Sympathetic neuroimaging

Getting to the heart of Lewy body disease

Early identification of neurodegenerative diseases before extensive neuronal loss or disabling symptoms have occurred is imperative for effective use of disease-modifying therapies. Emerging data indicate that central Lewy body diseases - Parkinson disease and dementia with Lewy bodies - can begin in the peripheral nervous system, opening up a therapeutic window before central involvement. In this issue of the JCI, Goldstein et al. report that cardiac 18F-dopamine positron emission tomography reveals lower activity selectively in individuals with several self-reported Parkinson disease risk factors who later develop Parkinson disease or dementia with Lewy bodies. Accurately identifying which at-risk individuals will develop central Lewy body disease will optimize early patient selection for disease-modifying therapies.

The Pathogenesis and Treatment of Cardiovascular Autonomic Dysfunction in Parkinson's Disease: What We Know and Where to Go

Cardiovascular autonomic dysfunctions (CAD) are prevalent in Parkinson’s disease (PD). It contributes to the development of cognitive dysfunction, falls and even mortality. Significant progress has been achieved in the last decade. However, the underlying mechanisms and effective treatments for CAD have not been established yet. This review aims to help clinicians to better understand the pathogenesis and therapeutic strategies. The literatures about CAD in patients with PD were reviewed. References for this review were identified by searches of PubMed between 1972 and March 2021, with the search term “cardiovascular autonomic dysfunctions, postural hypotension, orthostatic hypotension (OH), supine hypertension (SH), postprandial hypotension, and nondipping”. The pathogenesis, including the neurogenic and non-neurogenic mechanisms, and the current pharmaceutical and non-pharmaceutical treatment for CAD, were analyzed. CAD mainly includes four aspects, which are OH, SH, postprandial hypotension and nondipping, among them, OH is the main component. Both non-neurogenic and neurogenic mechanisms are involved in CAD. Failure of the baroreflex circulate, which includes the lesions at the afferent, efferent or central components, is an important pathogenesis of CAD. Both non-pharmacological and pharmacological treatment alleviate CAD-related symptoms by acting on the baroreflex reflex circulate. However, pharmacological strategy has the limitation of failing to enhance baroreflex sensitivity and life quality. Novel OH treatment drugs, such as pyridostigmine and atomoxetine, can effectively improve OH-related symptoms via enhancing residual sympathetic tone, without adverse reactions of supine hypertension. Baroreflex impairment is a crucial pathological mechanism associated with CAD in PD. Currently, non-pharmacological strategy was the preferred option for its advantage of enhancing baroreflex sensitivity. Pharmacological treatment is a second-line option. Therefore, to find drugs that can enhance baroreflex sensitivity, especially via acting on its central components, is urgently needed in the scientific research and clinical practice.

Improved Synthesis of [11C]COU and [11C]PHXY, Evaluation of Neurotoxicity, and Imaging of MAOs in Rodent Heart

The radiotracers [¹¹C]COU and [¹¹C]PHXY are potential PET imaging agents for in vivo studies of monoamine oxidases (MAOs), as previously shown in rodent and primate brain. One-pot, automated methods for the radiosynthesis of [¹¹C]PHXY and [¹¹C]COU were developed to provide reliable and improved radiochemical yields. Although derived from the structure of the neurotoxin MPTP, COU did not exhibit in vivo neurotoxicity to dopaminergic nerve terminals in the mouse brain as assayed by losses of VMAT2 radioligand binding. PET imaging studies in rats demonstrated that both [¹¹C]COU and [¹¹C]PHXY exhibit retention in cardiac tissues that can be blocked by pretreatment with the MAO inhibitors deprenyl (MAO-B) and pargyline (MAO-A and -B). In addition to prior neuroimaging applications, [¹¹C]COU and [¹¹C]PHXY are thus also of interest for studies of MAO enzymatic activity and imaging of sympathetic nerve density in heart.

