Day 7 (2): Anatomy of the Internal Eye Flashcards
What are the parts of the Internal Eye/Posterior Segment?
- Posterior Lens Capsule
- Uvea: Iris, Ciliary Body, Choroid
- Vitreous Gel
- Retina
- Optic Nerve
- Posterior Sclera
What are the boundaries of the Posterior Segment?
Anterior most structure: Lens
Posterior most structure: Fovea
Imaginary line delineating the superior from the inferior half of the retina
Horizontal Raphe
- passes through the optic nerve
Most posterior portion of the retina
Fovea/Foveola
Junction of the retina and the ciliary body and the anterior most portion of the retina
Ora Serrata
Landmarks when drawing the retina
4 Inner Concentric Circles
- differentiated based on histologic findings and organization of retinal cells
- no clear demarcation in between areas
- Foveola
- corresponds to the Foveal Avascular Zone
- at the center of the Fovea
- bordered by the Clivus or Foveal Slope
- diameter: 0.35 mm
- ROD-free zone
- highest density of CONE receptors: 199,000 cones/sq. mm
- decreasing cone density the farther away from the foveola - Fovea/Fovea Centralis
- diameter: 1.5 mm (~ optic disc size)
- located 3 mm from temporal border of optic disc, slightly below the horizontal raphe
- cone density: 20,000 cones/sq. mm
- (+) macula lutea: made of xanthophylls, a protective pigment, concentrated in this area - Macula/Posterior Pole/Area Centralis
- area encompassing the Foveola, Fovea, Parafovea and Perifovea
- diameter: 4.5 - 6.0 mm
- cone density: 5,000 cones/sq. mm
- central and photopic vision
- PARAfovea: 0.50 mm area around the fovea
- PERIfovea: 1.5 mm area around the parafovea - Optic Disc
- corresponds to the optic nerve head
- nasal to the fovea
3 Outer Concentric Circles:
- Equator: innermost
- circle connecting the vortex veins - Ora Serrata: middle
- anterior termination of the retina and the junction between the retina and the ciliary body - Pars Plana: outermost
- located between the Ora Serrata and the Limbus
2 Sets of Lines:
- Major Arterioles or Vascular Arcades
- 4 in number at 2, 4, 8 and 10 o’ clock positions - Vortex Veins
- along the equator
What is the Spiral of Tillaux?
- External landmark of the Ora Serrata
- Corresponds to the insertions of all four recti muscles
- Any injury posterior to this area will highly involve the retina
What is the Pars Plana?
- Located midway between the Spiral of Tillaux/Ora Serrata and the Limbus
- 3 - 4 mm from the limbal edge
- Safe zone for retinal surgery
1. Pars Plana Vitrectomy
2. Intravitreal Injections
What is the Foveal Avascular Zone?
- 500 um dark area at the center of the fovea/macula
- NO retinal capillaries (retinal circulation)
- Relies only on choriocapillaries
+ from CHOROIDAL circulation: via Posterior Ciliary Arteries
+ INNER retinal layers (INL, IPL, GCL, NFL) which are supplied by the RETINAL circulation (via Central Retinal Artery), are ABSENT from the fovea - High concentration of xanthophylls
- RPE: tall and dense
Appearance:
1. Fundus Photo: dark area at the center of the macula
2. Fluorescein Angiography: hypofluorescent (dark)
- leak outside the FAZ: EXTRA-foveal
- leak within the FAZ: SUB-foveal
3. Indocyanine Green Angiography: cherry red spot
What are the cell types found in the retina?
Neuronal Cells: signal transduction of light to vision
- Vertical
- Photoreceptors (Rods and Cones)
- Bipolar Cells
- Ganglion Cells - Horizontal/Interneurons
- Horizontal Cells
- Amacrine Cells
Glial Cells: supportive cells
Why is the retina considered to be an inverted structure?
- Light rays must traverse the entire thickness of the retina to reach the photoreceptors at the OUTERMOST layer to begin signal transduction
- Light stimulus is then converted into chemical and electrical signals which are propagated back INWARDS, passing again through the entire thickness of the retina
What is the 3 Cell Pathway of phototransduction?
