Posterior Segment | The Ophthalmology Survival Guide

The Posterior Segment

The posterior segment includes the structures in the eye posterior to the lens and zonules, including the vitreous, retina, choroid, and optic nerve.

Deep Dive: Posterior Segment

The posterior segment includes the structures in the eye posterior to the lens and zonules, including the vitreous, retina, choroid, and optic nerve. Except for the anterior vitreous, which can be directly examined with the slit lamp, the examination of the posterior segment requires the use of additional lenses or devices, such as the direct or indirect ophthalmoscopes. It can also be assessed with a variety of imaging modalities including photography, scanning laser ophthalmoscopy, optic coherence tomography, ultrasonography, and others.

The Vitreous

The vitreous (or hyaloid) is an avascular, acellular, transparent gel that fills the space in the center of the eye between the retina and lens. As we age the fibrillar collagenous structure of the vitreous gel begins to break down, a process called syneresis, leading to small opacities that can be seen as dark “floaters” by patients. Points of adhesion between the vitreous and retina can sometimes be the cause of retinal tears and detachment in areas where the retina is pathologically weakened (e.g. in lattice degeneration), or there are sudden forceful movements of the vitreous gel as can occur with ocular trauma or surgery.

The anterior vitreous, in close proximity to the lens, can generally be viewed directly at the slit lamp with no additional lenses. Careful examination of the mid and posterior vitreous, however, requires the use of additional lenses. Examination of the vitreous is important for the detection of cells, opacities, blood, and its anatomic relationship with the retina.

The Retina

Numerous techniques may be used to examine the retina. The most common is binocular indirect ophthalmoscopy, as it affords clear, stereoscopic views of large areas, and the capability to directly examine the most peripheral structures. When using indirect ophthalmoscopic lenses it is critically important to remember that the image you are seeing is inverted (upside down and backwards) from its actual orientation in the eye. This can be quite confusing for the beginner.

When examining the retina it is particularly important to make careful observations of the fovea and macula, the retinal blood vessels, the color and consistency of the underlying pigmentation, and the ability to visualize the underlying choroidal vasculature. Examining the retina is NOT easy. It takes time to develop proficiency and the only way to achieve proficiency is by understanding the basic optical principles, using the equipment, and using it often. Issues such as media opacities (like corneal scars, cataract, or vitreous blood) or poor patient cooperation can make it even more challenging, or impossible. Good pupillary dilation is extremely helpful, as is conducting the examination in a darkened room to minimize the glare and reflections from room lights.

Histologically the retina consists of 9 layers, with a combined maximum normal thickness of approximately 0.25 mm. These layers, beginning at the vitreoretinal interface (the innermost layer) and moving outward toward the RPE are:

Internal limiting membrane (ILM, in continuity with the vitreous/posterior hyaloid)
Nerve fiber layer (NFL, ganglion cell axons)
Ganglion cell layer
Inner plexifom layer
Inner nuclear layer
Outer plexiform layer
Outer nuclear layer
External limiting membrane
Photoreceptors (in continuity with the retinal pigment epithelium, or RPE)

Although OCT can provide extremely good visualization of these layers (Photo P1), their relative transparency makes them hard to discern on ophthalmoscopy.

Photo P1: Retina layers via OCT

Click to enlarge.

Mohandass, G., et al., CC BY 4.0, via Biomedical and Pharmacology Journal

The nerve fiber layer also contains the major retinal arterioles and venules. This is an important thing to keep in mind as it helps to identify the depth of lesions. If the retinal vessels are clearly visible then the lesion is not superficial to the retina. Similarly, lesions located within the nerve fiber layer, such as nerve fiber layer infarcts (e.g. cotton wool spots), or myelinated nerve fibers, can be seen directly adjacent to the major vessels, but do not fully obscure them.

The Retinal Pigment Epithelium (aka The RPE)

The single layer of pigmented cells beneath the retinal photoreceptors is the retinal pigment epithelium, or RPE. Critically important from a functional standpoint, the RPE can be difficult to visualize ophthalmoscopically, even at high magnification. Pathological changes in the RPE are most easily recognized by alterations in pigment density, clumping or mottling, atrophy, or the presence of extracellular deposits, such as drusen, beneath it (P2). It is important to specifically ask yourself when examining the retina if the RPE appears “normal” or not.

Photo P2: Drusen in the macula

Click to enlarge.

Ipoliker, CC BY-SA 3.0, via Wikimedia Commons

The Choroid

The choroid is the highly vascular layer between the RPE and sclera. Its relatively large vessels can only be indistinctly visualized on indirect ophthalmoscopy, particularly in lightly pigmented eyes. It is approximately 0.2 mm in thickness under the posterior retina. Though not as frequently a site of pathology as the retina, it can be involved in a number of conditions including inflammatory lesions, neoplasm, infection, hemorrhage, and effusion. As with the retina, the use of ancillary imaging technologies such as ultrasound and OCT are invaluable tools for evaluating the choroid.

The Optic Nerve (aka The Nerve)

The optic nerve is comprised of the 1.2 million axons from the retinal ganglion cells, the central retinal artery and vein, and glial tissue. It is a little over 1mm in diameter. The nerve is positioned approximately 4.5mm nasal to the fovea, but when viewed by indirect ophthalmoscopy, the inverted image you see makes it appear temporal to the fovea. Its appearance can be quite variable, with many “normal” variants, a feature that can make it quite difficult to differentiate normal from pathological. This can be further complicated by the fact that a normal appearance does not rule out a lesion, since lesions affecting the posterior portions of the nerve behind the eye (i.e. the retrobulbar segment), may not produce ophthalmoscopically visible abnormalities, especially early on. It is important, however, to try to identify the following abnormalities of the nerve (P3):

Swelling
Atrophy (diffuse or focal)
Cupping
Hypoplasia
Coloboma
Drusen (pseudopapilledema)

Some of these abnormalities are red flags of potentially serious underlying conditions and their presence requires further investigation. In general, when examining the optic nerve it is important to note its size, contour, color, and shape, and the symmetry in appearance between the two eyes. Developing a systematic process for examining the nerve and noting its features are key survival skills.