The Effect of Adrenergic Blocker Therapy on Cardiac and Striatal Transporter Uptake in Pre-Motor and Symptomatic Parkinson's Disease

Idiopathic Parkinson's disease (PD) is a progressive neurodegenerative disorder of unknown etiology, characterized by bradykinesia (slowness of movements) associated with tremor at rest and/or muscle rigidity. PD is typically associated with a significant loss of dopaminergic neurons in the substantia nigra pars compacta(SNpc).The resulting nigro-striatal degeneration can be detected and quantified using a dopamine transporter (DAT) single photon emission computerized tomography (SPECT) imaging technique. This type of imaging, recently approved for clinical use in the United States, uses a labeled ligand (123I-Ioflupane) with high affinity to the DAT in the striatum. The amount of transporter, which plays a crucial role in the health of the presynaptic dopaminergic neurons, is then visualized by SPECT. 123I-Ioflupane uptake is reduced 50-70% in patients with early PD.

In addition to cardinal motor symptoms, PD is characterized by a large number of "non-motor" symptoms (NMS), which add to the overall morbidity burden. Importantly, non--motor features may precede the diagnosis of PD, sometimes by several years. They include autonomic (gastrointestinal dysfunction, cardiovascular dysfunction with orthostatic hypotension (OH), urinary and sexual dysfunction, and hyperhidrosis), sleep (impaired sleep initiation and maintenance, rapid eye movement behavior disorder (RBD), sleep apnea and excessive daytime sleepiness), sensory (pain, hyposmia, and visual dysfunction), and neuropsychiatric disturbances (anhedonia, apathy, anxiety, depression, panic attacks, dementia, and psychosis).

Among NMS, cardiac dysautonomia is a common feature of PD, manifesting in 30% of patients as orthostatic hypotension, a symptom that is correlated to disease duration and severity. Cardiac sympathetic innervation (CSI) is also affected in PD and other synucleinopathies. Lewy Body (LB) pathology, widely considered a marker of PD when detected in the SNpc, is also found in the sinoatrial nodal ganglion and myocardium of PD patients at autopsy. Despite the high prevalence of both conditions, the relationship between CSI impairment and cardiac dysautonomia is still poorly understood.

Iodine-123 meta-iodobenzylguanidine (123I-MIBG) is an inactive physiological analogue that mimics the kinetics of norepinephrine (NE) and competes with NE for active cardiac uptake into the postganglionic sympathetic nerve terminal, where it is stored into granules by NE transport proteins. 123I-MIBG myocardial scintigraphy, an accepted measure of CSI, is abnormal in PD patients, with a characteristic reduction of Heart/Mediastinum (H/M) ratio (early and late uptake) and an increased Washout Rate (WR). 123I-MIBG uptake impairment is specific to PD and other synucleinopathies and can be used to differentiate PD and dementia with Lewy bodies (DLB) from other disorders with similar neurological phenomenology such as multiple system atrophy (MSA), progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD).

123I-MIBG uptake deficit in PD is attributed to cardiac sympathetic denervation, based on neuropathological studies using tyrosine hydroxylase (TH) immunostaining in epicardial nerves. There is evidence of alpha-synuclein aggregation in the epicardial nerve fascicles - the distal axons of the cardiac sympathetic nerve - in subjects with incidental Lewy Body Disease (ILBD) at stage 2 or 3 of Braak staging with preserved TH immunoreactive axons, suggesting a preliminary stage in the development of cardiac sympathetic denervation. However, while 123I-MIBG myocardial scintigraphy abnormalities have been correlated with pre-motor symptoms like RBD, hyposmia and constipation, there is no pathological evidence of cardiac sympathetic denervation in subjects with signs of 123I-MIBG myocardial scintigraphy abnormality and recognized pre-motor symptoms of PD. Finally, despite the specific association with PD diagnosis, the relationship between CSI impairment and nigrostriatal degeneration is poorly understood. Two studies found a strong correlation between nigrostriatal dopaminergic degeneration, as measured by 123I Ioflupane SPECT, and CSI impairment at different stages of disease. 123I-MIBG uptake deficits have been correlated with the progression of the disease.