Abstract
Purpose
To compare transthoracic echocardiography (TTE) and cardiac MRI measurements of left ventricular mass (LVM) and maximum wall thickness (MWT) in patients with Fabry disease and evaluate the clinical significance of discrepancies between modalities.
Materials and Methods
Seventy-eight patients with Fabry disease (mean age, 46 years ± 14 [standard deviation]; 63% female) who underwent TTE and cardiac MRI within a 6-month interval between 2008 and 2018 were included in this retrospective cohort study. The clinical significance of measurement discrepancies was evaluated with respect to diagnosis of left ventricular hypertrophy (LVH), eligibility for disease-specific therapy, and prognosis. Statistical analysis included paired-sample t test, Cox proportional hazard models, Akaike information criterion (AIC), and intraclass correlation coefficients.
Results
LVM indexed to body surface area (LVMI) and MWT were significantly higher at TTE compared with MRI (105 g/m2 ± 48 vs 78 g/m2 ± 36, P < .001 and 14 mm ± 4 vs 13 mm ± 5, P = .008, respectively). LVH classification was discordant between modalities in 23 patients (29%) (P < .001). Eligibility for disease-specific therapy based on MWT was discordant between modalities in 20 patients (26%) (P < .001). LVMI assessed with MRI was a better predictor of the combined endpoint compared with LVMI assessed with TTE (AIC, 127 vs 131). Interobserver agreement for LVMI and MWT was higher for MRI (intraclass correlation coefficient, 0.951 and 0.912, respectively) compared with TTE (intraclass correlation coefficient, 0.940 and 0.871; respectively).
Conclusion
TTE overestimates LVM and MWT and has lower reproducibility compared with cardiac MRI in Fabry disease. Measurement discrepancies between modalities are clinically significant with respect to diagnosis of LVH, prognosis, and treatment decisions.
© RSNA, 2020
Summary
Measurement discrepancies in left ventricular mass and maximum wall thickness between transthoracic echocardiography and cardiac MRI are clinically significant in patients with Fabry disease.
Key Points
■ Transthoracic echocardiography overestimates left ventricular mass and maximum wall thickness and has lower reproducibility compared with cardiac MRI in patients with Fabry disease.
■ Measurement discrepancies of left ventricular mass and maximum wall thickness between modalities are clinically significant in patients with Fabry disease because use of one modality over the other could affect the diagnosis of left ventricular hypertrophy in 29% of patients, eligibility for disease-specific therapy in 26% of patients, and prognosis.
Introduction
Fabry disease is an X-linked lysosomal storage disorder that results in accumulation of glycosphingolipids in multiple organs, including the heart. Cardiac involvement is the leading cause of mortality in patients with Fabry disease and is characterized by progressive left ventricular hypertrophy (LVH), myocardial inflammation, and fibrosis (1).
Disease-specific therapy is available with biweekly intravenous infusion of the missing enzyme or oral pharmacologic chaperone therapy. Treatment has been shown to slow progression of disease and result in improvement of existing changes if initiated before the development of irreversible organ damage (2). However, disease-specific therapy is expensive, and there are potential risks including development of infusion reactions, thus justification is needed to start treatment (3). LVH has been identified as an important predictor of adverse cardiac events for men and women with Fabry disease (4). Therefore, accurate and reproducible assessment of LVH is important in Fabry disease for early detection of cardiac involvement, assessment for treatment eligibility, and prognostication (4–6).
Transthoracic echocardiography (TTE) and cardiac MRI are both used in routine clinical practice to evaluate for LVH in patients with Fabry disease, including assessment of left ventricular mass (LVM) and maximum wall thickness (MWT) (5). However, discordance between TTE and cardiac MRI measurements of LVM and MWT have been reported in patients with other conditions, including hypertrophic cardiomyopathy and aortic stenosis (7–10). A prior study in a small cohort of patients demonstrated that TTE overestimates LVM compared with cardiac MRI in Fabry disease (11).
The purpose of our study was to compare TTE and cardiac MRI measurements of LVM, LVH, and MWT in patients with Fabry disease and to evaluate the clinical significance of discrepancies between modalities. We hypothesized that there would be clinically significant differences in LVM, LVH, and MWT measurements between modalities.
Materials and Methods
Patient Population
This retrospective cohort study was approved by the institutional research ethics board. The requirement for written informed consent was waived. All patients with gene-positive Fabry disease who had undergone both cardiac MRI and TTE at our institution within a 6-month interval between March 2008 and March 2018 were included in this study. Clinical outcomes in relation to LVH and late gadolinium enhancement (LGE) assessed with cardiac MRI have previously been reported in all patients included in the current study (4,12). The current study expands on this analysis by comparing the prognostic significance of LVM measurements assessed by using TTE and cardiac MRI.
Cardiac MRI
Cardiac MRI studies were performed using 1.5-T (MAGNETOM Avanto or Avanto fit; Siemens Healthineers, Erlangen, Germany) or 3-T (MAGNETOM Skyra or Skyra fit; Siemens Healthineers) scanners. Retrospectively gated cine steady-state free precession (SSFP) images were obtained in multiple planes including a stack of short-axis slices with coverage from the left ventricular (LV) base to the apex (6–8-mm slice thickness, 0–2-mm interslice gap, and temporal resolution of 30–40 msec). LGE imaging was performed 12–15 minutes following administration of intravenous contrast material (0.15–0.20 mmol per kilogram of body weight of gadobutrol [Gadovist; Bayer Healthcare, Berlin, Germany] or gadopentetate dimeglumine [Magnevist; Bayer Healthcare Pharmaceuticals, Berlin, Germany]) using a two-dimensional inversion recovery gradient-recalled echo sequence in multiple planes including a short-axis stack (slice thickness of 6–8 mm and 0–2-mm interslice gap).
Cardiac MRI analysis was performed by a fellowship-trained radiologist (C.O., 2 years of cardiac imaging experience) blinded to all clinical and identifying information using commercially available software (Circle cmr42 release 5.9; Circle Cardiovascular Imaging, Calgary, Alberta, Canada) according to current guidelines (13). LV endocardial and epicardial borders were contoured on short-axis SSFP images to assess for LV volumes, ejection fraction, and mass (Fig 1, A). LVM was assessed at end diastole and was indexed to body surface area (LVMI) calculated using the Mosteller formula (14). Papillary muscles were excluded from LVM. LV end-diastolic MWT was visually assessed and was measured on short-axis cine SSFP images at the thickest segment perpendicular to the LV long axis (Fig 1, B). LVH at cardiac MRI was defined as LVMI greater than 72 g/m2 in men and greater than 55 g/m2 in women (15). All LGE images were visually assessed for the presence of LGE.
Figure 1:
Midventricular short-axis cine steady-state free precession cardiac MR images in a 47-year-old man with Fabry disease demonstrate left ventricular (LV) mass and maximum wall thickness measurements. A, LV endocardial contour (red) and epicardial contour (green) for LV mass quantification. B, Maximum LV wall thickness measurement (yellow).
Transthoracic Echocardiography
TTE studies were scanned using standard commercially available two-dimensional imaging platforms (Philips Epiq and IE 33, Philips Healthcare; GE E95, E9 and Vivid7, General Electric; and Siemens Acuson SC2000, Siemens Healthcare) with 2.5-MHz and 3.5-MHz transducers according to current guidelines (16). TTE analysis was performed by a fellowship-trained cardiologist (I.B., 2 years of cardiac imaging experience) blinded to all clinical and identifying information including sonographer measurements using commercially available software (Syngo Dynamics, VA20E_HF02; Siemens Healthcare, Erlangen, Germany). Linear LV dimensions, including interventricular septum diameter (IVS), LV end-diastolic dimension (LVEDD), and posterior wall thickness (PWT) were measured as per current guidelines in the parasternal long-axis view at end diastole (Fig 2, A) (16). LVM was calculated according to the method recommended by the American Society of Echocardiography (ASE), using the following formula: LVM = 0.8 ∙ 1.04 ∙ ([IVS + LVEDD + PWT]3 – LVEDD3) + 0.6, and was indexed to body surface area calculated using the Mosteller formula (16). LVH at TTE was defined as LVMI greater than 115 g/m2 in men and greater than 95 g/m2 in women (16). LV end-diastolic MWT was visually assessed on all available images and was measured at the thickest segment perpendicular to the LV long axis (Fig 2, B).
