Probable 4‐Repeat Tauopathy Criteria Predict Brain Amyloid Negativity, Distinct Clinical Features, and FDG‐PET/MRI Neurodegeneneration Patterns in Corticobasal Syndrome

ABSTRACT

Background

Corticobasal syndrome (CBS) is associated with diverse underlying pathologies, including the four‐repeat (4R)‐tauopathies. The Movement Disorders Society (MDS) criteria for progressive supranuclear palsy (PSP) proposed the novel category “probable 4R‐tauopathy” to address the phenotypic overlap between PSP and corticobasal degeneration (CBD).

Objectives

To investigate the clinical ability of the MDS‐PSP criteria for probable 4R‐tauopathy in predicting a negative amyloid‐PET in CBS. Additionally, this study aims to explore CBS patients classified as 4R‐tauopathy concerning their clinical features and neuroimaging degeneration patterns.

Methods

Thirty‐two patients with probable CBS were prospectively evaluated and split into those who fulfilled or did not fulfill the 4R‐tauopathy criteria (CBS‐4RT+ vs. CBS‐4RT−). All patients underwent positron emission tomographies (PET) with [¹⁸F]fluorodeoxyglucose and [¹¹C]Pittsburgh Compound‐B (PIB) on a hybrid PET‐MRI scanner to perform multimodal quantitative comparisons with a control group.

Results

Eleven patients were clinically classified as CBS‐4RT+, and only one had a positive PIB‐PET. The CBS‐4RT+ classification had 92% specificity, 52% sensitivity, and 69% accuracy in predicting a negative PIB‐PET. The CBS‐4RT+ group presented with dysarthria and perseveration more often than the CBS‐4RT− group. Moreover, the CBS‐4RT+ group showed a prominent frontal hypometabolism extending to the supplementary motor area and striatum, and brain atrophy at the anterior cingulate and bilateral striata.

Conclusions

The 4R‐tauopathy criteria were highly specific in predicting a negative amyloid‐PET in CBS. Patients classified as 4R‐tauopathy presented distinct clinical aspects, as well as brain metabolism and atrophy patterns previously associated with tauopathies.

Corticobasal syndrome (CBS) is an atypical parkinsonian syndrome related to heterogeneous etiologies, including the four‐repeat (4R) tauopathies corticobasal degeneration (CBD) and progressive supranuclear palsy (PSP). ¹ , ² Clinical features cannot reliably distinguish between underlying pathologies. ¹ , ² , ³ , ⁴

The 4R‐tauopathies are neurodegenerative diseases with a predominance of 4R tau isoform aggregation in neuropathology, causing various motor and cognitive symptoms determined by their neuroanatomical distribution. ⁵ Potential therapeutic trials are being proposed for these entities. ⁶ , ⁷ However, validated biomarkers, such as second‐generation tau‐PET, are still under development and have not yet been approved for routine use. ⁸ , ⁹

Recently, the Movement Disorder Society (MDS) criteria for the clinical diagnosis of PSP ¹⁰ proposed a novel diagnostic category entitled “probable 4R‐tauopathy” to address the phenotypic overlap between PSP and CBD, aiming for a clinical prediction of an underlying 4R‐tauopathy antemortem. These criteria include cases of probable PSP categories, in addition to possible PSP with speech‐language phenotype and possible PSP with CBS (PSP‐CBS) phenotype. The CBS phenotype should present with ocular motor dysfunction, such as vertical supranuclear gaze palsy or slow vertical saccades. ¹⁰ The 4R‐tauopathy criteria were highly specific in a previous retrospective study with autopsy‐confirmed PSP and CBD cases. ¹¹ However, prospective studies have not yet evaluated the clinical usefulness of this diagnostic concept in a real‐world scenario.

The present study aims to investigate the clinical ability of the probable 4R‐tauopathy criteria to predict a negative amyloid‐PET in CBS patients. Also, we sought to explore the clinical features, as well as the brain metabolism and atrophy patterns of patients with CBS classified as probable 4R‐tauopathy, which may provide clinically useful insights among CBS cohorts.

Methods

Participants

Thirty‐two patients diagnosed with probable CBS according to Armstrong criteria ¹² and the MDS‐PSP criteria ¹⁰ were prospectively enrolled at the movement disorders and cognitive neurology clinics at Hospital das Clínicas, University of São Paulo School of Medicine, between February 2017 and February 2020.

Subsequently, all individuals were evaluated regarding their clinical profile by two board‐certified neurologists (JBP and SMDB). All patients showed a progressive disease course with a duration of at least 1 year and a half. These individuals were part of a prospective project studying neuroimaging biomarkers in CBS, and enrollment procedures were previously reported in detail. ¹³ Exclusion criteria included relevant non‐degenerative brain lesions such as stroke sequelae, tumors, hydrocephalus, and significant premorbid psychiatric disease. The Ethics Committee of our institution approved the research project, and all volunteers consented to participate by signing a consent form.

