Diagnosis & Biomarkers

1. The Diagnostic Pathway

The diagnosis of Parkinson’s disease is fundamentally a clinical diagnosis, supported by ancillary investigations only when needed. The work-up follows a stepwise logic:

1. Establish parkinsonism — bradykinesia plus rest tremor or rigidity (MDS criteria).
2. Exclude secondary causes — drugs (neuroleptics, metoclopramide, valproate), vascular parkinsonism (lower-body, stepwise), normal-pressure hydrocephalus, structural lesions, Wilson disease (in young onset).
3. Look for red flags of atypical parkinsonism — supranuclear gaze palsy (PSP), early severe autonomic failure (MSA), cortical signs (CBD), early hallucinations + cognition (DLB).
4. Apply MDS criteria for clinically established or clinically probable PD.
5. Confirm levodopa response — supportive criterion; sustained >30% improvement in motor scores on adequate trial.
6. Add ancillary tests in atypical or uncertain presentations: DaTscan, MRI, MIBG, autonomic testing, SAA, genetic panel.

Even in expert hands the clinical diagnosis is ~80–90% accurate against autopsy gold standard (Hughes et al., Brain 2002; Rizzo et al., Neurology 2016). The introduction of CSF and skin SAA is on track to improve this substantially in the coming years.

2. The MDS 2015 Clinical Diagnostic Criteria

The Movement Disorder Society Task Force criteria (Postuma et al., Mov Disord 2015) replaced the UK Brain Bank criteria as the modern standard. Two levels of certainty:

Step 1 — establish parkinsonism

Bradykinesia (decrement on repetitive movement) plus at least one of: 4–6 Hz rest tremor or rigidity.

Supportive criteria (positive features)

• Clear and dramatic beneficial response to dopaminergic therapy (sustained, ~UPDRS-III drop >30%).
• Levodopa-induced dyskinesias.
• Rest tremor of a limb (current or historical).
• Olfactory loss (UPSIT/Sniffin’ Sticks below 5th percentile) or cardiac sympathetic denervation on MIBG.

Absolute exclusion criteria (any one rules out PD)

• Unequivocal cerebellar signs (cerebellar gait, limb ataxia, oculomotor signs)
• Downward vertical supranuclear gaze palsy or selective slowing of downward saccades
• Probable bvFTD or PPA within first 5 years
• Parkinsonism restricted to lower limbs >3 years (vascular parkinsonism)
• Treatment with a dopamine receptor blocker / depleter at onset (drug-induced)
• Absence of any response to high-dose (≥600 mg/day) levodopa despite at least moderate severity
• Unequivocal cortical sensory loss, ideomotor apraxia, or progressive aphasia (CBS)
• Normal functional dopaminergic imaging in the presynaptic system
• Documented alternative condition known to produce parkinsonism that plausibly explains the syndrome

Red flags (each is a soft warning)

• Rapid progression of gait impairment requiring wheelchair within 5 years
• Complete absence of motor progression over 5 or more years
• Early bulbar dysfunction within 5 years
• Inspiratory respiratory dysfunction (stridor or sighing)
• Severe autonomic failure (orthostatic hypotension, severe urinary retention) within 5 years
• Recurrent falls due to balance impairment within 3 years
• Anterocollis (dystonic) or contractures within 10 years
• Absence of any common non-motor feature after 5 years
• Otherwise unexplained pyramidal signs
• Bilateral symmetric parkinsonism throughout course

3. DaTscan — ¹²³I-FP-CIT SPECT

DaTscan (ioflupane, ¹²³I-FP-CIT) is a SPECT radiotracer that binds presynaptic dopamine transporters (DAT) on striatal terminals. In healthy adults, DAT density gives a symmetric, comma-shaped caudate-putaminal signal. In PD, the putamen contralateral to the more affected side shows reduced uptake first — the “period” appearance — and progresses through “tail- missing” (caudate preserved, putamen lost) to global striatal loss in advanced disease.