Autonomic Dysfunction in the Synucleinopathies

Autonomic dysfunction is a characteristic feature in the synucleinopathies. Differences in cellular deposition and neuronal populations affected by α-synuclein aggregation influence the manifestations and severity of autonomic failure in the different synucleinopathy disorders. The Lewy body disorders (Parkinson's disease, dementia with Lewy bodies, and pure autonomic failure) have predominantly peripheral involvement, whereas multiple system atrophy chiefly manifests as central autonomic failure. Clinical and laboratory features may be useful in distinguishing the different synucleinopathies based on the pattern and severity of autonomic failure. Treatment recommendations are aimed at the underlying pathophysiology and utilize non-pharmacologic and pharmacologic approaches. This review will focus on pathophysiology, clinical manifestations, and management recommendations for autonomic failure including neurogenic orthostatic hypotension, thermoregulatory dysfunction, genitourinary dysfunction, and gastrointestinal dysfunction in the synucleinopathies.

Pure Autonomic Failure

Pure autonomic failure (PAF) is a neurodegenerative disorder of the autonomic nervous system clinically characterized by orthostatic hypotension. The disorder has also been known as Bradbury-Eggleston syndrome, named for the authors of the 1925 seminal description. Patients typically present in midlife or later with orthostatic hypotension or syncope. Autonomic failure may also manifest as genitourinary, bowel, and thermoregulatory dysfunction. With widespread involvement, patients may present to a variety of different specialties and require multidisciplinary treatment approaches. Pathologically, PAF is characterized by predominantly peripheral deposition of α-synuclein. However, patients with PAF may progress into other synucleinopathies with central nervous system involvement.

Head Rules Over the Heart: Cardiac Manifestations of Cerebral Disorders

The Brain-Heart interaction is becoming increasingly important as the underlying pathophysiological mechanisms become better understood. "Neurocardiology" is a new field which explores the pathophysiological interplay of the brain and cardiovascular systems. Brain-heart cross-talk presents as a result of direct stimulation of some areas of the brain, leading to a sympathetic or parasympathetic response or it may present as a result of a neuroendocrine response attributing to a clinical picture of a sympathetic storm. It manifests as cardiac rhythm disturbances, hemodynamic perturbations and in the worst scenarios as cardiac failure and death. Brain-Heart interaction (BHI) is most commonly encountered in traumatic brain injury and subarachnoid hemorrhage presenting as dramatic electrocardiographic changes, neurogenic stunned myocardium or even as ventricular fibrillation. A well-known example of BHI is the panic disorders and emotional stress resulting in Tako-tsubo syndrome giving rise to supraventricular and ventricular tachycardias and transient left ventricular dysfunction. In this review article, we will discuss cardiovascular changes caused due to the disorders of specific brain regions such as the insular cortex, brainstem, prefrontal cortex, hippocampus and the hypothalamus; neuro-cardiac reflexes namely the Cushing's reflex, the Trigemino-cardiac reflex and the Vagal reflex; and other pathological states such as neurogenic stunned myocardium /Takotsubo cardiomyopathy. There is a growing interest among intensivists and anesthesiologists in brain heart interactions as there are an increasing number of cases being reported and there is a need to address unanswered questions, such as the incidence of these interactions, the multifactorial pathogenesis, individual susceptibility, the role of medications, and optimal management.

KEY MESSAGES

BHI contribute in a significant way to the morbidity and mortality of neurological conditions such as traumatic brain injury, subarachnoid hemorrhage, cerebral infarction and status epilepticus. Constant vigilance and a high index of suspicion have to be exercised by clinicians to avoid misdiagnosis or delayed recognition. The entire clinical team involved in patient care should be aware of brain heart interaction to recognize these potentially life-threatening scenarios.

HOW TO CITE THIS ARTICLE

Hrishi AP, Lionel KR, Prathapadas U. Head Rules Over the Heart: Cardiac Manifestations of Cerebral Disorders. Indian J Crit Care Med 2019;23(7):329-335.

