Autonomic dysfunction in mental retardation and spastic paraparesis with MECP2 mutation

Cardiac autonomic control in Rett syndrome: Insights from heart rate variability analysis

Rett syndrome (RTT) is a rare and severe neurological disorder mainly affecting females, usually linked to methyl-CpG-binding protein 2 (MECP2) gene mutations. Manifestations of RTT typically include loss of purposeful hand skills, gait and motor abnormalities, loss of spoken language, stereotypic hand movements, epilepsy, and autonomic dysfunction. Patients with RTT have a higher incidence of sudden death than the general population. Literature data indicate an uncoupling between measures of breathing and heart rate control that could offer insight into the mechanisms that lead to greater vulnerability to sudden death. Understanding the neural mechanisms of autonomic dysfunction and its correlation with sudden death is essential for patient care. Experimental evidence for increased sympathetic or reduced vagal modulation to the heart has spurred efforts to develop quantitative markers of cardiac autonomic profile. Heart rate variability (HRV) has emerged as a valuable non-invasive test to estimate the modulation of sympathetic and parasympathetic branches of the autonomic nervous system (ANS) to the heart. This review aims to provide an overview of the current knowledge on autonomic dysfunction and, in particular, to assess whether HRV parameters can help unravel patterns of cardiac autonomic dysregulation in patients with RTT. Literature data show reduced global HRV (total spectral power and R-R mean) and a shifted sympatho-vagal balance toward sympathetic predominance and vagal withdrawal in patients with RTT compared to controls. In addition, correlations between HRV and genotype and phenotype features or neurochemical changes were investigated. The data reported in this review suggest an important impairment in sympatho-vagal balance, supporting possible future research scenarios, targeting ANS.

An Observational Study of Heart Rate Variability Using Wearable Sensors Provides a Target for Therapeutic Monitoring of Autonomic Dysregulation in Patients with Rett Syndrome

Rett Syndrome (RTT) is a complex neurodevelopmental disorder that has multi-system involvement with co-occurring epilepsy, breathing problems and autonomic dysregulation. Autonomic dysregulation can increase the risk of cardiorespiratory vulnerability in this patient group. Assessment of heart rate variability (HRV) provides an overview of autonomic health in RTT and offers insight into how the sympathetic and parasympathetic components of the nervous system function. However, to our knowledge, no study has evaluated HRV in Rett patients to assess how the dynamics of autonomic function vary with age and changes during the day and/or night. Using non-invasive wearable sensors, we measured HRV in 45 patients with RTT and examined the time and frequency domain sympathetic and parasympathetic indices. Among the HRV indices assessed, heart rate decreases with age and is lower in the night across all ages studied. The sympathetic index (SDNN) and the parasympathetic indices (RMSSD and pNN50) are not seen to change with age. Nevertheless, these indices were all higher during the day when compared to the night. Our findings appear to show that Rett patients are less adaptable to autonomic changes during the night. In the clinical setting, this might be more relevant for patients with severe psychopathology.

Autonomic dysfunction and sudden death in patients with Rett syndrome: a systematic review

BACKGROUND

Rett syndrome (RTT), a debilitating neuropsychiatric disorder that begins in early childhood, is characterized by impairments in the autonomic nervous system that can lead to sudden unexpected death. This study explores the mechanisms of autonomic dysfunction to identify potential risk factors for sudden death in patients with RTT.

METHODS

Following the Reporting Items for Systematic Review and Meta-Analyses (PRISMA) criteria, we undertook comprehensive systematic reviews using the PubMed, Scopus, Cochrane, PsycINFO, Embase and Web of Science databases.

RESULTS

We identified and critically appraised 39 articles for autonomic dysfunction and 5 for sudden death that satisfied the eligibility criteria. Following thematic analysis, we identified 7 themes: breathing irregularities, abnormal spontaneous brainstem activations, heart rate variability metrics, QTc changes, vagal imbalance, fluctuation in peptides and serotonergic neurotransmission. We grouped these 7 themes into 3 final themes: (A) brainstem modulation of breathing, (B) electrical instability of the cardiovascular system and (C) neurochemical changes contributing to autonomic decline. We described key evidence relating to each theme and identified important areas that could improve the clinical management of patients with RTT.

LIMITATIONS

The heterogeneity of the methods used to assess autonomic function increased the difficulty of making inferences from the different studies.

CONCLUSION

This study identified the important mediators of autonomic dysfunction and sudden death in patients with RTT. We proposed brainstem mechanisms and emphasized risk factors that increase brainstem vulnerability. We discussed clinical management to reduce sudden death and future directions for this vulnerable population.

