Hydroxychloroquine is one of the most widely used and best-tolerated medicines in rheumatology, and for many people with lupus or rheumatoid arthritis it is a cornerstone of care. This article explains a rare but serious complication of long-term use: a toxic cardiomyopathy, a disease of the heart muscle, often accompanied by disturbances in the heart's electrical wiring. It covers how the drug builds up inside heart cells, what the evidence shows about who is affected, which conduction abnormalities should prompt a closer look, and why a heart muscle biopsy examined under the electron microscope remains the most definitive way to confirm the diagnosis. The aim is to help patients and families understand the risk accurately, neither dismissing it nor overstating it.

A Medicine With a Long History and a Clear Purpose

Hydroxychloroquine descends from quinine, the bitter alkaloid in the bark of the South American cinchona tree, which was used against malarial fevers for centuries before anyone understood why it worked. Chemists refined that natural template into chloroquine in the 1930s and then into hydroxychloroquine, a less toxic derivative introduced in the 1950s. It is a fair example of how much healing has been drawn from the created world, waiting to be noticed and studied with care.

Today hydroxychloroquine is approved for treating and preventing certain forms of malaria, for systemic and discoid lupus erythematosus, and for rheumatoid arthritis. It is also prescribed off-label for conditions such as Sjögren's disease and some other inflammatory disorders. Its benefits in lupus are well established. In a randomised withdrawal trial published by the Canadian Hydroxychloroquine Study Group in the New England Journal of Medicine in 1991, patients switched to placebo were roughly two and a half times more likely to suffer a flare than those who continued the drug. Observational cohorts since then have associated continued use with fewer organ complications and better survival. That context matters: the cardiac complication discussed here is uncommon, and it has to be weighed against substantial, proven benefits.

How Lysosomal Accumulation Injures Heart Muscle

Every cell contains lysosomes, small compartments that act as the cell's recycling centres. They are kept strongly acidic so their enzymes can break down worn-out membranes, proteins and other debris. This system, called autophagy when the cell digests its own components, is one of the quiet marvels of human design, and heart muscle cells depend on it heavily because they rarely divide and must maintain themselves for decades.

Hydroxychloroquine is a weak base. In the neutral environment of the bloodstream it crosses membranes easily, but once it enters an acidic lysosome it picks up a proton, becomes charged and can no longer leave. This "ion trapping" concentrates the drug inside lysosomes to levels far higher than in the blood. Its very long half-life, measured in weeks, and its large volume of distribution in tissues reflect this behaviour. Two consequences follow:

Over years, heart muscle cells fill with these inclusions. The heart walls thicken and stiffen, producing a pattern that usually looks restrictive or hypertrophic: thick ventricles that fill poorly, with pumping strength often preserved until later stages. The conduction system, the specialised fibres that carry each electrical impulse from the upper to the lower chambers, appears particularly vulnerable, which is why heart block is often the first sign. Much of the mechanistic detail comes from cell culture and animal work showing that chloroquine-type drugs inhibit autophagy. Human evidence comes chiefly from biopsy and autopsy tissue showing the same storage changes, which makes the explanation biologically convincing, though it has not been mapped out in controlled human studies.

This chronic, cumulative injury is different from the short-term effect of hydroxychloroquine on the heart's ion channels, which can lengthen the QT interval on an ECG. QT prolongation can occur within days, particularly at high doses or alongside other QT-prolonging drugs, and became a widely discussed concern in 2020 when hydroxychloroquine was studied for COVID-19. It is not approved for COVID-19, and large randomised trials, including the UK RECOVERY trial published in the New England Journal of Medicine, found no survival benefit for hospitalised patients. The cardiomyopathy discussed here is a separate problem that typically develops over many years.

What the Evidence Shows About Frequency and Risk

Antimalarial cardiomyopathy is rare, and nearly everything known about it comes from case reports and case series. That is a real limitation. No one has a precise incidence figure, and the condition is likely under-recognised, because thick-walled hearts and heart block are common in older adults and may not be traced back to a medication.

The most frequently cited synthesis is a systematic review published in Drug Safety in 2013 that gathered 127 published cases of chloroquine or hydroxychloroquine cardiotoxicity. Conduction disorders were the dominant finding, reported in about 85 percent of patients, with ventricular hypertrophy, heart failure and valve abnormalities also common. Patients had typically taken the drug for many years, and a large share were women, which reflects who takes the medicine for lupus and rheumatoid arthritis rather than any proven sex-specific susceptibility. Stopping the drug was followed by improvement in some patients, but a substantial proportion had irreversible damage or died. A later review in the same journal in 2018, drawing on a broader search of published reports and pharmacovigilance data, reached similar conclusions. Because published cases tend to be the more severe and dramatic ones, these figures describe recognised disease, not the risk for a typical patient.

