Updates in Diagnostic Testing in Parkinson Disease

Several ancillary tests have become available to support a diagnosis of Parkinson disease (PD). These tests serve as supportive tools, but PD and forms of atypical parkinsonism are defined by clinical diagnostic criteria, underscoring the importance of the physical examination and patient history. However, given limited access to movement disorder specialists, neurologists may find such ancillary tests helpful to support a diagnosis.¹ Methods for detection of α-synuclein (αSyn) in skin and cerebrospinal fluid (CSF) samples have become commercially available, enhancing diagnostic accuracy in synucleinopathies. This article outlines the context, interpretation, and strengths and limitations of diagnostic testing in PD.

Dopamine Transporter Single-Photon Emission CT

Dopamine transporter single-photon emission CT (DAT-SPECT) is a radiotracer neuroimaging technique that uses SPECT to visualize DAT levels in the basal ganglia. The procedure involves intravenous injection of a radiotracer, followed in 3 to 6 hours by SPECT imaging to assess DAT binding in the striatum. Commonly referred to as “DaTscan”, the name for its patented radiotracer Ioflupane-123, this imaging modality received Food and Drug Administration (FDA) approval in 2011 for the detection of DAT in those with suspected parkinsonian syndromes.²

Interpretation

The majority of DaTscan results are interpreted qualitatively and provide a visual indication of the number of dopamine-secreting cells. In a positive scan, loss of the usual comma-shaped pattern of radiotracer uptake in the striatum is indicative of reduced DAT binding, consistent with the dopaminergic deficit seen in neurodegenerative parkinsonism (Figure).

DaTscan images demonstrate presynaptic dopaminergic innervation, distinguishing people with PD or other forms of neurodegenerative parkinsonism from those with essential tremor (ET), those with drug-induced parkinsonism, or unaffected individuals.³ Because atypical parkinsonism also results in a presynaptic dopaminergic deficit, DaTscan images cannot differentiate PD from atypical parkinsonism (ie, multiple system atrophy [MSA], progressive supranuclear palsy [PSP], corticobasal degeneration [CBD]).⁴ DaTscan results may be abnormal in patients with idiopathic normal pressure hydrocephalus, and these results have been shown to normalize after shunting, suggesting reversible striatal dysfunction.⁵ In rare cases, a DaTscan may have normal results in a person with parkinsonian symptoms meeting diagnostic criteria for PD, which is known as a scan without evidence of dopamine deficit (SWEDD).

Sensitivity and specificity of DaTscan results vary depending on disease stage and certainty of the clinical diagnosis. Assuming clinical diagnosis is the gold standard in PD, DaTscan has a reported sensitivity of 79% to 100% in individuals with possible or probable PD, which increases with disease progression.^6,7 DaTscan specificity has been reported as 97% to 98%.⁷ Consequently, a negative DaTscan result does not rule out PD. The sensitivity of DAT imaging may be lower in atypical parkinsonian syndromes.⁶

Uses

DaTscan imaging may provide additional diagnostic clarity in several situations (see Table). First, the DaTscan serves as a tool for distinguishing parkinsonian tremor from ET, as specified in its FDA indication. This distinction may inform appropriate symptomatic treatments and assist in selecting the optimal anatomic target for deep brain stimulation candidates. DaTscan imaging can also distinguish drug-induced parkinsonism from neurodegenerative forms of parkinsonism. In cases of dementia with psychosis, DaTscan imaging serves as a valuable tool for helping to distinguish dementia with Lewy bodies (DLB) from Alzheimer dementia, guiding treatment approaches.

Advantages and Limitations

The quality of DaTscan images may differ among imaging centers, and interpretation of imaging results may vary among radiologists. Results may also be affected by a patient’s concomitant use of medications, including certain antidepressants, anticholinergics, and amphetamines.⁸ Medications that should ideally be withheld before DaTscan imaging include bupropion, paroxetine, venlafaxine, benztropine, and methylphenidate, among others.⁹

DaTscan imaging provides valuable insights into the neurochemical changes associated with PD, but its routine use in diagnosis remains controversial. In most cases, clinical evaluation coupled with response to dopaminergic therapy is sufficient for PD diagnosis, making DaTscan unnecessary. However, in certain cases of diagnostic uncertainty, DaTscan results can offer clarification and help to guide treatment decisions. Ongoing research is exploring potential uses of DaTscan in monitoring disease progression and evaluating treatment response.¹⁰

α-Synuclein Testing: A Novel Diagnostic Approach

Synucleinopathies—including PD, MSA, DLB, and pure autonomic failure—are characterized pathologically by the accumulation of phosphorylated αSyn (αSynP) aggregates in the central and peripheral nervous systems.¹¹ Detection of αSynP in cutaneous nerve fibers and CSF has been developed as a novel diagnostic biomarker for these synucleinopathies. Plasma αSynP assays remain under development.¹²

Performance and Interpretation of α-Synuclein Tests

Commercially available tests for detection of αSyn in biosamples include αSyn skin biopsy and CSF αSyn testing by seed amplification assay.

α-Synuclein Skin Biopsy

Advanced immunohistofluorescence techniques have enabled αSynP identification in cutaneous samples.¹¹ The Syn-One Test (CND Life Sciences; Scottsdale, AZ) is a Clinical Laboratory Improvement Amendments–certified laboratory-developed test in which 3 samples are taken by punch biopsy from the neck, distal thigh, and distal leg. This test can be performed in the clinic setting using a local anesthetic. Samples are sent to a laboratory and immunohistochemical staining is performed, and results are returned within 3 weeks of the specimen’s arrival at the laboratory. The Syn-One pathology report includes a positive or negative result for each biopsy site with a summary of diagnostic conclusions as well as microscopic and macroscopic review of the specimen and a review of the pathology.

