Unclassified cardiomyopathies

Unclassified cardiomyopathies comprise a heterogeneous group of diseases of the cardiac muscle that cannot be linearly assigned to the traditional phenotypes of dilated, hypertrophic, restrictive or arrhythmogenic cardiomyopathy, or that cross several morphological categories during their natural history. They do not represent a single diagnosis, nor a residual group without clinical logic: rather, they represent the area in which transient, metabolic, toxic, endomyocardial, neuromuscular, autoimmune, eosinophilic and mitochondrial forms converge, together with complex anatomical phenotypes in which morphology alone is not sufficient to define cause, risk and treatment.

Many of these conditions are initially recognized as heart failure, arrhythmia, chest pain, intracardiac thrombus, hypertrophy, dilation, diastolic restriction or an imaging finding, but the correct diagnosis requires a further step: connecting the cardiac phenotype to a specific biological cause. Acute ventricular dysfunction after neurovegetative stress does not have the same pathophysiology as alcohol-related dilation, mitochondrial defect cardiomyopathy, endomyocardial fibrosis, ventricular noncompaction, a lysosomal disease such as Danon disease or eosinophil-mediated myocardial injury. The decisive clinical point is to recognize what has altered the myocardium, not merely to describe how the ventricle appears.

From an epidemiological standpoint, unclassified cardiomyopathies are difficult to quantify because they include rare and underdiagnosed conditions, often recognized only in specialist centers or confused with more common cardiomyopathies. Some, such as Takotsubo cardiomyopathy, are relatively frequent in acute coronary syndrome pathways; others, such as mitochondrial cardiomyopathies, Danon disease and hypereosinophilic forms, belong to the medicine of rare or multisystemic diseases; still others, such as toxic cardiomyopathies, may be more common than registry data suggest because the causal exposure is not always investigated. Classification should therefore not stop at the shape of the ventricle, but should reach the mechanism that produced that shape.

The modern classification of cardiomyopathies is no longer a rigidly anatomical system. Contemporary guidelines and nosologies distinguish the morphofunctional phenotype, familial or sporadic distribution, genetic or acquired cause, extracardiac involvement and functional stage. This approach is essential in unclassified cardiomyopathies because many of them are not without identity, but have an identity that does not coincide with a single ventricular geometry. The same condition may present as hypertrophy, dilation, restriction, arrhythmia, thrombosis or endocardial injury at different stages.

The concept of unclassified cardiomyopathy must therefore be used with precision. It does not mean that the disease is unknown, indeterminate or less important. It means that the phenotype is not sufficiently described by the main cardiomyopathic categories, or that the prevailing cause is not contained within the simple distinction between a dilated, thickened, stiff or arrhythmogenic ventricle. In many of these conditions the heart is the target of an extracardiac mechanism, such as a toxin, a metabolic defect, a neuromuscular disorder, an autoimmune response, an eosinophilic proliferation or a systemic disease.

This framework has immediate practical consequences. If a cardiomyopathy is described only as “dilated”, treatment tends to focus on heart failure. If it is recognized as toxic, removal of the exposure becomes an essential part of therapy. If it is recognized as mitochondrial, multisystem surveillance, genetics, prevention of metabolic crises and attention to potentially problematic medications are required. If it is recognized as hypereosinophilic, the priority becomes suppressing eosinophilic activity and preventing thrombosis and fibrosis. If it is recognized as Danon disease, the problem shifts to genetics, lysosomal accumulation, arrhythmias, early heart failure and family screening.

The clinical value of the category is particularly evident in young patients, in patients with family history, in patients with extracardiac manifestations and in cases in which cardiac disease appears disproportionate to common risk factors. Cardiomyopathy in a young person with muscle weakness, conduction disturbances, hypertrophy, hearing loss, neuropathy, retinopathy, eosinophilia, asthma, exposure to substances, oncological therapies or family history should not be filed as idiopathic. Every extracardiac element is a diagnostic clue.

Unclassified cardiomyopathies share a fundamental feature: risk does not depend only on left ventricular ejection fraction. Left ventricular ejection fraction (LVEF) remains a central parameter, but it is not sufficient. Takotsubo may be transient but may be complicated by shock, thrombus or arrhythmias. Ventricular noncompaction may be benign when isolated, but high-risk when associated with fibrosis or a pathogenic genotype. Mitochondrial disease may show apparently preserved function and dangerous conduction blocks. An eosinophilic form may present with elevated troponin and thrombosis before fibrosis. Antimalarial toxicity may present with conduction abnormalities and a restrictive phenotype rather than obvious dilation.

Clinical reasoning must therefore integrate morphology, function, rhythm, tissue, genetics, exposures, inflammation, metabolism and extracardiac organs. Echocardiography defines structure and function; cardiac magnetic resonance (CMR) adds edema, fibrosis, thrombi, infiltration and tissue patterns; the electrocardiogram (ECG) and rhythm monitoring define electrical risk; biomarkers clarify injury and wall stress; genetics identifies hereditary causes; toxicological, infectious, immunological and family history closes the etiological circle. None of these levels is sufficient on its own.

