Cornea - Telenet Users

Retina
10 lagen
1. RPE
2. Fotoreceptors (buiten- en binnensegment)
3. Membrana limitans externa
4. Outer nuclear layer (= laag met cellichamen van staafjes en kegels)
5. Outer plexiform layer (= synaps tussen foto-R’s en bipolaire cellen)
6. Inner nuclear layer
7. Inner plexiform layer (= synaps tussen bipolaire en ganglion cellen)
8. Ganglioncellen
9. Laag van ganglioncelvezels
10. Membrana limitans interna
Fovea: 0,3 mm
115 miljoen staafjes = 30000 / mm2
6,5 miljoen kegels = 150000 / mm2
1,2 miljoen ganglioncellen
Vezels van bovenkwadranten naar med. CGL
Vezels van onderkwadranten naar lat. CGL.
Neurotransmitters
Exciterend: Glutamaat, acetylcholine
Inhiberend: GABA, Glycine
Neuromodulator: Dopamine
Glutamaat is de belangrijkste NT in de synaps tussen fotoreceptor en bipolaire/horizontale
cellen. Er zijn 4 verschillende soorten Glu-receptors [196].
Horizontale cellen kunnen exciterende (Glu) en inhiberende (GABA) NT vrijstellen.
Amacriene cellen stellen ook exciterende (ACh) en inhiberende NT (GABA en Gly) vrij.
Deze 2 soorten cellen beinvloeden de frequentie waarmee bipolaire cellen afvuren.
Amacriene cellen spelen een belangrijke rol in het bepalen van de grootte van het receptieve
veld van de individuele ganglioncellen.
Ganglioncellen gebruiken vooral Glu als NT.
Horizontale cellen: fotoreceptor – bipolaire cel
Amacriene cellen: bipolaire cel – ganglion cel
Fotoreceptors
Donker: depolarisatie
Toename licht: toenemende hyperpolarisatie (en afbraak rhodopsine)
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Bloed-retina barriere
Wordt gevormd door tight-junctions thv. het RPE en de endotheelcellen van de retinale vaten.
Thv. deze vaten geen passage van molecuelen > 20000 – 30000 Da. Kleine moleculen, zoals
glucose en ascorbaat, worden vervoerd door gefaciliteerde diffusie (GLUT-3 voor glucose).
Metabolisme en turn-over: zie blz. 185.
Retinaal pigment epitheel (RPE)
Choroid
5 lagen:
1. Bruch
2. chorio-capillaris
3. laag van Haller: grote vaten
4. laag van Sattler: middelgrote vaten
5. suprachoroid
Bevloeiing:
1. vooral van a. ciliaris post. (brevis en longa)
2. recurrente takken van a. ciliaris ant.
Drainage via de vortexvenen (Vv. vorticosae) naar de v. ophthalmica (sup. en inf.)
Conjunctiva
Mucine secretors:
1. Goblet cellen
: inferonasaal in epitheel
2. Crypten van Henle : tarsale conjunctiva thv. fornix
3. Klieren van Manz : rond limbus
Traanfilm
De pH van normale tranen is 6,5 – 7,6 (gemiddeld 7,5)
98% H2O
Volume = 6 – 9 ul
Proteinen: lysozyme, lactoferrine, IgA
3 lagen:
1. lipiden
2. water
3. mucus
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De mucus-laag wordt gevormd door intra-epitheliale gobletcellen van de conjunctiva.
Gezondheidstoestand van de epitheelcellen en gobletcellen is afhankelijk van de
aanwezigheid van vitamine A.
Cornea
Buitenzijde convex (+49 D)
Binnenzijde concaaf (-6 D)
Totaal: +43 D
Opgebouwd uit keratine, chondroitine en chondroitine sulfaat.
Bestaat uit 5 lagen:
1. Epitheel
2. Bowman = Lamina limitans anterior:
Geen regeneratie
3. Stroma
4. Descemet = Lamina limitans posterior
Wel regeneratie.
Gevormd door endotheel.
Stopt aan lijn van Schwalbe.
5. Endotheel
Normale gemiddelde dikte = 555 um
Oculaire hypertensie: gemiddelde dikte = 577 um
Normotensie glaucoom: gemiddelde dikte = 515 um
http://www.chynn.com/ewc-article_cornea6.htm:
primary open angle glaucoma (POAG),
normal tension glaucoma (NTG),
ocular hypertension (OHT)
Mean corneal pachymetry was 555 um in normals, 551 um in POAG, and 577 um in OHT, versus only
515 um in NTG.