Parkinson's Disease: Biomarkers, Treatment, and Risk Factors

Parkinson's disease (PD) is a progressive neurodegenerative disorder caused mainly by lack of dopamine in the brain. Dopamine is a neurotransmitter involved in movement, motivation, memory, and other functions; its level is decreased in PD brain as a result of dopaminergic cell death. Dopamine loss in PD brain is a cause of motor deficiency and, possibly, a reason of the cognitive deficit observed in some PD patients. PD is mostly not recognized in its early stage because of a long latency between the first damage to dopaminergic cells and the onset of clinical symptoms. Therefore, it is very important to find reliable molecular biomarkers that can distinguish PD from other conditions, monitor its progression, or give an indication of a positive response to a therapeutic intervention. PD biomarkers can be subdivided into four main types: clinical, imaging, biochemical, and genetic. For a long time protein biomarkers, dopamine metabolites, amino acids, etc. in blood, serum, cerebrospinal liquid (CSF) were considered the most promising. Among the candidate biomarkers that have been tested, various forms of α-synuclein (α-syn), i.e., soluble, aggregated, post-translationally modified, etc. were considered potentially the most efficient. However, the encouraging recent results suggest that microRNA-based analysis may bring considerable progress, especially if it is combined with α-syn data. Another promising analysis is the advanced metabolite profiling of body fluids, called "metabolomics" which may uncover metabolic fingerprints specific for various stages of PD. Conventional pharmacological treatment of PD is based on the replacement of dopamine using dopamine precursors (levodopa, L-DOPA, L-3,4 dihydroxyphenylalanine), dopamine agonists (amantadine, apomorphine) and MAO-B inhibitors (selegiline, rasagiline), which can be used alone or in combination with each other. Potential risk factors include environmental toxins, drugs, pesticides, brain microtrauma, focal cerebrovascular damage, and genomic defects. This review covers molecules that might act as the biomarkers of PD. Then, PD risk factors (including genetics and non-genetic factors) and PD treatment options are discussed.

Seronegative and seropositive autoimmune autonomic ganglionopathy (AAG): Same clinical picture, same response to immunotherapy

Two patients with a syndrome of pandisautonomia with clinical criteria of AAG are provided. Both patients present a similar clinical picture and response to immunosuppressive treatment. One of them has positive antibodies against the ganglionic nicotinic acetylcholine (gAChr) and the other does not. This brief article serves to reflect the spectrum of AAG, at a clinical level, in laboratory tests and in the response to immunotherapy, independently of the presence of positive gAChr antibodies.

Small Molecule PET Tracers for Transporter Imaging

As the field of PET has expanded and an ever-increasing number and variety of compounds have been radiolabeled as potential in vivo tracers of biochemistry, transporters have become important primary targets or facilitators of radiotracer uptake and distribution. A transporter can be the primary target through the development of a specific high-affinity radioligand: examples are the multiple high-affinity radioligands for the neuronal membrane neurotransmitter or vesicular transporters, used to image nerve terminals in the brain. The goal of a radiotracer might be to study the function of a transporter through the use of a radiolabeled substrate, such as the application of 3-O-[¹¹C]methyl]glucose to measure rates of glucose transport through the blood-brain barrier. In many cases, transporters are required for radiotracer distributions, but the targeted biochemistries might be unrelated: an example is the use of 2-deoxy-2-[¹⁸F]FDG for imaging glucose metabolism, where initial passage of the radiotracer through cell membranes requires the action of specific glucose transporters. Finally, there are transporters such as p-glycoprotein that function to extrude small molecules from tissues, and can effectively work against successful uptake of radiotracers. The diversity of structures and functions of transporters, their importance in human health and disease, and their role in therapeutic drug disposition suggest that in vivo imaging of transporter location and function will continue to be a point of emphasis in PET radiopharmaceutical development. In this review, the variety of transporters and their importance for in vivo PET radiotracer development and application are discussed. Transporters have thus joined the other major protein targets such as G-protein coupled receptors, ligand-gated ion channels, enzymes, and aggregated proteins as of high interest for understanding human health and disease.

Neurocardiology: Cardiovascular Changes and Specific Brain Region Infarcts

There are complex and dynamic reflex control networks between the heart and the brain, including cardiac and intrathoracic ganglia, spinal cord, brainstem, and central nucleus. Recent literature based on animal model and clinical trials indicates a close link between cardiac function and nervous systems. It is noteworthy that the autonomic nervous-based therapeutics has shown great potential in the management of atrial fibrillation, ventricular arrhythmia, and myocardial remodeling. However, the potential mechanisms of postoperative brain injury and cardiovascular changes, particularly heart rate variability and the presence of arrhythmias, are not understood. In this chapter, we will describe mechanisms of brain damage undergoing cardiac surgery and focus on the interaction between cardiovascular changes and damage to specific brain regions.

