Patient Presentation: A 23-year-old obese female was diagnosed with idiopathic intracranial hypertension (IIH) and referred to neurosurgery for ventriculoperitoneal shunt. A baseline ocular examination was performed prior to the procedure.
On examination, vision was 20/200 in the right eye, and 20/40 in the left eye. There was a right relative afferent pupillary defect. Slit lamp examination was normal.
A dilated fundus examination was performed demonstrating the following:
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This section will provide an overview to retinal and corneal anatomy using optical coherence tomography (OCT).
Retinal Anatomy
Optical Coherence Tomography (OCT) is a rapid, non-invasive, in-office optical imaging technology that utilizes light waves to capture high resolution images of various structures of the eye. With the advent of higher resolution OCT technology, the layers of the retina that were once only able to be visualized histologically, can now be visualized in high resolution as well allowing analysis of tissues qualitatively and quantitatively.
A normal OCT macula is seen below. Pay attention to the labelled structures. Compare it with the histological specimen below.
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Retinal Layers and Function
A thorough understanding of retinal anatomy and function will aid in the interpretation of OCT images. The layers of the retina MUST be memorized.
The layers of the retina from inner to outer segments are outlined below:
Function of Each Layer
This section will help apply your knowledge of retinal anatomy to OCT images. As you move through the retinal layers, you will realize there are more distinct layers on OCT that are not highlighted from a histological standpoint, creating difficulties in anatomical correlates.
Nerve Fibre Layer:: Axons of the retinal ganglion cells.
Ganglion Cell Layer:: Low coherence scatter leads to a dark appearance compared to RNFL layer above. Includes superficial retinal capillary plexus.
Inner Plexiform Layer:: Hyper-reflective layer containing synapses between ganglion cells, bipolar cells, and amacrine cells.
Inner Nuclear Layer: Low coherence scatter leads to a dark appearance consisting of amacrine, bipolar and horizontal cells. Includes deep retinal capillary plexus.
Outer Plexiform Layer: Hyper-reflective layer consisting of synapses between photoreceptors, bipolar, and horizontal cells.
Outer Nuclear Layer: Low coherence scatter leads to a dark appearance consisting of photoreceptor cell bodies.
External Limiting Membrane: Hyper-reflective layer indicating zonular attachment between photoreceptors and Müller cells.
Ellipsoid Zone: Previously named inner segment/outer segment (IS/OS) junction. Hyper-reflective layer attributed to densely packed mitochondria.
Interdigitation Zone: Hyper-reflective band representing the junction between photoreceptors and RPE.
Retinal Pigment Epithelium (RPE)
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Monolayer of cells which contain melanosomes that absorb light to ensure any signals unconverted into an action potential do not pass through.
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Phagocytose photoreceptor waste products through lysosome-mediated enzymatic degradation to maintain photoreceptor integrity.
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Loss of RPE integrity is the precursor to a number of retinal diseases.
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Forms outer blood-retinal barrier.
Test yourself now! Hover your mouse on the figure below and work through the layers of the retina!
Review Questions
1. Which of the following is a function of label "X" as indicated by this OCT image?
2. A patient with a 12-year history of diabetic macular edema presents with decreased visual acuity. You compare the patient's OCT images over time. Which of the following layers or zones is the best predictor of visual outcome in this patient?
3. What layer of the retina is the yellow arrow pointing to?
Vitreous
OCT imaging has allowed for further understanding of the vitreous structures and associated diseases such as vitreomacular traction and epiretinal membranes. Various structures associated with the vitreous, such as the posterior cortical vitreous and retro-hyaloid space can be seen on OCT imaging.
The adherent posterior hyaloid is represented by the thickened hyper-reflective band anterior to the retina. The posterior hyaloid is attached to the retina, causing mild vitreomacular traction.
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Choroid
Enhanced depth imaging (EDI) on SD-OCT and SS-OCT allows for high quality images of the choroid. The swept-source OCT provides the best quality and resolution for choroidal imaging.
