Objective: We analyze the discriminatory diagnostic value of the Magnetic Resonance Parkinsonism Index (MRPI) and MRPI 2.0 in progressive supranuclear palsy (PSP) and normal pressure hydrocephalus (NPH) patients.

Background: PSP and NPH share significant clinical and radiographic overlap, often making diagnoses challenging when presentations are more ambiguous. MRPI has demonstrated strong diagnostic performance in distinguishing PSP from Parkinson’s disease and healthy controls.[1] MRPI 2.0 incorporates third ventricular (V3) and frontal horn (FH) width.[2] However, neither has been evaluated in NPH and compared to PSP patients.

Method: This is a retrospective analysis of 210 patients with clinically diagnosed PSP (n=95) and NPH (n=115). MRPI was calculated as (pons area/midbrain area) × (middle cerebellar peduncle width/superior cerebellar peduncle width).[1] MRPI 2.0 was calculated as MRPI × (V3/FH).[2]

Results: MRPI was significantly higher in PSP compared to NPH (median 14.37 [IQR 13.02–16.15] vs. 11.15 [9.98–14.40]; p<0.0001), consistent with the proposed PSP cutoff of 13.55. However, MRPI 2.0 did not differ significant between the two groups (PSP 2.38 [2.17–2.64] vs. NPH 2.39 [2.22–2.67]; p=0.9364). The proposed PSP cutoffs for MRPI 2.0 (2.18–2.50) overlapped substantially with findings in our NPH cohort.

Conclusion: This is the first study evaluating MRPI and MRPI 2.0 in NPH. MRPI reliably differentiated PSP from NPH, whereas MRPI 2.0 did not. We posit that, since PSP is also often characterized by ventriculomegaly, the incorporation of ventricular metrics in MRPI 2.0 may reduce its discriminatory value. Midbrain and cerebellar peduncle measurements appear to more robustly differentiate these clinically overlapping disease processes.

References: 1. Quattrone A, Nicoletti G, Messina D, et al. MR imaging index for differentiation of progressive supranuclear palsy from Parkinson disease and the Parkinson variant of multiple system atrophy. Radiology. 2008;246(1):214-221. doi:10.1148/radiol.2453061703

2. Quattrone A, Morelli M, Nigro S, et al. A new MR imaging index for differentiation of progressive supranuclear palsy-parkinsonism from Parkinson’s disease. Parkinsonism Relat Disord. 2018;54:3-8. doi:10.1016/j.parkreldis.2018.07.016

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MDS Abstracts - https://www.mdsabstracts.org/abstract/evaluating-magnetic-resonance-parkinsonism-index-mrpi-and-mrpi-2-0-as-radiographic-metrics-to-differentiate-progressive-supranuclear-palsy-psp-from-normal-pressure-hydrocephalus-nph/