Parkinson's disease (PD) currently affects 10 million people worldwide and its prevalence is projected to exponentially rise further in the absence of disease-modifying therapies. A scarcity of symptomatic treatments is available and the mainstay of therapy has been levodopa for over half a century. Although this treatment suffices for many patients in early phases of PD, treatment burden is significant, as are the adverse effects, wearing-off and dyskinesia that develop with disease progression. Therefore, additional treatment modalities are needed.

Preclinical studies have suggested that moderate hypoxia provokes release of survival-enhancing neurotransmitters, such as dopamine release from the substantia nigra. Clinical and preclinical evidence suggests the effects of hypoxia seem especially robust when applied using intermittent hypoxia therapy (IHT) compared to continuous hypoxia. IHT means that hypoxia is present for relatively short periods (i.e. minutes), interspersed with short periods of recovery at normoxia (i.e. sea-level). The precise working mechanism of IHT on the short term remains unclear, but the immediate clinical effects appear to be related to augmented dopamine release from the substantia nigra. Specifically, IHT may improve parkinsonian symptoms via activation of the Hypoxia Inducible Factor 1 (HIF-1) pathway, which in turn activates tyrosine hydroxylase (TH), which is the main rate-limiting enzyme in the production of dopamine. Several studies have demonstrated that HIF-1 stabilization leads to an increase in TH production, and consequently a rise in cellular dopamine content. IHT is a therapy proven safe and effective in a variety of disciplines, including fragile populations such as individuals with chronic obstructive pulmonary disorder (COPD), cardiac morbidity and spinal cord injury. Long-term application of IHT protocols was associated with improved oxidative stress response and adaptive plasticity in the dopaminergic system of rodents, suggesting that in addition to the acute symptomatic effects, repeated exposure to (intermittent) hypoxia might also exert some long-term neuroprotective effects. The general concept behind a possible (long-term) neuroprotective effect of IHT is the phenomenon of hypoxic conditioning: induction of a sub-toxic hypoxic stimulus to improve the (systemic) tolerance of cells and tissues to subsequent more severe stimuli, either in dose or duration. In this way, key adaptive mechanisms are induced that allow maintenance of cellular homeostasis under low-oxygen conditions. Among these adaptive mechanisms, activation of HIF-1 is the most prominent and most extensively described mechanism. Interestingly, IHT protocols also blocked the neurotoxic effect of agents that induce PD in rodents, preventing development of locomotor deficits, again suggesting some neuroprotective effects. Furthermore, circumstantial anecdotal evidence from individuals with PD suggests that ascending to high-altitude areas (e.g. on holidays) improves motor symptoms of PD, which the investigators recently confirmed in a survey conducted in the holiday context (https://doi.org/10.1002/mdc3.13597). The investigators hypothesize that the positive effect of altitude on the symptoms of PD result from decreased oxygen pressure at high altitude, which serves as an acute bodily stressor that releases survival-enhancing neurotransmitters such as dopamine and noradrenaline and might induce neuroprotective mechanisms.

The investigators will assess the potential of IHT in PD by assessing symptomatic effects of intermittent hypoxia therapy in an exploratory phase I trial. Primary objectives are the safety and feasibility of intermittent hypoxia in PD and assessing the responsiveness of subjective and standardized symptom scales to this intervention. This trial will exploit an aggregated N-of-1 approach, which allows testing multiple high-altitude simulation protocols and outcome measures, analysis of the treatment effect in individuals as it can account for random variation for treatment effects in the individual and enhances methodological power due to repeated treatment pairs.

During a screening procedure, participants undergo pulmonary function testing, carbon monoxide diffusion capacity testing and electrocardiography. If no cardiorespiratory abnormalities are demonstrated, individuals undergo a hypoxic intervention with gradually decreasing FiO2 levels from room air to either FiO2 0.127 or an arterial oxygen saturation (SaO2) of 80%, under vital parameter and blood gas monitoring. If a participant reaches FiO2 0.127 without SaO2 \<80%, the most intense active interventions will contain that FiO2. If a participant has an SaO2 \<80% before FiO2 0.127 is reached but still has an SaO2 of 80% or higher at FiO2 0.133, the most intense active intervention will be FiO2 0.133 instead of 0.127 (see Interventions)