In patients with acute ventricular dysfunction, chest pain and non-obstructive coronary arteries, Takotsubo cardiomyopathy becomes a diagnosis to consider when the clinical picture is consistent with transient myocardial stunning due to neurocardiac stress. In patients with prominent endocardial trabeculations, noncompaction cardiomyopathy instead requires distinguishing physiological hypertrabeculation, acquired phenotype and true genetic cardiomyopathy. When cardiac disease is associated with muscle weakness, hearing loss, diabetes, ophthalmoplegia or metabolic crises, mitochondrial cardiomyopathies shift attention from the ventricle to cellular bioenergetics.

The presence of eosinophilia, asthma, skin manifestations, neuropathy or intracavitary thrombi points toward hypereosinophilic cardiomyopathy, in which inflammation, thrombosis and fibrosis may represent successive phases of the same endomyocardial aggression. When alcohol, stimulants, anabolic androgenic steroids, chemotherapy, radiotherapy, antimalarial agents or metals emerge, toxic cardiomyopathies require a precise etiological diagnosis, because stopping the exposure may be the most important therapeutic step.

A stiff ventricle, with elevated filling pressures and relatively preserved systolic function, may lead to idiopathic restrictive cardiomyopathy when no sufficient infiltrative, endomyocardial or systemic cause is demonstrated. If instead endocardial injury, organized thrombosis, cavity obliteration and valvular regurgitation predominate, endomyocardial fibrosis becomes the most coherent pathophysiological model. In patients with severe hypertrophy, pre-excitation, myopathy or rapid deterioration, Danon disease must be recognized as a lysosomal genetic cardiomyopathy, not as a simple hypertrophic variant.

When cardiac disease is associated with muscle weakness, respiratory failure, conduction disturbances or skeletal myopathy, cardiomyopathies associated with neuromuscular disorders require joint assessment of the heart, muscle and ventilatory function. In patients with vasculitis, connective tissue diseases, autoantibodies, myocarditis, microvasculopathy or systemic fibrosis, cardiomyopathies associated with autoimmune disorders show how myocardial damage may be part of systemic inflammation, not an isolated cardiac event.

Classic cardiomyopathies have historically been defined through the dominant phenotype: dilation and systolic dysfunction in dilated cardiomyopathy, unexplained hypertrophy in hypertrophic cardiomyopathy, stiffness and impaired filling in restrictive cardiomyopathy, fibro-adipose replacement and arrhythmias in arrhythmogenic cardiomyopathy. This framework remains useful because it rapidly directs the diagnostic pathway. However, some diseases cross several phenotypes, change over time or derive from mechanisms so specific that assigning them to a single anatomical category reduces their clinical meaning.

Takotsubo cardiomyopathy is the clearest example of a non-static phenotype. At onset it may simulate acute myocardial infarction, show apical ballooning, mid-ventricular dysfunction, a basal form or a focal variant, but the fundamental feature is transient regional dysfunction mediated by neurovegetative and catecholaminergic stress. Describing it as a simple dilated cardiomyopathy would be incorrect, because the cavity may not be chronically dilated, dysfunction is often reversible and the acute clinical problem concerns shock, dynamic obstruction, thrombi and arrhythmias. The classic category does not capture the mechanism.

Noncompaction cardiomyopathy demonstrates the opposite limit: a morphological image may be very evident and yet not always pathological. Left ventricular noncompaction may be associated with genetic cardiomyopathy, heart failure, arrhythmias and thromboembolism, but it may also represent isolated hypertrabeculation in athletes, pregnancy or volume overload. Observing prominent trabeculae is therefore not sufficient to define a cardiomyopathy. Correct assessment must distinguish isolated phenotype, familial disease, association with dilated or hypertrophic cardiomyopathy, genetic variants, fibrosis and electrical risk.

Mitochondrial cardiomyopathies do not fit classic phenotypes because the same bioenergetic defect may produce hypertrophy, dilation, noncompaction, conduction disturbances, arrhythmias or heart failure at different ages. The ventricle may appear hypertrophic in one phase and dilated in another. Moreover, the heart is almost never the only relevant organ: the nervous system, skeletal muscle, hearing, eye, kidney, endocrine system and systemic metabolism contribute to diagnosis and prognosis. An exclusively morphological category does not describe maternal inheritance, heteroplasmy, oxidative phosphorylation deficiency or metabolic crises.

Hypereosinophilic cardiomyopathy is even more dynamic because it passes through distinct biological phases. In the necrotic phase it behaves like myocarditis; in the thrombotic phase it produces mural thrombi and embolic events; in the fibrotic phase it produces restriction, apical obliteration and atrioventricular valvular disease. If seen late, it may look like restrictive cardiomyopathy, but the true cause is eosinophilic toxicity on the endocardium and myocardium. Correct diagnosis does not consist only in describing a stiff ventricle, but in recognizing whether the eosinophilic process is still active or already scarred.

