Longitudinal Dynamics of Physical Function With Anxiety and Depression in Parkinson's Disease: A Cross‐Lagged Panel Analysis of the PPMI Dataset

Introduction Parkinson's disease (PD) is characterized by a complex interplay of motor and non‐motor symptoms. While cross‐sectional studies have shown a link between psychological symptoms and self‐reported physical function, the longitudinal and directional nature of this relationship remains unclear. This study aimed to clarify the temporal relationships between psychological symptoms (depression and anxiety) and physical function (both participant‐reported and clinician‐rated) in individuals with PD. Methods We used a rigorous longitudinal analytical approach, random‐intercept cross‐lagged panel modeling (RI‐CLPM), on data from 1128 individuals with PD from the Parkinson's Progression Markers Initiative (PPMI) dataset. We examined directional cross‐lagged paths between psychological symptoms (via GDS and STAI) and physical function measures (via MDS‐UPDRS Parts II and III). Results Our analysis revealed significant bidirectional cross‐lagged paths between depressive symptoms and participant‐reported physical function. A worsening of depression scores predicted a subsequent decline in participant‐reported physical function (via MDS‐UPDRS Part II), and vice versa. In contrast, no significant cross‐lagged paths were found between psychological symptoms and clinician‐rated motor function (via MDS‐UPDRS Part III). The analysis also showed a significant unidirectional path from participant‐reported function to future clinician‐rated function, suggesting that a participant's self‐perception of disability may precede objective motor decline. Conclusions Our findings reveal a bidirectional relationship between depressive symptoms and an individual's own reporting of their physical function. This emphasizes the important role of patient‐reported outcomes as an indicator of PD progression. Therefore, we advocate for an integrated, multidisciplinary approach in clinical practice, where mental health screening and support are included in standard PD care.