A new definition of PSP

When you design a research project in PSP, it’s important to make sure that everyone in the subject group with PSP in fact has PSP. Otherwise, you degrade the statistical power of the trial to detect any benefit of the treatment. The standard diagnostic criteria for PSP (called the “NINDS-SPSP Criteria” and spearheaded by Irene Litvan, MD, now of UCSD) were published in 1996 and have worked well for that purpose; Their positive predictive value (the fraction of patients satisfying the criteria who actually have PSP) and specificity (the fraction of those without PSP who fail to satisfy the criteria) are close to 100%.
But as we now enter the new era of trials of experimental neuroprotective treatment for PSP, we would like to diagnose the disease at an earlier stage, when such interventions are most likely to be effective, and the NINDS-SPSP Criteria don’t do that so well, with a sensitivity of about 80% overall, certainly lower in early cases. Another shortcoming is that the various phenotypes of PSP that have been described since 2005 won’t in many cases satisfy the criteria, which were designed for the “original flavor,” now called PSP-Richardson’s syndrome.
So time has marched on and we need a new set of criteria. Günter Höglinger, MD, Professor at the German Center for Neurodegenerative Disorders in Munich and probably the world’s leading clinical researcher in PSP, organized an international effort to revise the criteria. I’m privileged to serve on the four-person Steering Committee. A year ago we started to hash things out by email and conference calls, using the published articles on clinical features of PSP that use either autopsy or the NINDS-SPSP Criteria as a gold standard. The group, comprising 33 people from 11 countries, met in Munich on March 9 and 10 to turn our rough draft into a final version suitable for submission to a journal for peer review.
The new criteria recognize the various phenotypes of PSP. They are PSP-Richardson syndrome (about 55% of all PSP), PSP-parkinsonism (30%), PSP-frontal dementia (5%), PSP-ocular motor (1%), PSP- pure akinesia with gait freezing (1%), PSP-corticobasal syndrome (1%), PSP-progressive non-fluent aphasia (1%), and PSP-cerebellar (<1%). The remaining few percent are combinations of these or still-unrecognized forms.
The new criteria also delineate various “oligosymptomatic” or “prodromal” (the wording remains unsettled) forms, which may or may not develop into one of the diagnosable phenotypes. For example, there is now evidence that someone in the PSP age group with gradually progressive gait freezing for several years and a normal MRI, even without other abnormalities, will almost always prove to have PSP. The same is true for someone with bilateral rigidity and bradykinesia who fails to respond to levodopa and has some sort of nonspecific, undiagnosable visual symptoms or dizziness. Neither of these patients would satisfy the proposed new criteria for any of the PSP phenotypes, but they may still be worth identifying for inclusion in a longitudinal cohort study of people who are at risk of developing PSP. Our new criteria do that.
I’ll keep you updated.

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Not Your Father’s PSP

As it turns out, PSP comes in many clinical flavors. Back in the 80s I remember some patients whose illness looked like Parkinson’s until I realized that they weren’t responding to my levodopa prescriptions, at which point I repeated a careful ocular motor exam and found square wave jerks and slow downward saccades. I also remember one member of my first series of 41 patients with PSP from 1988 with severe gait apraxia and freezing as his most disabling feature.
Then, in 2005, David Williams and colleagues, mentored by Andrew Lees at Queen Square, published what is probably the most important clinical paper on PSP in the half-century since Steele, Richardson and Olszewski. That work delineated and named PSP-Richardson syndrome (PSP-RS ) and PSP-parkinsonism (PSP-P). This wasn’t just a new way to slice a clinical spectrum sharing the same basic pathology; the two variants actually had statistical differences by cluster analysis. This suggests that they differ at the pathoanatomic level. They even differed in the ratio of 4R/3R tau. (It turns out that the predilection of PSP’s tangles for 4R tau is driven by RS.)

Since then, a cornucopia of low-frequency clinical variants meeting pathoanatomic criteria for PSP has been described. In approximate descending order of prevalence after RS and PSP-P are corticobasal syndrome, postural Instability, pure akinesia with gait freezing, frontotemporal dementia, ocular motor predominance, progressive non-fluent aphasia, semantic dementia, and a cerebellar variant.
The clinicopathologic studies are only starting to appear, but it’s likely that they will all turn out to emphasize different cells, nuclei and brain regions. We will also probably see some subtle molecular differences among them (presaged by the 4R/3R difference between RS and PSP-P).

That sounds like different diseases to me. Different diseases shouldn’t be combined in treatment trials, genetic analyses or descriptive studies. What a mess.

Or is it? Maybe we don’t need to find causes and cures for each PSP variant individually. As they’re all tau aggregation disorders, maybe they will all yield to the same prevention. Maybe the mechanism of prion-like spread, by now pretty much a textbook verity, will apply not only to all of the “pure tauopathies” (and it’s not yet clear that all of the PSP variants are in fact pure tauopathies) but to all of the protein-aggregation-based neurodegenerative disorders. If it does, then poisoning that process could be the grand unified answer to Alzheimer’s, Parkinson’s, ALS, and PSP in all its malign variety.