Biomarkers & Differential Diagnosis of Lewy Body Dementia

The diagnosis of Lewy body dementia (LBD) has been transformed by advances in biomarker technology, evolving from a purely clinical syndrome defined by core features to one increasingly anchored in biological evidence of alpha-synuclein pathology. The 2017 fourth consensus report of the DLB Consortium formally recognized indicative biomarkers (dopamine transporter SPECT, polysomnography, MIBG myocardial scintigraphy) and supportive biomarkers (MRI, FDG-PET, EEG) as integral to diagnostic criteria. More recently, the development of alpha-synuclein seed amplification assays (SAA) in CSF and phosphorylated alpha-synuclein detection in skin biopsy has enabled direct measurement of the underlying proteinopathy, representing a paradigm shift from indirect markers of neurodegeneration toward disease-specific biological diagnosis. These advances are critical not only for distinguishing DLB from Alzheimer disease (AD), frontotemporal dementia (FTD), and Parkinson disease dementia (PDD) but also for enabling earlier detection in prodromal stages and facilitating enrollment in disease-modifying clinical trials.

Alpha-Synuclein Biomarkers

CSF Seed Amplification Assay (SAA)

Alpha-synuclein can spread from cell to cell and accumulate in a prion-like fashion, and this capacity for self-propagation has enabled the development of seed amplification assays (SAA) that detect aggregated alpha-synuclein in biospecimens. Two primary methodologies have emerged: real-time quaking-induced conversion (RT-QuIC) and protein-misfolding cyclic amplification (PMCA). Both exploit the ability of misfolded alpha-synuclein "seeds" to recruit and convert native monomeric alpha-synuclein into aggregated forms, amplifying the signal to detectable levels.

An FDA-cleared commercial CSF assay is now available with the following performance characteristics:

Recent clinicopathologic correlation studies have confirmed that the commercial CSF SAA performs very well in cases with high neocortical and limbic Lewy body burden, but sensitivity declines in cases with pathology restricted to the amygdala or brainstem. In the prodromal stages such as isolated RBD, the seeding assays may have lower sensitivity and specificity, reflecting the more limited distribution of alpha-synuclein aggregates early in the disease course.

Skin Biopsy for Phosphorylated Alpha-Synuclein

A dermal biopsy provides sweat and sebaceous gland tissue as well as intraepidermal and subdermal nerve fibers where Lewy pathology can be detected. An FDA-cleared commercial assay examines phosphorylated alpha-synuclein at serine 129 (pSer129) using immunofluorescence in skin punch biopsies from three standardized sites.

Plasma Alpha-Synuclein Biomarkers

Several platforms are being developed to measure total alpha-synuclein and phosphorylated alpha-synuclein in plasma using immunohistochemical, mass spectrometry, and immunomagnetic reduction (IMR) techniques. Preliminary studies show that plasma levels of total alpha-synuclein and pSer129 alpha-synuclein are significantly higher in patients with PD compared with controls. The ratio of phosphorylated to non-phosphorylated synuclein may help discriminate PD from healthy controls, and pSer129 levels correlate with Hoehn and Yahr motor stages and MDS-UPDRS Part III scores. Preliminary evidence suggests total alpha-synuclein may predict cognitive decline while pSer129 predicts motor progression. However, most laboratories have had difficulty replicating these findings, and blood-based alpha-synuclein markers are not yet clinically available.

Dopamine Transporter Imaging (DAT-SPECT)

Dopamine transporter single-photon emission computed tomography (DAT-SPECT), commonly known as DaTscan, uses iodine-123-ioflupane to visualize the integrity of the nigrostriatal dopaminergic pathway. It is the most established indicative biomarker for LBD and has an FDA-approved indication for the diagnosis of PD, PDD, and DLB.

DAT-SPECT was upgraded from a suggestive feature in the third DLB consensus report to an indicative biomarker in the fourth consensus report (2017). The scan is particularly valuable when clinical suspicion for DLB is high but core clinical features are insufficient for a confident diagnosis. However, it is critical to recognize that an abnormal DAT-SPECT alone, in the absence of core clinical features, is not sufficient to diagnose probable DLB — it reflects dopaminergic neurodegeneration, which is shared across multiple synucleinopathies and atypical parkinsonian disorders.

