A three-step pathway
The diagnosis of CADASIL is built on three pillars:
- Clinical pattern — recognising the constellation of symptoms and family history that should raise suspicion.
- Brain MRI — confirming a small-vessel imaging signature consistent with CADASIL.
- NOTCH3 genetic testing — confirming a pathogenic variant in the NOTCH3 gene.
A skin biopsy is occasionally added when genetic results are ambiguous (for example, when a NOTCH3 variant of uncertain significance is found).
The most important moment is the first one
The single highest-impact diagnostic step is thinking of CADASIL. Once it is on the differential, the rest of the pathway is straightforward.
Step 1 · Recognize the clinical pattern
CADASIL is a clinical syndrome before it is a genetic test result. Suspicion grows when several of the following appear together:
- Stroke or TIA before age 60 — particularly without conventional risk factors
- Migraine with aura starting in the 20s–30s, often with prolonged or atypical features
- Mood changes, apathy, or executive dysfunction earlier than expected
- Family history of stroke, vascular dementia, or unexplained neurological illness across multiple generations
- White-matter changes on a brain MRI obtained for any reason
The differential diagnosis at this stage includes multiple sclerosis, sporadic small-vessel disease, CARASIL, dominant HTRA1-related CSVD, Fabry disease, MELAS, and primary CNS vasculitis. Each of these can be teased apart with imaging, family history, and targeted testing.
Step 2 · Brain MRI: the imaging signature
Brain MRI is essential — and remarkably specific. Once you know what to look for, CADASIL has a recognisable imaging "fingerprint."
Sequences that matter
- FLAIR — best for white-matter hyperintensities
- T2 — confirms lesion characteristics
- T1 — identifies lacunar infarcts
- SWI / GRE — detects cerebral microbleeds
- DWI — identifies acute or subacute ischemic lesions
Findings that point to CADASIL
| MRI finding | Why it matters |
|---|---|
| Confluent T2/FLAIR hyperintensities in the deep and periventricular white matter | The earliest and most consistent imaging finding; typically present by the 30s. |
| Anterior temporal pole involvement (FLAIR hyperintensity) | Highly suggestive of CADASIL — sensitivity moderate, specificity high. |
| External capsule involvement | Classic location; helps distinguish from MS and sporadic small-vessel disease. |
| Lacunar infarcts in basal ganglia, thalamus, brainstem, and centrum semiovale | Reflect the small-vessel arteriopathy. |
| Cerebral microbleeds on SWI/GRE | Increase with disease severity; have implications for antithrombotic decisions. |
| Brain atrophy | Develops over time; correlates with cognitive impairment. |
Often misread as MS
White-matter changes in a younger adult are commonly attributed to multiple sclerosis. CADASIL spares the corpus callosum and U-fibers, classically involves the anterior temporal poles and external capsules, and is genetically defined. CSF studies are negative for oligoclonal bands. When in doubt, image carefully and ask the question.
Step 3 · NOTCH3 genetic testing
NOTCH3 sequencing is the gold-standard confirmatory test. A single blood draw is sufficient. In practice, testing is increasingly delivered via small-vessel disease or stroke gene panels rather than NOTCH3 alone — useful because the differential includes other genes (e.g., HTRA1, COL4A1/A2, GLA).
What is being tested?
Sequencing the protein-coding regions of NOTCH3 — particularly the EGFr-encoding exons (exons 2–24, with the most action in exons 2–6) — identifies the cysteine-altering variants that cause classical CADASIL.
How results are reported
- Pathogenic / likely pathogenic variant identified — in the right clinical context, this confirms the diagnosis.
- Variant of uncertain significance (VUS) — found in some patients; may require additional testing (e.g., skin biopsy) and re-evaluation as evidence accumulates.
- No variant identified — may suggest an alternative diagnosis or a variant outside the regions sequenced. Reassessment with a broader panel is sometimes appropriate.
Skin biopsy and ultrastructure
Before genetic testing was widely available, skin biopsy was the cornerstone of CADASIL diagnosis. It remains useful when genetic results are ambiguous. Two key findings are sought:
- Granular osmiophilic material (GOM) on electron microscopy in the basement membrane around small dermal arteries — pathognomonic for CADASIL.
- NOTCH3 immunohistochemistry — abnormal NOTCH3 ectodomain accumulation in vessel walls.
Genetic counselling
Genetic testing for CADASIL should always be paired with genetic counselling. A counselling visit before testing helps people:
- Understand what the test can and cannot tell them
- Think through implications for siblings, parents, and children
- Anticipate the emotional impact of a positive — or negative — result
- Plan downstream care: monitoring, vascular risk management, family planning
- Discuss issues like insurance, disclosure, and workplace accommodations
Predictive testing in unaffected relatives
When a pathogenic NOTCH3 variant is identified in one family member, asymptomatic relatives may consider predictive testing. This is a deeply personal decision. Most international guidelines recommend:
- Pre-test and post-test genetic counselling
- Predictive testing only in adults (≥18 years), unless there is a clear medical indication
- Sufficient time for reflection between counselling and testing
- A care plan in place — regardless of result
Why test if there is no cure?
A diagnosis changes everything that can be optimised — blood pressure, smoking, lipid management, migraine strategy, mental health support, and trial enrolment. Many patients also describe relief at finally having a name for what they have lived with — and a plan for the years ahead.
After diagnosis: what comes next
A CADASIL diagnosis is the start of structured care, not the end of a question. Immediate priorities typically include:
- Comprehensive vascular risk assessment (BP, lipids, glucose, lifestyle)
- Migraine management plan
- Mood, sleep, and cognitive screening — with referrals as needed
- Family history mapping and counselling for relatives
- Connection to peer community and the CADASIL Global Foundation
- Discussion of registry enrolment and research participation
Our treatment & management page walks through this in detail.