Branch Retinal Vein Occlusion

Primary Mechanism — Arteriovenous Crossing Compression

BRVO almost invariably occurs at sites where a retinal arteriole crosses over a retinal venule. At these AV crossing points, both vessels share a common adventitial sheath. In the presence of arteriosclerosis, the thickened and rigid arteriole compresses the underlying venule, reducing luminal diameter and promoting turbulent flow, endothelial damage, and thrombosis. This mechanism accounts for the strong association of BRVO with systemic hypertension and cardiovascular risk factors.

• Superotemporal vein (most common, ~60%): The AV crossing anatomy here creates the greatest mechanical compression; macular proximity means visual consequences are most severe
• Inferotemporal vein (~35%): Second most common; macular oedema also common but may have slightly different visual trajectory
• Nasal branch veins (~5%): Less common; usually have better prognosis as the macula may be spared

Virchow's Triad Applied to BRVO

The pathological thrombosis forming at the AV crossing can be understood through Virchow's triad:

• Endothelial damage: Turbulent flow and shear stress at the AV crossing injure the venular endothelium, exposing prothrombotic subendothelial matrix
• Stasis: External compression reduces luminal diameter, slowing blood flow and promoting local thrombus formation
• Hypercoagulability: Systemic prothrombotic states (hyperviscosity, thrombophilia) compound local mechanical compression

Less Common Causes

• Inflammatory vasculitis: Behçet disease, sarcoidosis, systemic lupus erythematosus — periphlebitis causes venous occlusion independent of AV crossing; more common in younger patients without cardiovascular risk factors
• Haematological disorders: Polycythaemia vera, essential thrombocythaemia, leukaemia, hyperviscosity syndromes — increased whole blood viscosity promotes thrombosis
• Inherited thrombophilias: Factor V Leiden, prothrombin gene mutation, protein C/S deficiency, antiphospholipid syndrome — particularly relevant in younger patients (<50 years)
• Hyperlipidaemia: Dyslipidaemia promotes arteriosclerosis and endothelial dysfunction, accelerating AV crossing damage
• Raised intraocular pressure / glaucoma: Elevated IOP increases the pressure differential at the lamina cribrosa region and AV crossings, predisposing to venous compression
• Optic disc anomalies: Buried drusen, tilted disc, crowded optic nerve head — structural factors may predispose to venous compression
• Oral contraceptive pill / hormone therapy: Oestrogen-mediated hypercoagulable states — particularly relevant in younger women

Step 1 — Venous Occlusion and Haemodynamic Disruption

Thrombosis at the AV crossing abruptly obstructs venous outflow from the drained retinal segment. Venous pressure rises acutely proximal to the occlusion, causing hydrostatic extravasation of blood components through the vessel wall. Red blood cells, plasma proteins, and lipids leak into the intraretinal and sub-internal limiting membrane (ILM) spaces, producing the characteristic haemorrhages, oedema, and exudates seen clinically.

Step 2 — Blood-Retinal Barrier Breakdown

Venous stasis and ischaemia trigger upregulation of vascular endothelial growth factor (VEGF), particularly VEGF-A. VEGF disrupts tight junctions between retinal pigment epithelium (RPE) cells and inner endothelial cells, breaching the inner and outer blood-retinal barriers. The resulting increase in vascular permeability drives fluid accumulation in the outer plexiform layer, the inner nuclear layer, and beneath the fovea — producing cystoid macular oedema (CMO), the primary driver of visual loss in BRVO.

Step 3 — Retinal Ischaemia and VEGF-Driven Neovascularisation

In cases where the occluded segment drains a large capillary bed, significant retinal ischaemia ensues. Non-perfusion of capillaries — demonstrable on fluorescein angiography (FA) as areas of capillary dropout — creates a chronic ischaemic stimulus for VEGF production. Sustained VEGF upregulation drives neovascularisation, which may appear on the disc (NVD), on the retinal surface (NVE), or at the disc margin. These fragile new vessels are at risk of haemorrhage into the vitreous.

