Parkinson's disease (PD) is a chronic neurodegenerative disorder affecting almost 2% of all people over the age of 65 worldwide. Based on records from the nationwide Danish Hospital Register, 1,931 patients with a first-time diagnosis of PD were seen in Danish hospitals between 2001 and 2006, and according to the Danish Parkinson Association, more than 8,000 people live in Denmark with PD, but many more people, including relatives and caregivers, are affected by the burden of the disease. The pathological hallmark of PD is the loss of dopaminergic projection neurons in the substantia nigra, manifesting as the classic triad of bradykinesia, rigidity and tremor. These symptoms can be effectively treated with dopamine replacement therapy, at least in the initial stages of the disease. However, as the disease progresses, more debilitating symptoms occur, including gait problems, postural instability, and falls. Unfortunately, the occurrence of these symptoms represents a major milestone in PD progression, resulting in loss of independence, worsened quality of life, and markedly increased mortality (from the consequences of falls e.g. hip fractures). Average survival is reduced to 7 years once a PD patient starts having falls. The socioeconomic cost of falls in PD is also significant, with 80% of spending on PD care arising from acute admissions, primarily falls related. Gait problems, postural instability, and falls in PD, like in the non-PD elderly population, are multi-factorial. However, there are PD-specific factors that contribute to the onset of these symptoms, including deficits of central sensory processing and motor deficits such as freezing of gait (FoG). Critically, these disabling symptoms often respond poorly to dopaminergic drugs, and advanced therapy, including subthalamic nucleus deep brain stimulation (DBS). Recent work has implicated cholinergic dysfunction in PD secondary to degeneration of brainstem locomotor regions such as the Pedunculopontine nucleus (PPN), which is involved in the control of movement initiation and body equilibrium. However, so far, the response of postural and locomotor symptoms to interventions such as cholinesterase inhibitors or PPN DBS (that both enhance cholinergic neurotransmission) has been disappointing with a great deal of variability in reported responses among patients. This variability in treatment response to therapy is probably related to the heterogeneity of mechanisms of postural and gait abnormalities across the PD population, suggesting the importance of phenotyping PD patients with postural and gait problems when starting a therapeutic agent or recruiting patients in clinical studies to investigate new strategies for these problems.

Spinal cord stimulation (SCS) is an established therapy to treat chronic back and neuropathic pain.

Several studies have shown an improvement in gait function in PD patients following SCS for back pain. More recently, a small number of PD patients with gait dysfunction (without back pain) were treated with encouraging initial results on gait function and with few adverse events. However, the trials performed so far have left a number of unanswered questions with SCS that need to be addressed before this procedure can be used more widely in PD patients with gait problems. Firstly, all the published studies are unblinded and carried out in small cohorts of PD patients. Secondly, while these studies have shown that, overall, SCS seems to have a beneficial effect on gait in PD, they have also shown a heterogeneous outcome, as some patients had a poor response to treatment. Thirdly, patient selection and gait characterisation in these studies was limited, and this lack of clinical phenotyping could have been responsible for the heterogeneous outcome of these studies. Fourthly, mechanisms of actions of SCS are uncertain/unstudied in these papers. Therefore, a prospective, double-blind clinical trial with a scientifically sound study protocol in larger cohort of well-characterised patients is required in order to provide clear Class I evidence whether SCS is effective in improving gait function in PD. Additionally, the exact mechanisms of action of SCS in PD patients with gait problems are uncertain, as they have not been fully investigated so far. Animal models of PD, including non-human primates, show that SCS improves locomotion by activating the dorsal column-medial lemniscal pathway which in turn desynchronizes abnormal corticostriatal oscillations. Inputs from ascending leminiscal and extralemniscal pathways to the brainstem and thalamus that may modulate the supplementary motor area (SMA) are also highly connected to the cholinergic PPN in the brainstem. In turn, the SMA has cortifugal projections to PPN, as part of the circuit that controls anticipatory postural adjustments. Therefore, SCS might modulate the activity of SMA, globus pallidus and PPN that are impaired in patients with FoG.

Positron Emission Tomography (PET) can be used to assess in vivo these changes induced by SCS on the brain cholinergic function and the motor and associative cortical-subcortical loops.

(\[18F\]FEOBV) PET is an in vivo marker of the brain vesicular acetylcholine transporter (VAChT) and provides information of the functional integrity of the brain cholinergic neurotransmitter system. Using (\[18F\]FEOBV) PET we have recently showed reduced striatal and limbic archicortical VAChT binding in patients who suffer from FoG compared to participants who do not suffer from this. 18F-deoxyglucose (18F-FDG) PET is a marker of regional cerebral glucose metabolic rate (rCMRglc), Over the last three decades, studies of regional cerebral glucose metabolism have provided insight into the pathophysiology of the cerebral dysfunction underlying PD and other movement disorders. 18F-FDG PET has also been extensively used to assess the effects of pallidotomy and DBS on the motor and associative cortical-subcortical loops.

Therefore, PET imaging with (\[18F\]FEOBV) PET and 18F-FDG before and after SCS treatment could significantly improve the understanding of the mechanisms of actions of SCS and its effects on brain cholinergic neurotransmission and resting metabolic brain networks. This knowledge may be helpful in selecting the right patient group for the procedure.