Advanced MRI can finally tell apart two Parkinson's-like diseases that have stumped clinicians for decades

Two diseases hiding behind Parkinson's symptoms

For a significant portion of patients who arrive at a neurology clinic with balance problems, falls, stiffness, and difficulty with speech and movement, the initial working diagnosis is Parkinson's disease. In some of those cases, the diagnosis is correct. In others, the patient has one of two different conditions - progressive supranuclear palsy (PSP) or corticobasal degeneration (CBD) - that produce overlapping symptoms but progress differently, respond differently to treatment, and require different clinical management.

The misdiagnosis problem is not a failure of clinical skill. Until recently, objective tools to distinguish PSP and CBD from each other and from Parkinson's disease in living patients were simply not available. An international study led by researchers from the Sant Pau Research Institute (IR Sant Pau) in Barcelona describes a way forward: advanced magnetic resonance imaging protocols that can identify the specific brain changes characteristic of each condition with substantially greater accuracy than clinical assessment alone.

The findings, published in The Journal of Prevention of Alzheimer's Disease, carry implications for individual patient care and for the broader challenge of developing treatments for diseases that currently have none.

What makes PSP and CBD so difficult to diagnose

"These are diseases that cause balance problems, falls, stiffness, or difficulties with speech and movement. Many patients initially present as if they had Parkinson's disease or are simply older adults with mobility difficulties," explained Dr. Jesus Garcia-Castro, first author of the study and neurologist at Hospital de Sant Pau. "This means they are greatly underdiagnosed, and for years we have not known with enough certainty which disease each patient actually had."

PSP and CBD belong to the tauopathies - neurodegenerative diseases characterized by abnormal accumulation of tau protein, the same protein whose pathological behavior defines Alzheimer's disease. But PSP and CBD are four-repeat tauopathies, with distinct biological characteristics that set them apart from Alzheimer's. The different distribution of tau deposits in the brain produces the distinctive - though overlapping - symptom patterns of each condition.

In PSP, tau accumulates particularly in areas controlling eye movement, posture, and balance. A characteristic sign is difficulty moving the eyes vertically - a finding specific enough that neurologists use it diagnostically, but which often does not appear until the disease has progressed. In CBD, tau pathology affects cortical and subcortical regions producing a syndrome that can include involuntary movement of a limb and characteristic difficulties with purposeful movement. Both conditions can also present with dementia, complicating the clinical picture further.

Dr. Ignacio Illan-Gala, senior author of the study, framed the diagnostic challenge: "These diseases are, in a way, halfway between Alzheimer's and Parkinson's. They resemble Parkinson's because of their motor symptoms, but they share with Alzheimer's the fact that they are caused by tau pathology. The problem is that, until now, we did not have reliable tools to distinguish them properly."

What MRI can see that clinical assessment cannot

The study demonstrates that specific MRI-based measurements of brain structure - particularly patterns of atrophy in regions differentially affected by PSP versus CBD pathology - can serve as objective biomarkers that substantially improve diagnostic accuracy. Advanced MRI protocols go beyond standard brain scans used in routine clinical care, capturing subtle volumetric differences and structural changes in regions like the midbrain, subthalamic nucleus, and specific cortical areas.

These measurements provide what clinical observation often cannot: an objective, quantifiable signal that reflects the underlying pathology rather than a symptom pattern that multiple diseases can produce. The improvement in diagnostic accuracy has direct practical value for individual patients - earlier and more accurate diagnosis enables more appropriate management, better prognostic counseling, and timely access to clinical trials that might offer experimental treatments.

The clinical trial problem

The implications for clinical trial design may be even more significant. PSP and CBD are rare conditions, and every previous attempt to test potential disease-modifying treatments has run into a fundamental obstacle: without reliable tools to confirm that enrolled patients actually have the target disease rather than a clinically similar condition, trials include contaminated samples that reduce the signal-to-noise ratio and make it harder to detect treatment effects even when they exist.

Several promising drug candidates for tauopathies have failed in clinical trials. While the drugs themselves may have been ineffective, it is also possible that some failures were partly attributable to imprecise patient selection - treating a mixed population in which only a fraction actually had the disease being targeted. MRI-based patient stratification could substantially improve the quality of patient selection in future trials, increasing the probability of detecting a genuine treatment effect if one exists.

"This strategy improves early diagnosis and transforms the design of clinical trials, making them more precise and feasible for diseases for which no disease-modifying treatment currently exists," the study notes. For rare diseases where the patient populations available for trials are small, this kind of methodological improvement can be the difference between a trial that can answer its question and one that cannot.

PSP affects an estimated 5-6 people per 100,000 globally; CBD is thought to be of similar or slightly lower frequency. International collaboration and patient registries capable of pooling data across clinical sites are essential components of the research infrastructure needed to make treatment development feasible.