We included 30 healthy participants as a control group for the statistical parametric mapping (SPM) group comparisons. The control group was matched to the CBS patients by age and same scanner type.

Clinical Assessment

All patients received a standardized predefined clinical evaluation, also previously described. ¹³ , ¹⁴ Global cognitive impairment was assessed with the Addenbrooke's Cognitive Examination—Revised (ACE‐R) ¹⁵ , ¹⁶ , ¹⁷ and the Mini‐Mental State Examination (MMSE), ¹⁸ validated in Brazilian cohorts. Episodic memory was investigated with the memory figure test from Brief Cognitive Screening Battery (BCSB), ¹⁹ working memory with the backward digit span, attention by forward digit span, and verbal fluency by letters and animals. Staging of dementia was measured by the Clinical Dementia Rating (CDR) scale, ²⁰ and the functional decline was assessed with the Functional Activities Questionnaire. ²¹ Neuropsychiatric symptoms were evaluated by Neuropsychiatry Inventory. ²²

Cortical functions were clinically evaluated by the presence of limb or orobuccal apraxia, cortical sensory deficits, alien limb phenomena, and Balint syndrome. We characterized the presence of limb apraxia by imitation of meaningful and meaningless gestures and with imaginary tool use, and orobuccal apraxia by meaningless orobuccal gestures. ²³ A speech‐language therapist (IJA) performed a comprehensive language assessment, including tests for motor speech disorders (dysarthria and apraxia of speech), agrammatism, confrontation naming, repetition, single‐word comprehension, reading, and spelling.

A comprehensive motor examination was conducted to identify the presence of parkinsonism, dystonia, myoclonus, pyramidal signs, postural instability, tremor, ocular motor dysfunction, and motor perseveration. The level of motor impairment was assessed by the Hoehn and Yahr scale. ²⁴

We applied the MDS‐PSP criteria for probable 4R‐tauopathies, ¹⁰ in which a patient was classified as a probable 4R‐tauopathy when, in addition to the CBS phenotype, they presented with vertical supranuclear gaze palsy or slowness of vertical saccades. ¹⁰ , ²⁵ Subsequently, we split the patients into two groups: those who fulfilled the criteria (CBS‐4RT+) and those who did not fulfill (CBS‐4RT−).

Neuroimaging Data Acquisition

Both [¹¹C]Pittsburgh Compound‐B (PIB) and [¹⁸F]fluorodeoxyglucose (FDG) were produced in an on‐site cyclotron (PET trace 880, GE Healthcare) at the Nuclear Medicine Center of the Institute of Radiology of our Hospital. PIB‐Positron Emission Tomography (PET) and Magnetic Resonance Imaging (MRI) images were simultaneously acquired on a hybrid 3.0‐Tesla SIGNA PET/MRI scanner (GE Healthcare, Milwaukee, WI, USA). The MRI protocol included volumetric sequences weighted on T1, T2, and T2/FLAIR and diffusion‐weighted imaging.

FDG‐PET was acquired in a Discovery 710 PET/Computerized Tomography (CT) scanner (GE Healthcare, Milwaukee, WI, USA). The FDG radiotracer was injected intravenously in bolus with a mean activity of 185–222 MBq (5–6 mCi). Before the injection of FDG, the subjects fasted for at least 6 h, and their blood glucose level was <180 mg/dL. The interval between injection and scan start was at least 30 min, and the scan duration was 15 min.

Each PET scan was corrected for attenuation using CT (FDG‐PET) or MRI data (PIB‐PET). Images were reconstructed using an ordered subset expectation maximization (OSEM) algorithm.

The radiopharmaceutical compound PIB production was previously validated and entirely carried out by the cyclotron facility. ²⁶ Cortical amyloid deposition images were analyzed using the acquisition time of 30 min, obtained in rest conditions, 40–70 min after intravenous administration of 370–555 MBq (10–15 mCi) of the tracer. Further data regarding imaging acquisition were previously detailed. ¹³ , ¹⁴ FDG‐PET was performed within 1 month after the clinical examination, and the time between FDG‐PET and PIB‐PET/MRI varied from 2 days to 6 months.

