Objective: The Magnetic Resonance Parkinsonism Index (MRPI) has been proposed as an enhancement of the Midbrain to Pons Ratio (MTPR) as a diagnostic tool for differentiating Progressive Supranuclear Palsy (PSP) and idiopathic Parkinson’s disease (IPD). Our objective was to evaluate the diagnostic value of the MRPI in discriminating both diseases.

Background: The differentiation of PSP from IPD is mostly depending on clinical assessment. Due to a considerable overlap of symptoms in both diseases, especially at early stages, accurate differential diagnosis is challenging. Searching for objectively measurable distinction criteria, parameters quantifying disease-specific brain atrophy patterns have been established, such as the MTPR. The more recently introduced MRPI is an extension of the MTPR that takes into account not only the midbrain but also the superior cerebellar peduncle (SCP) – both structures known to be typically atrophying in PSP. Evidence suggests that a MRPI of more than 13.55 is specific for PSP.

Method: We examined T1-weighted MRIs of 40 patients with either PSP (n = 15) or IPD (n = 25), to manually measure the midbrain area (MBA), pons area, SCP diameter, and middle cerebellar peduncle (MCP) diameter. Based on those measurements, we calculated the MTPR as well as the MRPI for each subject.

Results: PSP patients showed a mean MRPI of 17.59, which was significantly larger compared to IPD patients (p ≤ 0.001). This was associated with a significantly reduced MBA in PSP patients (p ≤ 0.001), which is consistent with PSP-specific midbrain atrophy. ROC analyses confirmed excellent diagnostic accuracy for the MRPI (AUC = 0.99; cutoff ≥ 12.10; sensitivity = 0.933; specificity = 1.00), when comparing PSP and IPD patients. The diagnostic accuracy of the earlier introduced MTPR was also excellent for both groups (AUC = 0.98; cutoff ≤ 0.540; sensitivity = 0.933; specificity = 0.97). However, there was no significant difference for the diagnostic accuracy between MTPR and MRPI (D = -0.41, p = 0.562).

Conclusion: The MRPI was confirmed to be an additional tool to distinguish PSP and IPD, though there was no significant enhancement in diagnostic value compared to the earlier established MTPR. Further research is needed to evaluate parameters that can significantly enhance early differential diagnosis.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/the-magnetic-resonance-parkinsonism-index-as-a-diagnostic-marker-for-differentiating-psp-from-ipd/