Step 4 — Collateral Formation and Natural Remodelling

Over weeks to months, the occluded thrombus may partially or fully recanalise. Collateral vessels also form between the occluded and adjacent patent venous systems, bypassing the occlusion site. These collaterals often cross the horizontal raphe. Spontaneous recanalization accounts for the variable natural history of BRVO and explains why some patients recover useful vision without treatment. However, persistent macular oedema beyond 3 months is unlikely to resolve spontaneously and requires intervention.

By Ischaemic Status

The most clinically important classification divides BRVO into non-ischaemic (perfused) and ischaemic (non-perfused) subtypes, based on fluorescein angiographic assessment of capillary perfusion:

By Anatomical Location

• Major BRVO: Occlusion of one of the four main branch veins draining a complete quadrant; produces quadrant-spanning haemorrhages and typically more severe macular oedema
• Macular BRVO (hemispheric variant): Occlusion of a small venule draining the macular capillary bed directly; haemorrhages confined to the macular region; may mimic other maculopathies; prognosis depends entirely on macular perfusion
• Peripheral BRVO: Occlusion of a small peripheral venule well away from the macula; often asymptomatic; may only present with vitreous haemorrhage from subsequent neovascularisation

By Temporal Stage

• Acute BRVO (<3 months): Active haemorrhages, fresh oedema, cotton wool spots; may have spontaneous improvement; anti-VEGF treatment typically initiated if VA ≤6/12
• Subacute BRVO (3–12 months): Resolving haemorrhages; persistent macular oedema; collateral development; treatment decisions guided by OCT central subfield thickness and VA
• Chronic BRVO (>12 months): Haemorrhages largely absorbed; established collaterals; hard exudates; fibrotic macular changes; chronic oedema or macular atrophy may limit visual recovery despite treatment

Cardiovascular / Systemic (Major)

• Systemic hypertension: The single most important risk factor; present in 50–70% of BRVO cases; promotes arteriosclerosis and AV crossing compression; hypertension control is the most important secondary prevention measure
• Hyperlipidaemia / dyslipidaemia: Elevated LDL-cholesterol and triglycerides contribute to arteriosclerosis and endothelial dysfunction
• Diabetes mellitus: Endothelial dysfunction and hypercoagulability; however, BRVO is less strongly associated with diabetes than CRVO
• Cardiovascular disease: Atherosclerosis is a shared pathological process; BRVO may be a marker of underlying carotid or coronary artery disease
• Obesity: Independent risk via hypertension, dyslipidaemia, and pro-inflammatory state
• Smoking: Promotes endothelial damage, platelet aggregation, and hyperviscosity

Ocular Risk Factors

• Elevated intraocular pressure / glaucoma: Elevated IOP — even within the normal range in the upper quartile — is an independent risk factor; glaucoma significantly increases BRVO risk
• High axial myopia: Larger AV crossing angle and posterior pole crowding may predispose
• Arteriovenous crossing ratio: Greater vessel calibre discrepancy at AV crossings increases mechanical compression risk
• Retinal arterial narrowing: Direct reflection of systemic arteriosclerosis

Haematological / Thrombophilic Factors

• Hyperviscosity syndromes: Polycythaemia vera, multiple myeloma, Waldenström macroglobulinaemia
• Inherited thrombophilias: Factor V Leiden, prothrombin G20210A mutation, protein C/S deficiency, antithrombin III deficiency — screen in patients under 50 with no vascular risk factors
• Antiphospholipid syndrome: Lupus anticoagulant, anti-cardiolipin antibodies — particularly in younger women; recurrence risk is high
• Hyperhomocysteinaemia: Endothelial toxicity and prothrombotic effect; correctable with B-vitamin supplementation
• Oral contraceptive pill: Oestrogen-mediated coagulation activation — should be discontinued after BRVO diagnosis

Demographic Factors

• Age: Incidence rises sharply after 50 years; peak presentation in the sixth to eighth decade
• Sex: Approximately equal incidence in men and women; oestrogen-related states may create increased risk in women of reproductive age
• Ethnicity: Higher rates reported in Asian and Hispanic populations in some studies, likely reflecting higher cardiovascular risk burden

