Highlights
Myocardial fibrosis burden emerges as a significant predictor of adverse outcomes in asymptomatic patients with severe aortic stenosis, including increased risk of death and unplanned hospitalizations. Early intervention with transcatheter or surgical aortic valve replacement demonstrates a notable reduction in aortic stenosis-related hospitalizations specifically among patients with high fibrosis burden above the median. However, the EVOLVED trial finds no statistically significant heterogeneity in overall treatment effects based on fibrosis degree, challenging the notion that myocardial fibrosis is the definitive marker for timing of intervention in all patients.
Background: The Clinical Challenge of Asymptomatic Aortic Stenosis
Severe aortic stenosis represents one of the most prevalent valvular heart diseases globally, particularly affecting elderly populations. While symptomatic patients with aortic stenosis have well-established indications for valve replacement, the management of asymptomatic individuals remains considerably more nuanced. The traditional approach has emphasized watchful waiting until symptom onset, yet accumulating evidence suggests that irreversible myocardial damage may occur during this asymptomatic phase, potentially diminishing the benefits of subsequent intervention.
Myocardial fibrosis has emerged as a critical pathological hallmark in aortic stenosis pathophysiology. The progressive pressure overload imposed by the stenotic valve triggers a cascade of adaptive mechanisms within the myocardium, including the development of diffuse and focal fibrotic changes. Cardiac magnetic resonance imaging has become the gold standard for detecting and quantifying these fibrotic changes, particularly midwall fibrosis, which represents interstitial fibrosis that develops in the myocardial layer between the epicardium and endocardium.
Previous observational studies have consistently demonstrated associations between myocardial fibrosis burden and adverse clinical outcomes in symptomatic aortic stenosis patients. However, whether these associations hold true in asymptomatic populations, and critically, whether fibrosis burden can serve as a biomarker to guide timing of intervention in patients who have not yet developed symptoms, remained substantively unanswered questions that the EVOLVED trial sought to address.
Study Design and Population
The EVOLVED trial conducted a post hoc analysis of a randomized clinical trial performed across 24 cardiac centers in the United Kingdom and Australia between August 2017 and October 2022. The study enrolled asymptomatic patients with severe aortic stenosis who demonstrated evidence of midwall fibrosis on cardiac magnetic resonance imaging. Severe aortic stenosis was defined by aortic valve peak velocity, with participants in this analysis having a mean peak velocity of 4.3 meters per second with a standard deviation of 0.5 meters per second.
The study population consisted of 224 participants with a mean age of 73 years (standard deviation 9 years), comprising 63 women and 161 men. All participants were asymptomatic at enrollment, having no reported exertional symptoms such as dyspnea, angina, or syncope that would typically prompt clinical intervention. Randomization occurred in a 1:1 fashion to either early intervention with transcatheter aortic valve replacement or surgical aortic valve replacement, versus guideline-directed conservative management involving clinical surveillance with eventual intervention upon symptom development.
The primary outcome of interest was a composite of all-cause death or unplanned aortic stenosis-related hospitalization. Secondary outcomes included the individual components of this composite endpoint. Participants were followed for a median duration of 42 months, with data analysis performed between October 2024 and June 2025.
Key Findings: Fibrosis Burden and Treatment Effects
The primary analysis revealed that fibrosis burden, expressed as a continuous variable per 1% increase, was significantly associated with the primary composite endpoint of all-cause death or unplanned aortic stenosis-related hospitalization. The hazard ratio of 1.23 with a 95% confidence interval of 1.08 to 1.37 indicates that each percentage point increase in fibrosis burden corresponded to a 23% increase in the hazard of experiencing the primary outcome. This association was primarily driven by the hospitalization component, with a hazard ratio of 1.22 (95% CI, 1.03-1.40) for unplanned aortic stenosis-related hospitalizations. Notably, the association with all-cause death alone did not reach statistical significance, with a hazard ratio of 1.17 (95% CI, 0.98-1.35).
A critical finding of the study was the lack of statistically significant interaction between randomization arm and midwall fibrosis burden for either the primary endpoint (P for interaction = 0.39) or secondary endpoints. This suggests that the treatment effect of early intervention compared to conservative management did not fundamentally differ based on the degree of myocardial fibrosis present.
However, when examining the data through a prespecified subgroup analysis comparing patients above and below the median fibrosis burden, intriguing patterns emerged. In the high fibrosis burden subgroup (fibrosis above the median), the primary endpoint occurred in 12 of 59 patients (20%) randomized to early intervention compared to 17 of 53 patients (32%) randomized to conservative management, yielding a hazard ratio of 0.62 (95% CI, 0.29-1.28). While this trend favored early intervention, the wide confidence interval crossing unity indicates this difference was not statistically significant.
