Metabolic and storage cardiomyopathies

Article written and reviewed by Dr Luca Moretti

Metabolic and storage cardiomyopathies comprise conditions in which the heart is damaged primarily not by a sarcomeric protein, but by a defect in lysosomal degradation, glycogen metabolism, mitochondrial energy production or other intracellular pathways. The result may resemble hypertrophic, dilated, non-dilated or restrictive cardiomyopathy, but the identity of the disease lies in its biochemical mechanism. Recognizing it changes surveillance of other organs, family counseling and, in some forms, enables causal therapy.

These diseases are individually rare, but are important in children with severe cardiomyopathy, adults with unexplained thick ventricular walls and families in which heart disease is associated with weakness, nephropathy, neuropathy, hearing loss, diabetes or cognitive abnormalities. Defining them simply as phenocopies of hypertrophic cardiomyopathy helps the differential diagnosis, but should not reduce them to imitations: they have their own natural history, specific electrical complications and multidisciplinary needs. Integrated etiology therefore takes precedence over ventricular shape alone.

Cardiomyopathy in Fabry disease is an X-linked sphingolipidosis with vascular, renal and neurologic involvement; Danon disease results from LAMP2 deficiency and combines impaired autophagy, severe heart disease and muscular or cognitive manifestations. In Pompe disease, glycogen accumulates in lysosomes because of acid alpha-glucosidase deficiency. Thus, three lysosomal diseases generate very different hearts because substrate, sex, age and treatment response differ. Defect specificity prevents the use of a single therapeutic algorithm.

PRKAG2 syndrome alters the gamma-2 subunit of AMP-activated protein kinase and combines hypertrophy, pre-excitation and conduction disease; glycogen storage disease cardiomyopathies, by contrast, include heterogeneous defects of glycogen synthesis or breakdown, often involving the liver and skeletal muscle. Mitochondrial cardiomyopathies arise from mitochondrial or nuclear DNA and reflect the inability to sustain the myocardium's exceptionally high energy demand. The common language of storage does not erase this diversity.

A classification based on the cellular compartment

Within the lysosome, enzymes and membrane proteins degrade macromolecules. Alpha-galactosidase A deficiency in Fabry disease causes accumulation of globotriaosylceramide and globotriaosylsphingosine; acid alpha-glucosidase deficiency in Pompe disease prevents glycogen clearance; loss of LAMP2 in Danon disease blocks stages of autophagy and produces vacuoles rich in material. The organ is not merely filled: signaling, inflammation, cellular trafficking and survival change. Lysosomal pathology explains why early substrate reduction is more effective than therapy begun after advanced scarring.

Cytosolic glycogen storage diseases depend on enzymes that synthesize, branch or mobilize glycogen. AGL in GSD III, GBE1 in GSD IV and other genes generate quantitatively or qualitatively abnormal material, with different combinations of hypoglycemia, liver disease, myopathy and cardiomyopathy. PRKAG2 is not a classic glycogen storage disease in every taxonomy, but it alters the AMPK energy sensor and produces a cardiac glycogen-storage phenotype. Enzymatic topography distinguishes these forms from Pompe disease, which stores glycogen in lysosomes.

Mitochondria produce ATP through oxidative phosphorylation and participate in calcium handling, apoptosis and reactive species. Mitochondrial DNA variants follow maternal inheritance and may be present at different proportions among tissues; nuclear variants instead follow Mendelian inheritance. The same mitochondrial variant can cause different phenotypes because of heteroplasmy and threshold effects, whereas variants in different genes can converge on the same cardiac phenotype. Bioenergetic genetics is therefore more complex than a linear one-gene-one-phenotype relationship.

Not all metabolic diseases accumulate visible material. Fatty-acid oxidation, carnitine transport or organic metabolism defects may present with cardiomyopathy, arrhythmias and acute crises without stable pseudohypertrophy. Endocrinopathies and nutritional deficiencies also cause acquired metabolic phenotypes, but follow distinct pathways. The clinical category serves to recall treatable pathways, not to propose a uniform histologic container.

Age, sex and inheritance as diagnostic tools

A newborn with massive hypertrophy, hypotonia and macroglossia suggests infantile Pompe disease; a male child or adolescent with extreme hypertrophy, pre-excitation, and elevated CK and transaminases suggests Danon disease; a young person with familial pre-excitation followed by heart block points toward PRKAG2. In adults, concentric hypertrophy associated with neuropathic pain, angiokeratomas or nephropathy makes Fabry disease plausible, whereas hearing loss, diabetes and maternal inheritance point toward mitochondrial disease. Syndromic chronology selects tests far more effectively than an indiscriminate panel.