• ~95% sensitivity and specificity for distinguishing degenerative parkinsonism (PD, MSA, PSP, CBD, DLB) from non-degenerative (essential tremor, drug-induced, dystonic tremor, vascular parkinsonism, psychogenic).
• Cannot distinguish PD from MSA/PSP/DLB — all are presynaptic dopaminergic.
• Useful for resolving the “essential tremor vs PD” question, the “drug-induced parkinsonism vs PD” question, and SWEDDs (scans without evidence of dopaminergic deficit) — ~10% of clinically diagnosed PD have normal DaTscans, mostly representing dystonic tremor or essential tremor mimics.
• FDA-approved (2011) for differentiating PD-spectrum disorders from essential tremor.
• Quantitative analysis (specific binding ratio, SBR) supports clinical visual reads.

PET DAT tracers (¹⁸F-FP-CIT,¹¹C-PE2I; and the more dopamine-synthesis-specific ¹⁸F-FDOPA) offer higher resolution and quantitation but are less widely available. New tracers targeting VMAT2 (¹⁸F-DTBZ) measure intra-vesicular dopamine handling and are increasingly used in research.

4. MRI in Parkinson’s and the Atypicals

Conventional 1.5/3 T MRI is typically normal in idiopathic PD — its role is mainly to exclude structural mimics (vascular parkinsonism, tumour, NPH, Wilson disease). High-field (3 T+) and susceptibility-weighted imaging (SWI / QSM) have introduced two PD-specific signs:

• Loss of the “swallow-tail” sign on SWI — nigrosome 1 is normally hyperintense (low iron) within the surrounding hypointense SNr, giving a swallow-tail appearance on axial SWI. Loss of N1 hyperintensity ≈ nigral degeneration. Sensitivity ~90%, specificity ~85% for PD vs controls (Schwarz et al., PLoS One 2014).
• Neuromelanin-MRI — T1-weighted with spectral suppression visualises the SNc directly; reduced volume/intensity in PD.

MRI is far more useful in the atypicals, where each disorder has characteristic red-flag signs:

These signs are imperfect; clinical context dominates. A normal MRI does not exclude an atypical syndrome, especially early in disease.

5. MIBG Cardiac Scintigraphy

¹²³I-MIBG (metaiodobenzylguanidine)is a noradrenaline analogue taken up by sympathetic nerve terminals. The heart-to-mediastinum ratio (H/M) measures cardiac sympathetic innervation. In PD, cardiac sympathetic denervation occurs early and is severe — the post- ganglionic sympathetic axons of the heart are an early target of α-synuclein pathology. Crucially, in MSA the cardiac sympathetic system is largely preserved (the lesion is preganglionic, central). MIBG is therefore one of the best PD-vs-MSA discriminators:

• ~90% sensitivity, ~80% specificity for PD vs MSA (Treglia et al., Mov Disord 2012).
• Reduced H/M also seen in DLB and pure autonomic failure (also synucleinopathies).
• Preserved in PSP, CBD, vascular parkinsonism, drug-induced — useful when these are in the differential.
• Available routinely in Japan and select European centres; less so in North America.

6. Seed Amplification Assays — the New Molecular Diagnostic

The most consequential development of the past decade in PD diagnostics: seed amplification assays (SAA), also known as RT-QuIC (real-time quaking-induced conversion) or PMCA. A small sample (CSF, skin biopsy, submandibular gland, olfactory mucosa) is incubated with recombinant α-synuclein monomer + ThT in a shaking plate; pre-formed seeds in the sample template fibril growth, producing a fluorescent kinetic curve.

• CSF α-synuclein SAA — ~95% sensitivity, ~98% specificity for PD vs healthy controls; positive years before motor onset; positive in ~99% of pure RBD; positive in nearly all DLB. The Parkinson Progression Markers Initiative (PPMI; Siderowf et al., Lancet Neurol 2023) demonstrated robust performance across >1,100 participants.
• Skin α-synuclein SAA — from a 3 mm punch biopsy of cervical or thigh skin; less invasive than LP; ~93% sensitivity, ~96% specificity; emerging Phase 3 data (Gibbons et al., JAMA 2024).
• Submandibular gland biopsy — high yield, more invasive; mainly research.
• SAA distinguishes PD from PSP/CBD (4R-tauopathies; SAA-negative) but cannot reliably distinguish PD from DLB or MSA (all SAA-positive). MSA SAA shows different kinetic signatures, and strain-specific assays are in development.

