Monogenic and Other Small-Vessel Diseases

Why the broader spectrum matters

The guide focuses on sporadic CAA and brain arteriolosclerosis, but a credible cSVD resource must show where that focus ends. Genetic, inflammatory, infectious, venous, toxic, radiation-associated, metabolic, and other disorders can produce overlapping imaging and clinical phenotypes. The purpose of this chapter is orientation and differential structure, not comprehensive diagnosis R036 R064.

Young age, family history, unusual systemic features, recurrent strokes, or a striking MRI pattern should raise a genetic question. None is individually required, and common vascular risks do not exclude a monogenic disorder.

1. A phenotype-first genetic approach

The practical guide to genetic cSVD organizes evaluation around ischemic stroke, hemorrhage, and nonstroke neurological or systemic presentations R064. Useful information includes:

Age at first symptom and longitudinal course.
Three-generation family history with specific manifestations.
Ischemic versus hemorrhagic events.
Migraine, psychiatric symptoms, seizures, encephalopathy, neuropathy, renal, ocular, skin, muscle, or systemic findings.
MRI distribution, lacunes, microbleeds, PVS, enhancement, calcification, cysts, mass-like lesions, and atrophy.
Common acquired causes and treatment exposures.
Inheritance model and variant classification.

The absence of an obvious family history does not exclude de novo disease, recessive inheritance, small families, reduced penetrance, misdiagnosis, or incomplete history.

2. CADASIL and NOTCH3

CADASIL is the most recognized monogenic cSVD. Clinical features can include migraine with aura, recurrent subcortical ischemic events, cognitive decline, mood disturbance, gait dysfunction, and other manifestations. MRI often shows extensive WMH and lacunes, with anterior temporal pole and external capsule involvement classically discussed, but patterns vary and are not by themselves diagnostic R064.

The genetic target is a pathogenic NOTCH3 variant interpreted in clinical context. Variant position, cysteine alteration, population frequency, family segregation, and phenotype influence interpretation. A variant of uncertain significance should not be converted into a diagnosis solely because WMH are present.

Biallelic HTRA1 variants cause the recessive CARASIL phenotype, while heterozygous pathogenic variants can cause autosomal dominant small-vessel disease with variable penetrance and later onset. Ischemic events, cognitive and gait impairment, alopecia, and spondylosis can appear in different combinations. Clinical overlap with sporadic cSVD is substantial R064.

4. COL4A1 and COL4A2 disorders

COL4A1/2-related disease can involve small-vessel hemorrhage, porencephaly, WMH, microbleeds, ocular abnormalities, renal or muscular involvement, and other systemic features. Perinatal and adult presentations differ. Trauma, anticoagulation, and other modifiers may influence hemorrhagic risk. Multisystem assessment and expert variant interpretation are central R064.

5. Fabry disease

Fabry disease is a systemic lysosomal disorder that can include cerebrovascular events, neuropathic pain, renal disease, cardiac disease, skin findings, and other manifestations. White-matter lesions are common but nonspecific; historical imaging signs are not sufficiently sensitive or specific to replace biochemical and genetic evaluation R064.

Retinal vasculopathy with cerebral leukoencephalopathy and systemic manifestations can cause retinal disease, enhancing or mass-like brain lesions, systemic organ involvement, and progressive neurological symptoms. CTSA-related CARASAL, ADA2 deficiency, mitochondrial and metabolic conditions, and other rare disorders expand the differential. Rarity, variable expression, and evolving gene-disease evidence demand specialized sources and genetics consultation R064.

7. Hereditary CAA

Hereditary cerebral amyloid angiopathies involve different amyloid proteins or APP-related mutations and can present earlier than sporadic CAA. They provide valuable natural-history and mechanistic models, but their timing, molecular species, vessel distribution, and co-pathology can differ. Findings should not be transported automatically to sporadic late-life CAA R056.

8. Inflammatory and immune-mediated small-vessel disease

Primary CNS vasculitis, systemic inflammatory disease, CAA-related inflammation, infection-associated vasculopathy, and other immune processes can produce infarcts, hemorrhage, enhancement, edema, seizures, or encephalopathy. The key distinction is that these are not all forms of sporadic arteriolosclerosis. Clinical tempo, vessel imaging, CSF, systemic testing, biopsy, and treatment response may be relevant in specialist evaluation.

CAA-ri has its own clinicoradiologic evidence base R008 R057 R061. It should not become a template for diagnosing all inflammatory-appearing white-matter disease.

9. Venous and venular disease

Small venules and veins contribute to drainage and white-matter physiology. Venous collagenosis and other venular changes have been proposed in age-related white-matter disease, but terminology, sampling, and in-vivo markers are less mature than arterial frameworks. A comprehensive future edition needs dedicated venous pathology and imaging review rather than treating the venous side as an afterthought.

10. Toxic, radiation, treatment, and infection-associated injury

Radiation, chemotherapy, immune therapy, infection, toxins, and severe systemic illness can injure endothelium, white matter, and small vessels. Temporal relationship to exposure, enhancement, lesion distribution, systemic findings, and competing diagnoses matter. These conditions are currently mapped rather than deeply reviewed in v0.6.

11. Red flags that should broaden the differential

Stroke or extensive cSVD at an unexpectedly young age.
Recurrent events across generations.
Migraine with aura plus characteristic familial stroke history.
Renal, ocular, skin, cardiac, muscular, or retinal manifestations.
Symmetric or strikingly patterned WMH.
Enhancement, mass effect, calcification, cysts, or tumor-like lesions.
Recurrent fever, systemic inflammation, immunodeficiency, or infection risk.
Rapid progression or subacute encephalopathy.
Disease severity poorly explained by age and acquired risks.

These are prompts for broader evaluation, not a scoring system.

12. What genetic testing can and cannot do

Testing can identify a pathogenic variant that explains disease in the right phenotype and inheritance context. It can also return a variant of uncertain significance, secondary finding, or negative result despite a genetic disorder. Panel composition, copy-number detection, mitochondrial coverage, mosaicism, and reanalysis matter.

The resource should record gene, transcript, variant nomenclature, classification framework, laboratory, date, segregation, and phenotype fit. It should not reinterpret a clinical genetic result without appropriate expertise.

13. Coverage boundary for v0.6

This chapter closes a major conceptual gap by making the broader spectrum visible. It is not yet a disease-by-disease monograph. The coverage map labels each topic so readers can distinguish deep modules, core orientations, mapped areas, and absent evidence.

Verification and further study

Practical guide to genetic cSVD: R064
Hereditary CAA systematic review: R056
General cSVD clinical and mechanistic overview: R036
CAA-related inflammation criteria: R008
CAA-ri systematic review: R057

Sources cited in this chapter

Open the original or authoritative record to verify wording, population, methods, and limitations.