Figure 2:
Transthoracic echocardiographic images in a 47-year-old man with Fabry disease demonstrate left ventricular (LV) mass and maximum wall thickness measurements. A, Parasternal long-axis image at end diastole demonstrates linear echocardiographic measurements at the basal level of the LV: interventricular septum diameter (1), LV end-diastolic diameter (2), and posterior wall thickness (3). B, Midventricular parasternal short-axis image demonstrates LV wall thickness measurements.
Intra- and Interobserver Agreement
To assess intraobserver agreement for LVM and MWT measurements, a random subset of 20 studies was reanalyzed by the same cardiac MRI and TTE readers following a minimum 4-week interval after the first analysis, blinded to the results of the initial assessment and all identifying data. To assess interobserver agreement for LVM and MWT measurements, measurement values recorded in MRI and TTE clinical reports were compared with the initial set of blinded study measurements. All clinical MRI and TTE studies were reported by experienced cardiologists or radiologists with dedicated cardiovascular imaging experience.
Clinical Significance
The clinical significance of measurement discrepancies between modalities was evaluated with respect to diagnosis of LVH, eligibility for disease-specific therapy, and prognosis. We assessed whether differences in LVM measurements would affect classification of LVH and prognosis given that LVH is typically defined based on LVMI above a normal reference range and LVMI is associated with increased risk of adverse cardiac events in patients with Fabry disease (4). We also assessed whether differences in MWT measurements could theoretically affect eligibility for disease-specific therapy given that most treatment guidelines include increased MWT as a criterion for initiation of therapy (6,17). With respect to eligibility for disease-specific therapy, we evaluated discrepancies between modalities at MWT cut points of greater than 12 mm in men and greater than 11 mm in women for both MRI and TTE (17). To evaluate prognostic significance, the association between LVMI and adverse cardiac events assessed as a composite endpoint was evaluated for both TTE and MRI. The composite endpoint was defined by the development of one or more of the following events: (a) nonsustained or sustained ventricular tachycardia, defined as three or more consecutive beats arising below the atrioventricular node with an R-R interval of greater than 100 beats per minute lasting less than 30 seconds or 30 seconds or longer, respectively; (b) bradycardia, defined as a heart rate of less than 60 beats per minute requiring device implantation for pacing; (c) severe heart failure, defined as the development of New York Heart Association functional class III or IV symptoms; or (d) cardiac death, classified as sudden or heart failure–related death, as described previously (4,12). Only new events from the time of cardiac MRI and TTE were considered in outcome analyses. Patients without events were censored at the time of their last clinical follow-up.
Statistical Analysis
Statistical analysis was performed using STATA v14.1 (Stata, College Station, Tex). A two-tailed P value of < .05 was considered statistically significant. Continuous variables were described using mean and standard deviation and categorical variables using numbers and percentage. The normality of all continuous data was evaluated graphically and using the Shapiro-Wilk test. Comparisons between MRI and TTE were made by paired-sample t test for continuous variables with normal distribution, Wilcoxon signed rank test for continuous variables with nonnormal distribution, and Fisher exact test for categorical variables. We also performed sensitivity analysis, restricting analysis to the subgroup of patients with no LGE. Time-to-event survival analysis using Cox proportional hazard models was used to evaluate the effect of LVMI assessed with TTE and MRI on time to the composite endpoint. The Akaike information criterion (AIC) was used to compare model fit between predictors, with smaller AIC values indicating better model fit. Bias and precision between techniques were evaluated using Bland-Altman plots. Intra- and interobserver agreement were assessed using individual intraclass correlation coefficients with one-way random effects models and two-way random effects models, respectively.