Mean corneal thickness was significantly less in NTG patients compared to patients with either POAG
or OHT, as well as normals. Conversely, patients who had been classified with OHT had the largest
mean corneal thickness of any group.
These results have two clear implications. First, some patients who have been classified with NTG
may in fact have POAG, with spuriously “low” or “normal” measured IOPs, secondary to their thinner
corneas, which require less force to applanate. Similarly, a subset of patients previously classified with
OHT may, in fact, have truly normal pressures, and not be at risk for developing glaucoma--their “high”
measured IOPs merely being a function of their thicker corneas. (Meer kracht nodig om de cornea’s
plat te drukken (?))
Only direct intraocular pressure measurement will be able to finally resolve these questions.
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But for now, beware of those patients with a corneal thickness greater than 585 um without typical
visual field or disc progression. These patients may carry an incorrect diagnosis of OHT, according to
Dr. Shah. Such patients, in his opinion, actually have a very low risk of suffering visual loss, their
“high” IOPs being purely the result of measurement error.
Conversely, it is those subset of patients classified with NTG who have a corneal thickness greater
than 540 um that may truly have this disease, rather than falsely low pressures due to measurement
error because of their thin corneas
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http://www.dog.org/2000/e-abstract_2000/669.html:
Effect of corneal thickness on applanation tonometry in glaucoma patients
B. Roesen, S. J. Fröhlich, C. Niederdellmann, S. Ullrich, M. Bechmann
Purpose: Goldmann applanation tonometry is based on the Imbert-Fick law, which assumes that the
surface of the cornea is perfectly elasitc and thin so the intraocular pressure could be measured by
applanation of the cornea. The design of the tonometer was based on the central corneal thickness
(CCT) of 520µm. Thinner or thicker CCT’s could lead to an under- or overestimation of the IOP.
Methods: In addition to the clinical examination we determined in 132 consecutive controls and
patientes with ocular hypertension (OHT), primary open-angle glaucoma (POAG) and normal tension
glaucoma (NTG) the CCT by ultrasonic pachymetry.
Results: Mean CCT of the control eyes was 573 µm  34, the mean CCT of the eyes with POAG was
570  39. The eyes with LTG had with a CCT of 548 µm  44 a significant lower CCT than the POAG
group. The eyes with OH had a significant thicker cornea than the POAG group (627 µm  28, p
0,001).
Conclusion: In patients with OHT, NTG and in patients with POAG with regulated IOP and
deterioration of the visual field the CCT can help to interpred the results of the Goldmann applanation
tonometry
Endotheel
Leeftijd
Geboorte
Middelbaar
Bejaard
Aantal cellen
3000 – 4000
2500
2000
Als < 800, dan vlug oedeem en zwelling.
Donorcornea moet >= 1500 hebben, anders te weinig voor transplantatie.
Glasvocht
4,5 ml
Zouten, proteinen en hyaluronzuur.
Vast aan:
1. ora serrata
2. pars plana
3. rand van papil
Kamerwater
Hypertoon t.o.v. plasma.
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Proteine-arm.
Afvoer via : Schlemm, trabekels en suprachoroidale ruimte.
Innervatie
N. ciliaris brevis en longus: sensiebele bezenuwing van cornea, iris en corpus ciliare.
N. ciliaris brevis bezenuwt ook: choroid, sfincter en corpus ciliare.
Accessoire traanklieren onder orthosympatische controle.
Traanklier: a. lacrimalis; n. facialis
Spieren
eMedicine:
A working knowledge of the relationships between the insertions of the rectus muscles is essential to
perform effective strabismus surgery. The tendon of the medial rectus muscle inserts 5.5 mm posterior
to the limbus along the medial aspect of the globe. Next most posterior in its insertion is the inferior
rectus, which inserts 6.5 mm posterior to the inferior limbus. Continuing counterclockwise around the
globe, the lateral rectus inserts 6.9 mm posterior to the lateral limbus, and the superior rectus inserts
7.7 mm posterior from the superior limbus. An imaginary line connecting these insertion points creates
a configuration known as the spiral of Tillaux.
( Herpes zoster: atropine, predforte, zelitrex, zovirax oogzalf)
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