Interpeduncular Angle: A Possible Marker to Differentiate Atypical Parkinsonism from Idiopathic Parkinson’s Disease

Atypical parkinsonian disorders (APD), such as progressive supranuclear palsy (PSP), multiple system atrophy (MSA), corticobasal syndrome (CBS), and Lewy body dementia (LBD) are frequently misdiagnosed, commonly as Parkinson's disease (PD). The study aims to assess the utility of the interpeduncular angle (IPA) in distinguishing APD from PD across age ranges. This retrospective study was conducted with 225 patients (75 with APD, 75 with PD, and 75 healthy controls). Patients were categorized into three age groups: (51-60, 61-70, and 71-80 years). Two independent raters measured the IPA from T1-weighted axial brain magnetic resonance images (MRIs) at a level below the mammillary bodies using standardized measurement techniques. The APD group included 51 (68%) with PSP, 16 (21.33%) with MSA, 5 (6.67%) with LBD, and 3 (4%) with CBS. Bland-Altman analysis for angle measurement suggested good to excellent agreement between raters ( P < 0.001). IPA measurements among the different diagnostic groups showed that PSP was higher than controls ( P < 0.001) and PD ( P < 0.001), and MSA was higher than controls ( P < 0.001) and PD ( P = 0.003). There was no significant association between IPA and age in the APD phenotypes. With increasing age, the significance between APD and IPD groups decreased ( P < 0.001 in 51-60 years to P = 0.686 in 71-80 years). Receiver Operating Characteristic (ROC) analysis revealed increasing IPA thresholds for PSP versus PD (67.66° in 51-60 years to 75.71° in 71-80 years). IPA is not reliable in differentiating APD, particularly PSP and MSA, from PD and controls.