Toxic cardiomyopathies are heterogeneous because the toxic agent may produce direct myocyte injury, ischemia, vasospasm, hypertension, arrhythmias, lysosomal accumulation, immune-mediated myocarditis or mitochondrial dysfunction. Anthracyclines, alcohol, methamphetamine, cocaine, anabolic androgenic steroids, hydroxychloroquine, cobalt and immune checkpoint inhibitors do not act in the same way. The final phenotype may be similar, but etiological therapy changes completely. Removing alcohol, stopping a stimulant substance, modifying oncological therapy or revising a metal-on-metal prosthesis are not interchangeable interventions.

Idiopathic restrictive and endomyocardial forms show that hemodynamic physiology may be similar despite different substrates. Idiopathic restriction is defined by ventricular stiffness not explained by infiltrative, hypertrophic or constrictive causes. Fibro-thrombotic endomyocardial disease mainly involves the endocardium, apices, valvular apparatuses and ventricular cavities. Both may produce dilated atria, elevated filling pressures, congestion and arrhythmias, but the primary anatomical site of injury and clinical history do not coincide. The distinction is essential for prognosis and therapeutic options.

Danon disease, neuromuscular cardiomyopathies and autoimmune forms go beyond the boundaries of traditional classification because the heart is part of a systemic disease. In Danon disease the lysosomal defect may produce severe hypertrophy, pre-excitation, arrhythmias, rapid deterioration and skeletal or cognitive involvement. In neuromuscular disorders, cardiomyopathy is conditioned by respiratory weakness, conduction disturbances, myopathy and genetics. In autoimmune diseases, damage may derive from myocarditis, microvasculopathy, fibrosis, antibodies, vasculitis, thrombosis or therapies. Cardiac morphology is only one part of the disease.

The practical result is that these conditions require a diagnostic pathway not standardized only on the ventricle. Each has a specific etiological treatment or risk strategy. If they are confused with idiopathic cardiomyopathies, the opportunity is lost to stop a toxic agent, treat an eosinophilic clone, identify a genetic variant, protect relatives, prevent conduction blocks, recognize a systemic disease or avoid harmful exposures. The definition of unclassified does not reduce diagnostic precision: it makes it more necessary.

Although different, unclassified cardiomyopathies share several recurring pathobiological mechanisms. The first is cardiomyocyte energy dysfunction. The heart continuously consumes adenosine triphosphate (ATP) for contraction, relaxation, maintenance of ionic gradients and calcium handling. Any defect that reduces energy production or increases its cost may produce dysfunction. This is evident in mitochondrial cardiomyopathies, but it also appears in toxic forms due to anthracyclines, alcohol, methamphetamine, antimalarial agents and metals, as well as in lysosomal diseases such as Danon disease. Loss of energy reserve makes the myocardium vulnerable to fever, tachycardia, ischemia, hypoxia, fasting, exercise and systemic stress.

The second mechanism is inflammatory injury. In autoimmune, eosinophilic and immunotherapy-related forms, and in some toxicities, the heart is affected by immune cells, cytokines, autoantibodies, cytotoxic mediators or vasculitis. Inflammation may be acute, as in eosinophilic myocarditis or myocarditis from immune checkpoint inhibitors, or chronic and fibrosing, as in some systemic autoimmune diseases. The diagnostic point is to recognize whether there is still treatable inflammatory activity. Edema on CMR, elevated troponin, systemic symptoms, eosinophilia, active autoimmunity or rapid deterioration change the therapeutic threshold.

The third mechanism is fibrosis. Fibrosis may be replacement fibrosis after necrosis, interstitial fibrosis from chronic remodeling, endomyocardial fibrosis after eosinophilic, lysosomal or toxic injury, or secondary to microischemia and inflammation. Clinically, fibrosis is important because it reduces compliance, worsens diastolic function, creates a substrate for arrhythmias, limits reversibility and worsens prognosis. CMR with late gadolinium enhancement (LGE) and tissue mapping is therefore central in stratification. Two patients with similar LVEF may have very different risk if one has extensive fibrosis and the other does not.

The fourth mechanism is intracardiac thrombosis. Some unclassified forms have a particular predisposition to thrombus formation. Extensive apical Takotsubo may generate stasis and ventricular thrombus. Noncompaction may favor stasis within recesses when systolic dysfunction or atrial fibrillation coexists. Hypereosinophilic cardiomyopathy directly damages the endocardium and activates coagulation. Methamphetamine may produce severely dilated ventricles with thrombi. Fibro-thrombotic endomyocardial forms may organize thrombi and transform them into scar. Embolic risk must therefore be assessed actively, not inferred generically from the diagnosis.