In X-linked conditions, hemizygous men and heterozygous women are not simply severe and mild versions. In Fabry disease, random X-chromosome inactivation produces variability in women that may include severe organ disease; in Danon disease, women tend to present later but may develop advanced hypertrophic or dilated cardiomyopathy. Sex bias can therefore delay diagnosis precisely in symptomatic female carriers.

Consanguinity increases the likelihood of autosomal recessive diseases, but its absence does not exclude them. A maternal lineage with cardiomyopathy, diabetes, deafness or neurologic disease suggests mitochondrial DNA, although expression may vary greatly; vertical transmission in both sexes suggests a dominant disorder such as PRKAG2. The three-generation pedigree should include transplants, early pacemakers, infant deaths and extracardiac diagnoses, not just the word cardiomyopathy.

Red flags in and outside the heart

Metabolic hypertrophy is often concentric, but may be asymmetric or obstructive and cannot be distinguished by geometry alone. True pre-excitation, a short PR interval, early blocks, exceptionally high voltages or progressive loss of voltage deserve particular attention. An apparently sarcomeric pattern accompanied by elevated CK, hypotonia or cognitive deterioration changes the probability. Electromorphologic discordance is one of the most productive red flags.

Fabry disease may announce cardiac involvement through acroparesthesias, pain crises, hypohidrosis, heat intolerance, angiokeratomas, cornea verticillata, proteinuria or stroke. Pompe disease is associated with axial and respiratory weakness, scapular winging and motor difficulties; Danon disease presents with myopathy, learning difficulties and retinopathy; hepatic glycogen storage diseases cause hypoglycemia, hepatomegaly and metabolic abnormalities. Mitochondrial diseases combine, in variable ways, ptosis, ophthalmoplegia, deafness, epilepsy, ataxia, diabetes and short stature. The extracardiac phenotype is often more specific than the LGE pattern.

The absence of red flags does not exclude a predominantly cardiac form. Late-onset GLA variants may remain limited to the heart and kidneys for decades; PRKAG2 may appear to be familial pre-excitation; some mitochondrial variants cause nearly isolated cardiac disease. Conversely, a mildly abnormal biomarker does not prove the diagnosis: reduced enzyme activity on a dried blood spot requires confirmation and pseudodeficiency is possible. Graded probability avoids both omission and overdiagnosis.

Electrocardiography, imaging and tissue

The ECG is a central phenotypic test. A short PR in Fabry disease may result from accelerated atrioventricular conduction and does not always imply an accessory pathway; Danon and PRKAG2 more typically show pre-excitation, although the mechanism may be complex. With age, PRKAG2 evolves toward sinus-node or atrioventricular disease, whereas Fabry may progress from accelerated conduction to block. The electrical trajectory observed over time has independent diagnostic value.

Echocardiography defines wall thickness, cavities, obstruction, function and strain. Prominent papillary muscles and concentric hypertrophy may support Fabry disease; extreme and rapidly progressive hypertrophy characterizes many males with Danon disease; infantile Pompe disease may produce obstruction and a small cavity. These signs are not pathognomonic and must be correlated with age and the extracardiac system. Serial morphology reveals the rate of progression and response much better than a single maximal wall thickness.

Magnetic resonance adds tissue composition and scar information. In Fabry disease, native T1 is often low before fibrosis and mid-wall inferolateral LGE appears in advanced stages; in Danon disease, LGE may be extensive and does not necessarily spare the septum; in mitochondrial disorders, patterns are variable. Normal T1 does not exclude Fabry in women or in the presence of fibrosis, and low T1 also occurs with iron. The tissue signature is a probability multiplier, not a genetic test.

Muscle or cardiac biopsy is now selective. Vacuoles and PAS-positive material may suggest glycogen storage disease or Danon disease, but treatment, sampling site and processing affect the result; in mitochondrial disease, histology and enzymology may be decisive when blood and genetics are inconclusive. Endomyocardial biopsy is not justified merely to confirm a cardiomyopathy already defined by less invasive methods. The informative specimen is chosen after discussion among clinician, geneticist, biochemist and pathologist.

Biochemistry and genetics: a two-way pathway

Clinical suspicion directs specific tests: alpha-galactosidase A and lyso-Gb3 for Fabry disease, acid alpha-glucosidase and urinary tetrasaccharide for Pompe disease, CK and liver profile in Danon disease, lactate and metabolic investigations in mitochondrial disorders. No biomarker is perfect. In women with Fabry disease, enzyme activity may be normal; normal lactate does not exclude mitochondrial disease; normal CK does not eliminate a predominantly cardiac form. Targeted biochemistry gains meaning from sex and clinical timing.