MIBG Myocardial Scintigraphy

Iodine-123 metaiodobenzylguanidine (¹²³I-MIBG) myocardial scintigraphy is an indicative biomarker that measures postganglionic cardiac sympathetic innervation. In LBD, alpha-synuclein aggregation damages postganglionic sympathetic neurons innervating the heart, resulting in reduced MIBG uptake. The heart-to-mediastinum (H/M) ratio on delayed images is the standard quantitative measure, with an H/M ratio <1.6 generally considered abnormal.

• Sensitivity: 69%–100% for DLB (varies by study and cutoff)
• Specificity: 87%–100% vs AD (very high for distinguishing LBD from AD)
• False positives: Diabetes mellitus with autonomic neuropathy, heart failure, ischemic heart disease, and medications affecting norepinephrine reuptake
• Clinical use: Rarely used in the United States; more commonly used in Japan where it is established in clinical practice
• Advantage: Abnormal early in LBD, often before prominent clinical features; reflects peripheral alpha-synuclein pathology

Polysomnography for RBD Confirmation

Polysomnography (PSG) demonstrating REM sleep without atonia (RSWA) is an indicative biomarker in the DLB diagnostic criteria. While RBD can often be suspected clinically from the patient and bed partner history, PSG provides objective confirmation by documenting excessive electromyographic activity during REM sleep.

Neuroimaging Biomarkers

FDG-PET Patterns

Fludeoxyglucose positron emission tomography (FDG-PET) is a supportive biomarker in the DLB diagnostic criteria. While posterior parietal and temporal hypometabolism can be seen in both LBD and AD, two additional patterns help distinguish LBD:

• Occipital hypometabolism: More common in LBD than AD, especially in patients with visual hallucinations; correlates with the visuoperceptual deficits characteristic of LBD
• Cingulate island sign (CIS): Relative preservation of FDG activity in the posterior cingulate cortex amid surrounding occipital and parietal hypometabolism; the posterior cingulate is typically affected early in AD but relatively spared in DLB. The CIS ratio (posterior cingulate metabolism divided by precuneus or cuneus metabolism) has been shown to reliably differentiate DLB from AD

Reduced occipital metabolism on FDG-PET is particularly useful when visual hallucinations are present, as it provides a neurometabolic correlate for this core clinical feature. An optimized cingulate island ratio improves differentiation between LBD and AD, and the combination of occipital hypometabolism with the cingulate island sign has high specificity for LBD.

MRI Findings

Structural MRI is classified as a supportive biomarker for DLB. The most important finding is the relative preservation of hippocampal and medial temporal lobe volumes in DLB compared with AD, where hippocampal atrophy is a hallmark feature:

• Medial temporal lobe atrophy: Mild or absent in DLB; prominent in AD (Scheltens visual rating scale can quantify)
• Cortical atrophy: Present in both DLB and AD; not a distinguishing feature
• Posterior cortical involvement: Occipital and posterior parietal atrophy may be more prominent in DLB, correlating with visuospatial deficits
• Volumetric analysis: Automated hippocampal volumetry tools (e.g., NeuroQuant) can provide quantitative data supporting the distinction
• Limitation: MRI cannot distinguish DLB from other non-AD dementias; mixed AD/LBD pathology (present in ~80% of DLB cases) may show intermediate atrophy patterns

EEG Features

Prominent posterior slow-wave activity on EEG is a supportive biomarker for DLB. Patients with DLB characteristically show slowing of the dominant posterior rhythm below the alpha range (<8 Hz) with increased theta and delta activity, particularly over posterior regions. Quantitative EEG (qEEG) shows a shift from alpha to pre-alpha/theta dominant frequency with increased variability. These changes may correlate with the cognitive fluctuations that are a hallmark of DLB. EEG findings are most useful as supportive rather than diagnostic evidence and should be interpreted alongside other clinical and biomarker data.

Amyloid and Tau PET in LBD

Although no PET tracers that bind to alpha-synuclein aggregates are available for clinical use, PET studies using ligands that characterize AD pathology are informative given the high rate of concomitant AD pathology in LBD.