The fifth mechanism is electrical instability. Conduction disturbances, pre-excitation, atrial fibrillation, ventricular tachycardias, torsades de pointes, atrioventricular blocks and sudden death may be central in many of these diseases. Danon disease may present with pre-excitation and arrhythmias; mitochondrial diseases may produce progressive conduction blocks; antimalarial toxicity may alter conduction and produce a restrictive phenotype; stimulants may trigger adrenergic arrhythmias; autoimmune and inflammatory forms may affect the conduction system and myocardium. Rhythm monitoring is therefore a structural part of the diagnostic pathway.

The sixth mechanism is mechanical remodeling. Initially cellular or endocardial injury becomes hemodynamic disease when the ventricle dilates, hypertrophies, becomes stiff, develops functional valvular regurgitation or loses synchronization. Restrictive physiology may derive from endocardial fibrosis, infiltration, lysosomal injury or myocardial stiffness; dilation may derive from toxicity, metabolic defect, neuromuscular disease or chronic inflammation; hypertrophy may derive from accumulation, energy defect, anabolic steroids or genetic diseases. The observed phenotype is often the final phase of a deeper process.

The seventh mechanism is systemic vulnerability. In unclassified cardiomyopathies, the heart may worsen because of apparently extracardiac events: metabolic crisis, infection, autoimmune flare, eosinophilic relapse, resumption of alcohol or stimulants, oncological treatment, anesthesia, pregnancy, respiratory failure, malnutrition, nephropathy or cumulative toxicity. Cardiac stability therefore also depends on control of the underlying disease and of the patient’s biological environment.

These mechanisms are not alternatives. A patient with alcohol-related toxic cardiomyopathy may have mitochondrial dysfunction, fibrosis, arrhythmias and nutritional deficiencies. A patient with eosinophilia may have inflammation, thrombosis and restriction. A patient with mitochondrial disease may have hypertrophy, blocks, fibrosis and metabolic crises. A patient with autoimmune disease may have myocarditis, microvasculopathy, thrombosis and toxicity from previous therapies. Diagnosis must therefore be multidimensional: identify the dominant mechanism, but do not ignore associated mechanisms.

The clinical manifestations of unclassified cardiomyopathies are broad because the heart may be affected as the primary target, as an organ involved in systemic disease or as the victim of an exposure. History-taking must start from the cardiac symptom, but must immediately broaden to age, family history, exposures, extracardiac diseases, therapies, alcohol or substance use, oncological history, neurological, muscular, respiratory, cutaneous, immunological, hematological and metabolic symptoms. In this context, history-taking is not a formality: it is often the most important examination for directing the diagnosis.

The most common symptom is dyspnea. It may appear acutely, as in Takotsubo, eosinophilic myocarditis or immunotherapy-related myocarditis; it may be progressive, as in chronic toxic, mitochondrial, neuromuscular or restrictive cardiomyopathies; it may be intermittent, when it depends on arrhythmias or metabolic crises. Dyspnea must be interpreted together with orthopnea, edema, reduced exercise tolerance, asthenia, chest pain, palpitations, syncope and systemic symptoms. The same dyspnea in an older woman after stress, in a young person with muscle weakness, in an oncological patient or in a subject with eosinophilia has completely different meanings.

Chest pain is frequent in forms that simulate acute coronary syndrome. Takotsubo may present with pain, ECG abnormalities and elevated troponin. Eosinophilic myocarditis may mimic infarction with unobstructed coronary arteries. Cocaine, methamphetamine, fluoropyrimidines and thoracic radiotherapy may cause true ischemia, vasospasm or coronary damage. Autoimmune forms may cause myopericarditis or microvasculopathy. For this reason, chest pain should not be prematurely attributed to a rare diagnosis: ischemic, aortic, embolic and myocarditic emergencies are excluded first, then etiology is refined.

Palpitations, presyncope and syncope have particular value because many unclassified cardiomyopathies are arrhythmogenic. Syncope in a patient with mitochondrial disease may indicate conduction block; in Danon disease it may indicate tachyarrhythmia or pre-excitation; in antimalarial toxicity it may indicate atrioventricular block; in neuromuscular forms it may signal disease of the conduction system; in Takotsubo it may reflect torsades de pointes or ventricular tachycardia; in autoimmune forms it may derive from myocarditis or sarcoidosis to be considered in the differential diagnosis. Unexplained syncope always requires rhythm monitoring.

Embolic events may be the first manifestation. Ischemic stroke, transient ischemic attack, peripheral ischemia, renal or splenic infarctions may derive from ventricular thrombi in Takotsubo, noncompaction, hypereosinophilia, methamphetamine-associated disease or toxic dilated cardiomyopathies. In a patient with cryptogenic embolism and atypical cardiac imaging, the ventricle must be studied carefully, using contrast echocardiography or CMR if necessary. The intracardiac thrombus may be small, apical, layered or masked by trabeculae.