Funduscopic Signs — Acute Phase

• Flame-shaped intraretinal haemorrhages: Classic hallmark; located in the nerve fibre layer, following the course of retinal nerve fibres; strictly confined to the drainage territory of the occluded vein — the most diagnostically useful feature of BRVO as it separates the affected quadrant from the normal fundus
• Dilated, tortuous venule: The occluded branch vein appears dark, engorged, and tortuous proximal to the occlusion site; contrast with the normal-calibre unaffected veins
• Cotton wool spots (CWS): White, fluffy superficial lesions representing localised axoplasmic stasis in ischaemic nerve fibre bundles; indicate significant retinal ischaemia in the affected segment; typically fade within 6–8 weeks
• Arteriovenous nicking: May be visible at or near the occlusion site; represents the compressive AV crossing anatomy; importantly, AV nicking in the non-occluded quadrants reflects underlying arteriosclerosis as a systemic risk factor
• Disc margin blurring: Mild disc oedema may be present if the occlusion is close to the disc, due to venous congestion; not a consistent feature

Macular Signs

• Macular oedema (cystoid pattern): Fluid accumulates in the outer plexiform and inner nuclear layers forming intraretinal cysts visible on OCT; the fovea may be lifted off Bruch's membrane in severe cases (subretinal fluid); the hallmark of vision-threatening BRVO
• Hard exudates: Yellowish, well-demarcated lipid deposits at the outer edges of resolving oedema; indicate chronic or recurrent macular leakage; may migrate towards the foveal centre
• Foveal haemorrhage: Intraretinal or sub-ILM haemorrhage overlying the fovea; associated with worse visual prognosis; may take months to clear
• Macular ischaemia: Enlargement of the foveal avascular zone (FAZ) on OCT angiography or fluorescein angiography; limits the visual recovery achievable with any treatment; associated with a relative afferent pupillary defect in severe cases

Chronic Phase Signs

• Collateral vessels: Fine, non-leaking shunt vessels crossing the horizontal raphe from the affected to the unaffected quadrant; may develop within weeks; distinguish from neovascularisation — collaterals do not leak on FA
• Neovascularisation (NVE/NVD): Abnormal new vessel fronds arising at the boundary of ischaemic and perfused retina; leak on FA; precede vitreous haemorrhage — prompt sectoral laser panretinal photocoagulation (PRP) or anti-VEGF indicated
• Pigmentary changes: RPE disturbance at the macula from chronic oedema; associated with poor visual prognosis if involving the fovea
• Epiretinal membrane: Fibrocellular membrane on the inner retinal surface; a common sequela of BRVO; may cause metamorphopsia and distortion even after oedema resolution
• Vitreous haemorrhage: Haemorrhage from ruptured neovascular fronds into the vitreous cavity; presents as sudden floaters or profound visual loss

Anterior Segment Signs

• Rubeosis iridis (iris neovascularisation): Rare in isolated BRVO; occurs only in severe ischaemic BRVO; if present, indicates extensive posterior segment ischaemia and high risk of neovascular glaucoma
• Relative afferent pupillary defect (RAPD): May be present in severe ischaemic BRVO with extensive macular or quadrant involvement

Visual Symptoms

• Sudden onset blurred or reduced vision: The most common presenting complaint when the macula is affected; may be described as a foggy or dim patch in the visual field rather than a total blackout; onset is typically rapid, occurring over hours to a day
• Sectoral visual field loss: Loss of part of the peripheral or central visual field corresponding to the drainage territory; may be the only symptom if the macula is spared (e.g., nasal or peripheral BRVO); may only be noticed during formal testing
• Metamorphopsia: Distortion of straight lines and images, arising from macular oedema displacing photoreceptors; may persist after oedema resolution if epiretinal membrane or macular scarring develops
• Micropsia: Perception that objects appear smaller than normal; caused by oedematous photoreceptor displacement increasing the effective inter-receptor distance
• Floaters and vitreous haemorrhage: Sudden onset of floaters, cobwebs, or severe visual loss (red haze or complete blackout) indicates vitreous haemorrhage from rupture of neovascular vessels; requires urgent ophthalmic assessment
• Paracentral scotoma: A fixed visual gap just off-centre, often noticed when reading; caused by macular ischaemia or persistent oedema involving the perifoveal region

Asymptomatic Presentations

A substantial proportion of BRVO cases — particularly those involving the nasal retina or peripheral branches away from the macula — are entirely asymptomatic and discovered only on routine retinal examination or fundus photography. These patients may present months after the occlusion, when haemorrhages are partially resolved and collateral vessels have formed, making timing of the acute event difficult to establish.