In a recent study, the proportions of individuals with cutaneous αSynP detected by skin biopsy were 92.7% with PD, 98.2% with MSA, 96.0% with DLB, 100% with pure autonomic failure, and 3.3% with no history of synucleinopathy (controls).¹⁴ Studies indicated that skin-based detection of αSynP using immunofluorescence demonstrated 90% to 96% sensitivity and 95% to 100% specificity for PD.^13-15 Although evidence is emerging that patterns of cutaneous αSynP may vary between individuals with MSA and PD,¹⁶ the Syn-One test does not differentiate between synucleinopathies.

CSF α-Synuclein Testing by Seed Amplification Assay

Misfolded αSynP has prion-like aggregation seeding activity, enabling its detection by seed amplification assays (SAAs), which are real-time quaking-induced conversion and protein misfolding cyclic amplification assays from CSF and skin samples.^17,18 SYNTap (Amprion, San Diego, CA), a CSF-based αSyn SAA, was granted breakthrough device designation by the FDA in 2019 and became commercially available in 2021.

SYNTap requires a CSF sample obtained by lumbar puncture. The SYNTap assay amplifies minute amounts of αSyn and provides a qualitative readout specifying its presence or absence.

A recent large analysis from the Parkinson’s Progression Markers Initiative cohort demonstrated that αSyn SAA in CSF had an 87.7% sensitivity in people with PD and 96.3% specificity in healthy controls.¹⁹

Utility of α-Synuclein Testing

Coupled with a thorough clinical evaluation, αSyn tests aid in the differentiation of synucleinopathies (PD and MSA) from other proteinopathies, including those involving accretion of tau (eg, PSP and CBD) or amyloid-β aggregation (eg, Alzheimer disease). Diagnostic clarity helps patients and their caregivers set expectations and plan for the future. A positive synuclein assay supports the diagnosis of PD or other synucleinopathies, but a negative test does not exclude these diagnoses.

Advantages and Limitations of α-Synuclein Testing

Skin and CSF-based testing for αSyn has high sensitivity and specificity for α-synucleinopathies including PD, adding pathologic information to the diagnostic arsenal. Despite its diagnostic value, αSyn testing remains in its early stages and has limitations. Further longitudinal studies and autopsy data are required to improve the diagnostic use of these biomarkers. Expert consensus maintains that additional research is needed before a biologic definition of PD can be established.²⁰

Genetic Testing for PD

Whereas the majority of cases of PD are sporadic, an estimated 5% to 10% of people with PD have identifiable high-penetrance monogenic sequence variations.²¹ Genetic testing options are expanding, with clinical, research, and direct-to-consumer avenues becoming increasingly available. Several companies offer PD-targeted gene panels, and the affordability of whole-exome sequencing is improving. In addition, research-based genetic testing can be pursued at no cost to the individual, with some projects providing genetic counseling. Research initiatives offering free genetic testing include PD GENEration, the Parkinson’s Progression Markers Initiative, and the Rostock International Parkinson’s Disease Study.

Certain genetic forms of PD may portend a specific prognosis or symptom profile. For example, individuals with PD with GBA variations tend to have more rapid cognitive decline, and some LRRK2 sequence variations predict slower motor progression.^21,22 However, additional research is needed to uncover the genotype–phenotype correlations in PD.

Uses

There is no consensus on when genetic testing is appropriate in PD, and genetic testing practices differ among health care centers. The diagnosis of a genetic form of PD does not alter symptomatic management. Ongoing research efforts are dedicated to developing targeted therapies for specific genetic forms of PD, although these approaches remain in investigational stages.^23,24

In general, genetic testing may be considered in individuals with early onset PD (age ≤50 years at diagnosis), a first-degree relative with PD, or ethnic background considered to be high risk (ie, Ashkenazi Jewish or North African Berber).^21,24 Because genetic testing results are not actionable in terms of treatment, careful consideration should be given before pursuing genetic testing for PD. Genetic testing in patients with PD should be guided by clinical indication and accompanied by pretest and posttest genetic counseling. Assessment of the individual’s goals and motivation for testing, psychosocial readiness for results, family dynamics, and insurability and legal ramifications should be examined before performing genetic testing.

Interpretation

Since the associated genes for several familial forms of PD remain unidentified, a negative genetic test result does not definitively exclude a genetic basis for PD. Involvement of genetic counselors in interpreting testing results, if available, is recommended to navigate complexities related to variable penetrance and different inheritance patterns, as well as help individuals cope with uncertainty surrounding their genetic risk for PD. Tests may yield variants of undetermined significance, which might later be reclassified as pathogenic sequence variations as more evidence accumulates in genetic databases.

Advantages and Limitations

Whereas genetic testing results may carry prognostic significance, much remains unknown about the genetics of PD, and factors such as incomplete penetrance in several key genes associated with PD complicate counseling around test results. Should familial PD be suspected, consultation with a genetic counselor or movement disorders specialist is recommended.

Conclusion

From αSyn detection in biosamples to multigene panels, the diagnostic arsenal in PD continues to expand. These tests enhance diagnostic accuracy and aid in distinguishing among various parkinsonian disorders, but they are not standalone diagnostic tools, and should be interpreted within the context of a thorough clinical evaluation, often with the support or guidance of a movement disorders specialist. Additional research is needed to further validate the use of newer biomarkers and refine their interpretation. This area of investigation remains a topic of interest in PD research. Researchers continue to work toward a more precise biologic understanding of PD, laying the groundwork for anticipated precision medicine approaches.