Results
Clinical and imaging information is summarized in Table 1. A total of 78 patients were included (mean age, 46 years ± 14 [standard deviation]; 63% female). The median interval between MRI and TTE was 29 days (interquartile range: 17–76 days). There was no significant difference in heart rate between MRI and TEE (65 beats per minute ± 11 vs 68 beats per minute ± 14, P = .15). No patient had disease-specific therapy initiated in the interval between MRI and TTE. LGE was present in 31 of 76 patients (41%) who had undergone LGE imaging.
Table 1:
Clinical and Imaging Characteristics
Difference in LVM
LVMI was significantly higher at TTE compared with MRI (105 g/m2 ± 48 vs 78 g/m2 ± 36, P < .001). On Bland-Altman analysis, mean LVMI difference was 27 g/m2 (95% confidence interval [CI]: −28 g/m2, 82 g/m2) between TTE and MRI, with greater discrepancy with increasing LVMI (Fig 3, A).
Figure 3:
Bland-Altman plots of the mean differences between transthoracic echocardiography (TTE) and cardiac MRI measurements. A, Left ventricular mass indexed to body surface area (LVMI) comparing TTE and cardiac MRI. B, Maximum left ventricular wall thickness (MWT) comparing TTE and cardiac MRI. The average of measurements from both modalities is plotted on the x-axis and the difference between modalities is plotted on the y-axis. The solid red horizontal line plots the mean difference and the solid black lines indicate the limits of agreement (differences from the mean of 1.96 standard deviations) for each parameter.
Difference in MWT
MWT was significantly higher at TTE compared with MRI (14 mm ± 4 vs 13 mm ± 5, P = .008). MWT was identical between TTE and MRI in four patients (5%), larger at TTE compared with MRI in 48 patients (62%), and smaller at TTE compared with MRI in 26 patients (33%). On Bland-Altman analysis, mean MWT difference between TTE and MRI was 0.7 mm (95% CI: −3.9 mm, 5.2 mm), with relatively equal distribution of discrepancy across the range of measured MWT values (Fig 3, B).
Clinical Significance: Diagnosis of LVH
Significant differences existed in the number of patients classified as having LVH between modalities. Thirty-four patients met criteria for LVH at TTE (44%), compared with 53 at MRI (68%, P < .001). Twenty-three patients (29%) had a discordant classification of LVH between TTE and MRI (two met criteria at TTE but not MRI and 21 met criteria at MRI but not TTE). More patients met criteria for LVH at MRI despite the fact that LVMI was higher at TTE because of differences in normal reference ranges between modalities.
This discrepancy remained significant in the subgroup of patients with no LGE. In this subgroup, 16 of 45 (36%) had a discordant classification of LVH between TTE and MRI (one met criteria at TTE but not MRI and 15 met criteria at MRI but not TTE) (P = .02).
Clinical Significance: Eligibility for Treatment
Significant differences existed in the number of patients meeting eligibility criteria for treatment based on MWT (P < .001). Twenty patients (26%) had discordant recommendations between TTE and MRI (19 patients met treatment criteria based on TTE but not MRI and one patient met treatment criteria based on MRI but not TTE).
This discrepancy remained significant in the subgroup of patients with no LGE. In this subgroup, 16 of 45 (36%) had discordant recommendations between TTE and MRI (15 patients met treatment criteria based on TTE but not MRI and one patient met treatment criteria based on MRI but not TTE) (P = .01).
Clinical Significance: Prognosis
Overall, 20 patients (26%) reached the composite endpoint with an incidence rate of 8.6% per year (median follow-up duration, 3.0 years; interquartile range: 1.3–5.7 years). Nonmutually exclusive events were nonsustained ventricular tachycardia in 18 patients (23%), sustained ventricular tachycardia in one (1%), severe bradycardia in five (6%), heart failure in two (3%), and death in one (1%). LVMI was associated with higher risk for the composite endpoint when assessed with both cardiac MRI (hazard ratio [HR], 1.02; 95% CI: 1.01, 1.03, P < .0001) and TTE (HR, 1.02; 95% CI: 1.01, 1.03, P < .0001). LVMI assessed with cardiac MRI was a better predictor of the combined endpoint compared with LVMI assessed with TTE based on model goodness-of-fit (AIC 127 vs 131).
Intra- and Interobserver Agreement
Intra- and interobserver agreement were good to excellent for LVMI and MWT for both modalities. However, agreement was higher for MRI compared with TTE for all measurements (Table 2).
Table 2:
Intra- and Interobserver Agreement of Echocardiography and Cardiac MRI Measurements
Discussion
The results of this study demonstrated that TTE overestimates LVM and MWT and has lower reproducibility compared with cardiac MRI in patients with Fabry disease. These differences in measurements between modalities were clinically significant with respect to diagnosis of LVH, prognosis, and treatment decisions.
Cardiac MRI is considered the reference standard for assessment of ventricular volumes, function, and mass (18,19). However, TTE measurements of LVM and MWT are often used in clinical practice because of widespread availability and lower cost. Discrepancies between TTE and MRI measures of LVH have been reported in other diseases, with potentially important clinical implications with respect to diagnosis, prognosis, and eligibility for treatment (7–10). However, there are limited data on the agreement between TTE and cardiac MRI measurements of LVH in Fabry disease. Given differences in the pattern and extent of LVH in patients with Fabry disease compared with hypertrophic cardiomyopathy, we sought to evaluate the clinical significance of discrepancies between TTE and MRI in this patient population (20).
Our results are consistent with a few prior studies that demonstrated that TTE tends to overestimate MWT compared with cardiac MRI in patients with hypertrophic cardiomyopathy (8,9,21). Hindieh et al reported mean bias between modalities of 0.5 mm with equal distribution of discrepancy across the range of MWT, similar to our results (8). Phelen et al reported a mean bias between modalities of 1.1 mm when measured at the anteroseptum (21). Corona-Villalobos et al reported even larger discrepancies between modalities with mean biases of 1.1–1.7 mm depending on the MRI measurement technique (9). In contrast, Bois et al reported that MWT was approximately 3 mm larger at cardiac MRI compared with TTE, although values were extracted from clinical reports and therefore a standardized measurement approach may not have been used in all cases (7).
Our findings were also consistent with a prior study that demonstrated that TTE overestimates LVMI compared with MRI in a small cohort of patients with Fabry disease (11). Mean bias between modalities was 16.7 g/m2, which is slightly smaller than the bias we found in our study. TTE has also been shown to overestimate LVMI compared with cardiac MRI in patients with aortic stenosis and LVH (mean bias of 31.2 g/m2 using the ASE method) (10). Overestimation of LVM at TTE is most likely the result of two factors: (a) the ASE method of LVM calculation is based on the geometric assumption of the LV cavity as a prolate ellipsoid of revolution and (b) the ASE method of LVM calculation assumes a symmetric distribution of wall thickening (22). However, these assumptions may not be valid in all hearts, particularly if there is LVH (10,23). Cardiac MRI allows for assessment of LVM without these geometric assumptions and demonstrates better definition of the border between blood pool and endocardium (22). The use of three-dimensional echocardiography or contrast material–enhanced echocardiography may improve measurement accuracy although these techniques may not be routinely available at all centers (9,24,25).
