Cardiac MIBG Scan for Parkinsonism

Introduction

The cardiac I-123 MIBG (metaiodobenzylguanidine) scan is a nuclear medicine imaging test that assesses sympathetic cardiac innervation. MIBG is a norepinephrine analog that is taken up by sympathetic nerve endings via the norepinephrine transporter. In neurodegenerative diseases, loss of sympathetic innervation produces characteristic patterns that help differentiate between tauopathies and synucleinopathies[@goldstein2000].

Mechanism

How MIBG Works

Radioactive Tracer: I-123 labeled MIBG is injected intravenously

Uptake: MIBG is taken up by sympathetic nerve terminals through the norepinephrine transporter (NET)

Imaging: Single-photon emission computed tomography (SPECT) images the distribution of MIBG in the heart

Quantification: Early and delayed images are compared to calculate the heart-to-mediastinum (H/M) ratio

Sympathetic Denervation in Neurodegeneration

• Synucleinopathies (PD, DLB, MSA): Loss of postganglionic sympathetic neurons → reduced MIBG uptake
• Tauopathies (PSP, CBS, AD): Relative preservation of cardiac sympathetic innervation → normal or near-normal uptake

Clinical Utility

Differentiating Tauopathies from Synucleinopathies

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Introduction

The cardiac I-123 MIBG (metaiodobenzylguanidine) scan is a nuclear medicine imaging test that assesses sympathetic cardiac innervation. MIBG is a norepinephrine analog that is taken up by sympathetic nerve endings via the norepinephrine transporter. In neurodegenerative diseases, loss of sympathetic innervation produces characteristic patterns that help differentiate between tauopathies and synucleinopathies[@goldstein2000].

Mechanism

How MIBG Works

Radioactive Tracer: I-123 labeled MIBG is injected intravenously

Uptake: MIBG is taken up by sympathetic nerve terminals through the norepinephrine transporter (NET)

Imaging: Single-photon emission computed tomography (SPECT) images the distribution of MIBG in the heart

Quantification: Early and delayed images are compared to calculate the heart-to-mediastinum (H/M) ratio

Sympathetic Denervation in Neurodegeneration

• Synucleinopathies (PD, DLB, MSA): Loss of postganglionic sympathetic neurons → reduced MIBG uptake
• Tauopathies (PSP, CBS, AD): Relative preservation of cardiac sympathetic innervation → normal or near-normal uptake

Clinical Utility

Differentiating Tauopathies from Synucleinopathies

| Pattern | Disease | H/M Ratio | Interpretation |
|---------|---------|-----------|----------------|
| Reduced | Parkinson's disease | <1.6 | Sympathetic denervation — synucleinopathy |
| Reduced | Dementia with Lewy bodies | <1.6 | Sympathetic denervation — synucleinopathy |
| Reduced | Multiple system atrophy | <1.6 | Sympathetic denervation — synucleinopathy |
| Normal | Corticobasal syndrome | >1.8 | Intact sympathetic — tauopathy |
| Normal | Progressive supranuclear palsy | >1.8 | Intact sympathetic — tauopathy |
| Normal | Alzheimer's disease | >1.8 | Intact sympathetic |

Sensitivity and Specificity

• Sensitivity for synucleinopathy: 80-90%
• Specificity for synucleinopathy: 85-95%
• Positive predictive value: High when combined with clinical assessment
• Negative predictive value: High for ruling out synucleinopathy in CBS/PSP

Interpretation

Early vs Delayed Imaging

| Parameter | Normal | Abnormal |
|-----------|--------|----------|
| Early H/M ratio | >1.8 | <1.6 |
| Delayed H/M ratio | >1.8 | <1.6 |
| Washout rate | <10%/hr | May be elevated |

Clinical Interpretation

• Preserved uptake (H/M >1.8): Suggests tauopathy (CBS, PSP) rather than synucleinopathy
• Reduced uptake (H/M <1.6): Suggests synucleinopathy (PD, DLB, MSA)
• Borderline (1.6-1.8): Correlate with clinical presentation; consider other tests

For This Patient

Given this patient's:

• Negative alpha-synuclein seed amplification assay
• Clinical presentation suspicious for CBS or PSP
• DAT scan confirmed dopamine neuron loss

Cardiac MIBG scan would be expected to show:

• Normal/near-normal uptake if CBS/PSP (tauopathy)
• Reduced uptake if there is an synucleinopathy component

This test can help determine if there is mixed pathology (tau + synuclein) if uptake is borderline.

