Evaluation of the Autonomic Nervous System in Chronic Chagasic Cardiopathy: A Systematic Review of the Literature

Chagas disease is a significant public health problem in most countries in Latin America. Although occurring mainly in rural areas, in the last decades, the disease has spread to non-endemic cities and countries, mainly as a result of migration of infected people.1 An increasing number of cases has been identified in the United States, Spain and other countries, making the diagnosis and management of Chagas disease of growing interest worldwide.1 During an antimalarial campaign in Lassance (Minas Gerais, Brazil) in 1909, Carlos Chagas identified the parasite Trypanosoma cruzi, 648

carried out in the bibliographic bases: Medical Literature Analysis and Retrieval System Online (Medline) via Pubmed, Latin America and Caribbean Health Sciences Literature (Lilacs) and Scientific Eletronic Library Online (SciELO). The studies included in the review were freely available online in the databases used.
The inclusion criteria for the studies were: full-text original articles, the study population included patients with Chagas disease, the study assessed dysautonomia using diagnostic methods of chagasic cardiomyopathy, had clear criteria for patient inclusion, exclusion and discontinuation (if appropriate).
Duplicate articles, studies on patients aged from 0 to 10 years, animals, in vitro experiments, case reports, editorials, theses, dissertations, and other types of articles that did not meet the objective of the research were excluded. First eligible studies were retrieved from the databases. Subsequently, the studies were selected by reading the title and abstract. If necessary, the full text was read to confirm whether the study met the inclusion criteria of the review. According to Proença and Silva,9 this type of survey is considered systematic by using heuristics to eliminate biases resulting from the consultation and use of sources.

Statistical Analysis
The high heterogeneity among the reviewed studies regarding the methods used to assess dysautonomia, the groups of patients studied, and classification of patients with Chagas disease prevented us from performing a meta-analysis. Thus, the description of the findings in the articles was made qualitatively.

Results
Of the 281 articles retrieved from the three databases, 20 articles published between 1990 and 2018 were selected for this review. Nine of the 20 were available in the PubMed database, which indexes the largest number of publications, especially when using cross-descriptors. After analysis for inclusion and exclusion criteria, 10 articles were excluded, and 10 articles composed the study sample. Figure 1 illustrates the search process, and Table 1 describes the main characteristics of the articles included in the systematic review.
Regarding the temporal distribution of the articles, half of the articles were published in the penultimate and the other half in the last ten years (2008-2018).
its transmission vector -triatomines (named differently in each country: kissing bugs, "barbers", vinchuca, bed bugs, etc.) -and described the initial cases of the disease. 2 Although defined as a new morbid entity, Chagas disease was not uncommon, afflicting millions of patients. In Brazil, the main probable forms of transmission are oral transmission (72%), vector transmission (9%) and unknown mode of transmission (18%). 3 In the acute phase of the disease, a large number of parasites is found in the blood, and in the chronic phase, parasitemia decreases while serology becomes positive. Cardiac involvement is considered the most severe manifestation and can affect up to 30% of chronically infected patients. 4 In a study published by Marin-Neto et al., 5 heart rate responses to tilt test were used to evaluate parasympathetic and sympathetic chronotropic control of the heart during the initial 10-second and late 5-minute phases, respectively. Other works have shown that changes in the sympathetic system precede changes in perfusion and contractility. 5 Cardiac autonomic impairment and right heart failure are prominent features of Chagas disease; however, no causal relationship between these phenomena has been defined so far, and the pathophysiology of such manifestations is unclear. 6 Cases of sudden death in asymptomatic patients with fibrosis and denervation have been confirmed by anatomopathological studies. 7 Regional sympathetic denervation was observed in areas without contractile abnormalities in a high percentage of patients and was the first evidence of ventricular sympathetic system disorder in chronic chagasic heart disease. 8 The aim of this article is to review available data on dysautonomia (sympathetic and parasympathetic branches) in chronic chagasic heart disease.

Methods
A systematic review of the literature was performed to assess articles investigating dysautonomia in Chagas heart disease. This type of review is an authorial contribution that presents the status of the literature about a subject. 9-10 The systematic review was carried out in five stages: definition of search strategy, selection of descriptors, definition of inclusion and exclusion criteria, identification of pre-selected and selected studies.
The following keywords were used in the search: ' Chagas' cardiomyopathy', 'arrhythmia' and 'dysautonomia'. These keywords were used in English, Portuguese and Spanish. The search was automated and  Of the 16 articles included in the study, seven (46.6%) were in English, nine in Portuguese, and none in Spanish. This was probably due to the journals selected by the authors to submit their articles. Of the 16 selected publications, four had a qualitative approach.
The diagnosis of chronic Chagas' disease in the studies was established by positivity in two different serological methods. The patients were aged between 18 and 80 years. In most studies, there was no record of the time of disease progression. Most participants of the studies were recruited from a cardiology, cardiac arrhythmia, chagasic heart disease or heart failure outpatient clinics. In the studies by Marin-Neto et al., 4 and Simões et al., 5 patients were selected after testing positive for Chagas disease in a screening among blood donors.
Regarding the study design, there was only one longitudinal prospective study comparing heart rate variability (HRV) assessed by Holter monitoring before and after a cardiac rehabilitation program in patients with Chagas' disease and heart failure. 11 The other studies had a cross-sectional design.
The search for patients in almost all articles was not limited by period in most studies, except in three studies: the study by Landessman et al., 12 in which patients were studied from October 2003 to November 2006; in the study by Souza et al., 11 in which patients were studied between April 2009 and November 2010, and the study by Marino et al. 13 , where patients were selected from March 2014 to February 2016. All selected articles included both men and women, with no restrictions on gender. Among the methods used for assessment of autonomic denervation of patients with Chagas disease, Marin-Neto et al., 6 submitted the participants to the Valsalva maneuver, tilt test with head elevation and assessment of baroreflex sensitivity through intravenous injection of phenylephrine. Landessman et al., 12 and Marino et al., 13 used myocardial scintigraphy with metaiodobenzylguanidine (MIBG) to verify denervation of autonomic nervous system. Simões et al., 5 Miranda et al., 14 Barizon et al., 15 and Gadioli et al., 16 compared myocardial scintigraphy with MIBG to assess denervation, and myocardial perfusion scintigraphy using the sestamibi technetium (Tc-MIBI). In order to correlate the areas of myocardial fibrosis with the anomalous extension of the myocardial denervation Regarding the results, one histopathology study compared patients with heart failure caused by Chagas RIBEIRO  disease or not. Both groups showed sympathetic and parasympathetic denervation. Parasympathetic denervation was more evident in the group with Chagas disease than in those without Chagas disease, while the degree of sympathetic denervation was similar in both groups. 7 The two studies that used Holter monitoring involved different populations: one included only patients with heart failure 11 and the other included patients with different stages of Chagas disease and left ventricular (LV) systolic dysfunction and one control group. 17 The first study performed a longitudinal analysis of patients and found no changes in dysautonomia after a cardiac rehabilitation program. 11 The second study found that, after adjusting for covariates, the values of the short-term 24-hour HRV index were consistently lower in groups with Chagas disease. The values of the beta slope index (derived from the analysis of the HRV power law) were also lower in the groups with Chagas disease (group 1: normal echocardiograms, group 2: segmental alteration on the echocardiogram without LV systolic dysfunction, group 3: dysfunction LV systolic) than controls. This decomposition of the long-range fractal correlation of the RR interval dynamics, a strong predictor of mortality in other cardiomyopathies, may reflect cardiac dysautonomia that may have been detected in the long-term analysis in the time domain.
The study that used Valsalva's maneuver and tilt test showed that there were no significant differences between controls and patients with indeterminate form of Chagas disease, while patients with the digestive form had dysautonomia. This study did not include patients with cardiac form. 6 The studies that used MIBG myocardial scintigraphy had a heterogeneous study population, since one included patients with indeterminate Chagas disease 12 and the other patients with heart failure. 13 The first study suggested the presence of sympathetic denervation in patients with the indeterminate form 12 and the second failed to demonstrate a higher degree of sympathetic denervation in patients with Chagas' disease and heart failure than in patients with heart failure of other etiologies. 13 Four studies used myocardial scintigraphy with MIBG to assess denervation, and MIBI to assess myocardial perfusion. The first also included MRI to assess patients with cardiac form of Chagas disease. 15 This study demonstrated a strong correlation between areas of denervation, hypoperfusion and fibrosis, but the areas of denervation were larger than areas of hypoperfusion or fibrosis. 15 Another study included patients with indeterminate form and cardiac form with or without systolic dysfunction and demonstrated that changes in sympathetic denervation occurred early in Chagas' disease and that they correlated with perfusion disorders and could occur before contractile changes were evident. 5 Finally, two of these studies compared the results of myocardial scintigraphy between patients with and without sustained ventricular tachycardia. One study showed that the denervated but viable myocardial area was associated with previous history of sustained ventricular tachycardia. The presence of three or more segments with "mismatch" between perfusion and sympathetic innervation was strongly associated with a history of sustained ventricular tachycardia. 14 The other study confirmed that the size of the "mismatch" area between perfusion and sympathetic innervation was greater in patients with a previous history of sustained ventricular tachycardia than in those with non-sustained ventricular tachycardia and controls. 16