• Amyloid PET: Three commercially available tracers (florbetapir, florbetaben, flutemetamol) bind fibrillar amyloid-beta but do not significantly bind to neurofibrillary tangles or Lewy bodies. Cortical amyloid burden: DLB > PDD, but slightly lower than pure AD. Amyloid PET is useful for distinguishing AD from PDD but less so for AD vs DLB given that ~80% of DLB cases are amyloid-positive. In MCI stages, amyloid PET is less useful than DaT-SPECT for identifying LBD
• Tau PET: Tau burden is variable in DLB but generally lower than in AD. A 2023 study found that 43% of amyloid-positive DLB cases were tau-positive vs only 8% of amyloid-negative DLB cases on flortaucipir imaging. Regional distribution in DLB is most notable in the inferior temporal gyrus and precuneus. PDD shows less tau binding than DLB, consistent with lower concomitant AD pathology. PD without dementia shows tau levels similar to healthy controls

Differential Diagnosis

DLB vs Alzheimer Disease

Distinguishing DLB from AD is the most common diagnostic challenge, particularly given that approximately 80% of DLB cases have concomitant AD copathology at autopsy.

DLB vs Parkinson Disease Dementia

DLB and PDD share the same underlying alpha-synuclein pathology and are distinguished solely by the 1-year rule: if dementia develops more than 12 months after established PD motor symptoms, PDD is diagnosed; if dementia precedes or is concurrent with parkinsonism, DLB is diagnosed. While this distinction is clinically useful, it has been increasingly challenged:

• Neuropathology: There is no pathologic distinction between PDD and DLB at autopsy — both show Lewy bodies and Lewy neurites in neocortical, limbic, and brainstem regions
• Neuropsychological profile: Very similar between PDD and DLB; both conditions are better distinguished from AD than from each other
• Biomarkers: CSF alpha-synuclein SAA, DAT-SPECT, and MIBG scintigraphy are abnormal in both; no biomarker reliably distinguishes PDD from DLB
• Tau PET: Tau binding tends to be slightly less in PDD than DLB, most notably in the inferior temporal gyrus and precuneus, but with significant overlap
• Proposed reclassification: A 2024 proposal to redefine PD, PDD, and DLB as "neuronal alpha-synuclein disease" using a biological definition (S anchor = CSF SAA positive; D anchor = abnormal DAT-SPECT) may eventually replace the 1-year rule with an integrated staging system

DLB vs Frontotemporal Dementia

Although less commonly confused than DLB and AD, the behavioral variant of FTD (bvFTD) may occasionally enter the differential diagnosis, particularly when early behavioral changes are prominent:

• Behavioral changes in DLB: Depression, anxiety, apathy, and hallucinations predominate; social comportment relatively preserved early
• Behavioral changes in bvFTD: Early loss of social comportment, disinhibition, apathy, loss of empathy, compulsive behaviors, and dietary changes; hallucinations are uncommon
• Cognitive profile: DLB shows visuospatial/executive deficits with relative memory preservation; bvFTD shows executive/social cognition deficits with relative visuospatial preservation
• Motor features: Parkinsonism supports DLB; motor neuron signs or progressive supranuclear palsy features support FTD spectrum disorders
• Imaging: DLB shows posterior-predominant changes (occipital hypometabolism, cingulate island sign); bvFTD shows frontal and anterior temporal atrophy/hypometabolism
• CSF SAA: Positive in DLB; negative in FTD

Recent Advances in Fluid Biomarkers

Blood-Based Biomarker Development

While blood-based biomarkers have revolutionized AD diagnosis (plasma p-tau217, Abeta42/40 ratio), progress in LBD-specific blood biomarkers has been more challenging. Several approaches are under investigation:

Alpha-Synuclein PET Tracers

There are currently no clinically available PET ligands for alpha-synuclein, but several tracers are in development. The PET ligand [¹⁸F]ACI-12589 demonstrated good binding in the cerebellar white matter of patients with multiple system atrophy (MSA) with limited binding in PD in early studies. Other libraries of PET tracers are being developed with early evidence for selective binding to alpha-synuclein pathology and the ability to cross the blood-brain barrier. The development of an effective alpha-synuclein PET tracer would represent a transformative advance, enabling in vivo visualization of the defining pathology of LBD.

Neuropathologic Correlation

There is no pathologic distinction between PDD and DLB. The core pathologic features of LBD are Lewy bodies (intraneuronal cytoplasmic inclusions) and Lewy neurites (pathologic aggregates in neuronal processes), with alpha-synuclein as the primary building block.