Cell 1: Photoreceptors
- photons strike the photopigments in the photoreceptors
- photopigments split creating a chemical signal
Cell 2: Bipolar Cells
- intermediary/interneurons
- connects the two cells
Cell 3: Ganglion Cells
- axons carry signal through the optic nerve into the brain
What are the 10 layers of the retina?
Innermost to Outermost:
- Internal Limiting Membrane (ILM)
- inner/basal footplate of the Muller cells
- separates neurosensory retina from the vitreous - Nerve Fiber Layer (RNFL): axons of ganglion cells
- Ganglion Cell Layer (GCL): cell bodies of ganglion cells
- Inner Plexiform Layer (IPL): axons of BC synapsing with GC
- Inner Nuclear Layer (INL): cell bodies of bipolar cells
- Outer Plexiform Layer (OPL): axons of PRs synapsing with BC
- Outer Nuclear Layer (ONL): cell bodies of photoreceptors
- Outer Limiting Membrane (OLM)
- outer/apical footplate of the Muller cells
- between inner segment of PRL and ONL - Photoreceptor Layer (PRL): inner and outer segments of PRs
- Retinal Pigment Epithelium (RPE)
What are photoreceptors and their parts?
- Axons (LIGHT Band) = Outer Plexiform Layer
- synapses of photoreceptors with bipolar cells - Nucleus (DARK Band) = Outer Nuclear Layer
- cell body/nucleus of photoreceptors
——————-ELM (LIGHT Band)——————- - Inner Segment = Photoreceptor Layer
- contains cytoplasm
- Myoid Zone (DARK Band): other organelles
- Ellipsoid Zone (LIGHT Band): mitochondria; junction of inner and outer segments - Outer Segment (DARK Band) = Photoreceptor Layer
- comprised of disks or modified cilia containing opsin-retinal complex
- closer to the choroid and sclera
What are the FOUR HYPERreflective outer retinal bands seen in the SS-OCT?
(DARK) Outer Nuclear Layer
(LIGHT) External Limiting Membrane : apical processes of Muller Cells
(DARK) Myoid Zone of Inner Segment
(LIGHT) Ellipsoid Zone of Inner Segment
- densely packed with mitochondria causing increased backscattering of light and high refractive index
- junction of inner segment and outer segment
(DARK) Outer Segment
(LIGHT) Interdigitation Zone : junction of outer segment and RPE
(DARK + LIGHT) RPE/Bruch’s Membrane Complex
What are Bipolar Cells?
- Interneurons
- Connects Photoreceptors to Ganglion Cells
1. Horizontal Cells
2. Amacrine Cells
3. Interplexiform Cells - Corresponds to 3 layers of the neurosensory retina:
1. Outer Plexiform Layer: axons of PRs synapsing with bipolar cells
2. Inner Nuclear Layer: cell bodies of bipolar cells
3. Inner Plexiform Layer: axons of BCs synapsing with ganglion cells
What are Ganglion Cells?
- Connects the neurosensory retina to the brain
- 1.2 M in number
- Density depends on the area of the retina:
+ Foveola: Absent
+ PERIfovea: 8 - 10 rows; greatest thickness
+ Beyond the macula: 1 row only - Corresponds to 3 layers of the neurosensory retina:
1. Inner Plexiform Layer: axons of BCs synapsing with ganglion cells
2. Ganglion Cell Layer: cell bodies of ganglion cells
3. Nerve Fiber Layer: axons of ganglion cells passing into the ON
What is the Nerve Fiber Layer?
- Axons of the ganglion cells which coalesce to form the Optic Nerve
- Connects the retina to the brain
Optic Disc
- convergence point of all ganglion cell axons from the macula and peripheral retina
- corresponds to the optic nerve head
Papillo-Macular Bundle
- dense concentration of axons coming from the macula
- severe vision loss if damaged
What are glial cells?