[ ¹¹C]PIB‐PET Visual and Semi‐Quantitative Classification

Two board‐certified nuclear medicine physicians (AMC, CRO), blinded for clinical data (including criteria group), performed a visual assessment of the PIB‐PET images assisted by a 3D‐SSP semi‐quantitative software (Cortex ID Suite, GE Healthcare). Each PIB‐PET scan was classified as “amyloid positive” if there was a loss of gray and white matter contrast, with increased uptake in cortical gray matter in at least two of six areas: frontal, temporal, lateral parietal, precuneus, anterior cingulate, and posterior cingulate. The image was also classified as positive if only a single large cortical area had a strong tracer uptake. Conversely, the image was rated as “amyloid negative” when there was a clear contrast between gray and white matter, with strong uptake in the white matter and no significant activity in the cortex. ²⁷

Semi‐quantitative analysis was performed with standard uptake values ratio (SUVr) of the cortical areas normalized to the cerebellar gray matter. A cutoff point of 1.42 for the SUVr was considered the positivity standard in addition to the visual analysis. A previous study from our group observed a high interrater agreement and similar amyloid positivity rates in amnestic patients from the literature. ²⁸

Quantitative FDG‐PET Analysis

After splitting CBS patients into two groups using the clinical criteria of probable 4R‐tauopathies (CBS‐4RT+ and CBS‐4RT−), we performed a quantitative FDG‐PET group analyses to investigate the areas of regional brain glucose metabolism (rBGM) reduction in both groups compared to age‐matched healthy controls. This analysis excluded the patient classified as CBS‐4RT+ with a positive amyloid‐PET.

PET images were co‐registered with their respective MRI images and spatially normalized using the Statistical Parametric Mapping 12 (SPM12) software (Wellcome Department of Cognitive Neurology, Functional Imaging Laboratory, London, UK) into an anatomic template. ²⁹ The images were flipped to represent the hemisphere contralateral to the most affected limbs on the right side of the image because of CBS's asymmetric nature.

The FDG‐PET spatial normalization was performed using a dementia‐optimized brain FDG‐PET template. ²⁹ Scans were smoothed with an 8 mm full width at half maximum Gaussian kernel to reduce misregistration into the template space and improve the signal‐to‐noise ratio. A default threshold of 0.8 of the mean uptake inside the brain was selected to ensure the analysis only included voxels mapping cerebral tissue. Global uptake differences were adjusted using the “proportional scaling” SPM12 option.

For the group analyses, statistical parametric maps were generated with an SPM12 threshold at the voxel level at p uncorrected (p unc) = 0.001, with a minimum extension of 100 voxels in the cluster. Statistical results were considered valid when they survived correction for multiple comparisons with the familywise error (FWE) or false discovery rate (FDR) methods (pFWE/FDR ≤0.05). Relevant peak voxels from the statistical parametric maps were identified in the Montreal Neurologic Institute (MNI) coordinates system.

Voxel‐Based Morphometry (VBM) Analysis

We performed quantitative T1w volumetric MRI group analyses (voxel‐based morphometry —VBM) to investigate brain atrophy patterns in both groups (CBS‐4RT+ with negative amyloid‐PET and CBS‐4RT−) compared to healthy controls. Images were also flipped to represent the hemisphere contralateral to the most affected limbs on the right side of the image. MRI T1‐weighted volumetric images were processed using VBM on SPM12 using the SPM toolbox Diffeomorphic Anatomical Registration using Exponentiated Lie algebra (DARTEL) algorithm.

Study Design

Firstly, we performed a prospective clinical evaluation and assessed the frequencies of the CBS patients that clinically fulfilled the MDS‐PSP criteria for 4R‐tauopathies (CBS‐4RT+), blinded to PIB‐PET results, and investigated the ability of the criteria to predict a negative amyloid‐PET. Subsequently, to explore the characteristics of the CBS‐4RT+ group, quantitative group analyses of the FDG‐PET hypometabolism and the VBM‐MRI brain atrophy patterns were obtained from both CBS‐4RT+ and CBS‐4RT− groups and compared to age‐matched healthy controls. The patient classified as CBS‐4RT+ with positive amyloid‐PET was excluded before performing the quantitative neuroimaging analyses. Only CBS‐4RT+ patients with negative amyloid‐PET were compared with healthy controls because the MDS‐PSP criteria consider a positive amyloid‐PET an exclusion criterion. ¹⁰

Statistical Analysis

For descriptive purposes, categorical variables were presented using relative and absolute frequencies and compared using the Chi‐squared or Fisher's exact test. Continuous variable distributions were assessed for normality by skewness, kurtosis, and graphical methods. Those with normal distribution were presented as mean and standard deviations and compared with the independent samples Student t test. Otherwise, variables were presented using medians and quartiles and compared with the Mann–Whitney non‐parametric test.

The diagnostic accuracies of the 4R‐tauopathy predicting a negative amyloid‐PET were measured through sensitivity, specificity, accuracy, and predictive values, which were described with the respective confidence intervals.

All tests were two‐sided; final P‐values under 0.05 were considered statistically significant. All analyses were performed in R language (Version 4.1.1, R Core Team, 2020).