Factors that appear in these reports again and again include:

The retinal parallel is instructive. A large Kaiser Permanente study published in JAMA Ophthalmology in 2014 found retinal toxicity in about 7.5 percent of long-term users overall, rising to roughly 20 percent after 20 years. This finding led the American Academy of Ophthalmology in 2016 to recommend daily doses of no more than 5 mg per kilogram of actual body weight. The same lysosomal process drives both conditions, so a patient with retinal toxicity deserves a thoughtful look at the heart, and the reverse is also true.

Conduction Abnormalities and Other Signs to Watch For

Because the electrical system is often affected first, the humble electrocardiogram is one of the most useful screening tools. In someone who has taken hydroxychloroquine for years, the following findings justify a closer look, especially if they are new compared with an earlier tracing:

Symptoms, when present, are those of heart block or heart failure: fainting or near-fainting, unusual fatigue, breathlessness on exertion, swelling of the ankles, or reduced exercise tolerance. These are nonspecific and have many causes, which is exactly why a patient's own knowledge of the medicines they take is so valuable. A person who can tell a new cardiologist, "I have been on hydroxychloroquine for twelve years," may shorten the path to the right diagnosis considerably.

Further testing usually includes an echocardiogram, which may show thickened ventricular walls, enlarged atria and impaired filling. Blood tests may show persistently raised troponin or natriuretic peptides, and creatine kinase may be elevated if skeletal muscle is also involved. Cardiac MRI can characterise the thickened muscle, show patterns of scarring and help rule out other causes. However, no imaging pattern is specific enough to settle the question alone. Current guidance does not call for routine cardiac screening of every long-term user. Many clinicians do, however, consider a baseline ECG and periodic reassessment reasonable in higher-risk patients. That is a conversation worth having with one's own physician.

How Endomyocardial Biopsy Confirms the Diagnosis

The central diagnostic problem is that several conditions produce a thick, stiff heart with conduction disease. The most important alternatives are cardiac amyloidosis, Fabry disease (an inherited lysosomal storage disorder), other glycogen or lysosomal storage diseases, sarcoidosis and hypertensive or hypertrophic heart disease. Since the treatments differ completely, and since stopping a drug that controls lupus is not a trivial decision, tissue confirmation is often sought.

In an endomyocardial biopsy, a cardiologist passes a thin catheter-mounted instrument, a bioptome, through a vein in the neck or groin into the right ventricle and takes several tiny samples from the septal wall. In experienced centres the procedure carries a low but real risk of complications such as perforation or arrhythmia, which is part of the informed-consent discussion.

Under the ordinary light microscope, affected heart cells look swollen and vacuolated, with a lacy, bubbly appearance. Special stains are used to exclude amyloid. The decisive findings, however, come from electron microscopy, which shows two characteristic types of inclusion:

A small pathology series from the Mayo Clinic, published in Cardiovascular Pathology in 2002, compared chloroquine cardiotoxicity with Fabry disease and highlighted curvilinear bodies as a key distinguishing feature. Because the two conditions can look alike, many experts also recommend measuring alpha-galactosidase A activity or performing genetic testing for Fabry disease, especially in women, whose enzyme levels can appear normal despite carrying the condition. When biopsy is not feasible, the diagnosis rests on the overall picture: long exposure, compatible imaging, exclusion of alternatives and, in some cases, improvement after the drug is stopped.

Management centres on discontinuing hydroxychloroquine under specialist guidance, treating heart failure, and implanting a pacemaker when heart block is advanced. Recovery, when it occurs, can take months because the drug leaves tissues slowly. Some patients improve substantially, while others do not, and in rare cases heart transplantation has been required. The underlying autoimmune disease must then be controlled by other means, which calls for close cooperation between rheumatologist and cardiologist.

Key takeaway: Hydroxychloroquine cardiomyopathy is rare but real, arising when years of lysosomal drug accumulation stiffen the heart and disrupt its wiring, so long-term users and their physicians should watch for new conduction changes, keep doses within weight-based limits, and rely on biopsy with electron microscopy when the diagnosis must be certain.