Optic Nerve Physiology Flashcards
Combien de retinal ganglion cells (RGC) sur la rétine?
Approximativement 1-1,2 million RGC in each retina.
One axon per cell projects into ON
Combien de retinal ganglion cells (RBC) perd-on avec l’âge?
Gradual loss during aging : 5000/year
Où se situe la lower rate of loss de retinal ganglion cells (RBC) sur la rétine
Rate of loss is lower for macular RGC
Que se produit-il avec les RGC dans le glaucome avancé?
In very advanced glaucoma, most RGC are already lost
Yearly age-related decline can lead to field loss even with good IOP control
Quel est le NT impliqué dans les RGC?
Glutamate
Épaisseur de la RGC layer a/n de la périphérie versus macula versus fovéa?
Périphérie : 10-20 um thick with single row of cells
Macula : 60-80 um thick with rows of cells
Fovéa : No ganglion cells
Quand l’OCT mac est-il pertinent dans le glaucome?
Macula usually has RGC remaining in ADVANCED glaucoma
Macular OCT may become more beneficial for advanced disease
Qu’est-ce que le RNFL?
Thicknest near disc
Axons range from 0,6-2,0 um in diameter
Axons form bundles in RNFL
Par quelles ¢ sont entourées les axones dans le RNFL?
Glial cells : Muller cells and astrocytes
Les axones du RNFL sont-ils ou non myélinisés?
Usually unmyelinated
(Possibilité que certains pts aient des axones myélinisés, mais aucune importante clinique)
Quels sont les cellules du NO?
Axons (cell bodies in RGC layer of retina)
Oligodendrocytes (and myelin)
Astrocytes
Microglia
Meninges
Combien d’axones y a t il dans le NO?
Environ 1 million in ON
Cell bodies in retina (RGC) so ON is pure white matter
Fascicles of 50-300 axons surrounded by septa (pia matter)
Quel est le rôle des oligodendrocytes?
20-30 processes per oligodendrocyte, each myelinating part of an axon
Myelin - Fatty, multilayered, insulating structure
Speeds conduction of impulse
Quel est le rôle des astrocytes?
Major supporting cell
Energy source for axon
Ionic homeostasis
Absorb glutamate
Prelaminar : glial tubes for axons
Lamilar : form collagen/elastin beams
First cells that respond to damage : affects support of axons, affects lamina cribs physiology, affects axoplasmic flow
Quel est le rôle des microglia?
Resident macrophages of optic nerve/CNS
Quels sont les 3 méninges et leur rôle?
Dura:
- Thick fibrovascular tissue
- Continuous with sclera, periorbita, dural layer of brain
Arachnoid
- Middle layer
- Loose, thin, fibrovascular
Pia :
- Very thin vascular layer
- Forms septa between fascicles
- Blood supply to infraorbital and intracranial nerve
Quel espace entre les méninges est en continuité avec le NO et le cerveau?
Subdural space of ON is NOT continuous with subdural space of the brain
SUBARACHNOID space of ON is CONTINUOUS with rest of CNS
Clinical correlation with the fact that subarachnoid space of ON is continuous with rest of CNS (aka 1 signe clinique).
Papilloedème
Increased intracranial presses transmitted around ON
Axoplasmic flow impeded distal to LC : swelling of disc head
Dimensions de l’Optic Disc
Vertical : 1,9 mm (1,0-3,0 mm)
Horizontal : 1,7 mm (0,9-2,6 mm)
Disc area : 2,7 mm^2 (0,8-5,5 mm^2)
Neuroretinal rim area : 2,0 mm^2 (0,8-4,7 mm^2)
Does OCT includes disc parameters?
Yes
OCT often includes disc parameters
Knowing averages can be helpful in noting outliers
Quel est l’impact des fibres non myélinisées sur les dimensions du NO?
At disc, nerve is unmyelinated
Axons leave globe through lamina cribosa
Posterior to disc, diameter doubles dues to myelination
Quel est le blood supply de la rétine, du NO, du infraorbital and optic canal ?
Retina :
- RGC layer and RNFL : retinal vascular circulation (central retinal artery)
ONH :
- Mostly Posterior Ciliary Arteries : comes from ophthalmic artery, branches form circle of Zinn-Haller
- Also central retinal artery
- Vessels to optic nerve are NOT fenestrated, can autoregulate
Intraorbital and optic canal :
- Pial circulation : direct from ophthalmic artery, central retinal artery, short posterior ciliary arteries
Peut-on opérer un méningiome du NO?
Non. Meningiomas of ON often inoperable because removal would remove/disrupt pia.
Pia = importante pour la vascularisation du NO.
Qu’est-ce qu’occasionne la perte des ganglions dans le glaucome?