Evaluation of QTc in Rett syndrome: Correlation with age, severity, and genotype

Rett syndrome (RTT) is caused by MECP2 mutations, resulting in various neurological symptoms. Prolonged corrected QT interval (QTc) is also reported and is a speculated cause of sudden death in RTT. The purpose of this study was to correlate QTc in RTT patients with age, clinical severity, and genotype. 100 RTT patients (98 females, 2 males) with MECP2 mutations underwent baseline neurological evaluation (KKI-RTT Severity Scale) and QTc measurement (standard 12 lead electrocardiogram) as part of our prospective natural history study. Mean QTc of the cohort was 422.6 msec, which did not exceed the normal values for age. 7/100 patients (7%) had QTc prolongation (>450 msec). There was a trend for increasing QTc with age and clinical severity (P = 0.09). No patients with R106C, R106W, R133C, R168*, R270*, R294*, R306C, R306H, and R306P mutations demonstrated QTc prolongation. There was a relatively high proportion of QTc prolongation in patients with R255* mutations (2/8, 25%) and large deletions (1/4, 25%). The overall presence of QTc prolongation did not correlate with mutation category (P = 0.52). Our findings demonstrate that in RTT, the prevalence of QTc prolongation is lower than previously reported. Hence, all RTT patients warrant baseline ECG; if QTc is prolonged, then cardiac followup is warranted. If initial QTc is normal, then annual ECGs, particularly in younger patients, may not be necessary. However, larger sample sizes are needed to solidify the association between QTc and age and clinical severity. The biological and clinical significance of mild QTc prolongation above the normative data remains undetermined.

Progressive noradrenergic deficits in the locus coeruleus of Mecp2 deficient mice

Methyl-CpG binding protein 2 (MeCP2) is a transcriptional regulator. Mutations in this gene cause a wide range of neurological disorders. Mecp2 deficiency has been previously associated to catecholaminergic dysfunctions leading to autonomic defects in the brainstem and the sympathoadrenergic system of the mouse. The present study was undertaken to determine if the locus coeruleus (LC), the main noradrenergic cell group of the brain, is affected. Using real type PCR, we found a reduction of the tyrosine hydroxylase (Th) mRNA level, the rate-limiting enzyme in catecholamine synthesis, in the whole pons of P15 (-36%), P30 (-47%) and P50 (-42%) Mecp2 null male as well as in adult heterozygous female (-44%) mice. Using immunoquantification we did not observe any difference of the Th staining level in P30 null male mice. However at P50, we demonstrated a significant decrease in both the Th staining level (-24%), and the number of Th-positive neurons (-23%). We subsequently characterized a reduction (-28%) of the dendritic density of the Th-positive fibers surrounding the LC in P50 null male mice. In heterozygous female mice immunoquantification did not revealed significant modifications, but only a tendency towards reduction. Finally, we did not found any apoptotic neurons in the pons indicating that LC neurons are not dying but are more likely loosing their catecholaminergic phenotype. In conclusion, our results showing a progressive catecholaminergic deficit in the LC of Mecp2 deficient null male mice could open new perspectives to better understand the autonomic and cognitive deficits due to the lack of Mecp2.

Biogenic amines in Rett syndrome: the usual suspects

Rett syndrome (RTT) is a severe postnatal neurological disorder caused by mutations in the methyl-CpG binding protein 2 (MECP2) gene. In affected children, most biological parameters, including brain structure, are normal (although acquired microcephaly is usually present). However, in recent years, a deficit in bioaminergic metabolism has been identified at the cellular and molecular levels, in more than 200 patients. Recently available transgenic mouse strains with a defective Mecp2 gene also show abnormalities, strongly suggesting that there is a direct link between the function of the MECP2 protein and the metabolism of biogenic amines. Biogenic amines appear to have an important role in the pathophysiology of Rett syndrome, for several reasons. Firstly, biogenic amines modulate a large number of autonomic and cognitive functions. Secondly, many of these functions are affected in RTT patients. Thirdly, biogenic amines are the only neurotransmitters that have repeatedly been found to be altered in RTT patients. Importantly, pharmacological interventions can be envisaged to try to counteract the deficits observed. Here, we review the available human and mouse data and present how they have been and could be used in the development of pharmacological treatments for children affected by the syndrome. Given our current knowledge and the tools available, modulating biogenic amine metabolism may prove to be the most promising strategy for improving the life quality of Rett syndrome patients in the short term.