Parkinson’s disease is heavily overdiagnosed by general neurologists, whereas parkinsonism plus syndromes are underdiagnosed. Despite improvements in the diagnostic methods during recent decades and the development of diagnostic clinical criteria for parkinsonian syndromes, the diagnostic accuracy of Parkinson’s disease remains relatively low, and 1/4 of diagnoses are incorrect (J Joutsa).
|Sagittal T1-weighted volumetric spoiled gradient-echo|
MR images show MCP width (arrow)
Massey et colleagues from IoN (London) developed a measure of the midbrain and pons, and midbrain/pons ratio to identify patients with progressive supranuclear palsy (PSP).  These measurements were significantly different among PSP, Parkinson disease (PD), and multiple system atrophy (MSA) groups. However, these measurements could not distinguish patients affected by these different diseases at the individual level. Nicoletti et colleagues from different Italian centres investigated MRI-based brainstem measurements and found that middle cerebellar peduncle (MCP) width accurately differentiated patients with MSA from those with PD with a sensitivity, specificity, and positive predictive value of 100%.  They also described a new imaging measure, the Magnetic Resonance Parkinsonism Index (MRPI), which distinguished patients with PSP from those affected by PD and MSA, at the individual level.  MRPI was calculated multiplying pons/midbrain areas ratio by MCP/superior cerebellar peduncle (SCP) widths. These latter brain structures have been selectively involved in MSA and PSP, respectively. Similarly to the British group,  the Italian group also showed [3-5] midbrain/pons ratio values overlapped among patients with PSP and those with PD and MSA, demonstrating that MCP and SCP width measurements are needed to accurately identify patients with MSA or PSP, on an individual basis. While the m/p-d-ratio was the best index in differentiating PSP from non-PSP and distinguishing between PSP and PD, the MRPI was the most accurate measurement in differing PSP from MSA .
This text was taken and addapted from Neurology
1. Massey LA, Jager HR, Paviour DC, et al. The midbrain to pons ratio: a simple and specific MRI sign of progressive supranuclear palsy. Neurology 2013;80:1856-1861.
2. Nicoletti G, Fera F, Condino F, et al. MR imaging of middle cerebellar peduncle width: differentiation of multiple system atrophy from Parkinson disease. Radiology 2006;239:825-830.
3. 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;246:214-221.
4. Arabia G, Quattrone A. MRI measurements of brainstem structures in patients with Richardson's syndrome, progressive supranuclear palsy- parkinsonism, and Parkinson's disease. Mov Disord 2011;26:1575-1576.
5. Morelli M, Arabia G, Salsone M et al. Accuracy of magnetic resonance parkinsonism index for differentiation of progressive supranuclear palsy from probable or possible Parkinson disease. Mov Disord 2011;26:527-533.
6. Reiter. The Potential Of Diameter And Planimetry Algorithm In The Differential Diagnosis Of PSP
Typical changes on MRI in patients with PSP are as follows:
- Midbrain atrophy (hummingbird appearance on sagittal MRI)
- Midbrain high signal (can be punctate)
- Putaminal atrophy
- Red nucleus atrophy
- Globus pallidus atrophy
- Enlarged aqueduct of sylvius
- Increased interpeduncular angle
- It is estimated that the rate of midbrain atrophy is seven times that of healthy controls
No study has looked prospectively at positive predictive value of these parameters for PSP diagnosis at a stage where the clinical diagnosis is not yet obvious. If clinical history is atypical; for example if the patient is young or has a family history, appropriate additional investigations should be performed.
- Accuracy of magnetic resonance parkinsonism index for differentiation of progressive supranuclear palsy from probable or possible Parkinson disease
- Diagnostic accuracy of the magnetic resonance Parkinsonism index and the midbrain-to-pontine area ratio to differentiate progressive supranuclear palsy from Parkinson's disease and the Parkinson variant of multiple system atrophy
- Overall predictive accuracy was similar for the m/p-ratio (87.0%) and the MRPI (80.5%) with a predictive accuracy for PSP from MSA-P being significantly better for the MRPI (87.5%) compared with the m/p-ratio (75%) as well as a predictive accuracy for PSP from PD being significantly better for the m/p-ratio (87.6%) compared with the MRPI (77.3%).
- Morphometric analysis of subcortical structures in Progressive Supranuclear Palsy: in vivo evidence of neostriatal and mesencephalic atrophy