Extracardiac symptoms are often the key. Asthma, rhinosinusitis, nasal polyposis, neuropathy and eosinophilia point toward eosinophilic granulomatosis with polyangiitis (EGPA) or hypereosinophilic syndrome. Muscle weakness, ptosis, ophthalmoplegia, hearing loss, diabetes, epileptic seizures or short stature point toward mitochondrial or neuromuscular disease. Cognitive delay, myopathy and severe hypertrophy suggest Danon disease. Rash, arthralgia, Raynaud phenomenon, ulcers, fever, serositis or nephropathy point toward autoimmune disease. A history of chemotherapy, radiotherapy, alcohol, stimulants, anabolic steroids, antimalarial agents or metals points toward toxicity. The cardiological examination must therefore include an active search for systemic signs.

On physical examination, the patient may be normal or show signs of heart failure, congestion, low output, arrhythmia, hypertension, valvular regurgitation, pulmonary hypertension or systemic disease. Crackles, third heart sound, edema, jugular venous distension, hepatomegaly and ascites indicate heart failure or restriction. Systolic murmurs may derive from functional mitral or tricuspid regurgitation, endomyocardial fibrosis or remodeling. Irregular rhythm suggests atrial fibrillation. Bradycardia, pauses or blocks indicate involvement of the conduction system. Cutaneous, neurological, muscular, ocular and respiratory signs must be sought systematically.

Presentation may be acute, subacute or chronic. Acute forms include Takotsubo, eosinophilic myocarditis, immunotherapy-related myocarditis, adrenergic toxicity, substance-related ischemia, autoimmune flares and heart failure from recent exposure. Subacute forms include early oncological toxicity, thrombotic eosinophilic progression, heart failure from alcohol, methamphetamine or neuromuscular disease. Chronic forms include endomyocardial fibrosis, idiopathic restriction, lysosomal accumulation, mitochondrial injury and antimalarial toxicity. Disease timing helps distinguish reversible inflammation from stabilized remodeling.

Severity should not be judged only from symptoms. Patients with low physical activity due to myopathy or systemic disease may not report major dyspnea despite significant heart disease. Patients receiving oncological therapy or immunosuppression may attribute fatigue and tachycardia to the underlying disease. Patients using substances may not report exposure. Patients with restrictive forms may have preserved LVEF and severe symptoms. Patients with conduction disease may have an almost normal echocardiogram and high sudden risk. Assessment must therefore be objective, multimodal and repeated over time.

The diagnostic pathway for unclassified cardiomyopathies must be orderly and progressive. The first step is to define the cardiac phenotype: dilated, hypertrophic, restrictive, arrhythmic, endomyocardial, trabecular, inflammatory, toxic or mixed. The second is to establish whether the phenotype is isolated or associated with systemic disease. The third is to identify the cause. The fourth is to assess the risk of heart failure, arrhythmias, sudden death, thromboembolism and progression. Diagnosis must not stop at the first descriptive label.

The first level includes complete history, family history over at least three generations when possible, cardiological and systemic examination, 12-lead ECG, transthoracic echocardiography, cardiac biomarkers, basic blood tests, renal function, liver function, electrolytes, complete blood count, metabolic profile and exposure assessment. ECG may show hypertrophy, pseudoinfarction Q waves, blocks, pre-excitation, repolarization abnormalities, long corrected QT interval (QTc), atrial fibrillation or ischemic signs. Echocardiography defines LVEF, volumes, wall thickness, diastolic function, atria, right ventricle, valves, pulmonary pressures, trabeculation, thrombi and restrictive pattern.

The second level includes CMR, Holter ECG, prolonged rhythm monitoring, exercise testing or cardiopulmonary exercise testing, coronary assessment, computed tomography, nuclear medicine, cardiac catheterization, genetics, immunology, hematology, infectious disease evaluation, neurology, toxicology or biopsy according to the clinical suspicion. CMR has a cross-cutting role because it identifies edema, fibrosis, LGE, accumulation, infiltration, thrombi, right ventricular involvement, noncompaction and myocarditic or ischemic patterns. In rare phenotypes, CMR does not replace etiological diagnosis, but reduces the area of uncertainty.

In the absence of unique diagnostic criteria for the entire category of unclassified cardiomyopathies, according to the ESC approach to cardiomyopathies and the phenotype-genotype-etiology logic, a clinically correct diagnosis requires:

integrated diagnostic pathway

• defining the morphofunctional phenotype with ECG, echocardiography and, when indicated, CMR;
• establishing whether the condition is acute, subacute, chronic, reversible, progressive, familial, sporadic or acquired;
• searching for extracardiac, familial, toxicological, hematological, neurological, immunological and metabolic red flags;
• excluding ischemic heart disease, primary valvular disease, uncontrolled hypertension, constrictive pericarditis, common myocarditis, tachycardiomyopathy and endocrine causes when clinically plausible;
• using genetic, immunological, infectious disease, toxicological or metabolic tests only when the clinical suspicion makes them interpretable;
• assessing the risk of arrhythmias, thromboembolism, heart failure, progression, sudden death and family involvement;
• reassessing uncertain cases over time, because some phenotypes become clear only during follow-up or after exposure withdrawal.