Mixed pathology has important clinical implications: patients with combined DLB and AD pathology tend to have worse memory performance than those with pure LBD pathology and may present with a more AD-like clinical phenotype, further complicating differential diagnosis during life. Genetic risk factors overlap: both APOE epsilon-4 and GBA mutations are risk factors for LBD, with heterozygous GBA mutations increasing risk approximately fivefold. Recent genome-wide studies have also identified SNCA, BIN1, and TMEM175 as additional risk loci.

References

1. Galvin JE. Lewy body dementia. Continuum (Minneap Minn). 2024;30(6):1673-1698.
2. McKeith IG, Boeve BF, Dickson DW, et al. Diagnosis and management of dementia with Lewy bodies: fourth consensus report of the DLB Consortium. Neurology. 2017;89(1):88-100.
3. Wang Y, Hu J, Chen X, et al. Real-time quaking-induced conversion assay is accurate for Lewy body diseases: a meta-analysis. Neurol Sci. 2022;43(7):4125-4132.
4. Samudra N, Fischer DL, Lenio S, et al. Clinicopathological correlation of cerebrospinal fluid alpha-synuclein seed amplification assay in a behavioral neurology autopsy cohort. Alzheimers Dement. 2024;20(5):3334-3341.
5. Iranzo A, Mammana A, Munoz-Lopetegi A, et al. Misfolded alpha-synuclein assessment in the skin and CSF by RT-QuIC in isolated REM sleep behavior disorder. Neurology. 2023;100(18):e1944-e1954.
6. Gibbons CH, Freeman R, Bellaire B, et al. Synuclein-One study: skin biopsy detection of phosphorylated alpha-synuclein for diagnosis of synucleinopathies. Biomark Med. 2022;16(7):499-509.
7. Lin CH, Yang SY, Horng HE, et al. Plasma biomarkers differentiate Parkinson's disease from atypical parkinsonism syndromes. Front Aging Neurosci. 2018;10:123.
8. Lin CH, Liu HC, Yang SY, et al. Plasma pS129-alpha-synuclein is a surrogate biofluid marker of motor severity and progression in Parkinson's disease. J Clin Med. 2019;8(10):1601.
9. Woyk K, Sahlmann CO, Hansen N, et al. Brain 18F-FDG-PET and an optimized cingulate island ratio to differentiate Lewy body dementia and Alzheimer's disease. J Neuroimaging. 2023;33(2):256-268.
10. Simuni T, Chahine LM, Poston K, et al. A biological definition of neuronal alpha-synuclein disease: towards an integrated staging system for research. Lancet Neurol. 2024;23(2):178-190.
11. Smith R, Capotosti F, Schain M, et al. The alpha-synuclein PET tracer [18F]ACI-12589 distinguishes multiple system atrophy from other neurodegenerative diseases. Nat Commun. 2023;14:6750.
12. Coomans EM, de Koning LA, Rikken RM, et al. Performance of a [18F]flortaucipir PET visual read method across the Alzheimer disease continuum and in dementia with Lewy bodies. Neurology. 2023;101(19):e1850-e1862.
13. Rossi M, Baiardi S, Teunissen CE, et al. Diagnostic value of the CSF alpha-synuclein real-time quaking-induced conversion assay at the prodromal MCI stage of dementia with Lewy bodies. Neurology. 2021;97(9):e930-e940.
14. Roberts G, Kane JPM, Lloyd J, et al. Can early phase cardiac [123I]mIBG images be used to diagnose Lewy body disease? Nucl Med Commun. 2022;43(7):770-777.
15. Thomas AJ, Donaghy P, Roberts G, et al. Diagnostic accuracy of dopaminergic imaging in prodromal dementia with Lewy bodies. Psychol Med. 2019;49(3):396-402.
16. Chia R, Sabir MS, Bandres-Ciga S, et al. Genome sequencing analysis identifies new loci associated with Lewy body dementia and provides insights into its genetic architecture. Nat Genet. 2021;53(3):294-303.
17. Toledo JB, Abdelnour C, Weil RS, et al. Dementia with Lewy bodies: impact of co-pathologies and implications for clinical trial design. Alzheimers Dement. 2023;19(1):318-332.
18. Kantarci K, Lowe VJ, Chen Q, et al. Beta-amyloid PET and neuropathology in dementia with Lewy bodies. Neurology. 2020;94(3):e282-e291.