Macular Complications (Most Common Cause of Permanent Visual Loss)

• Chronic cystoid macular oedema (CMO): Persistent oedema beyond 3–6 months is associated with progressive photoreceptor loss, RPE atrophy, and irreversible foveal damage; the single most important complication to treat promptly
• Macular ischaemia and foveal avascular zone (FAZ) enlargement: Ischaemic BRVO can produce capillary dropout within the macular capillary plexus, enlarging the FAZ; this represents irreversible photoreceptor loss not amenable to any current treatment; limits the visual acuity ceiling despite successful oedema resolution
• Epiretinal membrane (ERM) formation: Seen in approximately 20% of BRVO eyes; the inflammatory milieu promotes glial cell proliferation on the inner retinal surface; produces metamorphopsia, traction, and may contribute to persistent CMO; vitrectomy with membrane peeling is indicated in visually significant ERM
• Lamellar or full-thickness macular hole: Uncommon; may arise from chronic tractional forces from ERM or persistent CMO; requires vitrectomy
• Subretinal fibrosis: End-stage scarring from chronic oedema and RPE decompensation; associated with final visual acuity below 6/60

Neovascular Complications

• Retinal neovascularisation (NVE/NVD): Occurs in approximately 20–40% of ischaemic BRVO eyes within the first 18 months; vessels arise at the ischaemic border; recognised by leakage on FA and elevated frond-like vessels on fundoscopy; requires scatter photocoagulation or anti-VEGF
• Vitreous haemorrhage (VH): Rupture of fragile neovascular vessels into the vitreous; presents with sudden profound visual loss (red haze or complete obscuration); if dense and non-clearing within 3 months, vitrectomy is required; associated with risk of tractional retinal detachment
• Tractional retinal detachment (TRD): Fibrovascular proliferation associated with neovascularisation may cause retinal traction; more common in ischaemic BRVO complicated by vitreous haemorrhage; requires vitreoretinal surgery
• Rubeosis iridis and neovascular glaucoma (NVG): Rare in BRVO (more common in CRVO); occurs only in extensive ischaemic BRVO; iris neovascularisation may lead to synechial angle closure and intractable IOP elevation

Other Ocular Complications

• Retinal detachment: Exudative detachment from massive subretinal oedema (rare in acute BRVO); rhegmatogenous detachment may occur following vitreous haemorrhage and PVD
• Recurrent BRVO: Same or fellow eye; systemic risk factor recurrence is the primary driver; fellow eye involvement occurs in approximately 10% of cases
• Optociliary shunts: Late collateral channels that form at the disc between retinal and choroidal circulations in chronic cases; indicate long-standing retinal venous obstruction

Cardiovascular Disease

• Hypertension (most common systemic association): Present in 50–70% of BRVO cases; aggressive BP control is the most evidence-based secondary prevention strategy; target <130/80 mmHg in most guidelines
• Ischaemic heart disease: BRVO patients have approximately 2× the age-matched risk of myocardial infarction; reflects shared arteriosclerotic burden
• Stroke (cerebrovascular accident): BRVO is associated with increased stroke risk; retinal venous occlusion may be the first clinical manifestation of underlying cerebral small vessel disease; share common pathological pathways with lacunar stroke
• Peripheral arterial disease: Shared arteriosclerotic risk factor burden
• Heart failure: As a consequence of hypertensive and ischaemic cardiomyopathy

