Progressive Supranuclear Palsy and Normal Pressure Hydrocephalus: Can Neuroimaging Solve this Mysterious Diagnostic Overlap?

Common clinical experience and some anecdotic publications (Table 1) have over the years suggested that the co-occurrence of idiopathic normal pressure hydrocephalus (iNPH) and progressive supranuclear palsy (PSP) is more than just a chance or misdiagnosis. DESH associated with worse clinical outcomes Oculomotor and midbrain assessment important before diagnosis and shunting The most recent evidence comes from Shimada et al, who documented a remarkable 21.2% prevalence of PSP comorbidity among 85 iNPH patients.1 This finding fundamentally challenges previous assumptions that these conditions are mutually exclusive. Patients with concurrent iNPH and PSP exhibited distinctive clinical features including impaired vertical eye movement, axial-dominant parkinsonism, and increased propensity for backward falls, while radiologically showing shortened mesencephalic tegmentum length and asymmetrical dopamine transporter deficits on imaging.1 Comparative neuroimaging studies consistently demonstrate that PSP patients frequently exhibit NPH-like MRI features. Önder et al found that 90% of PSP patients showed Evans Index values greater than 0.3, traditionally considered diagnostic for ventricular enlargement in NPH.2 Moreover, PSP patients demonstrated significantly higher scores for dilated Sylvian fissures, focal sulcal dilatation, and overall DESH scores compared to healthy controls.2 The prevalence of disproportionately enlarged subarachnoid-space hydrocephalus (DESH) in PSP has been systematically evaluated by Fu et al, who demonstrated that approximately 15% of PSP patients exhibit imaging-suggestive hydrocephalus patterns.3 Among 181 PSP patients, 11% showed both enlarged ventricles and enlarged subarachnoid spaces, while 34% demonstrated enlarged subarachnoid spaces only. Importantly, PSP patients with DESH patterns showed worse clinical outcomes and greater midbrain atrophy.3 The diagnostic implications are profound, as suggested by clinicopathological studies revealing substantial misdiagnosis rates. Magdalinou et al reported that among four patients clinically diagnosed with iNPH during life, post-mortem examination revealed three had PSP pathology.4 However, pathology is seldom available and these recent neuroimaging studies have immediate clinical implications, necessitating a more nuanced approach to patients presenting with suspected NPH. The implications are also practical as there is emerging evidence that surgical intervention may benefit carefully selected patients with mixed presentations. The Shimada study demonstrated that lumboperitoneal shunt surgery significantly improved outcomes in patients with concurrent PSP and iNPH.1 This suggests that PSP pathology does not necessarily preclude benefit from CSF diversion procedures, challenging traditional contraindications to shunt surgery in neurodegenerative conditions. Nevertheless, careful patient selection and expectation management is key. In conclusion, clinicians should maintain heightened awareness of potential PSP comorbidity, particularly in patients with atypical features. The development of multimodal diagnostic algorithms will be essential for improving diagnostic accuracy and optimizing patient outcomes in this challenging clinical scenario. Future studies should explore the mechanisms underlying what seems to be more than a mere association. Research project: A. Conception, B. Organization, C. Execution; Statistical Analysis: A. Design, B. Execution, C. Review and Critique; Manuscript: A. Writing of the first draft, B. Review and Critique. S.A.: 1C, 3A. A.F.: 1A, 3B. Ethical Compliance Statement: No institutional review board or ethics committee approved the study given the nature of the work (editorial summarizing previously published works). Informed patient consent was not necessary for this work. All authors have read and complied with the Journal's Ethical Publication Guidelines. We confirm that we have read the Journal's position on issues involved in ethical publication and affirm that this work is consistent with those guidelines. Funding Sources and Conflict of Interest: No specific funding was received for this work. The authors declare that there are no conflicts of interest relevant to this work. Financial Disclosures for the previous 12 months: SA has nothing to report. AF reports receiving consultancies from Abbvie, Ceregate, Medtronic, Boston Scientific, Iota, Inbrain, Inbrain Pharma; honoraria from Abbvie, Medtronic, Boston Scientific, Sunovion, Chiesi farmaceutici, UCB, Ipsen; grants from University of Toronto, Weston foundation, Abbvie, Medtronic, Boston Scientific, CIHR. Data sharing not applicable to this article as no datasets were generated or analysed during the current study.

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