Examining the individual components within the high fibrosis subgroup provides particularly compelling insights. For all-cause death alone, event rates were 15% (9 patients) in the early intervention group versus 19% (10 patients) in the conservative management group, with a hazard ratio of 0.84 (95% CI, 0.33-2.07). The difference in mortality between treatment arms did not achieve statistical significance. In contrast, for unplanned aortic stenosis-related hospitalization, the benefit of early intervention became more apparent, with events occurring in only 4 patients (7%) in the early intervention group compared to 13 patients (25%) in the conservative management group, yielding a hazard ratio of 0.27 (95% CI, 0.08-0.77). This represents a statistically significant 73% reduction in hospitalization risk with early intervention in patients with high fibrosis burden.
In stark contrast, patients with low fibrosis burden below the median demonstrated no meaningful differences between treatment strategies. The hazard ratio for the primary outcome in this subgroup was 1.05 (95% CI, 0.39-2.86), effectively indicating equivalent outcomes regardless of whether patients underwent early intervention or were managed conservatively. Similarly, no significant differences were observed for the individual components of death or hospitalization in the low fibrosis subgroup.
Expert Commentary: Interpreting the Evidence
The EVOLVED findings present a nuanced picture that challenges simplistic interpretations of myocardial fibrosis as a universal decision-making biomarker. While the clear association between fibrosis burden and adverse outcomes confirms the pathological significance of fibrotic changes in asymptomatic aortic stenosis, the absence of a statistically significant interaction term suggests that fibrosis quantification alone may not be sufficient to identify all patients who would benefit from early intervention.
The differential effect observed for hospitalization versus mortality merits careful consideration. The significant reduction in unplanned aortic stenosis-related hospitalizations among high-fibrosis patients receiving early intervention may reflect the ability of valve replacement to prevent the progression to symptomatic disease that precipitates hospitalization. This finding aligns with physiological expectations—patients with established myocardial damage may be more likely to decompensate when challenged with the additional hemodynamic burden of uncorrected severe stenosis.
However, the lack of clear mortality benefit, even in the high-fibrosis subgroup, introduces important questions about the natural history of myocardial recovery following valve replacement and the timecourse over which procedural benefits manifest. With a median follow-up of 42 months, the study may have been underpowered to detect modest but clinically meaningful differences in survival, particularly given the relatively low event rates observed.
Several limitations warrant acknowledgment in interpreting these results. The post hoc nature of the analysis, while employing prospectively collected trial data, introduces inherent limitations in causal inference. The sample size of 224 participants, while adequate for demonstrating associations with the primary outcome, may have been insufficient to detect subgroup-specific differences with precision, as evidenced by the wide confidence intervals around key estimates. Additionally, the population consisted predominantly of male participants (161 versus 63 women), which may limit generalizability to female patients with potentially different fibrotic remodeling patterns.
The clinical implications suggest a potential role for cardiac magnetic resonance fibrosis quantification in risk stratification of asymptomatic severe aortic stenosis patients, particularly for predicting hospitalization risk. However, the absence of a clear interaction effect means that current guideline-based indications for intervention in truly asymptomatic patients should not be altered based solely on fibrosis burden findings from this analysis.
Conclusion
The EVOLVED trial provides important mechanistic insights into the relationship between myocardial fibrosis and clinical outcomes in asymptomatic severe aortic stenosis. Higher midwall fibrosis burden clearly predicts worse outcomes, with each incremental increase in fibrosis percentage conferring approximately 23% greater hazard for the composite of death or hospitalization. Early intervention demonstrated substantial benefit for preventing hospitalizations in patients with high fibrosis burden, with a 73% relative risk reduction, yet this did not translate into significantly improved survival or a universally demonstrable treatment-by-fibrosis interaction.
These findings suggest that while myocardial fibrosis identifies a higher-risk phenotype among asymptomatic aortic stenosis patients, the decision to pursue early intervention remains complex and should consider multiple factors beyond fibrosis quantification alone. Future research should focus on longer-term follow-up to assess whether mortality benefits emerge over extended time periods, and whether combining fibrosis assessment with additional biomarkers or imaging parameters might better identify patients most likely to benefit from early intervention. The current evidence supports incorporating cardiac magnetic resonance assessment of fibrosis into comprehensive risk evaluation of asymptomatic severe aortic stenosis patients, though this should complement rather than replace clinical judgment and existing guideline recommendations.
Funding and ClinicalTrials.gov
ClinicalTrials.gov Identifier: NCT03094143. The EVOLVED investigators acknowledge the collaborative funding support from research institutions across the United Kingdom and Australia that made this multicenter trial possible.
References
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4. Everett RJ, Tastet L, Clavel MA, et al. Progression of hypertrophy and myocardial fibrosis in aortic stenosis: a multicenter cardiac magnetic resonance study. Circ Cardiovasc Imaging. 2018.
