L06 - The eye as a window to the CNS Flashcards
How it the eye similar to the brain – neuronally and vascularly?
Neuronal – both have highly metabolically active tissues
Vasculature – Blood neural barriers (BBB in brain and Blood Retinal Barrier in eye)
What advantages does eye assessment have over brain assessment?
- Non-invasive
- Inexpensive relative to PET/MRI
- Simpler to analyse/interpret: relatively 2D instead of 3d
- Simple and rapid to conduct “photo” (in brain assessment, takes hours, can’t wear metal objects, no food etc.)
Besides the retina being part of the CNS (embryonically retinal extension of diencephalon), how are photoreceptors similar to CNS neurons?
- Both highly metabolically active tissue – photoreceptors most metabolically active tissue/weight
- Similar support cells = glia
- Similar NT: Glu, glycine, dopa, NMDA, GABA
- Similar age-related decline/neurodegeneration: Healthy aging, Alzheimer, Parkinsons
!Similarities between blood neural barriers between the eye and the brain? Mechanical barrier? Metabolic barrier
Both protect neurons, regulate substances from blood
Mechanical barrier: formed by tight junctions
1) Occludin
2) Claudins
3) Junctional adhesion molecules (JAM)
Metabolic barrier:
1) Glucose transport mechanisms: GLUT1 and 3
2) Specific a.acid protein transport systems
!The retina has an inner and outer blood supply and an inner and outer barrier - what are these 2 blood supplies and 2 barriers?
BLOOD SUPPLY
Inner (60-80%): central retinal artery/vein (thinner) - supplies GC & BC
Outer (20-30%): Choroid - supplies Photoreceptors
BARRIER
Inner: endothelial cell tight junctions in vasculature
Outer: RPE tight junctions that prevent blood from leaking into the neurons
Inner BS:
The tight junctions are located around the capillaries which are in between the endothelial cells of the capillaries
Outer BS:
Choriocapillaris have more gaps between the endothelial cells - more leaky
Inner BS:
The tight junctions are located in between the endothelial cells of the capillaries
Outer BS:
Choriocapillaris have more gaps between the endothelial cells - more leaky
What is fluorescein angiography?
Venous injection of fluorescent dye that highlights breakdown of blood retinal barrier
-> As the dye is a large molecule, it will stay within the retinal vasculature as it cannot penetrate through the retinal blood barrier
Similarities in vascular regulation (inner retinal circulation and cerebral circulation) between eye and brain
Similar regulatory processes: autoregulation (constant blood flow)
As the eye is the only place where microvascular system can be directly visualised, it is a marker for diseases such as?
Diabetes
- Loss of retinal pericytes (cells that are on the outside of the endothelial cells - this leads to leaking of blood retinal barrier)
- Basement membrane thickening
- Capillary microaneurysm (swelling) formation
- Increased vascular permeability such as exudation (fluid oozing) and tissue oedema
- Ischemia lead to retinal neovascularization (formation of new blood vessels - if they don’t have proper blood retinal barriers, they just leak and make things worse)
Hypertension
- generalized retinal arteriolar narrowing
- arteriovenous nicking (artery squashing overlapping vein due to increased BP) and arteriolar tortuosity (curly)
- blood retinal barrier breakdown (haemorrhages, exudates, nerve fibre ischemia - cotton wool spots)
Stroke
Predictor of stroke is quantitative by measuring vessel diameter
Multiple sclerosis: inflammatory demyelinating disease
- Retinal nerve fibre layer (where GC axons exit) thinning in MS patients
- Occurs with optic neuritis (also seen in MS patients that don’t have ocular disease)
- Can be assayed using optical coherence tomography (like US except shorter wavelengths - this means higher res using this technique)
Alzheimer’s - IN THE FUTURE
At the moment, current definitive diagnosis is post-mortem Alpha beta plaques or PET imaging
HOWEVER recently retinal Alpha Beta plaques are also seen in Alzheimer’s patients