The layers of the choroid can be identified on OCT below:
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Choriocapillaris (small blood vessels)
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Sattler’s layer (larger blood vessels)
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Haller’s layer (larger blood vessels)
Corneal Anatomy
Anterior Segment Optical Coherence Tomography (AS-OCT) is a rapid, non-contact method of in vivo imaging, using similar technology as posterior segment OCT. Advances in time-domain and spectral-domain OCT devices allow for better visualization of cross-sectional views, illuminating layers of conjunctival and corneal pathologies.
A normal AS-OCT image of the cornea is seen below. Compare it to the histological cross-section.
Corneal Layers and Function
A thorough understanding of corneal anatomy and function will help in interpreting AS-OCT findings. The layers of the cornea from anterior to posterior are outlined below:
Epithelium
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Histology: Non-keratinized stratified squamous epithelium composed of 5–7 cell layers: basal columnar cells (attached to basement membrane via hemidesmosomes), intermediate wing cells, and superficial squamous cells linked by tight junctions.
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Function: Provides smooth refractive surface, physical and microbial barrier, and maintains tear film stability. Rapid turnover aids wound healing.
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OCT Appearance: Smooth hyper-reflective anterior band.
Bowman’s Layer
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Histology: Acellular, dense collagen immediately beneath the epithelial basement membrane.
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Function: Structural rigidity, resistance to external injury. Does not regenerate; scarring may occur after damage.
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OCT Appearance: Thin hyper-reflective band beneath epithelium.
Corneal Stroma
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Histology: Parallel lamellae of type I collagen fibrils arranged orthogonally between layers, interspersed with keratocytes and glycosaminoglycans (GAGs).
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Function: Maintains transparency via precise fibril spacing and minimal light scatter; contributes to biomechanical strength and shape of cornea.
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OCT Appearance: Homogeneous hypo-reflective region.
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Edema appears as hypo-reflective swelling with reduced keratocyte visibility.
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Scarring causes localized hyper-reflectivity.
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Dua’s Layer
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Histology: Acellular layer 6 to 15 µm thick, composed of coarse type I collagen bundles arranged in transverse, longitudinal, and oblique orientations. Distinct from adjacent Descemet’s membrane, which has finer banded collagen and endothelium.
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Function: Provides posterior corneal strength and may act as a surgical cleavage plane in deep anterior lamellar keratoplasty (DALK). Involved in formation of “big-bubble” types, acute hydrops, Descemetoceles, and pre-Descemet’s dystrophies. Also reduces scrolling of donor endothelial tissue when retained in keratoplasty.
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OCT Appearance: Not visualized on clinical OCT due to thin nature; inferred indirectly during big-bubble DALK or by intrastromal clefts in hydrops.
Descemet Membrane
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Histology: Basement membrane secreted by endothelium; composed of anterior banded zone and posterior non-banded zone.
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Function: Anchors endothelium to cornea and has elastic properties which may help maintain posterior corneal curvature. Also modulates entry of growth factors into stroma to protect structure and transparency.
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Abnormal function of Descemet membrane leads to thickening of this layer with visible guttae (ex. Fuchs' dystrophy).
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OCT Appearance: normally poorly discernible compared to overlying stroma, unless a detachment is present. In the presence of fibrosis or guttae, it appears as a thin hyper-reflective posterior line.
Endothelium
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Histology: Single cell layer of hexagonal cells joined by tight junctions, containing numerous mitochondria to fuel active ion transport.
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Function: Maintains stromal dehydration via ions transport with Na⁺/K⁺ ATPase mechanism.
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Limited regenerative capacity. Cell loss is compensated by enlargement and migration of remaining cells.
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OCT Appearance: Not always individually resolved on standard AS-OCT; integrity inferred from posterior stromal clarity and absence of edema.
Functional Correlation on OCT
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Anterior changes (epithelial thickening, hyper-reflective Bowman’s layer) suggest surface trauma or early ectatic disease.
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Mid-stromal changes indicate keratopathy, scarring, or keratoconus.
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Posterior changes (Descemet folds, stromal hyporeflectivity) are common in endothelial dysfunction and microcystic edema.
Review Questions
1. Which corneal layer is responsible for maintaining corneal clarity?
2 . Which layer has the greatest impact on corneal biomechanics and refractive stability?