Genetic testing may be targeted when the phenotype is characteristic or may use a cardiomyopathy panel that includes phenocopy genes. Pathogenic or likely pathogenic variants must be distinguished from variants of uncertain significance, which do not justify predictive testing in relatives. Copy-number changes, rearrangements, low heteroplasmy and intronic variants may escape some techniques. Analytical competence includes what a test does not detect as well as what it reports.

In mitochondrial diseases, blood may lose some heteroplasmic variants over time; urine, buccal mucosa or muscle may offer greater sensitivity depending on the variant. In Mendelian forms, segregation analysis strengthens or weakens causality, but incomplete penetrance and X-chromosome inactivation complicate the picture. Returning to the phenotype after sequencing prevents a molecular coincidence from becoming a diagnosis.

Etiologic therapies and the limit of reversibility

Enzyme replacement therapy in Fabry disease reduces substrate and may stabilize or improve some parameters, especially when started before fibrosis and severe hypertrophy; migalastat can be used only for amenable variants and according to renal-function indications. In Pompe disease, recombinant enzyme has transformed infant survival and rapidly reduces ventricular mass, but muscle response, immunogenicity and CRIM status affect outcome. The biological timing of therapy is as important as the drug.

Danon disease and PRKAG2 currently have no approved etiologic therapy capable of correcting the defect. Gene therapy for LAMP2 and other platforms are under study and must be described as experimental. In glycogen storage diseases, diet, fasting prevention and metabolic treatment depend on the type; PGM1-CDG may respond to galactose within a specialist protocol. In mitochondrial disorders, few therapies are genotype-specific and many supplement combinations have limited evidence. The hierarchy of evidence protects against confusing biochemical plausibility with clinical benefit.

Heart failure, arrhythmias, thromboembolism and conduction disease are treated according to cardiologic principles adapted to the disorder. A beta-blocker may be useful but problematic with progressive block; an ICD does not stop mechanical progression in Danon disease; standard heart-failure therapy may require caution in mitochondrial disease with dysautonomia or renal involvement. Precision cardiology does not replace guidelines: it applies their benefits while considering mechanism and trajectory.

Surveillance, crises and transition of care

In newborns and infants, the priority is to recognize conditions in which hours or days change the outcome. Hypotonia, feeding difficulty, cardiomegaly and hypertrophy require urgent GAA activity testing; acidosis, hypoglycemia, arrhythmia and dysmorphic features simultaneously point toward mitochondrial defects, fatty-acid oxidation disorders and glycosylation disorders. Stabilization and diagnosis proceed in parallel, preserving samples before transfusions when relevant. The neonatal window does not tolerate a sequential pathway that waits for every result before beginning the next test.

In older children, growth, motor development, school performance and exercise tolerance add information that the heart does not show. Presumed HCM with declining school performance, proximal weakness or elevated muscle enzymes requires etiologic reassessment; conversely, training and puberty may modify mass and voltages without a storage disease. The pediatric trajectory uses individual curves and does not merely compare the patient with an adult threshold.

In adults with hypertrophy, the priority is to distinguish common, sarcomeric and treatable causes. Blood pressure measured over time, aortic stenosis and obesity are quantified, but a hemodynamic load does not close the case if wall thickness, LGE or systemic manifestations are disproportionate. Fabry disease, amyloidosis and glycogen disorders have different ages, ECG features and tissue signatures. The adult differential diagnosis accepts coexistence of hypertension and genetic disease without using one to cancel the other.

Biomarkers, screening and confirmation

High-sensitivity screening is not the same as a clinical diagnosis. Dried blood spots, gene panels and biomarkers may generate pseudodeficiencies, uncertain variants and borderline values; the next step confirms the finding in an independent sample and verifies organ involvement. Conversely, normal tests in a poorly sensitive tissue do not exclude every disease. The confirmation cascade must be defined before screening, otherwise broad analytical capacity produces more ambiguity than useful diagnoses.

Sampling during a crisis may show abnormal lactate, ammonia, acylcarnitines, organic acids, ketones and glucose that normalize after stabilization. Critical samples collected without delaying resuscitation may be impossible to reproduce. Time, temperature, fasting, drugs and fluids must accompany the result because they modify interpretation. Metabolic preanalytics are part of the scientific datum, not a laboratory formality.