Results

Frequencies of the Probable 4R‐Tauopathy Criteria among CBS Patients and its Ability to Predict a Negative amyloid‐PET

In total, 32 patients diagnosed as probable CBS were included, and all underwent clinical evaluation and neuroimaging procedures. Demographic data are shown in Table 1. Eleven (34.4%) patients presented with vertical gaze palsy or slowness of vertical saccades; consequently, they fulfilled the clinical criteria for probable 4R‐tauopathy and were classified in the CBS‐4RT+ group. Twenty‐one (65.7%) subjects did not fulfill the criteria (CBS‐4RT− group).

TABLE 1.

Comparison of demography and cognitive assessment of the whole patients with corticobasal syndrome and according to the MDS‐PSP criteria for probable 4‐repeat tauopathy

	CBS (n = 32)	CBS‐4RT+ (n = 11)	CBS‐4RT− (n = 21)	P‐value
Demography
Age at main assess, ya	65.9 ± 3.3	62.6 ± 6.4	67.7 ± 3.9	0.152
Age at symptom onset, ya	61.9 ± 3.2	58.3 ± 6.5	63.8 ± 3.8	0.123
Symptoms duration, yb	4.0 (2.75–4.25)	3.0 (2.50–4.50)	4.0 (3.0–4.0)	0.823
Male genderc	17/32 (53%)	5/11 (45%)	12/21 (57%)	0.712
Schooling, ya	10.8 ± 2.1	9.5 ± 3.8	11.6 ± 2.6	0.328
Right side involved limbsc	13/32 (40.6%)	5/11 (45.5%)	8/21 (38.1%)	0.721
Family historyc	3/32 (9.4%)	1/11 (9.1%)	2/21 (9.5%)	>0.999
CDRb	2.0 (1.75–2.0)	2.0 (1.0–2.0)	2.0 (2.0–3.0)	0.193
Functional activities questionnaireb	22.0 (14.0–26.5)	20.5 (15.5–24.7)	24.0 (13.0–27.0)	0.687
Hoehn &Yahrb	2.0 (2.0–3.0)	3.0 (2.50–4.0)	2.0 (2.0–3.0)	0.110
Cognitive assessment
ACE‐R Totala	46.2 ± 9.03	47,9 ± 17.4	45,3 ± 11.63	0.782
ACE‐R Attention	11.0 (9.0–15.0)	14.0 (11.0–16.0)	11.0 (8.25–12.0)	0.140
ACE‐R Memory	10.67 ± 2.79	13.11 ± 5.05	9.44 ± 3.56	0.202
ACE‐R Fluency	2.9 (± 2.9)	1.8 (± 2.1)	3.3 (± 3.1)	0.224
ACE‐R Language	17.96 ± 2.57	19.0 ± 3.79	17.44 ± 3.34	0.503
ACE‐R Visuospatial	7.0 (4.0–9.0)	8.0 (8.0–9.0)	6.0 (4.0–10.25)	0.133
MMSEa	17.6 ± 2.6	20.2 ± 4.7	16.2 ± 3.2	0.134
Delayed Recall (BCSB)b	3.0 (1.0–6.0)	5.0 (3.0–6.0)	2.0 (0–6.0)	0.356
Verbal Fluency (letter)a	4.03 ± 1.2	3.7 ± 2.3	4.15 ± 1.5	0.768
Verbal Fluency (animals)a	7.03 ± 1.7	6.3 ± 3.4	7.35 ± 2.18	0.584
Digit Span Forwardsb	6.50 (5.0–8.0)	6.0 (5.0–8.0)	7.0 (5.0–8.0)	0.812
Digit Span Backwardsb	2.0 (0.0–3.75)	2.0 (0.0–3.0)	2.0 (0.0–4.0)	0.795
NPIb	13.0 (8.0–18.75)	15.0 (7.5–19.5)	12.0 (8.0–18.25)	0.629

Note: Data presented as a: mean ± standard deviation, or b: median (1st – 3rd quartiles), or c: n (%).

Abbreviations: ACE‐R, Addenbrooke cognitive examination‐revised; BCSB, brief cognitive screening battery; CBS, corticobasal syndrome; CBS‐4RT+, patients who fulfilled criteria for probable four‐repeat tauopathy; CBS‐4RT−, patients who did not fulfill criteria for probable four‐repeat tauopathy; CDR, clinical dementia rating; L, left; MMSE, Mini‐mental state examination; NPI, neuropsychiatric inventory; R, right; y, years.

Regarding amyloid‐PET imaging, 13 (40.6%) among 32 patients had positive PIB‐PET results. Only one (9.0%) patient out of 11 clinically classified as CBS‐4RT+ had a positive PIB‐PET, compared to 12 (57.1%) out of 21 classified in the CBS‐4RT− group. The clinical classification of probable 4R‐tauopathy demonstrated 92% specificity, 52% sensitivity, 91% positive predictive value, 57% negative predictive value, and 69% accuracy in predicting a negative amyloid‐PET.