Family history is essential when the patient is young, has unexplained hypertrophy, non-ischemic dilation, conduction blocks, sudden death in the family, neuromuscular diseases, hearing loss, juvenile diabetes, intellectual disability, heart transplantation or implanted devices in relatives. Genetics must be used precisely: indiscriminate testing may produce variants of uncertain significance that are difficult to interpret, whereas targeted testing in the presence of red flags may identify diseases such as Danon disease, muscular dystrophies, laminopathies, mitochondrial diseases or familial phenotypes with prognostic implications.

Toxicological history must be systematic. It is necessary to ask about oncological therapies, thoracic radiotherapy, alcohol, cocaine, methamphetamine, anabolic androgenic steroids, antimalarial agents, antipsychotics, antidepressants, supplements, industrial substances, metals, solvents and metal-on-metal prostheses. This part of diagnosis requires direct and non-judgmental questions, because unreported exposure may turn a modifiable cardiomyopathy into an incorrect idiopathic diagnosis.

Hematological and immunological assessment becomes a priority when eosinophilia, asthma, nasal polyposis, rash, fever, arthralgia, neuropathy, proteinuria, cytopenias, splenomegaly, lymphadenopathy or constitutional symptoms are present. In these cases, hypereosinophilic syndromes, EGPA, systemic autoimmune diseases, vasculitis, myeloid neoplasms, lymphomas and parasitic causes are investigated. Elevated troponin, compatible CMR and eosinophilia should prompt consideration of eosinophilic myocarditis even if the initial picture resembles infarction or common myocarditis.

Endomyocardial biopsy is not a first-level test for every patient, but remains decisive in selected scenarios: fulminant myocarditis, shock, severe arrhythmias, suspected eosinophilic myocarditis, giant cell myocarditis, sarcoidosis, immunotherapy-related toxicity, unexplained infiltrative disease, antimalarial accumulation or a diagnosis that would radically change therapy. The main limitation is sampling, because many diseases are focal; its value increases when biopsy is imaging-guided and interpreted by expert centers.

The most difficult differential diagnosis is distinguishing unclassified cardiomyopathy from idiopathic cardiomyopathy. An idiopathic diagnosis is acceptable only after a reasonable search for relevant causes. This does not mean that every patient must undergo every possible test; it means that every red flag must be followed. A patient with ventricular dilation and chronic alcohol exposure requires a toxicological framework. A patient with hypertrophy and pre-excitation requires targeted genetics. A patient with restriction and eosinophilia requires endomyocardial and hematological assessment. A patient with heart failure and muscle weakness requires neuromuscular evaluation. The correct diagnosis arises from the right sign, not from an indiscriminate quantity of tests.

Takotsubo cardiomyopathy belongs to acute and potentially reversible functional forms. Ventricular dysfunction arises from a neurocardiac response to stress, with catecholaminergic toxicity, microvascular dysfunction, myocardial stunning and possible recovery. The clinical problem is not only recognizing it, but rapidly distinguishing it from infarction, myocarditis and other emergencies, and monitoring shock, dynamic obstruction, thrombus and arrhythmias.

Noncompaction cardiomyopathy belongs to complex morphological phenotypes. In this case the essential point is to establish whether hypertrabeculation is a variant, an adaptation, an acquired trait or a genetic cardiomyopathy. The anatomical image must be integrated with ventricular function, CMR, LGE, arrhythmias, family history, embolic events and genetics. Modern classification tends to avoid making every prominent trabecula automatically a disease, but at the same time must not miss patients who are truly at risk.

Mitochondrial cardiomyopathies and Danon disease belong to metabolic and bioenergetic forms. In the former, the central defect concerns oxidative phosphorylation, ATP, heteroplasmy, mitochondrial or nuclear genes and multisystem involvement. In Danon disease, the LAMP2-related lysosomal defect alters autophagy and intracellular accumulation, with an often hypertrophic, arrhythmic and progressive phenotype. In both, the cardiac diagnosis must be linked to neurology, muscle, genetics, metabolism and family.

Idiopathic restrictive cardiomyopathy and endomyocardial fibrosis share impaired filling physiology, but not the same substrate. The former is defined by ventricular stiffness not explained by more common causes, whereas the latter represents fibro-thrombotic endocardial injury with obliteration, valvular regurgitation and restriction. These conditions require distinction from amyloidosis, constrictive pericarditis, sarcoidosis, hemochromatosis, storage diseases, hypereosinophilia and primary valvular disease. Restrictive physiology is similar, but causes and therapeutic options are different.

Hypereosinophilic cardiomyopathy and cardiomyopathies associated with autoimmune disorders belong to inflammatory forms. In the former, eosinophilic infiltration and degranulation may evolve toward thrombosis and endomyocardial fibrosis. In the latter, damage may derive from myocarditis, microvasculopathy, vasculitis, fibrosis, thrombosis and systemic inflammation. In these forms it is essential to understand whether inflammation is still active, because immunological, hematological or rheumatological treatment may change prognosis only if applied in the correct phase.