Glaucomatous Damage :
Ganglion cell loss correlates with
- Augmentation du C:D ratio (loss of neuroretinal rim)
- VF loss
- RNFL thinning
- Post-synaptic atrophy/cell loss in LGN
Time to death of th RGC after axonal injury? (ex. glaucome)
Time to death of RGC can be days to months after axonal injury
When RGC dies, there is anterograde degeneration of axon
Apoptosis main mechanism
Qu’est-ce que l’apoptose?
Genetically programmed cell death
No inflammation, no necrosis
Tissue loss
Apoptosis can be triggered in neighbouring neurons when one is damaged
Site of damage in glaucomatous damage?
Site of damage : OPTIC DISC
ONH changes (neural tissue)
Bowing (courbure) of Lamina Cribrosa
Accumulation of organelles in axons (blocked transport)
Anterograde (Wallerian) degeneration distal to lamina cribosa
Quels sont les 2 théories du glaucomatous damage?
MECHANICAL theory : elevated IOP stretches laminar beams and damages RGC axons
VASCULAR theory : reduced ocular blood flow causing ONH damage/RGC loss
Qu’est-ce que impaired axonal transport dans le glaucome?
May precede RGC death
May be reversible at this stage (by lowering IOP)
Impaired may be due to :
- Impaired anatomy (mechanical strain at ONH)
- Metabolism (mitochondria and ATP)
- Damaged cytoskeleton
Qu’est-ce que la Lamina Cribosa?
Sieve-like opening in sclera at back of globe
Collagen and elastin extracellular matrix
Astrocytes and lamina cribosa cells
Overlapping and branching beams support axon bundles as they form ON
Lien entre la Lamina cribrosa et le glaucome
Superior and inferior LC have larger pores, and thinner beams = clinical correlation avec VF
Progressive glaucoma shows posterior bowing and thinning of Lamina cribrosa
Previously thought to be purely mechanical
More recent work suggests remodelling of Laminar extracellular matrix
Causes du cupping dans le glaucome
Due to loss of RGC’s and their axons
Due to bio-mechanical changes in Lamina Cribrosa
ISN’T the rule dans le disc rim thickness
Disc rim thickness in normal patient :
- Inferior >/= Superior >/= Nasal >/= Temporal
Quelle pathologie est associée à un non respect du ISN’T rule?
If rule violated, may have GLAUCOMA
Dimensions du disque optique (moyen, petit et large)
Average disc size : 2 mm
Smal disc : < 1,5 mm
Large disc : > 2,2 mm
Vrai ou Faux : « Normal » cup size depends on size of disc?
Vrai
Qu’est-ce que les zones Alpha et Beta parapapillaires?
Parapapillary Atrophy
Alpha-zone :
- Hyper/Hypo-pigment of RPE
- Normal and glaucoma eyes
Beta-zone :
- Area with ABSENT or ATROPHIED RPE
- Choriodal vessel/sclera visible
- Associated with GLAUCOMA progression
- Size of area and enlargement = progression
Apparence des Hg a/n du disque optique dans le glaucome?
Hemorrhage near disc margin in RNFL : splinter apparence
Théories derrière les Hg du disque optique
Vasculaire : vessel Hg at disc = axon ischemia and loss
Mechanical : pre-existing disc damage (ex. notch) gives collapse of tissue, stain on vessel then bleed
Gliosis : Glaucomatous damage causes localized gliosis. Contraction of scar causes capillary damage at junction of glial and normal tissue (border of RNFL defect)
Où sont principalement localisés les Hg du disque optique? Quelle est la durée d’une Hg du disque?
2/3 occur inferotemporal
Often occur in area of previous damage (notch, RNFL defect, PPA)
On average last 2-3 mois (donc si présence persistante d’un Hg au FO au suivi de 6 mois = nouvelle Hg et non la même)
Les Hg du disque optique sont ils associés à une progression du glaucome?
Oui.
Hemorrhage strongly associated with progression and active disease
Often subtle and easily missed on exam
Autres FdR associés au glaucome
Ocular Perfusion Pressure
- Perfusion pressure = Pa-Pv (IOP)
- Perfusion pressure is reduced when : arterial pressure reduced + increased IOP = lower blood flow
- Ex. Migraine = vasospasme = affecting ocular blood flow
CSF pressure
- Pressure difference between intra ocular and CSF space
- Lamina cribrosa divides the 2 spaces (différence entre les 2 compartiments)
- Low CSF pressure compared to IOP may contribute to damage at optic disc
- Pressure gradient - depends on pressure difference AND distance between the compartments
- Ocular hypertensives had significantly higher CSF pressure
- CSF pressure is correlated with blood pressure
CSF turnover