The implications are large. SAA is the first molecular biomarker for synucleinopathy, opening the door to: (1) biological diagnosis before motor onset, (2) eligibility-defining biomarker for disease-modifying trials, (3) the proposed NSD-ISS biological staging (Höglinger 2024). The MJFF and Parkinson’s UK have invested heavily in SAA validation and standardisation across labs.

7. Genetic Testing

See Part IVfor the underlying genetics. Practical recommendations (MDS Genetics Task Force 2021, plus the GP2/PD-GENEration era):

• Test all PD patients for LRRK2 G2019S and GBA panel where possible — even sporadic, late-onset, no family history. Both are common and trial-relevant.
• Early-onset (<50) — full panel including PRKN (with deletion analysis), PINK1, DJ-1, plus dominant SNCA + LRRK2 + GBA.
• Juvenile (<21) — expand to ATP13A2, PLA2G6, FBXO7, DNAJC6, SYNJ1, plus check for Wilson disease.
• Family history of PD — targeted to family pattern.
• High-risk populations — Ashkenazi Jewish, North African Berber: routine LRRK2/GBA.
• Atypical features (early dementia) — consider SNCA dosage (CMA or MLPA, not just sequencing).

Pre-test genetic counselling is mandatory: implications for relatives, insurance/employment (variable by jurisdiction), and the absence of disease- modifying therapy outside trials.

8. Prodromal and Pre-clinical PD

The MDS Research Criteria for Prodromal PD (Berg et al., Mov Disord 2015, updated 2019) provide a probability framework based on combinations of risk and prodromal markers. Key markers (each carries a likelihood ratio):

Combining markers via Bayesian probability gives a posterior probability that the individual will develop manifest PD. A ≥80% post-test probability defines prodromal PD per the MDS framework. The 2024 NSD-ISS proposal would simplify this by anchoring on SAA + DaTscan as biological markers, decoupling diagnosis from clinical syndrome.

9. Diagnostic Pitfalls

• Drug-induced parkinsonism (DIP) — neuroleptics (especially first-generation), metoclopramide, prochlorperazine, valproate, lithium, tetrabenazine. Symmetric, often without rest tremor, develops over weeks. DaTscan normal. Resolves over months after offending drug withdrawal — but may unmask pre-existing PD.
• Vascular parkinsonism — lower-body predominant gait disturbance, stepwise course, prominent cognitive/pseudobulbar features, MRI white-matter disease and basal ganglia lacunes. Limited levodopa response.
• Normal-pressure hydrocephalus (NPH) — gait apraxia (magnetic gait), urinary incontinence, cognitive decline. Ventriculomegaly with DESH on MRI. Therapeutic LP / shunt response.
• Wilson disease — in any parkinsonism <50 years, screen serum ceruloplasmin and urinary copper, slit-lamp for Kayser-Fleischer rings.
• Essential tremor — bilateral postural/action tremor, ~8–12 Hz, frequently family history; alcohol responsive; not resting; normal DaTscan. Some ET patients later develop PD (controversial “ET-PD” entity).
• Dystonic tremor — tremor in a dystonic posture; can mimic PD rest tremor.
• Functional (psychogenic) parkinsonism — abrupt onset, distractible, inconsistent, no decrement, normal DaTscan.
• SWEDDs — clinically diagnosed PD with normal DaTscan; ~10% of clinical PD; mostly proves to be dystonic tremor or essential tremor on follow-up.

Therapeutic implications branch from the diagnosis: management of true PD is the subject of Part VII; disease-modifying frontiers in Part VIII.