Overview
This lecture covers the topography and anatomy of the retina, focusing on its zonal divisions, layers, and special anatomical features, with clinical relevance and key definitions.
General Anatomy & Measurements
- The retina measures about 32 mm horizontally from ora serrata to ora serrata.
- Total retinal surface area is about 1100 mm²; average thickness is about 200 µm.
- Thickest at the posterior pole (near optic nerve head and macula), thinnest at the ora serrata and foveola.
Central Retina: Macula, Fovea, Foveola, and Related Zones
- Macula lutea (area centralis) is a 5–6 mm diameter circular zone, rich in cones, containing over 50% of retinal ganglion cells.
- Macula appears yellow due to carotenoids (lutein, zeaxanthin) that filter UV light.
- Macula covers about 15° of the visual field.
- Fovea, a 1.5 mm diameter area within the macula, contains only cones and provides the highest visual acuity and color vision.
- Foveola is the 0.35 mm diameter floor of the fovea, avascular, nourished by underlying choroid, and its cones are slender and densely packed.
- Inner retinal layers are displaced at the foveola, which contains only outer layers.
- Full foveola development occurs by age 4; foveolar reflex is a sign of normal anatomy.
Specialized Foveal Zones and Clinical Findings
- Umbo: central depression within the foveola seen as a light reflex.
- Foveal avascular zone (FAZ): 0.25–0.6 mm in diameter, area within fovea lacking blood vessels.
- Foveal pit: depression formed by the slope of the fovea meeting the foveola.
- Foveal hypoplasia: absence of normal foveal pit and reflex, often lacks lateral displacement of inner retinal layers and FAZ.
Surrounding Regions: Parafovea & Perifovea
- Parafovea: 0.5 mm wide ring around the fovea with thickest ganglion cell, inner nuclear, and outer plexiform layers (Henle’s fiber layer).
- Perifovea: 1.5 mm ring outside parafovea, marks outer boundary of macula.
Peripheral Retina: Near, Mid, and Far Periphery
- Retina outside macula is termed peripheral, rich in rods, with only a single layer of ganglion cells.
- Ora serrata: anterior edge of retina, 18.5 mm from optic disc nasally, 23–24 mm temporally.
- Near periphery: 1.5 mm ring beyond macula.
- Mid-periphery: 3 mm ring beyond near periphery.
- Far periphery: beyond mid-periphery, to ora serrata.
Ora Serrata: Anatomy and Clinical Features
- Ora serrata is more anterior and scalloped nasally; projects dentate processes (retinal projections) and oral bays (par plana projections).
- Enclosed oral bays can mimic retinal holes.
- Oral pearls (pars plana pearls): drusen-like structures at ora serrata, in ~20% of eyes.
- Meridional folds: radially oriented, full-thickness retinal folds, common in superior-nasal quadrant, present in ~26% of people.
- Meridional complexes: combination of enlarged dentate process, ciliary process, and meridional fold; can cause retinal excavation.
Peripheral Findings & Clinical Notes
- Pars plana cysts: bullous elevations formed by separation of non-pigmented and pigmented ciliary epithelia.
- Peripheral degenerations are common at ora serrata due to it being a watershed zone with minimal vascular supply.
- Variable RPE (retinal pigment epithelium) distribution in far periphery may mimic retinal detachment/retinoschisis.
Key Terms & Definitions
- Macula lutea — central yellow area of retina, rich in cones.
- Fovea — depression within macula, contains only cones.
- Foveola — center of fovea, avascular and highest acuity.
- Umbo — central depression in foveola, gives foveal reflex.
- FAZ (Foveal Avascular Zone) — central region without blood vessels.
- Parafovea — ring around fovea with thick neural layers.
- Perifovea — zone peripheral to parafovea, outer margin of macula.
- Ora serrata — anterior retina margin with scalloped edge.
- Dentate processes — tooth-like retinal projections at ora serrata.
- Oral bays — rounded pars plana regions between dentate processes.
- Meridional fold/complex — full-thickness retinal fold in periphery.
Action Items / Next Steps
- Review the anatomy diagrams of the retina, macula, and periphery.
- Watch the referenced videos on retinal layers and blood supply if suggested by your course.
- Memorize diameters and angular coverage for macula, fovea, and foveola for exam purposes.
- Understand clinical relevance of peripheral findings (e.g., ora serrata, foveal hypoplasia, meridional folds).