Imaging the Nonmotor Symptoms in Parkinson's Disease

Parkinson's disease is acknowledged to be a multisystem syndrome, manifesting as a result of multineuropeptide dysfunction, including dopaminergic, cholinergic, serotonergic, and noradrenergic deficits. This multisystem disorder ultimately leads to the presentation of a range of nonmotor symptoms, now appreciated to be an integral part of the disease-specific spectrum of symptoms, often preceding the diagnosis of motor Parkinson's disease. In this chapter, we review the dopaminergic and nondopaminergic basis of these symptoms by exploring the neuroimaging evidence based on several techniques including positron emission tomography, single-photon emission computed tomography molecular imaging, magnetic resonance imaging, functional magnetic resonance imaging, and diffusion tensor imaging. We discuss the role of these neuroimaging techniques in elucidating the underlying pathophysiology of NMS in Parkinson's disease.

(123)I-meta-iodobenzylguanidine (MIBG) cardiac scintigraphy in α-synucleinopathies

Cardiac meta-iodobenzylguanidine (MIBG) uptake on (123)I-MIBG cardiac scintigraphy is reduced in patients with Lewy body disease such as Parkinson's disease (PD), dementia with Lewy bodies (DLB), and pure autonomic failure, and has been reported to be useful for differentiating PD from other parkinsonian syndromes, as well as DLB from Alzheimer disease (AD). Postmortem studies have shown that the number of tyrosine hydroxylase (TH)-immunoreactive nerve fibers of the heart was decreased in pathologically-confirmed Lewy body disease, supporting the findings of reduced cardiac MIBG uptake in Lewy body diseases. Now, reduced cardiac MIBG uptake can be a potential biomarker for the presence of Lewy bodies in the nervous system. (123)I-MIBG cardiac scintigraphy can allow us to determine the presence of Lewy bodies.

Cardiac Iodine-123-Meta-Iodo-Benzylguanidine Uptake in Carotid Sinus Hypersensitivity

Background

Carotid sinus syndrome is the association of carotid sinus hypersensitivity with syncope, unexplained falls and drop attacks in generally older people. We evaluated cardiac sympathetic innervation in this disorder in individuals with carotid sinus syndrome, asymptomatic carotid sinus hypersensitivity and controls without carotid sinus hypersensitivity.

Methods

Consecutive patients diagnosed with carotid sinus syndrome at a specialist falls and syncope unit were recruited. Asymptomatic carotid sinus hypersensitivity and non-carotid sinus hypersensitivity control participants recruited from a community-dwelling cohort. Cardiac sympathetic innervation was determined using Iodine-123-metaiodobenzylguanidine (123-I-MIBG) scanning. Heart to mediastinal uptake ratio (H:M) were determined for early and late uptake on planar scintigraphy at 20 minutes and 3 hours following intravenous injection of 123-I-MIBG.

Results

Forty-two subjects: carotid sinus syndrome (n = 21), asymptomatic carotid sinus hypersensitivity (n = 12) and no carotid sinus hypersensitivity (n = 9) were included. Compared to the non- carotid sinus hypersensitivity control group, the carotid sinus syndrome group had significantly higher early H:M (estimated mean difference, B = 0.40; 95% confidence interval, CI = 0.13 to 0.67, p = 0.005) and late H:M (B = 0.32; 95%CI = 0.03 to 0.62, p = 0.032). There was, however, no significant difference in early H:M (p = 0.326) or late H:M (p = 0.351) between the asymptomatic carotid sinus hypersensitivity group and non- carotid sinus hypersensitivity controls.

Conclusions

Cardiac sympathetic neuronal activity is increased relative to age-matched controls in individuals with carotid sinus syndrome but not those with asymptomatic carotid sinus hypersensitivity. Blood pressure and heart rate measurements alone may therefore represent an over simplification in the assessment for carotid sinus syndrome and the relative increase in cardiac sympathetic innervation provides additional clues to understanding the mechanisms behind the symptomatic presentation of carotid sinus hypersensitivity.