Clinical Features According to the Criteria for Probable 4R‐Tauopathy

There were no demographic differences regarding age at onset or symptom duration between CBS patients classified according to the criteria for 4R‐tauopathy (Table 1). Concerning motor symptoms, both groups (CBS‐4RT+ and CBS‐4RT−) presented asymmetric akinetic‐rigid parkinsonism as the most prevalent motor sign, followed by myoclonus (Table 2). They also did not significantly differ concerning global cognitive assessment and functional impairment (Table 1). Apraxia was the most common cortical sign in both groups. Notably, patients classified as CBS‐4RT+ presented dysarthria and motor perseveration more often than the CBS‐4RT− group (Table 2).

TABLE 2.

Comparison of cortical and motor signs of the whole patients with corticobasal syndrome and according to the MDS‐PSP diagnostic criteria for probable 4‐repeat tauopathy

	CBS (n = 32)	CBS‐4RT+ (n = 11)	CBS‐4RT− (n = 21)	P value
Cortical
Limb apraxia	31 (96.9%)	10 (90.9%)	21 (100%)	0.344
Cortical sensory deficits	8 (25%)	1 (9.1%)	7 (33.3%)	0.209
Alien limb phenomena	8 (25%)	4 (36.4%)	4 (19.1%)	0.397
Motor perseveration	7 (21.9%)	5 (45.5%)	2 (9.5%)	0.032
Balint syndrome	3 (9.4%)	1 (9.1%)	2 (9.5%)	>0.999
Aphasia	5 (15.6%)	2 (18.2%)	3 (14.3%)	>0.999
Agrammatism	2 (6.25%)	0 (0%)	2 (9.5%)	0.676
Apraxia of speech	8 (25%)	2 (18%)	6 (28.6%)	0.097
Motor
Parkinsonism	32 (100%)	11 (100%)	21 (100%)	>0.999
Myoclonus	26 (72.2%)	5 (45.5%)	21 (65.6%)	0.123
Limb dystonia	11 (34.4%)	5 (45.5%)	6 (28.6%)	0.442
Cervical dystonia	4 (12.5%)	2 (18.2%)	2 (9.5%)	0.593
Postural instability	8 (25%)	5 (45.5%)	3 (14.3%)	0.088
Pyramidal	8 (25%)	4 (36.4%)	4 (19.1%)	0.397
Dysarthria	11 (34.4%)	7 (63.6%)	4 (19.1%)	0.020

Note: Bold faced values are statistically significant according to P‐values.

Abbreviations: CBS, corticobasal syndrome; CBS‐4RT+, CBS patients who fulfill criteria for probable four‐repeat tauopathy; CBS‐4RT−, CBS patients who did not fulfill criteria for probable four‐repeat tauopathy; MDS, Movement Disorders Society; PSP, progressive supranuclear palsy.

FDG‐PET and VBM‐MRI Quantitative Group Analyses According to the Criteria for Probable 4R‐Tauopathy

Compared to healthy controls, the CBS‐4RT+ group showed prominent hypometabolism contralateral to the most affected side, surviving correction for multiple comparisons at superior prefrontal regions, with a significant cluster at the inferior and middle frontal gyri extending to the supplementary motor area (SMA). This group also showed significant clusters of hypometabolism at the contralateral middle cingulate area and striatum (Fig. 1).

Figure 1.

Brain glucose metabolism in patients with corticobasal syndrome according to the criteria for probable 4‐repeat tauopathy. Surface projections of the reductions in brain glucose metabolism measured with FDG‐PET in comparison to a control group. Upper row (A): clusters of reduced FDG uptake (areas in blue) in individuals fulfilling criteria for 4R‐tauopathy (CBS‐4RT+). Lower row (B): clusters of reduced FDG uptake in individuals that did not fulfill criteria for 4R‐tauopathy (CBS‐4RT−). Parametric maps were generated with an unpaired t test (P < 0.001, uncorrected) in the SPM12 software and plotted on surface maps with the Surf Ice software. Bars on the right side indicate z scores, ranging from P = 10⁻³ (z score = 3.0) to P = 10⁻⁴ (z score = 4.0).

Conversely, the CBS‐4RT− group showed a significant cluster of rBGM reduction contralateral to the most affected side surviving correction for multiple comparisons at posterior temporoparietal areas, mainly at the superior parietal lobule, posterior temporal gyri, and the posterior cingulate cortex. Other significant clusters were the inferior frontal gyrus, SMA, thalamus, and striatum (Fig. 1). The SPM12 statistics and areas of reduced rBGM are disclosed in Tables S1. and S2.

