Retina Of The Eye

The retina is divided into 3 parts:

1. Central retina: It contains the optic disc and macula.
2. Mid-peripheral retina: The equator is marked by the vortex veins that drain the choroid tissues.
3. Peripheral retina: It has a dentate process called ora serrata.

• The diameter of the optic disc is 1.5mm, and the macula is 5.0 - 5.5mm.
• The macula is always temporal to the optic disc (opposite side of the nose) and vice versa, and the optic disc is always nasal to the macula.
• Ora serrata is spanned by a 3 - 4mm vitreous known as the vitreous base.
• Ora serrata is 6 - 6.5mm away from the limbus.
• Pars plicata is 2.2 - 2.5mm away from the limbus.
• Pars plana is 3.5 - 4mm away from the limbus. Intravitreal injections are given at pars plana.

Macula

• It contains the retina's most sensitive and thinnest part, the fovea centralis.
• The foveola is inside the fovea centralis, and there is a central depression over it known as the umbo.
• The diameter of foveola is 0.35mm and fovea is 1.5mm.
• There is no retinal blood supply in the fovea centralis or the foveola, so this area is known as the foveal avascular zone.
• Outside the fovea centralis are two areas - the parafoveal (2.5mm in diameter) area, and outside the parafoveal area is the perifoveal area (5.0-5.5mm in diameter).

Layers Of Retina

There are 10 layers of the retina. These are (outermost to innermost)-

1. Retinal pigment epithelium: Outermost layer
   • 1.1  Subretinal space
2. Subretinal space
3. Photoreceptor layer: Layers of rods and cones
4. External limiting membrane
5. Outer nuclear
6. Outer plexiform
7. Inner nuclear
8. Inner plexiform
9. Ganglion cell layer
10. Nerve fibre layer
11. Internal limiting membrane

• Inside this is the condensed part of the retina, known as the hyaloid membrane.
• Subretinal space is very important because it is very weak embryologically, so any retinal separation happens in this layer. 
• Retinal detachment is the separation of the outermost layer, the retinal pigment epithelium, from the rest of the layers to the internal limiting membrane, collectively called the neurosensory retina (NSR).
• The retinal detachment happens along the subretinal space.
• Bruch's membrane is the innermost layer of the choroid, between the choroid and the retinal pigment epithelium.

Blood Supply of the Retina

• Short posterior ciliary arteries supply the outer 4 layers, and the central retinal artery supplies the inner layers.
• The blood vessels or the capillary system are present at two levels -
  • OP and IN (outer plexiform and inner nuclear): The blood vessels are perpendicular to the retina's surface.
    • Any bleeding at this level looks like a dot and a blot (Dot and blot haemorrhages).
    • These are called deep haemorrhages.
• Along the nerve fibre layer: The blood vessels parallel the retina's surface.
  • Any bleeding at this layer looks like a flame-shaped haemorrhage.
  • These are called superficial haemorrhages.

EXUDATES FROM THE RETINA

Exudates in the retina are of two types:

1. Hard exudates: These have well-defined margins.
   • These are leaked lipids.
2. Soft exudates: Their margins are ill-defined.
   • These are also called cotton wool spots.
   • These are the axonal infarcts (dead axons).
   • It is a more serious condition as it happens due to hypoxia.

Blood Retinal Barrier

There are two blood-retinal barriers:

1. Inner: It is the capillary integrity.
   • When this barrier is broken, it causes leakage and thus leads to cystoid macular oedema.
2. Outer: It is made up of strong adhesion of retinal pigment epithelium.
   • It prevents any choroidal leakage in the retinal layers.
   • These strong adhesions are called zonula occludens.
   • If the barrier breaks (shallow detachment), there can be a mild collection of fluid, which is called central serous retinopathy (CSR).

Cells Of The Retinal Layers

The different types of cells are present on different layers of the retina.

• Rods and cones are present below the subretinal space.
• The outer nucleus contains the nuclei of rods and cones.
• The outer plexiform has the synapses of axons of rods and cones with the dendrites of bipolar cells/amacrine cells/horizontal cells.
• The inner nuclear layers contain the bipolar cells, amacrine cells, and horizontal cells.
• The inner plexiform layer contains the synapses of the ganglion cells.
• The ganglion cell layer contains ganglion cells.
• The nerve fibre layer is made up of axons of ganglion cells.
• The Muller cells are the supporting glial cells that span from the rods and cone layers and reach the internal limiting membrane.

Histology

• The retinal pigment layer looks like a red layer.
• The synapse layers have multiple deep purple dots.

Investigations Related To The Retina

1. Fundus Fluorescein Angiography (FFA)

• After dilating the pupil and through examination, the dye is injected into the antecubital vein.
• A vial of 10% 5ml fluorescein is injected into the blood, which reaches from the arm to the retina.
• The arm-retina time is 10-11 secs.
• It is first going to appear in choroid.
• It absorbs light from low wavelengths and emits light from high wavelengths.
• In flourescin angiography, a blue light is used, and the dye emits a green light. So, the fundus camera has blue and green filters to help study the blood vessels.
• In normal angiography, the path of the retinal veins is seen as solid white lines. The background is faint white due to the choroidal fluorescence.
• Centrally, a black spot represents the fovea and the foveola. It is dark due to the lack of blood vessels.
• Also, the choroid fluorescence fails to reach the foveal avascular zone as there is a dense collection of pigments in this area.
• The pigments present in the foveal or foveola are melanin and xanthophyll.
• Findings: It can show -
  • Hypofluorescence: It contains more dark areas. The causes are -
    • Capillary non-perfusion (CNP) or capillary block.
    • Blocked fluorescence can be due to any haemorrhage or exudate in that area.
  • Hyperfluorescence: It contains more white areas. The causes are -
    • Leakage due to breakage of the inner retinal barrier
    • Retinal pigment epithelial defect.
    • If the hyperfluorescence is due to leakage, it will increase in size; this is how it can be differentiated from retinal pigment epithelial defect.
    • The disadvantage of FFA is that fluorescein washes away very quickly, which gives less time for diagnosis, especially of the choroid.