Discussion
Dysautonomia in Chagas disease has been recognized for a long time, since the description of a decreased response to atropine in patients with Chagas disease. 18 In 1949, the first report of cardiac neuronal damage was published 19 and in 1959, neuronal stress depopulation was described in patients with Chagas disease. 20 However, the autoimmune theory prevailed for decades over the pathophysiology of Chagas disease and studies on dysautonomia became of secondary importance. However, the importance of dysautonomia in the pathophysiology and prognosis of Chagas disease has been revisited, especially after the possibility of assessing sympathetic denervation in vivo through the use of scintigraphy and the finding of the correlation between sympathetic denervation and the occurrence of malignant ventricular arrhythmias. Despite this, in this review, the number of selected publications was small, which indicates a clear need for further technical-scientific research in Chagas disease. 21 In fact, authors have pointed out that there are few studies on sympathetic denervation, with small samples or experimental animal models. 22 However, the cardiac dysautonomia found in Chagas disease may explain the increased risk of sudden arrhythmic death found in these patients, even in the absence of LV dysfunction. 17 The English language was the most used in publications and possibly reflects the international interest in the nuances of Chagas disease and, consequently, in the autonomic nervous system in Chagas cardiomyopathy. Regarding the identification of types of research, crosssectional studies were the most used. The choice of the research method was related to the objectives of the studies, since the research aims were to assess dysautonomia and to compare the findings across different groups of patients. Regarding clinical outcomes, only two studies correlated dysautonomia with the arrhythmic event, but not prospectively. There is a clear need for prospective studies that can elucidate the real prognostic value of identifying dysautonomia or sympathetic denervation in patients with Chagas disease. In addition, studies are needed on how to intervene in this process in a patientfriendly manner. It is known that the pathophysiology of sympathetic dysautonomia includes cardiac neuronal and nerve fiber destruction by inflammatory processes and parasitism and anti-β1 receptor autoantibodies, 23 leading to down-regulation of adrenergic receptors. Therefore, studies evaluating therapies in Chagas disease could also assess their effects on dysautonomia. In this review, we found only one study that evaluated the effect of an intervention, in this case, cardiac rehabilitation, on dysautonomia. 11 The study of the effect of other interventions, such as trypanocidal medications or inflammatory modulators, on dysautonomia or sympathetic denervation is clearly needed.
The articles analyzed in this review were highly heterogeneous, whether by the method used to assess dysautonomia, the group of patients studied, and the classification system used to characterize the patients. This prevented data from different studies from being combined for reanalysis. However, the data presented pointed to the presence of dysautonomia in the indeterminate form to cardiac forms with or without LV systolic dysfunction and also in the digestive form of Chagas disease, 4 which clearly showed that cardiac parasympathetic impairment can occur in the absence of any detectable LV function disorder. Sympathetic denervation has also been demonstrated in patients with the indetermate 5,12 and cardiac 5,14,16 forms of Chagas disease. There was a concordance between areas of denervation, hypoperfusion and fibrosis, however with areas of denervation larger than those of hypoperfusion, which characterized the "mismatch" 5,15 . The frequency of patients with sympathetic denervation and its extension gradually increased from the indeterminate form, to the cardiac form without LV dysfunction and with LV 5 systolic dysfunction. There was also a correlation between sympathetic denervation and degree of ventricular dysfunction 13 and an association between the degree of sympathetic denervation extension and the previous history of malignant ventricular arrhythmia. 14,16 In patients with chagasic heart disease, a history of sustained ventricular tachycardia correlated with the extent of sympathetic cardiac denervation, but not with the extent of fibrosis, indicating an important role of sympathetic cardiac denervation to trigger ventricular arrhythmia. 16 Marino et al., 13 demonstrated scintigraphic evidence of sympathetic hyperactivity, based on the findings of low uptake of 123I-MIBG (early and late heart-mediastinal relationship) by presynaptic endings in patients with Chagas disease and heart failure. The low uptake of 123I-MIBG indicates dysfunction of the receptors and loss of integrity of the presynaptic sympathetic fibers, reinforcing the theory of sympathetic hyperactivity in the pathogenesis of HF.

Conclusion
The evidence presented in this review supports that an early diagnosis of autonomic denervation in chronic Chagas' disease, even in patients without changes in the electrocardiogram and echocardiogram, may allow the identification of patients with an increased risk of sudden death.
Therefore, this issue must be studied extensively so that existing knowledge gaps are addressed. This systematic review contributed to the analysis of publications on the assessment of dysautonomia in chronic Chagas heart disease. It should be noted that the systematic review aims to demonstrate the periodicity and numbers of publications on the subject investigated, which was achieved by this study.
Finally, the knowledge gap on dysautonomia of chagasic cardiomyopathy is highlighted, and it is suggested that more studies on the topic be developed to provide scientific basis for health policies and interventions. It is also recommended that further research on autonomic nervous system in Chagas cardiomyopathy be undertaken, so that the best treatment for this condition can be found.

Author contributions
Conception and design of the research: Cunha AB. Acquisition of data: Rimolo LSM. Analysis and interpretation of the data: Souza AC, Saraiva RM. Statistical analysis: Cunha AB, Rimolo LSM. Writing of the manuscript: Cunha AB, Saraiva RM. Critical revision of the manuscript for intellectual content: Rimolo LSM, Saraiva RM.