Practical Considerations

Cost and Access

| Component | Cost (USD) |
|-----------|------------|
| Procedure | $1,500-2,500 |
| Insurance | Often requires pre-authorization |
| Availability | University hospitals with nuclear medicine |

Contraindications

• Absolute: Pregnancy, breastfeeding
• Relative: Recent beta-blocker use (interferes with uptake), clonidine

Test Procedure

Preparation: Fast for 4-6 hours; hold beta-blockers for 24 hours if possible

Injection: I-123 MIBG injected intravenously

Early imaging: 15-30 minutes post-injection

Delayed imaging: 3-4 hours post-injection

Total time: 4-5 hours

Comparison to Other Autonomic Tests

Complementary Diagnostic Approaches

Cardiac MIBG scintigraphy provides unique information about post-ganglionic sympathetic neurons that complements other autonomic assessments. Each test evaluates different components of the autonomic nervous system[@treister2023].

| Test | Target | What It Measures | MIBG Advantage |
|------|--------|------------------|----------------|
| Cardiac MIBG | Post-ganglionic sympathetic | Cardiac sympathetic innervation | Direct visualization of sympathetic nerve terminals |
| HRV Analysis | Parasympathetic + sympathetic | Heart rate variability | Non-invasive, no radiation |
| Orthostatic Hypotension | Central autonomic | BP response to standing | Identifies neurogenic OH severity |
| QSART/TST | Post-ganglionic sudomotor | Sweat response | Evaluates peripheral autonomic fibers |
| Urodynamic Studies | Pelvic autonomic | Bladder function | Direct assessment of detrusor function |

When to Choose MIBG

MIBG is particularly valuable when:

Differential diagnosis is unclear between tauopathies (CBS, PSP) and synucleinopathies (PD, MSA, DLB)

Autonomic symptoms are ambiguous — MIBG provides objective sympathetic denervation evidence

Patient cannot tolerate invasive testing — MIBG is well-tolerated

Need to assess disease severity — MIBG uptake correlates with sympathetic neuron loss

Consider other tests when:

• Patient has contraindications to nuclear imaging (pregnancy)
• Need comprehensive autonomic evaluation (use full battery)
• Resource-limited settings (HRV and OH testing are more accessible)

Diagnostic Algorithm Integration

Centers Offering MIBG Scanning

United States

| Center | Location | Notes |
|--------|----------|-------|
| Mayo Clinic | Rochester, MN; Phoenix, AZ; Jacksonville, FL | Leading center for movement disorders, offers MIBG |
| Massachusetts General Hospital | Boston, MA | Autonomic disorders program |
| University of Pennsylvania | Philadelphia, PA | Parkinson's disease center |
| Cleveland Clinic | Cleveland, OH | Movement disorders program |
| UCLA Medical Center | Los Angeles, CA | Lewy body dementia program |
| Mount Sinai | New York, NY | Autonomic testing available |
| Johns Hopkins | Baltimore, MD | Movement disorders center |

International

| Center | Location | Notes |
|--------|----------|-------|
| Royal Free Hospital | London, UK | Autonomic unit |
| University College London | London, UK | PSP Research Centre |
| Karolinska Institutet | Stockholm, Sweden | Movement disorders |
| Tokyo Metropolitan Institute | Tokyo, Japan | High-volume center |
| Università di Bologna | Italy | Autonomic research |
| University of Tübingen | Germany | Movement disorders |

Finding a Center

Movement disorder specialists — Ask neurologist for referral

Nuclear medicine departments — University hospitals typically offer MIBG

Autonomic disorder centers — Often have MIBG as part of evaluation

Clinical trials — Search [ClinicalTrials.gov](https://clinicaltrials.gov) for "MIBG Parkinson" trials

References

[Goldstein DS, et al, Cardiac sympathetic denervation in Parkinson disease (2000)](https://pubmed.ncbi.nlm.nih.gov/10979882/)

[Orimo S, et al, 123I-MIBG cardiac scintigraphy in Lewy body disease (2008)](https://pubmed.ncbi.nlm.nih.gov/18796596/)

[Treister R, et al, Autonomic function testing in neurodegenerative diseases (2023)](https://pubmed.ncbi.nlm.nih.gov/38041967/)

Pathway Diagram

The following diagram shows the key molecular relationships involving Cardiac MIBG Scan for Parkinsonism discovered through SciDEX knowledge graph analysis: