Retinal Layer Anatomy & Normal Findings

Retinal Layer Anatomy & Normal Findings

Before you can identify pathology on OCT, you need to know what normal looks like. This lesson walks through the 10 retinal layers visible on spectral-domain OCT and establishes the baseline architecture you'll reference for every scan you read.

The 10 Layers (Internal to External)

🔭 Clinical Reference — CC BY 4.0

Normal OCT B-scan (Fig.1). Ten retinal layers labeled from ILM through choroid. Ellipsoid zone (EZ/IS–OS junction) and RPE appear as bright hyperreflective bands. — Kulyabin M et al. Sci Data 11:365 (2024), CC BY 4.0

On a standard SD-OCT B-scan, the retina presents as alternating hyper-reflective (bright) and hypo-reflective (dark) bands. Each band corresponds to a specific anatomical structure:

The Foveal Contour

At the fovea, the inner retinal layers (RNFL through INL) are absent — they're displaced laterally to create the foveal pit. This is normal. What remains at the foveal center is the outer retina: ONL, ELM, EZ, IZ, and RPE.

Key measurements at the fovea:

• Central foveal thickness (CFT): ~250 μm (varies by device and ethnicity)
• Foveal pit depth: The pit should be symmetric and well-defined
• EZ band: Should be continuous and unbroken across the fovea

The Choroid

Below the RPE, the choroid appears as a heterogeneous band with varying reflectivity. Enhanced depth imaging (EDI-OCT) or swept-source OCT improves choroidal visualization. Normal subfoveal choroidal thickness is approximately 250-350 μm but decreases with age.

In en face imaging, the choroid becomes critically important — it's where you'll see:

• Pachychoroid spectrum disorders (thick choroid with dilated vessels)
• Haller's layer vessels (large choroidal vessels)
• Choriocapillaris flow patterns on OCTA

Vitreous Interface

Above the ILM, you may see the posterior vitreous face. In a complete posterior vitreous detachment (PVD), it appears as a hyper-reflective line separated from the retinal surface. Incomplete PVD with residual adhesion at the fovea is a setup for vitreomacular traction (VMT).

What to Look For on Every Scan

Before analyzing pathology, run through this mental checklist on every OCT:

1. Scan quality: Signal strength adequate? Any motion artifact?
2. Foveal contour: Symmetric pit? Flattened? Elevated?
3. Layer integrity: Is the EZ band continuous? Any disruptions in ELM?
4. Fluid: Intraretinal (cystic spaces)? Subretinal (between EZ and RPE)? Sub-RPE?
5. RPE: Smooth? Elevated (PED)? Disrupted? Attenuated?
6. Vitreous interface: Attached? Detached? Traction?

Key Takeaways

• The retina has 10 distinct layers on OCT, alternating in reflectivity
• The EZ band (ellipsoid zone) is your single most important layer for assessing photoreceptor health
• The foveal pit is normal — inner layers are absent at the center
• Always check: quality, contour, layers, fluid, RPE, vitreous
• Normal measurements vary by device — use normative databases, not memorized numbers