What is the stucture of the optic nerve? Which cranial nerve is it?
50mm long
1 million fibres
retina to chiasm
Cr. N II (2)
Intra-ocular 1mm
Intra-orbital 25-30mm
Intracanalicular 4-10mm
Intracrannial 10mm
ONH vs OD
OHN= 3D, cross section seen with oct- collection and exit point for axons of ret gang cells
OD= 2D, seen with opthalmoscopy
What is the lamina cribosa ?
Mesh of coll fibres
pathway for RNF to exit
structural support to OHN when IOP rises
Increased visibility of LC if glaucoma or shallow cup
Describe the distribution of nerve fibres in optic disc
Topographical
macular fibres- enter temporally.. form papillomacular bundle
nasal fibres- enter nasally
sup and inf fibres bend around macula
fibres x cross horizontal raphe
Where is the blind spot? Why does it occur?
at optic head
no photoreceptors
Blood supply to intraocular?
SPCA small vessels
if bld supply reduced, ONH becomes pale (pallor)
What are the 3 types of mater and their structure?
Protective layer around ON- start in intra-orbital
Dura- toughest
Arachnoid- vascular
Pia- thin
In the cranium, only pia mater.
Why is the ON myelinated?
insulation, prevents short circuiting
starts after OHN , nerves in ret should not be myelinated
if so, appears fluffy white patch
Glial cells structure and func
Found around NF axons
homeostasis and structural support
3 types: Astrocytes,
Oligodendrocytes
NG2
Describe blood supply in optic nerve
ICA》 OA》 LPCA+ CRA
CRA supplies retina only
SPCA supply choroid and ON
Optic disk parameters
Location: 10 degrees nasal + 25 degrees sup
Size: 7 degrees vertical + 5 degrees horizontal
Shape: tilted
How to estimate ONH depth
change in focus
higher change =deeper surface
OCT
CD ratio: 0.5 to 1
Structure of the optic disc
Cup - can be large or small
NNR- pale= loss of bld vessels
Margin
How big is the OD cup and NNR ?
With Volk super 66 = same magnification
direct ophthalmoscopy - use aperture size
notice colour change or bend of bld vessels
Width of NNR= ISNT rule
NNR in Glaucoma
curved vessel in cup rather than edge suggests cup has enlarged
circumlinear vessel
SVP
lOP varies with pulse, can be higher than CRV pressure
leads to spontaneous venous pulsation
Neovasc and haemorrhage indicate what?
Neovasc
diabetic ret
Finer vessels tend to leak
Splinter haemorrhages
Glaucoma
What does peripapillary mean? Name of adjacent tissues. Why does a cresent form?
Peripap- next to optic disc
Ret, choroid, sclera,
Sometimes crescents seen- layers don't reach end of cup
Crescents aka chorio-retinal atrophy
What is peripapillary atrophy?
Thinning of layers in retina
outerzone of hypo and hyper pig
PPA is not disc margin
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