Non-neuronal cells which provide supportive functions to the retina
1. Microglia
2. Oligodendrocytes
3. Astrocytes
4. Muller Cells: main supporting structure encompassing the entire retina
- corresponds to 2 layers of the neurosensory retina:
1. Internal Limiting Membrane
+ inner footplate of the Muller cells
+ separates the neurosensory retina from the vitreous
2. External Limiting Membrane
+ outer footplate of the Muller cells
+ separates PRL from ONL
What is the Retinal Pigment Epithelium?
- Outermost layer of the retina
- Supports the neurosensory retina and provides the substrates needed to initiate phototransduction process
- Able to proliferate under pathologic conditions
- (+) Lipofuscin: accumulates with age
- (+) Potential space: between the photoreceptor layer and RPE
- (+) Tight Junctions/Zonula Occludens: OUTER Blood Retina Barrier
What is the Bruch’s Membrane?
- Layer between the Retinal Pigment Epithelium and Choriocapillaris
- (+) Drusen: in ARMD
- Composed of 5 sublayers: innermost to outermost
1. Basement membrane of RPE
2. Inner collagenous layer
3. Central elastic layer
4. Outer collagenous layer
5. Basement membrane of choriocapillaris
Regional differences in the retinal structure of the eyes.
Macula
- all 10 layers are complete
Peripheral Retina
- all 10 layers are complete
Fovea
- innermost layers are ABSENT: INL, IPL, GCL, NFL
- (+) Henle’s Layer: part of OPL; high concentration of CONES
- NO RODS
- (+) Foveal Avascular Zone
Ora Serrata
- peripheral termination of the retina
- (+) Junctional complexes: between NSR and RPE; tightly adherent and NO potential space
- Neurosensory Retina –> NON-pigmented ciliary epithelium
- Retinal Pigment Epithelium –> PIGMENTED ciliary epithelium
Appearance of retinal layers in Optical Coherence Tomography?
Hyporeflective (dark) layers: nuclear layers (GCL, INL, ONL)
Hyperreflective (bright) layers: axons and synapses (NFL, IPL, OPL)
Red/orange layer: RPE
Hazy layer underneath the RPE: Choroid
What is Serous Neurosensory Detachment?
Central Serous Chorioretinopathy: if detachment is in the macula
Pathology: Extravasation or leakage of fluid due to a defect or dysfunction of the RPE (OUTER BRB)
- OCT: collection of hyporeflective fluid between the PRL and RPE
- FA: smokestack pattern (extravasating fluorescein rises to the top of the defect)
What is the appearance of Diabetic Retinopathy on OCT and FA?
- Dysfunction of the endothelium of the retinal vessels (INNER BRB)
- Extravasation or leakage of blood or intraluminal contents into the retinal stroma
OCT: stromal cystic spaces interspersed in between retinal layers
FA: multiple cloud-like blotches due to leakage of dye
What is a Macular Hole?
Absence of retinal layers at the macula
Full-thickness: complete absence of ALL layers
Partial-thickness: some layers are absent
What is the appearance of Choroidal Neovascular Membrane on OCT?
Fusiform hyperreflective irregular lesions (blood vessels) involving the RPE and choroid
Distribution of ocular blood flow.
Highly metabolic organ with increased blood flow
80%: Choroid
15%: Iris and Ciliary Body
5%: Retina
Describe the blood supply and drainage of the retina.
Internal Carotid Artery –> Ophthalmic Artery –> either:
- Retinal Circulation
- Anterior 2/3 (INNER) retina: NFL, GCL, IPL, INL
- INNER Blood Retina Barrier:
+ retinal vessel lining are endothelium with tight junctions/zonula occludens
+ prevents seepage of intraluminal contents like blood and proteins
- Central Retinal Artery –> Retinal Arterioles (vascular arcades) –> capillaries –> venules –> Central Retinal Vein –> Ophthalmic Vein
–> Cavernous Sinus - Choroidal Circulation
- Posterior 1/3 (OUTER) retina: OPL, ONL, PRL, RPE
- OUTER Blod Retina Barrier: Retinal Pigment Epithelium
- ONLY blood supply of the FAZ and extreme retinal periphery (ora serrata)
- blood vessels are fenestrated –> intraluminal contents can diffuse into the choroidal layer
- Posterior Ciliary Artery (Medial and Lateral) –> Long (1) and Short (multiple) Posterior Ciliary Arteries –> Choriocapillaries –> venules
–> Vortex Veins –> Ophthalmic Vein –> Cavernous Sinus
Differentiate the use of Fluorescein Angiography and Indocyanine Green Angiography?
