OPTM4102 Accommodation PDF

OPTM4102 Accommodation Dr. Jason Charng [email protected] Acknowledgement of country The University of Western Australia acknowledges that its campus is situated on Noongar land, and that Noongar people remain the spiritual and cultural custodia...

OPTM4102 Accommodation Dr. Jason Charng [email protected] Acknowledgement of country The University of Western Australia acknowledges that its campus is situated on Noongar land, and that Noongar people remain the spiritual and cultural custodians of their land, and continue to practise their values, languages, beliefs and knowledge. Artist: Dr Richard Barry Walley OAM Objectives Anatomy underpinning accommodation Accommodation Dioptre change in the optical power of the eye ~Cornea refractive power set ~Increase curvature of lens ~Focuses objects up close lens become bowldge thicker - and > Levin LA, et al. Adler’s Physiology of the Eye, 11th Edition. 2011. Saunders.  Accommodation – apparatus Ocular structurals : Ciliary muscle Zonules Lens capsule Lens http://www.images.missionforvisionusa.org/anatomy/uploaded_images/ASpelAtlas-751023.jpg Ciliary muscle => Smooth muscle controlled the autonomic nerve by m. (We syste can't control it) Arranged in 3 bundles ~ Outer longitudinal ~ Middle radial ~ Inner circular Attaches anteriorly to scleral spur and TM Attaches posteriorly to elastic network of Bruch’s membrane and the choroid Zonules Arise from the pars plana ~ Non-collagenous glycoprotein secreted by the ciliary epithelium Primary function is to stabilise the lens and facilitate accommodation Three groups of fibres: ~ Anterior zonules ~ Equatorial zonules ~ Posterior zonules https://pubmed.ncbi.nlm.nih.gov/17065477/ Lens = Capsule e http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2984119/  Lens capsule covers the whole lens - Continuous basement membrane with elastic properties that envelops the lens ↓ ↓ ↓ ~a meshwork structure Regional variations in thinest thickest thickness Serves as the point of attachment of the lens zonules Spithelial cell membrane. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2674021/ http://www.ncbi.nlm.nih.gov/pubmed/16639015 Accommodation – ciliary muscle - During accommodation the outer longitudinal - fibres contract (muscle - area decreases) Main mass of ciliary muscle moves forward along curved inner wall of sclera towards scleral spur This forces inner radial fibres and overlying ciliary processes to bulge inwards towards posterior chamber (muscle area increases) http://www.ncbi.nlm.nih.gov/pubmed/23504546 Accommodation – zonules Accommodation – theories Mechanism of accommodation still debated Original study ~https://pubmed.ncbi.nlm.nih.gov/23745002/ ~https://pubmed.ncbi.nlm.nih.gov/23745005/ Letter to Editor ~https://pubmed.ncbi.nlm.nih.gov/33591360/ Response to letter ~https://pubmed.ncbi.nlm.nih.gov/33591361/ Accommodation – capsule The lens capsule draws the (young) lens into a more accommodated form ~ Decapsulation results in flattening and an increase in focal length In young eyes the capsule determines the force required to change the lens diameter ~ In older eyes it is the lens contents that determine the force required https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2689386/  Lens shape and displacement the lens bowdge towards anterior chamber ↑ Anterior pole of the lens displaced during accommodation ~ Most displacement is nucleus, not cortex mus - Increased nucleus thickness with accommodation ~ Posterior pole unchanged Forward displacement leads to shallowing of the anterior chamber ~ Increased thickness of lens ~ Increased lens convexity ~ Closer proximity of cornea to anterior lens https://pubmed.ncbi.nlm.nih.gov/17997685/ Levin LA, et al. Adler’s Physiology of the Eye, 9th Edition. 1992. Saunders. Neural control of accommodation from the retina Retina V Blur signal → primary visual H cortex → pretectal area→ optic nerve # Edinger Westphal nucleus Optic Chasm Two signals are then sent H ~ One signal via parasympathetic Optictract pathway to the ciliary ganglion and ↓ ciliary muscle (accommodation), LGN also results in pupil constriction H ~ Other signal via CNIII to medial Visual Cortex rectus (convergence) Edinger Westphal nucleus Accommodation/near reflex (accommodation + pupil + convergence) https://www.youtube.com/wat ch?v=-BMW3vBTFLk Accommodation and convergence reflex Accommodation – neural control Parasympathetic nerves dominate ciliary muscle (muscarinic mediated contraction) m Efferent, preganglionic nerve fibres originate in Edinger- Westphal nucleus and travel via CN III to synapse in the ciliary ganglion Post-ganglionic fibres travel to ciliary muscle via short ciliary nerves (a few via long ciliary nerve) Levin LA, et al. Adler’s Physiology of the Eye, 9th Edition. 1992. Saunders.  Accommodation – neural control This pathway is miner compare to the parasympathetic pathway. Some sympathetic nerve activity (adrenergic mediated relaxation) - Originate in diencephalon - - Pre-ganglionic fibres synapse at superior cervical ganglion Post-ganglionic fibres travel to ciliary muscle via nasociliary nerve (mostly long ciliary nerve) supplement the Some post-ganglionic fibres Parasympathetic pass through ciliary ganglion pathway by providing without synapsing, then travel signal with short ciliary nerves to CM inhibitory to achieve sustain nearwork. Levin LA, et al. Adler’s Physiology of the Eye, 9th Edition. 1992. Saunders. Presbyopia => lost of accommodation with age - Common presentation in clinic, onset ~40 yo Gradual inability to accommodate Various contributors ~ Increased stiffness of crystalline lens (main driver) ~ Change in zonule insertion with age due to increased lens thickness. Decrease in effect on lens shape with zonular contraction/relaxation Optical correction https://pubmed.ncbi.nlm.nih.gov/18336584/ Summary Accommodative apparatus: ~ Lens ~ Capsule ~ Zonules ~ Ciliary muscle Anatomical basis underpinning accommodation