Redefining Non-Motor Symptoms in Parkinson's Disease

Abstract

Parkinson's disease involves widespread neurodegeneration that extends far beyond the basal ganglia, giving rise to a diverse range of non-motor symptoms that frequently emerge before motor onset. These include autonomic dysfunction, cognitive decline, neuropsychiatric disturbances, sleep-related disorders, and sensory deficits. Here, we synthesize current evidence on the anatomical, neurochemical, and network-level mechanisms that drive these symptoms, and we examine how they shape disease progression and clinical heterogeneity. We highlight the limitations of dopamine-centric models and advocate for a framework that treats non-motor symptoms as the disorder's primary, mechanistically distinct features. We also discuss how emerging technologies-such as multi-omic profiling, artificial intelligence, and network neuroscience-enable earlier identification, stratification of non-motor phenotypes, and the development of precision-based therapeutic strategies. Recognizing non-motor symptoms as central to Parkinson's disease redefines how the disorder should be diagnosed, studied, and treated.

Keywords: Parkinson’s disease; REM sleep behavior disorder; autonomic dysfunction; biomarkers; cognitive impairment; neuropsychiatric disorders; non-motor symptoms; pain; sensory dysfunction; sleep-related disorders.

Figure 1

Temporal emergence of non-motor symptoms in Parkinson’s disease relative to Braak staging. The evolving trajectory of non-motor symptoms across Braak stages 1 through 6, aligned with the ascending neuroanatomical propagation of α-synuclein pathology. Symptoms are grouped by domain—autonomic, sensory, sleep-related, neuropsychiatric, and cognitive—and plotted according to their earliest reported onset in prodromal, early, and advanced disease stages. A color gradient represents the intensification of each symptom over time. Early autonomic signs (e.g., constipation, orthostatic hypotension) and REM sleep behavior disorder typically precede motor onset, highlighting their diagnostic and predictive relevance. Mid-stage features such as apathy and visual hallucinations reflect advancing pathology, while cognitive decline and dementia dominate in later stages. This temporal map underscores the value of early symptom recognition for diagnosis, prognosis, and disease stratification. Abbreviations: EDS, excessive daytime sleepiness; NPDs, neuropsychiatric disorders; OSA, obstructive sleep apnea; RBD, REM sleep behavior disorder; RLS, restless leg syndrome. Figure created with BioRender.com, accessed on 5 February 2025. A color gradient represents the intensification of each symptom over time.

Figure 2

Neuroanatomical substrates and mechanistic underpinnings of non-motor symptoms in Parkinson’s disease. This illustration maps the brain and peripheral regions implicated in non-motor manifestations of Parkinson’s disease, integrating structural and neurochemical changes with clinical symptom domains. α-Synuclein accumulation and neuronal loss in brain stem nuclei (e.g., dorsal motor nucleus of the vagus, locus coeruleus), limbic areas (e.g., amygdala, orbitofrontal cortex), and cholinergic centers (e.g., nucleus basalis of Meynert) disrupt circuits involved in autonomic regulation, emotional processing, arousal, memory, and sleep–wake transitions. Degeneration in the hypothalamic and prefrontal regions contributes to apathy, anxiety, and thermoregulatory failure, while dysfunction in the olfactory bulb, spinal cord, and peripheral autonomic ganglia gives rise to sensory and visceral symptoms. The convergence of central and peripheral pathology reinforces the need to reconceptualize Parkinson’s disease as a network-level, multisystem disorder rather than a focal basal ganglia syndrome. Abbreviations: ↑ = Increased; ↓ = Decreased; α-syn, α-synuclein; EDS, excessive daytime sleepiness; FOp, frontal operculum; NBM, nucleus basalis of Meynert; OB, olfactory bulb; OFC, orbitofrontal cortex; OSA, obstructive sleep apnea; PFC, prefrontal cortex; RBD, REM sleep behavior disorder; RLS, restless leg syndrome; SC, spinal cord; SN, substantia nigra; VTA, ventral tegmental area. Figure created with BioRender.com, accessed on 5 February 2025.