FA
- delineates retinal vessels
- choroidal circulation NOT seen
- FAZ: dark area
IGA
- delineated BOTH retinal circulation and LARGE choroidal vessels
- FAZ: cherry red
How to differentiate arterioles and venules in a fundus photo?
Arterioles: smaller in caliber, bright red
Venules: larger, dark red
What are the two blood retina barriers in the eye?
- Regulates supply of oxygen and other molecules into the retina
- Pathology leads to extravasation of intraluminal contents into the retinal layers
- Inner BRB: endothelium of the retinal vessels
- Outer BRB: retinal pigment epithelium
Differentiate CRAO vs CRVO vs BRVO based on fundus photo.
Central Retinal Artery Occlusion
- whitening or pallor of the entire retina due to loss of blood supply
- cause: cardiac EMBOLUS
- fovea: cherry red or chico-brown spot because pallor of the entire retina accentuates the patent choroidal circulation underneath
Central Retinal Vein Occlusion
- occlusion of the central retinal vein at the level of the lamina cribrosa posterior to the optic disc/optic nerve head
- cause: THROMBOSIS
- (+) dot hemorrhages present on all quadrants
- (+) tortuous and dilated venules
Branch Retinal Vein Occlusion
- (+) segmental hemorrhagic area RESPECTING the horizontal raphe
- cause: venule compression by an adjacent thickened arteriole due to a common adventitial sheath
What is the Hyaloid Circulation?
- In-utero vascular circulation which involutes at around 12-36 weeks
- Usually with NO visual consequence
- Pathologies:
- Mittendorf Dot: on the lens
- Bergmeister’s Papilla: elevated membrane at the ONH
- Persistent Fetal Vasculature: membrane emanating from the disc to the posterior lens capsule/retinal periphery; remnant of the hyaloid artery
- Persistent Pupillary Membrane
Characteristics of the retinal blood supply and drainage.
- Dual BUT non-overlapping circulation
- Retinal (Central Retinal Artery): INNER retina (NFL, GCL, IPL, INL)
- Choroidal (Posterior Ciliary Arteries): OUTER retina (OPL, ONL, PRL, RPE)
- NO collateral circulation or anastomosis between the two thus doubly vulnerable to loss of blood supply - ENDARTERIAL arrangement
- a single retinal arteriole supplies a single capillary bed
- interruption of that supply causes a loss of supply to the area of the retina being supplied by the capillary bed - COMMON adventitial sheath between adjacent artery and vein
- pathology in either vessel will affect the other due to their close proximity - LOW venous hydrostatic pressure
- susceptible to compression and obstruction due to elevated IOP, intraorbital pressure or ICP
What is the vitreous gel?
Composition:
1. Water - most abundant component
2. Hyaluronan - primary solid component
3. Collagen fibrils - main structural framework
3 Parts:
1. Central Vitreous
2. Cortical Vitreous
3. Vitreous Base
Primary sites of attachment of the vitreous?
- strands of collagen fibrils are adherent to certain areas of the posterior segment
1. Vitreous Base: where vitreous is MOST adherent to the retina; most common location of retinal detachments and tears
2. Optic Nerve Head
3. Along vascular arcades
4. Macula
5. Posterior Lens Capsule
Common pathologies of the retina related to the vitreous.
Retinal Tear
- tearing of the retina due to traction by the vitreous most commonly at the vitreous base at the retinal periphery
VitreoMacular Traction –> Macular Hole
- posterior vitreous pulls on its attachment at the fovea resulting to cystic changes on OCT
Proliferative Diabetic Retinopathy
- (+) florid neovascularization
- (+) fibrous membrane formation: grow from the retinal surface to attach to the vitreous gel
- in later stages of the disease, traction by the vitreous causes retina to detach
- treatment: membranolysis/adhesiolysis to sever the attachment of retina to the vitreous and vitrectomy to prevent recurrence