Metabolic and Haematological Conditions

• Metabolic syndrome: The cluster of hypertension, dyslipidaemia, central obesity, and insulin resistance substantially increases BRVO risk
• Diabetes mellitus: Hyperglycaemia promotes endothelial dysfunction and hypercoagulability; patients with BRVO should be screened for T2DM if not already diagnosed
• Haematological malignancy: Myeloproliferative neoplasms and lymphoma may cause hyperviscosity-related BRVO; new BRVO in a young patient or without typical vascular risk factors warrants a full blood count and blood film
• Systemic inflammatory disease: Autoimmune conditions (SLE, Behçet, sarcoidosis) causing retinal vasculitis may present as BRVO; erythrocyte sedimentation rate (ESR), ANA, ANCA screening is indicated in atypical presentations

Investigations Recommended at Diagnosis

Clinical Diagnosis

BRVO is a clinical diagnosis based on characteristic funduscopic findings in the appropriate clinical context. The hallmark is intraretinal haemorrhages strictly confined to one quadrant or sector, with a dilated tortuous venule in the same territory. No additional imaging is strictly required for diagnosis, but multimodal imaging is essential for staging, treatment planning, and monitoring.

• Visual acuity: Best-corrected Snellen VA in both eyes; the most important baseline measurement; VA on presentation is the strongest predictor of final visual outcome
• Intraocular pressure: Elevated IOP is a risk factor and must be measured at baseline; glaucoma as an underlying cause should be excluded
• Fundus camera imaging: Non-mydriatic or mydriatic colour fundus photography documents baseline haemorrhage distribution, collateral formation, and neovascularisation for serial comparison

Optical Coherence Tomography (OCT)

OCT is the most important investigation for BRVO management and is indispensable for treatment decisions:

• Central subfield thickness (CST): The key quantitative metric for macular oedema severity; CST >300 µm is the threshold for treatment in most clinical trial protocols; serial CST guides retreatment decisions
• Intraretinal cysts (IRC): Hyporeflective fluid spaces in the outer plexiform and inner nuclear layers; the characteristic OCT pattern of macular oedema in BRVO; their disruption of the ellipsoid zone (IS/OS) is associated with poorer visual prognosis
• Subretinal fluid (SRF): Fluid beneath the neurosensory retina; may coexist with IRC; indicates more severe blood-retinal barrier disruption
• Ellipsoid zone (EZ) integrity: Disruption of the inner segment/outer segment (IS/OS) junction indicates photoreceptor damage; strongly predicts final VA ceiling; important for setting patient expectations
• Epiretinal membrane (ERM): Hyperreflective band on the inner retinal surface; if present, may contribute to persistent CMO and distortion
• Outer nuclear layer (ONL) thinning: Represents cumulative photoreceptor loss from chronic oedema; associated with poor visual prognosis

Fluorescein Angiography (FA)

• Perfusion status: The critical role of FA — distinguishing ischaemic from non-ischaemic BRVO based on areas of capillary non-perfusion; >5 disc areas of non-perfusion = ischaemic BRVO with high neovascularisation risk
• Leakage from neovascularisation: FA demonstrates hyperfluorescence and late leakage from NVE/NVD; guides timing and extent of sectoral laser treatment
• Foveal avascular zone (FAZ) assessment: FA delineates the FAZ to assess macular ischaemia, which limits prognosis regardless of oedema treatment
• Collateral vessels: Non-leaking crossing vessels at the horizontal raphe confirm diagnosis and chronicity; leaking vessels at the same location = neovascularisation
• Timing relative to haemorrhage: Dense haemorrhages block fluorescence, limiting FA utility in the acute phase; FA is most informative once haemorrhages partially clear (4–8 weeks)

OCT Angiography (OCTA)

• Non-invasive capillary-level imaging of retinal flow; delineates FAZ enlargement and capillary dropout without dye injection
• Useful for monitoring macular ischaemia progression and identifying areas of non-perfusion not apparent on conventional FA
• Does not show active leakage — FA remains necessary to assess exudation and NVE activity
• Deep capillary plexus (DCP) involvement on OCTA correlates with worse visual prognosis and greater risk of macular ischaemia

Visual Field Assessment

• Automated static perimetry (Humphrey 24-2 or 30-2): documents sectoral field loss corresponding to the occluded vein territory; useful for medicolegal driving fitness assessments
• Driving: patients with BRVO affecting the temporal field of the affected eye may fail binocular visual field standards for driving; formal assessment by an ophthalmologist or optometrist is required

