The paper presents a multicenter longitudinal study of 233 consecutive patients with clear-cut diagnosis by type of the three main forms of cardiac amyloidosis, seen at 2 large Italian centers (Bologna and Pavia) providing coordinated amyloidosis diagnosis/management facilities since 1990. The study enrolled 157 patients with (AL), 61 patients with ATTRm, 15 patients ATTRwt. The three groups of patients were compared in terms of clinical and instrumental (echocardiographic and electrocardiographic) profiles at baseline and in term of clinical outcome. In about 40% of study population hemodynamic data and endomyocardial biopsy data were also available for comparisons. Diagnosis of amyloidosis was defined by histological documentation of Congo Red staining and apple-green birefringence under cross-polarized light in at least 1 involved organ. Amyloidotic cardiomyopathy was defined echocardiographically as end-diastolic thickness of the interventricular septum >1.2 cm in the absence of any other cause of ventricular hypertrophy. Clear-cut distinction between TTR-related and AL amyloidosis was based on genotyping and/or immunohistochemistry.
In about 65% of cases, amyloidotic cardiomyopathy was detected at routine echocardiographic screening after diagnosis of systemic amyloidosis. In <2% of cases (2 AL; 1 ATTRwt) diagnosis of cardiac systemic amyloidosis was incidental. In the remaining 34% of cases, patients were initially diagnosed as having hypertrophic cardiomyopathy, heart failure, or arrhythmia at a secondary or tertiary cardiological center.
At presentation, the 3 groups of patients showed some expected clinical differences, including high prevalence of neurological impairment and carpal tunnel syndrome in ATTRm, kidney involvement in AL, and heart failure in both AL and ATTRwt (a disease thought to be confined to elderly men).
Regarding ECG, at univariate analysis left bundle branch block was more frequent in ATTRwt; ATTRm patients less often displayed low QRS voltage (25% versus 60% in AL; P<0.0001) or low voltage-to-mass ratio (1.1±0.5 versus 0.9±0.5; P<0.0001). At multivariate analysis low QRS voltages were negatively associated with ATTRm.
Highly significant differences were apparent for most echocardiographic morphological and functional descriptors: morphologically, ATTRwt group showed the highest average values, with 3 to 4 mm greater mean LV wall thickness (diastolic interventricular septum thickness 19.7±4 mm) than in ATTRm (16.6±3.8 mm) or AL (15.8±2.8 mm), p <0.0001; LV ejection fraction values varied considerably, tending to be normal in ATTRm (58±13%), around lower normal limits in AL (52.5±13%), and abnormally low in ATTRwt (44.2±15.4, p<0.0001).
The different etiologies showed relevant hemodynamic differences, with AL patients most often displaying abnormal values in the different measures of diastolic function.
With regard to clinical outcome, in terms of both overall and MACE-free survival, the group with the least morphological impairment— AL— had a rather aggressive clinical course; the group that showed the greatest LV wall thickness— ATTRwt — showed a less aggressive course despite the patients’ higher average age. On multivariate analysis, ATTRm was a strongly favorable predictor of survival, and ATTRwt predicted freedom from major cardiac events.
Comment
The paper regards the largest available follow-up study of cardiac amyloidosis and represents a relevant contribution at different levels ranging from disease classification to diagnosis and clinical management.
From a nosographic point of view, this study supports the concept that cardiomyopathies due to AL, ATTRm and ATTRwt should be considered three different cardiac diseases with different pathophysiologic substrates and clinical courses. Cardiac amyloidosis is commonly considered a form of restrictive cardiomyopathy (i.e., a myocardial disease with increased parietal stiffness, causing precipitous rises in ventricular pressure accompanied by only small increases in volume). Nevertheless, as many as one-fifth of the hemodynamically evaluated patients did not display any abnormal finding, and the majority of the overall patients did not display restrictive filling pattern, which is traditionally considered the key non-invasive marker of restrictive pathophysiology. Sub analysis of baseline hemodynamic data highlights the differences among the three etiologic forms, with only AL patients displaying abnormal values in the different measures of diastolic function in the vast majority of cases. The higher frequency of hemodynamic impairment in the AL patients contrasts with their lesser morphological involvement. So, within a group of infiltrative cardiomyopathies that are traditionally considered “restrictive”, the degree of infiltration (assessed by increased wall thickness) does not seem to be associated with the severity of restrictive hemodynamic impairment. This mismatch could plausibly be attributed to the well-documented direct toxicity of the immunoglobulin circulating immunoglobulin light-chains in AL, along with other plausible contributory factors.
For instance, it is reasonable to hypothesize that a higher frequency of vascular localization of amyloid deposition in AL could be responsible for myocardial ischemia, contributing to ventricular dysfunction. Furthermore, gradual deposition in the TTR-related forms might allow the organism time to develop local compensatory mechanisms (a rather less likely scenario in the rapidly developing amyloidotic cardiomyopathy of AL patients). Different types of amyloid substance could also lead to different degrees of myocardial damage (unfortunately, the study did not explore this aspect). Taken together, these observations may also explain why the clinical outcome of the three groups of patients, in terms of both overall and MACE-free survival, appeared to contrast with the degree of morphological involvement, where the group with the least morphological derangement (AL) had a rather aggressive clinical course. In contrast, the group that showed the greatest LV wall thickness values (ATTRwt) seemed to have a less aggressive course.
From a clinical point of view, a particularly important observation of the study regards standard ECG. Although the presence of low QRS voltages is considered a key player to orient diagnostic suspicion of cardiac amyloidosis, the prevalence of low QRS voltages at the time of diagnosis was lower than in other reports (~45% overall) and particularly low in the ATTRm subset (25%), despite a greater myocardial infiltration (as indicated by mean ventricular wall thickness values). A possible explanation for this finding could be greater myocardial cellular damage (regardless of wall thickness) induced by light-chain toxicity in AL. Interestingly the occurrence of left bundle-branch block was relatively frequent in ATTRwt type (up to 40% of cases). Appropriately, the study underscores the importance of not excluding a diagnosis of amyloidotic cardiomyopathy (especially of TTR-related forms) on the grounds of normal QRS voltage or left bundle-branch block.
Conclusion:
To sum up, the paper underscores the profound differences between the three most frequent etiological types of cardiac amyloidosis in terms of disease profile and long-term outcome. Awareness of this heterogeneity may help orient aspects of the diagnostic workup of patients with suspected cardiac amyloidosis and subsequent clinical management.
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