Furthermore, compared to healthy controls, group analyses on VBM showed that the CBS‐4RT+ group displayed volumetric brain atrophy, surviving correction for multiple comparisons at the anterior cingulate, contralateral to the most affected side, and bilateral striatum (Fig. 2).

Figure 2.

Areas of brain atrophy in patients with corticobasal syndrome according to the criteria for probable 4‐repeat tauopathy. Area of brain atrophy measured with MRI (voxel‐based morphometry—VBM) in comparison to a control group. Upper row (A): clusters of reduced volume in individuals fulfilling criteria for 4‐repeat tauopathy (CBS‐RT+). Lower row (B): clusters of reduced volume in individuals that did not fulfill criteria for 4‐repeat tauopathy (CBS‐RT−). Parametric maps were generated with an unpaired t test (P < 0.001, uncorrected) in the SPM12 software and plotted on a T1w‐MRI template. Bars on the right side indicate z scores, ranging from P = 10⁻³ (z score = 3.0) to P = 10⁻⁴ (z score = 4.0).

The CBS‐4RT− group, instead, demonstrated areas of volumetric brain atrophy in the superior temporal gyrus and thalamus, contralateral to the most affected side. They also displayed brain atrophy at the bilateral striatum and anterior cingulate (Fig. 2). The areas of brain atrophy are disclosed in Tables S3 and S4. Additionally, areas of brain amyloid deposition are disclosed in Tables S5 and S6.

Discussion

In this prospective study, we assessed the diagnostic ability of the MDS‐PSP criteria ¹⁰ for probable 4R‐tauopathy to predict a negative amyloid‐PET on a cohort of CBS patients. We also explored the clinical features and neuroimaging patterns of patients classified as probable 4R‐tauopathy to investigate if they would be congruent with prior observations from clinicopathological CBD and PSP studies.

As our main findings, the probable 4R‐tauopathy criteria were highly specific in predicting amyloid‐negative CBS, and such specificity was similar to a postmortem validation study ¹¹ in predicting 4R‐tauopathy pathology. This finding illustrates that the presence of ocular motor dysfunction suggestive of PSP possibly represents a clinical signature of 4R‐tauopathies among CBS patients. Moreover, the group classified as CBS‐4RT+ showed distinct clinical features and brain metabolic and volumetric patterns previously described in 4R‐tauopathies. ⁸ , ³⁰ , ³¹ , ³²

CBS is a clinical conundrum with diverse etiological substrates. ³³ Among underlying pathologies, CBD and PSP, followed by Alzheimer's disease (AD), stand out as the most common in autopsy series. ² Many clinical criteria have been proposed for CBD ¹⁰ , ¹² , ³⁴ , ³⁵ ; however, their sensitivity and specificity are considered suboptimal in validation studies, ³⁶ , ³⁷ and this topic remains a matter of debate. ³⁸ Meanwhile, there is a vast overlap in tau‐related neurodegenerative diseases, which share common phenotypes. ³⁹ Thus, we hypothesized that the clinical concept of probable 4R‐tauopathy, applied for joint diagnosis of PSP and CBD, would be a valuable tool for distinguishing CBS due to 4R‐tauopathy from CBS due to AD. In order to approach this matter, we opted to clinically classify patients as probable 4R‐tauopathy if they presented vertical gaze palsy or slow saccades, blinded to amyloid‐PET results, and evaluate this classification's ability to distinguish amyloid‐negative CBS patients.

Diagnostic accuracy for the probable 4R‐tauopathy criteria was assessed in autopsy‐confirmed CBD and PSP patients in a previous retrospective study. ¹¹ They found 23% sensitivity and 98% specificity in the third year after disease onset and 59% sensitivity and 88% specificity at the final record. ¹¹ These results are congruent with our findings, showing 52% sensitivity and 92% specificity to predict a negative amyloid‐PET in CBS patients. The aforementioned study did not include CBS‐AD in their analysis. ¹¹ Conversely, we included all CBS cases recruited prospectively regardless of biomarkers results, and, interestingly, the specificity of the criteria remained the same. Although we acknowledge that our work does not represent a validation criteria study and that the MDS‐PSP criteria stated the presence of amyloid biomarkers ³⁸ as an exclusion criterion, we aimed to investigate the clinical usefulness of the 4R‐tauopathy criteria in a real‐world scenario before using a high‐cost AD biomarker. In this regard, we demonstrated high specificity in identifying amyloid‐negative CBS patients, strengthening the clinical utility of the probable 4R‐tauopathy criteria.