Treatment of Macular Oedema — First Line

Treatment of Macular Oedema — Second Line / Adjunct

Laser Photocoagulation

• Sectoral grid laser (for macular oedema): Branch Vein Occlusion Study (BVOS, 1984) demonstrated benefit of grid laser for CMO in BRVO; now largely supplanted by anti-VEGF as first line due to superior visual acuity outcomes and non-invasiveness; grid laser may still have a role in recalcitrant cases or as adjunct to anti-VEGF in selected patients
• Sectoral scatter laser (for neovascularisation): Photocoagulation of the ischaemic retina in the affected sector reduces VEGF drive to neovascularisation; indicated when NVE/NVD is identified on FA; effective in inducing regression of new vessels; remains the standard of care for ischaemic BRVO with neovascularisation (BVOS 1986 trial evidence)

Surgical Management

• Vitrectomy for vitreous haemorrhage: Indicated if dense vitreous haemorrhage fails to clear within 1–3 months or if tractional retinal detachment is present; pars plana vitrectomy allows haemorrhage clearance and treatment of underlying neovascularisation with endolaser
• Vitrectomy with ERM peeling: For visually significant epiretinal membrane causing persistent CMO or significant metamorphopsia unresponsive to anti-VEGF; ILM peeling may improve anatomical outcomes
• Arteriovenous sheathotomy: Historical surgical technique involving incision of the adventitial sheath at the AV crossing to decompress the vein; not adopted into routine practice due to lack of robust RCT evidence and surgical risk
• Radial optic neurotomy: Decompression procedure via incision at the disc margin; investigated in CRVO but not BRVO; now largely abandoned

Systemic Risk Factor Management

Monitoring Schedule

Visual Outcome — With Modern Anti-VEGF Treatment

• Overall outcomes are good in non-ischaemic BRVO: BRAVO trial (ranibizumab) demonstrated that 55–61% of treated patients gained ≥15 ETDRS letters at 12 months vs. ~29% in the sham group; mean final VA of approximately 6/9.5 (20/32) in responders
• Ischaemic BRVO has a more guarded prognosis: Even with successful oedema control, macular ischaemia creates a ceiling on visual recovery; if the FAZ is significantly enlarged or if the EZ is disrupted on OCT, final VA below 6/60 is possible despite anatomical resolution
• Presentation VA is the strongest prognostic factor: Patients presenting with VA ≥6/12 have a significantly better prognosis than those presenting with VA <6/60; the duration of oedema before treatment also determines outcome — longer delay correlates with more photoreceptor loss
• Spontaneous resolution: A minority of patients (approximately 30%) improve spontaneously within the first 3 months; this has been used to justify a watchful waiting approach in patients with mild-to-moderate oedema and good VA at presentation, though most guidelines now recommend early treatment

Negative Prognostic Indicators

• Presenting VA <6/60
• Extensive macular ischaemia (FAZ enlargement >600 µm diameter on OCTA/FA)
• Disruption or loss of the ellipsoid zone (IS/OS line) on OCT at baseline
• Outer nuclear layer (ONL) thinning on OCT
• Duration of oedema >6 months before treatment
• Epiretinal membrane formation causing persistent traction
• Subretinal fibrosis or RPE atrophy
• Vitreous haemorrhage requiring vitrectomy
• Ischaemic BRVO with non-perfused area >10 disc areas

Fellow Eye and Recurrence Risk

• Recurrence in the same eye: Approximately 2–5% recurrence rate in the same vein; higher if systemic risk factors remain uncontrolled
• Fellow eye BRVO: Approximately 5–10% of patients develop BRVO in the fellow eye within 2–5 years; fellow eye has the same underlying arteriosclerotic risk
• BRVO to CRVO progression: Rare but possible if multiple branch occlusions occur, or if a separate CRVO develops; approximately 0.9% per year risk of CRVO in the fellow eye
• Systemic vascular events: 10-year cardiovascular event rate (MI, stroke) is meaningfully higher in BRVO patients than age-matched controls; systemic follow-up with GP/cardiologist is important