The midbrain‐to‐pons ratio distinguishes progressive supranuclear palsy from non‐fluent primary progressive aphasias

Background and purpose To determine the clinical utility of the midbrain‐to‐pons (M/P) ratio as a clinical biomarker of progressive supranuclear palsy ( PSP ) in patients with non‐fluent primary progressive aphasia syndromes. Methods Patients with PSP , progressive non‐fluent aphasia ( PNFA ) and logopenic progressive aphasia ( LPA ) were recruited. Patients were diagnosed clinically, but pathological confirmation was available in a proportion of patients. Midbrain and pons areas were measured using Osirix Lite, a free DICOM viewer. The M/P ratio and Magnetic Resonance Parkinsonism Index were calculated and their diagnostic utility compared. Results A total of 72 participants were included (16 PSP , 18 PNFA , 16 LPA and 22 controls). Patients with PSP had motor features typical of the syndrome. Both the M/P ratio and Magnetic Resonance Parkinsonism Index differed significantly in PSP compared with controls. The M/P ratio was disproportionately reduced in PSP compared with PNFA and LPA ( PSP, 0.182 ± 0.043; PNFA, 0.255 ± 0.034; LPA, 0.258 ± 0.033; controls, 0.292 ± 0.031; P < 0.001). An M/P ratio of ≤0.215 produced a positive predictive value of 77.8% for the diagnosis of PSP syndrome. Pathological examination revealed Alzheimer's disease in three cases (all LPA ), pathological PSP in two cases (one clinical PSP and one PNFA ) and corticobasal degeneration in one case ( PNFA ). The M/P ratio was ≤0.215 in both pathological cases of PSP . Conclusions The M/P ratio was disproportionately reduced in PSP , suggesting its potential as a clinical marker of the PSP syndrome. Larger studies of pathologically confirmed cases are needed to establish the M/P ratio as a biomarker of PSP pathology.