We demonstrated that measurement discrepancies in MWT between modalities could influence treatment decisions for patients with Fabry disease, with potentially detrimental clinical consequences if initiation of disease-specific therapy is delayed (26). We focused our analysis related to treatment decisions on discrepancies in MWT between modalities given that most treatment guidelines include increased MWT as a criterion for initiation of therapy (6,17). However, future research should be performed to clarify the best metric of increased LV wall thickness and/or mass to guide treatment decisions in patients with Fabry disease. We also found significant differences in the number of patients classified as having LVH between modalities, despite using modality and analysis-specific reference ranges. Although LVMI was higher with TTE compared with MRI, the proportion of patients who met criteria for LVH was higher with MRI given that categorization of LVH is dependent on the LVMI reference range used (27). Normal reference ranges for LVMI vary between modalities, with a much higher upper limit of normal for TTE compared with MRI in both men and women (15,16). Measurement discrepancies are also important with respect to prognosis because LVH and increased LVM are associated with elevated risk for adverse cardiac events in the general population and in patients with Fabry disease (4,28–31). Given the high cost of disease-specific therapy per year, use of one modality over another to assess for LVH may also affect health care costs on a population level.
Caution should be used when comparing measurements between modalities over time, given that apparent differences in measurements between modalities may not necessarily reflect actual changes in LVM or MWT. Therefore, consistent use of one technique to assess LVH is recommended for longitudinal follow-up in patients with Fabry disease. When available, MRI may be preferred for evaluation of LVH in Fabry disease given higher reproducibility and the ability to evaluate for myocardial tissue changes in the same study, including LGE, impaired myocardial strain, and sphingolipid deposition using T1 mapping (20,32–34).
Our study had several limitations. As a retrospective study, there were minor differences in cardiac MRI and TTE protocols between studies. Cardiac MRI and TTE studies were performed up to 6 months apart, which could potentially influence results. However, minimal changes are expected in LVM and wall thickness over such a short period. Fabry disease is a relatively rare condition, limiting the number of patients included. The study was performed at a single tertiary referral hospital, and therefore results may not be generalizable to all centers. Finally, a reference standard for assessment of LVM was not included in this study. However, SSFP cardiac MRI has previously been shown to accurately determine LVM compared with pathology-weighted explanted hearts (19).
In conclusion, the results of this study demonstrated that discrepancies between TTE and cardiac MRI measurements of LVM, LVH, and MWT in patients with Fabry disease have potentially important clinical consequences with respect to diagnosis of cardiac involvement, prognostication, and treatment decisions. When available, cardiac MRI is preferred over TTE for assessment of LVH and MWT in patients with Fabry disease because of its higher measurement reproducibility and the additional ability for myocardial tissue characterization.
Disclosures of Conflicts of Interest: C.O. disclosed no relevant relationships. I.B. disclosed no relevant relationships. G.R.K. disclosed no relevant relationships. R.M.I. disclosed no relevant relationships. C.F.M. Activities related to the present article: disclosed no relevant relationships. Activities not related to the present article: consultant for Shire, Amicus, and Genzyme for development of patient educational material (Genzyme), development of rare diseases dashboard (Shire) and advisory board meeting (Amicus); travel to Amicus-sponsored Fabry conference in October 2018. Other relationships: disclosed no relevant relationships. E.T.N. disclosed no relevant relationships. P.T. disclosed no relevant relationships. A.W. disclosed no relevant relationships. K.H. Activities related to the present article: disclosed no relevant relationships. Activities not related to the present article: travel expenses paid by Shire Pharmaceutical to attend Fabry meeting in Athens, Greece, in April 2018. Other relationships: disclosed no relevant relationships.
Abbreviations:
- AIC
- Akaike information criterion
- ASE
- American Society of Echocardiography
- CI
- confidence interval
- IVS
- interventricular septum diameter
- LGE
- late gadolinium enhancement
- LV
- left ventricular
- LVEDD
- left ventricular end diastolic dimension
- LVH
- left ventricular hypertrophy
- LVM
- left ventricular mass
- LVMI
- left ventricular mass index
- MWT
- maximum wall thickness
- PWT
- posterior wall thickness
- SSFP
- steady-state free precession
- TTE
- transthoracic echocardiography
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