Screening a population with hypertrophy has a different yield from familial or neonatal screening. Testing alpha-galactosidase in men with LVH may identify Fabry disease, but women require genetic testing; measuring GAA in an adult with respiratory weakness has a higher pre-test probability than doing so in every HCM; a broad panel without phenotype increases VUS findings. Pre-test probability determines predictive value and should be stated when communicating a weakly positive result.

Therapeutic decisions and outcome measurement

Before starting a specific therapy, it is necessary to define what should change and over what time. Ventricular mass, T1, LGE, glomerular filtration, proteinuria, strength, ventilation and quality of life do not respond at the same rate; scar may not regress even when substrate decreases. A predefined endpoint prevents judging failure from an irreversible parameter or success from a biomarker that does not translate into function.

Stopping therapy is more complex than starting it. Reactions, antibodies, progression despite treatment, pregnancy, organ failure and preferences require reassessment that distinguishes biological inefficacy, damage that is too advanced and inadequate administration. In rare diseases, slower worsening may represent benefit, but should not be assumed without longitudinal comparison. Shared decision-making makes evidence, uncertainties and the clinical cost of continuing or stopping explicit.

Disease registries, referral centers and standardized measures are essential because small trials and heterogeneous phenotypes leave many questions unanswered. Real-world data acquire value only if diagnosis, variant, dose and outcome are defined comparably. Cumulative knowledge arises from the quality of every visit and makes it possible to distinguish a therapeutic signal from selection, survival and regression to the mean.

Follow-up is built around the organ and the etiology. ECG, echocardiography, Holter monitoring and CMR intervals are determined by age, variant, wall thickness, LGE, symptoms and therapy. Kidneys and nervous system are central in Fabry disease, respiratory muscle in Pompe disease, rapid progression and transplantation in Danon disease, conduction in PRKAG2, and liver and glycemia in glycogen storage diseases. The multisystem map prevents a reassuring cardiac parameter from concealing deterioration in another organ.

Fever, fasting, vomiting, surgery and anesthesia can precipitate decompensation in energy disorders. The emergency plan specifies hydration, glucose, drugs to avoid, and monitoring of lactate, electrolytes and cardiac function without indiscriminately administering glucose in conditions where it may be inappropriate. The anesthesiologist and metabolic center should know the diagnosis and respiratory reserve. Catabolism prevention is a treatment in its own right.

Transition from pediatric to adult care is particularly vulnerable: heart disease may accelerate precisely when teams, adherence and responsibility change. A transition dossier should include the variant, enzyme activity, therapy, immune reactions, devices, respiratory function, anesthetic risks and emergency contacts. Document continuity reduces repetition, interruptions and misinterpretation of a rare phenotype.

Family, reproduction and prognosis

A definitive molecular diagnosis enables cascade screening and reproductive counseling, but the type of risk changes radically among X-linked, autosomal recessive, dominant and mitochondrial inheritance. A mother with an mtDNA variant transmitted maternally may pass it to her children with unpredictable heteroplasmy load and severity; some variants, particularly single large deletions, do not carry a 100% transmission risk, whereas a father does not transmit mtDNA. In X-linked forms, sex modifies probability and phenotype without making women unaffected. Mechanistic counseling avoids applying correct percentages to the wrong disease.

Screening asymptomatic relatives should use the classified familial variant rather than a new broad panel and should be linked to a surveillance protocol. In minors, the benefit of early diagnosis is clear when the disease begins in childhood or has a treatment; for adult-only conditions, autonomy and clinical utility are balanced. Presymptomatic diagnosis makes sense when it produces a proportionate action.

Prognosis should be communicated by disease, sex, stage and organs, not by category. Treated infantile Pompe disease no longer has the untreated natural history; males with Danon disease retain a high risk of advanced therapy; Fabry disease with fibrosis has a different cardiac response from the pre-hypertrophic phase; PRKAG2 may require pacing even with preserved function. Contemporary prognosis incorporates the effect of treatment and acknowledges the uncertainty of rare-disease cohorts.

The value of recognizing these cardiomyopathies ultimately lies in precision of the pathway. Thick walls are not a diagnosis, pre-excitation is not always isolated WPW, and weakness is not necessarily a consequence of heart failure. When the cardiologist reconstructs metabolism together with the geneticist, neurologist, nephrologist and pediatrician, a fragmented set of findings becomes a treatable or at least monitorable disease. A unifying diagnosis is useful only if it remains verifiable and leads to specific decisions.