Toxic cardiomyopathies derive from exposure to substances capable of damaging myocardium, endothelium, mitochondria, the conduction system or the microcirculation. The group includes oncological toxicity, alcohol, stimulants, anabolic androgenic steroids, antimalarial agents, metals and other exposures. Diagnosis is important because the main etiological therapy is to stop, reduce or modify the exposure, when possible. Without an accurate toxicological history, many forms are defined as idiopathic and continue to worsen.

Cardiomyopathies associated with neuromuscular disorders involve the heart together with skeletal muscle, breathing, the conduction system and sometimes metabolism. Dystrophinopathies, laminopathies, myotonic dystrophies and other neuromuscular diseases may produce dilation, fibrosis, arrhythmias, blocks and heart failure. Cardiological diagnosis must include respiratory and neurological assessment, because hypoventilation, diaphragmatic weakness and conduction disturbances may influence prognosis as much as LVEF.

The common denominator is the need to go beyond the generic description of the ventricle and reach the reason why that ventricle has changed. The same reduced LVEF may be the consequence of toxicity, genetic defect, inflammation, tachyarrhythmia, neuromuscular disease or metabolic injury. The same restriction may derive from endocardial fibrosis, accumulation, infiltration or idiopathic disease. The same arrhythmic risk may be produced by fibrosis, ion channels, conduction, inflammation or toxicity. The correct diagnosis arises from the integration of all these levels.

The treatment of unclassified cardiomyopathies cannot be unique because there is no single disease. There is, however, a common rule: treat the cardiac phenotype and, at the same time, the specific cause. Heart failure is treated with validated therapies when LVEF is reduced; congestion is treated with diuretics; arrhythmias are treated with monitoring, medications, ablation or devices when indicated; thrombi are treated with anticoagulation; but if the causal mechanism remains active, cardiological therapy remains incomplete. The cause must be sought and addressed.

In toxic forms, etiological therapy is removal of the exposure. Abstinence from alcohol, cessation of stimulants, discontinuation of anabolic androgenic steroids, modification of cardiotoxic oncological therapy, discontinuation of cardiotoxic antimalarial agents or removal of a cobalt source are therapeutic interventions as important as heart failure medications. Prognosis improves when discontinuation occurs before extensive fibrosis. If exposure continues, even optimal cardiological therapy may fail.

In inflammatory, autoimmune and eosinophilic forms, treatment must suppress the biological activity. Corticosteroids, immunosuppressants, biological therapies, targeted hematological therapies, antiparasitic treatment or control of vasculitis may be decisive, but they must be used after correct assessment. Eosinophilic myocarditis from hypereosinophilic syndrome, EGPA with cardiac involvement and immunotherapy-related myocarditis do not have the same therapy. Etiological diagnosis is therefore the prerequisite for therapy, not an academic detail.

In genetic, metabolic, mitochondrial, lysosomal and neuromuscular forms, treatment includes cardiological therapy, rhythm surveillance, multisystem management, genetic counseling and family screening. Some forms require lower thresholds for pacemaker or defibrillator implantation because electrical risk may precede severe dysfunction. Others require prevention of metabolic crises, attention to anesthesia, respiratory assessment, nutrition, neurology and rehabilitation. Prognosis depends on the systemic disease at least as much as on the heart.

In restrictive and endomyocardial forms, therapy is often more difficult because mechanical injury may be poorly reversible. Managing congestion requires careful balance: excessive diuretics may reduce preload too much, whereas insufficient diuretics leave the patient congested. Atrial arrhythmias, thrombi, valvular regurgitation and pulmonary hypertension must be treated specifically. In selected cases, surgery or transplantation is considered, but the decision depends on cause, extent, reversibility, age, involved organs and operative risk.

The prognosis of unclassified cardiomyopathies is extremely variable. Early diagnosis, identification of the cause, reversibility of damage, absence of fibrosis, preserved LVEF, control of arrhythmias, absence of thrombi, extracardiac stability and adherence to follow-up are favorable. Diagnostic delay, extensive fibrosis, right ventricular dysfunction, shock, ventricular arrhythmias, progressive conduction blocks, severe multisystem involvement, persistence of the toxic agent, uncontrolled inflammatory activity, a high-risk genetic form and relapse are unfavorable.

Follow-up must be built on the specific risk. A patient with Takotsubo requires monitoring of ventricular recovery, thrombi and arrhythmias. A patient with isolated hypertrabeculation requires proportionate surveillance, without excessive medicalization. A mitochondrial patient requires cardiology, genetics and metabolic medicine. An eosinophilic patient requires complete blood count, imaging and control of the cause. A toxic patient requires verification of abstinence or discontinuation of exposure. A neuromuscular patient also requires respiratory function and conduction assessment. Follow-up is therefore not the same for everyone: it must reflect the mechanism of the disease.