Moreover, the sensitivity obtained from our data was low, likely because vertical gaze palsy or slow saccades are required for its diagnosis, and our cohort is primarily a CBS, not PSP, clinical cohort. For example, among CBS patients who did not fulfill the 4R‐tauopathy criteria, nine out of 21 were PIB‐PET negative, and, therefore, most of them may have an underlying tau pathology. This observation indicates that clinical features are likely unable to identify 4R‐tauopathies with adequate sensitivity in CBS cohorts and that developing fluid biomarkers ⁴⁰ or specific tau‐PET ligands ⁸ may be needed, especially when recruiting for clinical trials.

In our cohort, only 34.4% presented with ocular motor dysfunction, such as vertical gaze palsy or slow saccades, required for a 4R‐tauopathy classification. Few studies have investigated the prevalence of ocular motor dysfunction among CBD pathologic‐proven cases. ³ , ⁴¹ A previous review showed that 60% percent had eye movement abnormalities, ¹² including increased saccadic latency, early slowing of horizontal saccade velocity, ⁴¹ delayed onset of vertical gaze palsy, and lesser predominant downgaze abnormalities compared to PSP. ³ In addition, a clinicopathological retrospective study showed that 8% of PSP and 22% of CBS cases had absent ocular motor dysfunction. ¹¹ Thus, those patients who did not fulfill the probable 4R‐tauopathy criteria and had negative amyloid‐PET might have a CBD underlying pathology without evident ocular motor dysfunction or, instead, a pathology different from a primary tauopathy or AD, such as, for example, a TDP‐43 underlying pathology. ³

The clinical profile of the CBS‐4RT+ group also presented with dysarthria and motor perseveration more often than patients who did not fulfill the 4R‐tauopathy criteria. Primary 4R‐tauopathies are a clinical continuum rather than divergent disease groups, sharing various clinical pictures. ⁵ In line with this, dysarthria has often been described in PSP, CBD, and primary progressive aphasia (PPA). ⁴² , ⁴³ A recent study from our group showed that the frequency of dysarthria was higher among amyloid‐negative CBS patients. ¹⁴ Of note, spastic dysarthria is considered a clinical clue in the MDS criteria for PSP. ¹⁰ Motor perseveration, in turn, represents a frontal sign translating into executive dysfunction and impulsivity often found in PSP. ⁴⁴ , ⁴⁵ These symptoms might constitute clinical signatures of primary tauopathies that can be helpful in the clinical setting.

Furthermore, although a previous study showed that memory and language ACE‐R sub‐domains could distinguish CBS due to 4R‐tauopathy from CBS due to AD, ⁴⁶ our results did not find those differences between CBS‐4RT+ or CBS‐4RT− groups (Table 1), possibly owing to the small number of patients classified in the CBS‐4RT+ group.

Metabolic patterns of CBS patients showed significant differences concerning the presence of the 4R‐tauopathy criteria. Previous studies investigated the neuroimaging patterns of different CBS underlying pathologies through biomarkers ¹³ , ⁴⁷ or clinicopathological cohorts. ⁴⁸ However, to our knowledge, no research has focused on this specific subgroup of patients presenting with CBS, ocular motor dysfunction, and negative amyloid‐PET, defined as probable 4R‐tauopathy. The CBS‐4RT+ group had a markedly anterior hypometabolism, extending to SMA and basal ganglia, in line with a previous study of CBS cases with autopsy‐confirmed PSP. ⁴⁸ Our observation, therefore, suggests that most patients in the CBS‐4RT+ group might have an underlying PSP pathology. Conversely, CBS‐4RT− patients displayed hypometabolism in posterior temporoparietal areas, possibly due to the presence of CBS‐AD cases. ¹³ , ⁴⁸

Additionally, CBS patients classified as 4R‐tauopathy displayed brain atrophy at the anterior cingulate and bilateral striatum. In line with this, a previous study with neuropathological data showed that CBS‐CBD and CBS‐PSP had atrophy patterns distinct from CBS‐AD, mainly located at frontal lobes and SMA. ³² Also, a recent clinicopathological study from the same group found that all PSP variants had a high tau burden at the striatum and that the PSP‐CBS variant had a more remarkable degree of frontal white matter abnormalities than other subcortical variants. ³¹ Similar distribution patterns of tau burden in PSP and CBD were confirmed in studies with a second‐generation tau PET tracer. ⁸ , ⁹

The main limitation of our study was the absence of neuropathological examination, precluding us from elucidating the diagnosis of the single case classified in the CBS‐4RT+ group with positive PIB‐PET. The possibility of coexisting pathologies, such as AD in 4R‐tauopathies, cannot be excluded. A previous study highlighted that AD‐related neuropathological changes had been identified in about 26% of PSP and CBD cases. ⁴⁹ Another study demonstrated that clinical phenotypes in tauopathies mainly correlate to tau deposition, with little influence on co‐pathologies. ⁵⁰

Moreover, in vivo biomarkers for tau pathology, such as second‐generation tau‐PET, could have provided valuable information for our study. Firstly, through demonstration of the topographical distribution of tau deposition in the CBS‐4RT+ group. Secondly, distinguishing among patients with negative PIB‐PET in the CBS‐4RT− group, which had an underlying tau pathology from the ones that were tau and amyloid negative. Therefore, this represents another limitation of our work.