2. Indocyanine Green Angiography (ICG)

• The fluorescence is seen in the infrared range.
• ICG is 98% bound to plasma protein, so it becomes a big molecule that is hard to leak out of the capillaries, and the dye remains in the choroid circulation for a longer time.
• It is the investigation of choice for occult CNV (Choroidal Neovascularization).

3. Optical Coherence Tomography (OCT)

• This test shows the cross-sections of the different retina layers with an accuracy of 10-15 microns.
• It is based on the principle of inferometry. It uses light waves instead of sound waves. 
• Thus, the media has to be very clear; this cannot be done in opaque media (like cataracts).
• A red-orange represents the RPE, an easily recognizable layer.
• There is a dip in the foveal area, as not all 10 layers are present in the fovea (6 layers) and the foveola (5 layers). The capillary layers are absent, resulting in reduced thickness in the foveal area.

4. Amsler-Grid test

• It is a subjective test where the patient’s visual field is tested.
• The patient is given a piece of paper with several grids/squares and a dot in the center.
• The patient is asked to hold the paper approximately 30 cm away from the eye, focus on the dot one eye at a time, and then ask about the different squares.
• In this, around 20° of the visual field is tested, so any pathology in the macular area is detected by this test.
• The patient can see wavy lines/distortion of the image which is known as metamorphopsia.
• The square can look bigger which is known as macropsia.
• The square can appear smaller which is known as micropsia.
• These are the different disturbance of vision.
• This test is very useful in patients with CSR or Age-related macular degeneration patients (AGMD).
• It is a very helpful follow-up test.

5. Photostress Test

• The principle of this test involves exposing the macula to a light source bright enough to bleach a significant proportion of the visual pigments.
• The return of normal retinal function and sensitivity depends on the regeneration of the visual pigments.
• The patient's visual equity is recorded before the test is performed. For example, it is 6/12.
• Now, a bright light is flashed into the patient;’s eyes and then immediately removed.
• The number of seconds needed by the patient to read at least 2 letters of the 6/12 line is recorded, and this is called the photo stress recovery time (PSRT).
• The normal value of PSRT is 30-50 secs.
• In the case of a macular lesion, PSRT increases to around 60 secs.
• However, in the optic nerve lesion, the PSRT will be normal.
• The photostress test is used to differentiate between macular lesions and optic nerve lesions.

6.Usg B Scan

• It helps to visualize the posterior segment of the eye.
• A lesion with a collar button appearance seen is the choroidal malignant melanoma..
• The frequency of the ultrasound used is 7.5-10MHz.

7. Electroretinogram(ERG)

• When the light is shown to the eye, it gives the activity of photoreceptor cells and bipolar cells.
• It shows an A wave, a B wave, and a small C wave.
  • A wave - represents photoreceptors (rods and cones).
  • B wave - represents bipolar muller cells.
  • C wave - represents RPE.
• Procedure: There are two electrodes, one is placed on the cornea/conjunctiva, the other is placed on the forehead and the electrical activity is taken.
• It gives the activity of both neuronal and non-neuronal cells.
• Special ERGs:
  • Pattern ERGs: These indicate the activity of ganglion cells and thus it is helpful for the diagnosis of glaucoma.
  • Multifocal ERGs: These are multiple recordings taken from different points of the central retina.
• This covers 40-50° of the visual field.
• It is helpful in the diagnosis of cone dystrophy and toxic neuropathy & maculopathy due to chloroquine and hydroxychloroquine.
• The frequency of ultrasonic probe is 7.5 – 10 megaHz

8. Electrooculogram (EOG)

• It measures the standing potential of the eye, i.e., the potential difference between the front and back of the eye.
• Two electrodes are placed in the medial and lateral canthus and the patient is asked to move the eyes right and left.
• The cornea is making the nearest electrode positive in respect to the other.
• The normal reading is 6mV.
• This test is very sensitive to any disease of the RPE.
• Two cycles are recorded, 15 mins in the dark and 15 mins in the light and the maximum potential in the light (Light peak) and minimum potential in the dark (dark trough) are noted.
• Light peak/dark trough = ARDEN RATIO.
• The normal value of the ardent ratio is ≥ 1.85.
• It is affected in RPE dystrophies.
• In Best disease, the ardent ratio becomes <1.5.
• This test is also helpful in the diagnosis of Stargardt's disease and chloroquine toxicity.
• Flat EOG: The ardent ratio is <1.25

Retinal Pathologies

• Retinal pathologies in different diseases are broadly classified as
  • Vascular diseases
  • Macular diseases
  • Hereditary fundus dystrophy
  • Retinal detachments
  • Retinoblastoma
• These pathologies are very important to understand in detail. It is highly recommended that you watch the video from the prepLadder application to get a better understanding of the topic.

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