Advanced therapies, such as implantable defibrillator, resynchronization, ventricular assist device or transplantation, must be assessed according to general guidelines and the specific context. In some forms, reversibility supports protected waiting and reassessment; in others, electrical risk requires early intervention; in still others, systemic involvement limits eligibility for transplantation or mechanical support. The decision should be made in centers with expertise in cardiomyopathies, because standard heart failure management does not always capture the specific features of these conditions.

The complications of unclassified cardiomyopathies include heart failure, shock, arrhythmias, sudden death, thromboembolism, fibrotic progression, functional valvular disease, pulmonary hypertension, multiorgan involvement and relapse. Their frequency changes according to the specific disease, but the common principle is that many complications become preventable if the mechanism is recognized early. The same complication may have different causes: a ventricular thrombus may derive from Takotsubo, eosinophilia, noncompaction or methamphetamine; ventricular tachycardia may derive from genetic fibrosis, toxicity, myocarditis or accumulation; restriction may derive from fibrotic endocardium, accumulation, infiltration or idiopathy.

Heart failure is the most cross-cutting complication. It may be acute, as in Takotsubo, eosinophilic myocarditis or immunotherapy-related toxicity; chronic, as in alcoholic, mitochondrial, neuromuscular or toxic forms; restrictive, as in endomyocardial forms; or mixed, when fibrosis, valvular disease and right ventricular dysfunction coexist. Management requires recognition of the hemodynamic phenotype. A restrictive patient is not treated like a purely dilated patient; a patient with dynamic obstruction in Takotsubo is not treated like conventional pump-failure shock; a patient with neuromuscular disease requires respiratory assessment.

Arrhythmias and sudden death are particularly important because they may precede or exceed the apparent severity of ventricular dysfunction. Conduction blocks in mitochondrial diseases, pre-excitation in Danon disease, ventricular tachycardias in fibrotic forms, long QTc in medication toxicities, atrial fibrillation in restrictive forms and adrenergic arrhythmias from stimulant substances are different examples of a single clinical problem: electrical risk is not always proportional to LVEF. For this reason, ECG, Holter and prolonged monitoring are not accessories, but essential tools.

Thromboembolism is a recurring complication in forms with stasis, endocardial injury or local hypercoagulability. Extensive apical Takotsubo, hypereosinophilic cardiomyopathy, noncompaction with dysfunction, methamphetamine-associated cardiomyopathy, severe dilated forms and endomyocardial fibrosis may produce intracardiac thrombi. Embolic events may involve the brain, kidneys, spleen, intestine or limbs. Thrombus must be actively sought in high-risk patients, because its absence on the first echocardiogram does not always exclude subsequent formation.

Fibrosis is both a complication and a marker of irreversibility. In inflammatory forms it represents the scar of injury; in toxic forms it indicates prolonged exposure; in genetic forms it signals remodeling and arrhythmic risk; in endomyocardial forms it becomes the main mechanism of restriction. A diagnosis made before fibrosis has a better prognosis than one made when the heart is already scarred, stiff or electrically unstable. CMR is therefore a prognostic tool as well as a diagnostic one.

Functional valvular disease and pulmonary hypertension worsen many forms. Ventricular dilation produces mitral and tricuspid regurgitation; endomyocardial fibrosis may trap valvular apparatuses; restriction increases atrial and pulmonary pressures; methamphetamine may be associated with pulmonary hypertension; neuromuscular and respiratory diseases increase the load on the right ventricle. These complications modify symptoms, therapy and prognosis, and must be assessed serially.

Multiorgan involvement is only apparently an extracardiac complication. In mitochondrial, neuromuscular, autoimmune, eosinophilic and toxic diseases, the kidney, liver, lung, nervous system, skeletal muscle, endocrine system and hematological system influence cardiac management. Renal failure limits medications and anticoagulants; respiratory weakness worsens heart failure; liver disease modifies coagulation and metabolism; autonomic neuropathy alters heart rate and blood pressure; active neoplasia conditions cardio-oncology. Cardiac prognosis cannot be separated from systemic prognosis.

Relapses are a specific complication of many forms. Takotsubo may relapse; eosinophilia and autoimmunity may reactivate; alcohol, stimulants and anabolic steroids may be resumed; oncological toxicity may recur with new treatment cycles; genetic diseases may progress after a stable phase. Follow-up must therefore search not only for residual damage, but also for the possibility that the pathogenic mechanism may reignite.

The most dangerous complication at the diagnostic level is the label “idiopathic” applied too early. Whenever a cardiomyopathy is defined without a cause, it is necessary to ask whether exposures, genetics, eosinophilia, autoimmunity, neuromuscular diseases, metabolic defects, medications, toxins, extracardiac signs and family history have been investigated. Not all causes are identifiable, but many are searchable. In unclassified cardiomyopathies, diagnostic precision is part of therapy: what is not recognized cannot be treated, stopped, monitored or prevented in relatives.