Optic Disc Flashcards
Name the 3 cells that information travels through on its way to the brain
Photoreceptors, Bipolar, and Retinal Ganglion Cells
How do Ganglion cells travel to the optic disc
They take the long way around, a curved route instead of the straight path. This is why there are little nerve fibres across the fovea - needs to remain place of clearest vision
What do Ganglion cell’s do when reaching the optic disc
They do a 90° turn and dive down the back of the eye
How many photoreceptors are there in the Optic disc?
None
What part of the retina has routes out and routes in
Routes out = Central retinal vein
Routes in = Central retinal artery
Name 3 conditions that affect the ONH and describe them briefly
Glaucoma - blurred margins and orange colour - Causes irreversible loss of retinal nerve fibres & is, in most cases, asymptomatic
Papilloedema - unclear margins, disc would be sticking out, & haem around the disc. Causes brain swelling and increased pressure, very serious
Papillitis - also causes unclear margins but only in one eye. Signifies inflammation of the Optic nerve
Approx how many nerve fibres are there in the optic disc and what is the variation
There are approx 1million nerve fibres that exit the eye, but inter-individual variation has been researched to be 50%
How many nerve fibres do we lose per year as part of aging
We lose about 3,400 a year due to aging
What are the dimensions of the Optic Disc
Average width = 1.7mm-1.8mm
Average height = 1.9mm
Average area = 2.7mm^2
What is the range of disc diameter and what factors can affect the size?
Normal range in disc diameter = 1.2-2.27mm
Often larger in myopic eyes and smaller in hypermetropic ones
Larger in eyes with a bigger Axial length
Larger in people of African descent
What does anisometropia mean and what does it mean for the discs?
Anisometropia is where the eyes have a different Rx. In eyes with a big different, one disc may be much larger than the other
What supplies blood to the inner retina
The central retinal artery
What is a cilioretinal artery and why is it good
The cilioretinal artery is and additional artery that hooks over the optic disc and has a different blood supply. This means that if there was a blockage in that ONH area the branches of the Cilioretinsl artery would not be affected and the px would be able to maintain some vision
What are the two main branches of blood supply to the eye?
The internal carotid artery & the ophthalmic artery
What two branches of the ophthalmic artery provide are we interested in, in relation to the optic disc
Tne short posterior ciliary artery an the central retinal artery
Describe the central retinal artery in relation to the ON
The central retinal artery pierces the ON and travels up through the middle, Ans is what you see when looking at the back of the eye, coming up to the surface
Describe the short posterior ciliary artery in relation to the ON
The short posterior ciliary artery comes up the sides of the optic nerve and branch into the choroid and then into the upper most part of the ONH to form a partial circle that supplies the top region
Where does the Optic nerve get its supply of blood from
The optic nerve gets its supply mostly from the pial plexus - a fine network of vessels that travels down the side of the ON where branches go in. Some people have it supplied by the CRA
What is the area outside the cup called?
The neuroretinal rim (NRR)
What is the ISNT rule?
The ISNT rule essentially states that the thickness of the neuralretinal rim changes as you go around it, going from thickest at the interior, then decreasing as you go to the superior, nasal, and temporal. It was said that if the disc didn’t follow this rule then there was something wrong with the eye. However this rule is a little disputed, not everyones disc fits into this.
What is the white ring sometimes seen around the disc called
The peripupillary disc or Ehlsching’s ring. It lies where the sclera meets the edge of the optic disc. Need to take this in account when measuring CD ratio
What is the slightly off colour pigment around the optic disc called?
The slightly off-colour pigment around the optic disc is called the peripapillary atrophy, and is due to the shrinking of RPE cells. Very normal but can be very intense in px’s with glaucoma
Are retinal nerve fibres normally Myelinated? And what is Myelin
No, Retinal nerve fibres aren’t normally myelinated, as myelin is a special coating around typically axons that prevents any leakage of signals and prevents crosstalk between adjacent ones. However it does not allow light to pass through meaning it cannot be in the retina as we wouldn’t be able to see. Although in some px’s, their ON has some slight myelination. This is generally inconsequential unless widespread
What is the area under the surface of the optic disc called
The area under the surface of the optic disc is called the optic nerve head
What are the three regions in the ONH?
The three regions in the ONH are:
1. Pre-laminar
2. Laminar
3. Post-Laminar
What happens in the pre-laminar region
In the pre-laminar region, nerve fibres firstly turn 90 degrees and then begin to gather themselves into ~ 1000 bundles. These bundles are seperated by a type of glial cells called astrocytes with capillary blood supply
What is the Lamina Cribrosa and what happens there
Information in needs to leave the Optic nerve and does so through the Lamina Cribrosa. Information is unable to pass through the sclera as it is very tough and strong. In the Lamina Cribrosa there are a series of ‘sieve-like’ plates continuous with the sclera. Bundles wiggle through the lamina to pass the sclera and get to the brain.
What happens to the Lamina Cribrosa in Glaucoma?
In cases of glaucoma, the Lamina Cribrosa becomes compressed, the holes of the Lamina which are typically small even and in regular order become the opposite
Can the Lamina be seen by looking at the Optic disc
Yes, sometimes the Lamina can be seen by looking at the optic disc. They are visible as what looks like pores on the surface of the disc. This can be normal, especially in px’s with large discs. Although pores that look more streaky can be a sign of glaucoma
What happens in the post-Laminar region
In the post-laminar region, the overall diameter increases. The nerve fibres can now become myelinated as light does not need to pass through. The outer layer becomes surrounded by meninges (protective layers) and this is known as the ‘optic nerve’ part of visual pathways
What is the structure of the optic nerve itself?
The optic nerve itself is not straight, it is flexible and is so to allow for eye movements, as the optic nerve moves when the eyes themselves do. The optic nerve portion is about 25-30mm long
Where does the optic nerve exit orbit
The optic nerve exits the orbit through the optic canal in the sphenoid bone, there is then another 16mm to the next part of the visual pathway known as the optic chiasm
