What is a posterior vitreous detachment (PVD) and is it dangerous?

A PVD occurs when the vitreous gel separates from the inner retinal surface as it liquefies with age. Most PVDs are benign and cause floaters (from condensed vitreous fibers) and brief flashes of light (from traction on the retina). However, if the vitreous is strongly adhered to the retina, separation can cause a retinal tear. Any patient with new floaters or flashes should be evaluated promptly to rule out a retinal tear that could progress to a retinal detachment.

Why is the fovea especially important for visual acuity?

The fovea has the highest concentration of cone photoreceptors (about 200,000 per mm squared) and virtually no rods. Cones in the fovea are individually wired to their own ganglion cells, providing one-to-one neural processing for maximum spatial resolution. Additionally, the inner retinal layers are displaced away from the foveal center, so light reaches the photoreceptors directly without passing through overlying neural tissue. Any disease affecting the fovea, such as AMD or macular edema, dramatically reduces central visual acuity.

What is a normal cup-to-disc ratio and what does an elevated ratio suggest?

The cup-to-disc (C/D) ratio is normally 0.3 or less. Ratios above 0.6, or asymmetry of more than 0.2 between the two eyes, raise concern for glaucomatous optic nerve damage. In glaucoma, elevated IOP causes the rim of neural tissue surrounding the cup to be lost, enlarging the cup relative to the disc. However, some people have naturally large cups without glaucoma, so C/D ratio must be interpreted alongside visual field testing and IOP measurements.

What is the ora serrata?

The ora serrata is the serrated anterior border of the sensory retina, where the retina transitions into the relatively featureless pars plana (part of the ciliary body). It is located approximately 8 mm posterior to the limbus on the temporal side and 7 mm on the nasal side. The ora serrata is clinically important because peripheral retinal tears most commonly occur near this transition zone, and intravitreal injections are administered through the pars plana just posterior to it.

Why do patients with macular disease lose central vision but retain peripheral vision?

The macula is responsible for central, high-resolution, and color vision. Disease processes like AMD, diabetic macular edema, or epiretinal membrane damage the densely packed cone photoreceptors and supporting cells in the macula. The peripheral retina, which is dominated by rods and is responsible for motion and low-light peripheral vision, remains intact. This explains the characteristic central scotoma (blind spot) with preserved peripheral vision seen in macular diseases.

Retina & Posterior Segment Anatomy for the CPO Exam

The posterior segment is the back two-thirds of the eye, encompassing the vitreous cavity, retina, choroid, and optic nerve. These structures are responsible for capturing light and transmitting visual information to the brain. Many sight-threatening conditions, including retinal detachment, macular degeneration, and diabetic retinopathy, originate here.

The Vitreous Humor

The vitreous humor is a transparent, gel-like substance that fills the large posterior segment of the eye between the lens and the retina. It consists primarily of water (about 99%) with a scaffolding of collagen fibers and hyaluronic acid that gives it its gel consistency. The vitreous:

Maintains the eye's spherical shape
Provides cushioning to protect the retina
Allows light to pass to the retina with minimal scattering

With age, the vitreous liquefies and can separate from the retina, a process called posterior vitreous detachment (PVD). PVD is common and usually benign, causing floaters and light flashes, but can occasionally cause a retinal tear if vitreoretinal adhesions are strong.

Floaters alone are common and benign. However, a sudden shower of floaters, a curtain or veil blocking vision, or persistent flashing lights may indicate a retinal tear or detachment and require urgent evaluation.

Retinal Anatomy

The retina is the thin, transparent, multilayered neural tissue lining the inner posterior globe. It extends from the ora serrata (the serrated anterior border of the retina) to the optic disc posteriorly. The retina processes light and converts it into electrical signals for transmission to the brain via the optic nerve.

Photoreceptors: Rods and Cones

The retina contains two types of photoreceptors:

Feature	Rods	Cones
Number	~120 million	~6 million
Distribution	Peripheral retina	Concentrated in macula/fovea
Function	Low-light (scotopic) vision; peripheral vision	Color and high-acuity (photopic) vision
Photopigment	Rhodopsin (one type)	Three types (short, medium, long wavelength)

Color blindness results from absent or defective cone photopigments. Night blindness (nyctalopia) results from rod dysfunction, which can occur in vitamin A deficiency or retinitis pigmentosa.

The Macula and Fovea

The macula is the specialized central region of the retina, approximately 5 mm in diameter, located temporal to and slightly below the optic disc. It is responsible for central vision and fine detail perception. Within the macula, the fovea is a small central pit (about 1.5 mm wide) containing the highest density of cone photoreceptors and no rods, providing the sharpest visual acuity.

The exact center of the fovea is the foveola, where ganglion cells and inner retinal layers are displaced aside, allowing light to reach photoreceptors without interference.

Age-related macular degeneration (AMD), diabetic macular edema, and epiretinal membrane all affect the macula, causing central vision loss while peripheral vision is preserved. This is why patients describe losing the center of their vision while still being able to see around the edges.

The Optic Disc

The optic disc (optic nerve head) is the circular structure at the posterior pole where the ganglion cell axons exit the eye to form the optic nerve. It is approximately 1.5 mm in diameter. Because the optic disc contains no photoreceptors, the corresponding area in the visual field is a blind spot.

The center of the optic disc contains a small depression called the optic cup. The cup-to-disc (C/D) ratio compares the diameter of the cup to the diameter of the entire disc. A normal C/D ratio is typically 0.3 or less. An enlarged cup (C/D ratio above 0.6 or asymmetric between eyes) raises concern for glaucomatous optic nerve damage.

The Choroid

The choroid is the vascular layer between the sclera and the retina. It is the primary blood supply for the outer retinal layers including the photoreceptors. The choroid also contains melanocytes that absorb scattered light, improving image quality.

Key Takeaways

The vitreous is a gel that fills the posterior segment; posterior vitreous detachment causes floaters and can lead to retinal tears.
Rods handle peripheral and low-light vision; cones in the macula handle color and high-acuity central vision.
The fovea within the macula has the highest cone density and provides the sharpest visual acuity.
The optic disc has no photoreceptors (physiologic blind spot); an enlarged cup-to-disc ratio suggests glaucoma.
The choroid supplies blood to the outer retina and photoreceptors.