On the other hand, as a prospective study, we had a standardized clinical assessment and imaging protocol, which is relevant since the clinical concept of 4R‐tauopathy aims to link a specific clinical syndrome to a molecular diagnosis. ⁵¹ Our findings strengthen this concept, as our data from amyloid‐PET and neurodegeneration markers signatures (brain metabolism and atrophy patterns) of the CBS‐4RT+ group matched previously described patterns in 4R‐tauopathies. ⁴⁷ , ⁴⁸ More validation studies, including PPA cases, are necessary.

Finally, our study demonstrated that the clinical criteria for probable 4R‐tauopathy have high specificity in predicting a negative amyloid‐PET among CBS patients. Also, our findings highlighted the clinical value of ocular motor dysfunction in CBS patients, showing that it might point towards a 4R‐tauopathy as underlying pathology. Therefore, this concept certainly brings advantages to physicians and clinical trial investigators when recruiting patients with primary 4R‐tauopathies, mainly concerning CBS and PPA cases presenting with ocular motor dysfunction. Besides that, these observations raise the possibility of incorporating eye movement abnormalities in future CBD criteria.

Although it is not possible to accurately predict CBD pathology among CBS cases, in a reasonable practical approach to differentiate 4RT from AD‐CBS, these criteria might be a useful tool allowing neurologists to classify CBS patients as probable 4R‐tauopathy. Further studies considering cost‐effectiveness are needed to determine if supporting biomarkers such as FDG‐PET and tau‐PET would be necessary and whether they should be incorporated into diagnostic criteria to achieve optimal sensitivity and specificity.

Author Roles

(1) Research project: A. Conception, B. Organization, C. Execution; (2) Statistical Analysis: A. Design, B. Execution, C. Review and Critique; (3) Manuscript: A. Writing of the first draft, B. Review and Critique.

J.B.P.: 1A, 1B, 1C, 2A, 2B, 3A.

C.d.G.C.: 1B, 1C, 3B.

I.J.d.A.: 1C, 3B.

M.C.B.d.O.: 1C, 3B.

P.M.B.: 1C, 3B.

A.S.‐N.: 1C, 3B.

C.R.O.: 1C, 3B.

R.N.: 1B, 2A, 3B.

E.R.B.: 1B, 2A, 3B.

C.A.B.: 1B, 2A, 3B.

S.M.D.B.: 1A, 1B, 1C, 2A, 3B.

A.M.C.: 1A, 1B, 1C, 2A, 2B, 3B.

Disclosures

Ethical Compliance Statement: The Ethics Committee of our institution (Hospital das Clínicas, Faculdade de Medicina da Universidade de São Paulo) approved the research project (under protocol number 67195517.4.0000.0068). All volunteers consented to participate by signing a consent form. 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 Conflicts of Interest: This work was supported by the São Paulo Research Foundation (FAPESP) in Brazil, reference number 2017/10033‐4. The authors report no conflicts of interest.

Financial Disclosures for the Previous 12 Months: J.B.P., C.d.G.C., I.J.d.A., M.C.B.d.O., P.M.B., A.S‐N., C.R.O., E.R.B., R.N., C.A.B., S.M.D., A.M.C. reports no disclosures.

Supporting information

Table S1. Areas of regional brain glucose metabolism (rGBM) reduction in corticobasal syndrome patients who fulfilled criteria for “probable four‐repeat tauopathy” (CBS‐4RT+) compared to age‐matched healthy controls.

Table S2. Areas of regional brain glucose metabolism (rGBM) reduction in corticobasal syndrome patients who did not fulfill criteria for “probable four‐repeat tauopathy” (CBS‐4RT−) compared to age‐matched healthy controls.

Table S3. Areas of volumetric brain atrophy in corticobasal syndrome patients who fulfilled criteria for “probable four‐repeat tauopathy” (CBS‐4RT+) compared to age‐matched healthy controls.

Table S4. Areas of brain atrophy in corticobasal syndrome patients who did not fulfill criteria for “probable four‐repeat tauopathy” (CBS‐4RT−) compared to age‐matched healthy controls.

Table S5. Areas of brain amyloid deposition in corticobasal syndrome patients who fulfilled criteria for “probable four‐repeat tauopathy” (CBS‐4RT+) compared to age‐matched healthy controls.

Table S6. Areas of brain amyloid deposition in corticobasal syndrome patients who did not fulfill criteria for “probable four‐repeat tauopathy” (CBS‐4RT−) compared to age‐matched healthy controls.