Last updated: October 2026
The blood-ocular barrier (BOB) is not a single structure but two functionally distinct barriers — understanding which one is compromised in a given disease directly explains the clinical findings and guides treatment (e.g., why certain drugs reach the retina poorly, or why inflammation in one compartment doesn't necessarily mean the other is involved).
Blood-aqueous barrier
- Formed by the non-pigmented ciliary epithelium (tight junctions) and the endothelium of iris blood vessels
- Regulates what enters the aqueous humor from the ciliary body's blood supply and the iris vasculature
- Breakdown produces the classic signs of anterior segment inflammation: aqueous flare (protein leakage) and cells (inflammatory cells) in the anterior chamber — the basis of anterior chamber grading in uveitis
- Disrupted by intraocular surgery, trauma, and anterior uveitis
Blood-retinal barrier (inner and outer)
| Component | Formed by | What breaks down when compromised |
|---|
| Inner blood-retinal barrier | Tight junctions between retinal capillary endothelial cells | Breakdown causes retinal vascular leakage — the mechanism behind diabetic macular edema, retinal vein occlusion-related edema, and leakage seen on fluorescein angiography |
| Outer blood-retinal barrier | Tight junctions between retinal pigment epithelium (RPE) cells | Breakdown allows fluid from the choroidal circulation to leak into the subretinal space — relevant in central serous chorioretinopathy and exudative AMD |
Why this distinction matters clinically
- Fluorescein angiography leakage patterns can localize which barrier has failed — helping distinguish, for example, a retinal vascular cause of edema (inner barrier) from an RPE-level cause (outer barrier)
- Systemic drug penetration into the eye depends heavily on which barrier a drug needs to cross — this partly explains why some systemically administered drugs reach the anterior segment more readily than the retina, and is part of the rationale for intravitreal (rather than oral/IV) delivery of many retinal therapeutics
- Breakdown of the blood-retinal barrier is a therapeutic target itself — anti-VEGF therapy works partly by reducing the vascular permeability that VEGF drives at the inner blood-retinal barrier
Revise with flip cards
Key sources
- American Academy of Ophthalmology. Basic and Clinical Science Course, Section 2: Fundamentals and Principles of Ophthalmology (current edition)
- Cunha-Vaz J, Bernardes R, Lobo C. Blood-retinal barrier. Eur J Ophthalmol. 2011;21(Suppl 6):S3-S9