despite therapy, invasive coronary functional testing provides the most comprehensive phenotyping.
The diagnosis must be reported explicitly: for example, ‘microvascular angina with CFR 1.7 and IMR 32, negative ACh’ or ‘vasospastic angina with provoked epicardial spasm’; this precision makes it clear why a beta-blocker or calcium channel blocker was chosen.
After therapy begins, follow-up assesses episode frequency, functional capacity, and quality of life. The Seattle Angina Questionnaire can quantify the response more objectively than a generic impression.
If symptoms persist, adherence and dose are checked first. Many patients receive drugs at very low doses or stop them because of adverse effects. Only after endotype treatment has been optimized is it rational to add further antianginal drugs.
New symptoms at rest, syncope, ECG changes, or abnormal biomarkers must be managed as a possible new event, not automatically attributed to known MVA. The coronary profile may change over time.
Risk-factor control is reassessed annually or more often. The presence of non-obstructive plaque requires prevention proportionate to the burden, whereas a patient without plaque but with vasospasm has a different atherosclerotic profile.
Cardiac rehabilitation or an exercise program may be particularly useful when the patient has drastically reduced activity for fear of pain. Supervised exercise helps rebuild confidence and improve endothelial function.
If the diagnosis remains uncertain, reassessment at a dedicated ANOCA/INOCA center may prevent repeated conventional coronary angiograms that continue to show the same lumen without interrogating physiology.
Communication must explain that ‘non-obstructive’ does not mean imaginary and, at the same time, that the risk is not identical to that of a critical stenosis; this balanced position improves the therapeutic relationship and reduces both undertreatment and excessive medicalization.
Classic Prinzmetal vasospasm is dominated by episodes at rest with transient ST-segment elevation and marked epicardial constriction. Historical syndrome X, by contrast, was often associated with positive exercise tests and angiographically normal coronary arteries. Today the two conditions can be distinguished with vasomotor provocation.
The slow coronary flow phenomenon describes delayed progression of contrast in the absence of significant stenosis. It may reflect elevated microvascular resistance but is not synonymous with MVA and requires contextual interpretation.
Myocardial bridging may cause systolic compression of an epicardial segment and ischemia in selected patients. It may coexist with vasospasm or endothelial dysfunction and must not automatically be classified as microvascular angina.
Spontaneous coronary artery dissection may heal, leaving non-obstructive coronary arteries but persistent symptoms. Post-SCAD pain is common and may have multiple mechanisms; the previous history modifies risk and the choice of provocative testing.
MINOCA requires a diagnostic pathway with CMR and, when indicated, intracoronary imaging and functional testing to identify the mechanism, because it may include coronary disorders and initially mimicking nonischemic causes. Once necrosis has been documented, management must not be generically reduced to syndrome X.
Takotsubo syndrome features transient ventricular dysfunction and often an emotional or physical trigger; the microcirculation may contribute to its pathophysiology but is not sufficient to classify the condition as MVA.
Hypertrophic cardiomyopathy may cause microvascular ischemia through increased mass, intramural compression, and capillary rarefaction; in this setting CMD is secondary to structural cardiomyopathy and therapy must first address the substrate.
In cardiac amyloidosis, infiltration and microvascular dysfunction may reduce reserve. Here too, low CFR does not mean primary syndrome X. Tissue characterization guides the diagnosis.
The distinction between primary CMD and CMD secondary to cardiomyopathy, valvular heart disease, or systemic disease is important because treatment and prognosis depend on the dominant disorder.
This classification prevents a single physiological datum from being interpreted out of context. The microcirculation is a common final pathway of many diseases, not a single diagnosis.
Research on ANOCA/INOCA is moving from small physiological studies to pragmatic outcome trials. WARRIOR's neutral result demonstrated the difficulty of showing benefit with uniform therapy in a heterogeneous and already treated population.
One direction is to enroll patients on the basis of a documented endotype. If a drug acts on microvascular resistance, including patients with pure vasospasm dilutes the effect. Precision medicine is therefore also a methodological strategy for trials.
Endothelin antagonists, modulators of the NO-cGMP pathway, and metabolic drugs are under investigation. Before clinical adoption, they must demonstrate not only improvement in CFR but also symptoms or meaningful outcomes.
Physiology based on continuous thermodilution and absolute measurements could identify new subtypes according to resting flow, resistance, and hyperemic capacity. Standardization among centers is needed before universal cutoffs can be established.
Quantitative PET and CMR imaging may make noninvasive phenotyping more accessible, reducing the need for invasive testing in some patients; vasospasm, however, remains difficult to diagnose without provocation.
Artificial intelligence applied to CCTA, ECG, and perfusion imaging may identify patterns associated with CMD, but predictive models must be prospectively validated and must not replace physiology on the basis of retrospective accuracy.
Circulating biomarkers of endothelial dysfunction, inflammation, and oxidative stress are being studied. None currently has sufficient specificity to diagnose MVA in an individual patient.
Research on biological sex and hormones must distinguish vascular effects, age, and social factors. The higher prevalence of INOCA in women must not lead to the disorder being considered a nonspecific consequence of menopause.
Patient involvement in defining outcomes is particularly important because angina, limitation, and quality of life dominate the burden. Trials that measure only death or myocardial infarction may fail to capture the most immediate clinical benefit.
Standardizing terminology is already progress. Moving from ‘syndrome X’ to a physiological diagnosis reduces ambiguity, improves research, and allows genuinely mechanistic therapies to be developed.
The coronary microcirculation comprises prearterioles, intramyocardial arterioles, and capillaries that are not visible on conventional coronary angiography; these vessels determine much of coronary resistance and adapt flow to metabolic demand. An angiographically normal epicardial coronary artery may therefore supply a territory in which vasodilatory capacity is profoundly reduced.
Regulation occurs through interaction among metabolic, myogenic, endothelial, and neurohumoral mechanisms. Adenosine and other metabolites contribute to hyperemia, while the endothelium modulates tone through nitric oxide, prostanoids, and endothelins. Dysfunction may involve one pathway or several components at once, generating different physiological phenotypes that may share the same symptom.
Structural CMD is characterized by arteriolar remodeling, an increased wall-to-lumen ratio, capillary rarefaction, and perivascular fibrosis; these changes reduce maximal capacity to increase flow and may be associated with hypertension, diabetes, cardiomyopathies, and aging. Functional CMD may instead result predominantly from impaired vasodilation or excessive vasoconstriction without severe anatomical loss of the network.
Coronary flow reserve is the ratio of hyperemic flow to resting flow. A reduced value may result from low maximal flow or unusually high resting flow; these two mechanisms are not equivalent. In the first, elevated minimal resistance may predominate; in the second, the problem may be an apparently reduced reserve despite a relatively preserved hyperemic response. Contemporary physiology seeks to distinguish these phenotypes.
IMR uses pressure and thermodilution during hyperemia to estimate microvascular resistance. An elevated value suggests a structural or functional microcirculatory abnormality independent of epicardial stenosis. CFR and IMR must be interpreted together because a patient may have low CFR with normal or elevated IMR, configurations that imply different mechanisms.
Continuous thermodilution allows absolute flow and resistance to be estimated through intracoronary saline infusion, offering a more direct quantitative approach. It is an evolving specialist method, and cutoffs and standardization are still under study. Its potential value lies in separating resting flow, hyperemic flow, and resistance with greater granularity.
Microvascular spasm represents a mechanism different from impaired adenosine-mediated vasodilation. During acetylcholine administration, the patient develops symptoms and ischemic changes without epicardial constriction above the threshold used to define large-vessel spasm. The test therefore demonstrates a pathological vasoconstrictor response of the microcirculation or clinically relevant endothelial dysfunction.
Acetylcholine provocation testing is performed in the catheterization laboratory with progressively increasing intracoronary doses while symptoms, ECG, and epicardial diameter are monitored. Epicardial spasm is defined by the combination of symptom reproduction, ischemic changes, and marked angiographic vasoconstriction. Diagnosis is not based on the chest sensation alone because symptoms may also occur without a documented vasomotor endotype.
In microvascular spasm, symptoms and ischemia occur without the epicardial constriction required for epicardial vasospasm; this distinction is clinically important because both may respond to calcium channel blockers, but their pathophysiological profiles and responses to beta-blockers or nitrates may differ. Some patients have mixed forms with both epicardial and microvascular abnormalities.
The test must be performed at experienced centers with monitoring and immediate availability of intracoronary nitrates and arrhythmia management. Bradycardia, atrioventricular block, prolonged spasm, and arrhythmias may occur, but the procedure is generally safe when performed using standardized protocols. Assessment of the risk-benefit ratio is particularly important in complex anatomy or clinical instability.
Adenosine predominantly assesses endothelium-independent vasodilatory capacity and is used for CFR and IMR, whereas acetylcholine interrogates endothelial vasoreactivity and propensity to spasm. Comprehensive coronary function testing therefore uses different stimuli to answer different questions. A single normal test does not necessarily exclude every form of dysfunction.
An intermediate epicardial stenosis must be assessed physiologically before symptoms are attributed to the microcirculation. FFR or nonhyperemic indices allow a flow-limiting lesion to be excluded. CMD may nevertheless coexist with obstructive atherosclerosis and persist after PCI, accounting for some residual angina despite technically satisfactory revascularization.
The 2024 ESC guidelines give greater importance to coronary function testing in patients with persistent ANOCA/INOCA and impaired quality of life. The objective is not merely to assign a label, but to identify a mechanism that guides therapy. The endotype-guided strategy improved symptoms and quality of life in the CorMicA trial, although that small study did not demonstrate fewer major events.
A useful report must state the measured values, method, behavior during acetylcholine administration, and integrated interpretation. Vague definitions such as “slow microcirculation” are insufficient. Standardizing the report makes it possible to compare follow-up and prevents tests performed with different methods from being interpreted as perfectly equivalent.
PET allows myocardial flow to be quantified at rest and during stress and myocardial flow reserve to be calculated. A global reduction may suggest CMD or diffuse atherosclerosis, whereas territorial defects point toward epicardial stenoses. PET has a robust quantitative basis, but availability and cost limit universal use.
Stress CMR can identify subendocardial ischemia and, with quantitative techniques, estimate myocardial perfusion. It simultaneously offers tissue characterization, allowing recognition of scar, myocarditis, or cardiomyopathies that might explain pain and electrocardiographic abnormalities. Nonischemic late gadolinium enhancement shifts the differential diagnosis away from primary syndrome X.
Doppler echocardiography of the left anterior descending artery may estimate CFR at experienced centers. It is noninvasive and involves no radiation but is operator-dependent and primarily assesses one territory. Normal CFR in the LAD does not completely exclude focal abnormalities or vasospasm, whereas a reduced value must be interpreted in light of heart rate, blood pressure, and acoustic-window quality.
CCTA documents non-obstructive plaque and helps exclude significant stenoses, but does not directly measure the microcirculation. Discovery of mild plaque must not automatically be interpreted as the explanation for every anginal episode. Non-obstructive atherosclerosis, CMD, and vasospasm may coexist and require different therapeutic components.
Myocardial bridging may cause ischemia through systolic compression, delayed diastolic relaxation, and proximal flow abnormalities. Its anatomical presence is common and often harmless, so it must be correlated with physiology and symptoms before causality is assigned. Treatment of significant forms differs from that of pure vasospasm.
Microvascular dysfunction secondary to ventricular hypertrophy, aortic stenosis, amyloidosis, or hypertrophic cardiomyopathy must not be confused with a primary microvascular syndrome. Treating the dominant structural disease may modify perfusion. An approach that ignores cardiac imaging and considers only the coronary arteries risks missing the principal diagnosis.
Baseline treatment includes control of blood pressure, lipids, diabetes, smoking, weight, and physical activity. Even without obstructive stenosis, non-obstructive plaque warrants prevention according to atherosclerotic risk. In patients without plaque, the choice of statins or antiplatelet agents must depend on general indications, not on chest pain alone.
In CMD with reduced CFR and increased resistance, beta-blockers may reduce demand and prolong diastole in appropriate patients. ACE inhibitors or ARBs may improve blood-pressure control and endothelial function. Statins are indicated when atherosclerosis or lipid risk is present. The response is heterogeneous and requires titration guided by symptoms and tolerability.
For vasospasm, calcium channel blockers are the foundation of treatment and may require high doses or combinations in severe forms. Long-acting nitrates may be added, taking tolerance and headache into account. Nonselective beta-blockers may worsen vasospasm in some patients, and their indication must be assessed carefully when spasm is the dominant mechanism.
Ranolazine may be used for persistent angina in some CMD endotypes, but studies have shown inconsistent results. Benefit appears greater in subgroups with particularly impaired reserve; this exemplifies why unstratified trials may appear neutral even when a biologically defined part of the population responds.
Persistent pain despite normalization of vascular tests requires consideration of nociceptive mechanisms and extracardiac diagnoses. Greater visceral sensitivity may amplify the perception of cardiac stimuli, but must not be used prematurely to psychologize the symptom. Before pain is attributed to a functional disorder, a cardiovascular assessment proportionate to risk must be completed.
Prognosis is not benign in an absolute sense. Reduced CFR is associated with a higher risk of events and mortality in several cohorts, especially when it reflects systemic disease or diffuse atherosclerosis; individual risk nevertheless varies greatly and is not equivalent to that of a critical left main stenosis. Communication must avoid both minimization and catastrophizing.
The WARRIOR trial did not demonstrate a significant reduction in events with a uniform strategy of a statin, ACE inhibitor or ARB, and aspirin versus usual care in women with suspected ANOCA/INOCA, in a context of high use of the same therapies in the control group. The result reinforces the need to distinguish atherosclerotic prevention from endotype-specific treatment and shows how difficult it is to demonstrate benefit with a nonselective package.
Follow-up should measure angina frequency, functional limitation, nitrate use, quality of life, and risk-factor control. Repeating coronary angiography without a relevant clinical change rarely resolves the problem. Reassessment should be guided by new risk signals, substantial worsening, or the need to redefine the mechanism.
Replacing the historical term “cardiac syndrome X” with diagnoses such as microvascular angina, vasospastic angina, and ANOCA/INOCA is not merely semantic. It turns a label of exclusion into a positive pathophysiological classification, reducing patient uncertainty and enabling more rational therapy; this conceptual evolution is one of the principal changes in contemporary ischemic cardiology.
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