Emmetropisation & Accommodation Flashcards
Explain trends in adult refractive error regarding myopic/hyperopic shift
hyperopic shift (40+): reduced accommodative ability in latent hyperopes
myopic shift (75+): primarily due to nuclear (age-related) cataract development causing a marked increase in lens n
Explain form deprivation and it’s implications
light blocked from forming sharp macular image via congenital cataracts, juvenile macular dystrophy, congenital ptsosis ~ typically develop high myopia
causes small ciliary body change but large axial elongation which causes myopic shift
How does axial length, corneal and lens power change in early emmetropisation?
increased axial length (k’) causes myopia increase (K’ dioptric length decrease)
reduced corneal power caused hyperopia increase (increased corneal radius)
lens power decreases as it starts steep then flattens so hyperopia increases
general sync shift from hyperopia (birth) to emmetropia
What is the common cause for large ametropias developing?
1 ocular component (usually k’ axial length) is outside emmetropic range
high myopes have longer k’, high hyperopes have shorter k’
How does astigmatism change with age?
horizontal corneal meridian typically steepens at faster rate than vertical causing general shift from WTR astig to ATR astig (45+)
possibly due to elasticity loss in orbicularis oculi
What are some serious consequences of developing pathological myopia?
PERMANENT VISION LOSS
RetDet, Glaucoma (retinal elongation damages RGC axons at ONH)
Lacquer cracks (RPE/Bruch’s membrane breaks at (para)central macular)
Choroidal neovascularisation
Staphyloma (backward bulging of sclera/choroid/RPE)
Describe 3 possible treatments for halting myopia progression
bifocal/dual-focus soft CLs (concentric, centre-distance design)
Orthokeratology (flattening central cornea via reverse-geometry RGP CLs
Muscarinic-receptor antagonists (Atropine (0.5,0.1,0.01% and Pirenzepine 2%)
Describe the impact of peripheral refraction on myopia progression
uncorrected, relative peripheral hyperopia could stimulate similar magnitudes of axial elongation (peripheral retina grows backwards) and subsequent myopia progression
What is the likely driving mechanism behind emmetropisation?
retinal image quality feedback system regulates ocular tissue growth to achieve overall refractive error close to emmetropia (from studies on Rhesus Monkeys, Chick eyes)
Explain 3 abrupt changes in corneal refraction
Ectasia: stromal thinning/forward protusion with decreased Rc causes increase in corneal power/irregular astigmatism (keratoconus/globus)
Oedema: corneal n/thickness change causes myopia increase (CL overwear/Keratitis/Ulcers)
Eyelid Pressure change: astigmatism increase de to tarsal plate abnormalities (chalazion)
Explain 4 abrupt changes in lens refraction
Age-related cataract: increases n causes myopic shift
Transient change: diabetes mellitus (blood glucose increase - increased n ~ transient myopic shift)
Lenticonus: bulging of either lens surface, lower Rc induces myopia/astigmatism
Subluxation (Marfan’s): high myopia, lens displaced with subseqent k’ elongation so risk of RetDet)
Explain 3 abrupt changes in the eye’s refraction
Pupil: abnormal diameters cause ‘apparent’ refractive change (impacts aberrations/DoF)
Ciliary body: accommodative spasm causes pseudo-myopia for distance vision
Axial length (k’): reduced due to retro-bulbar tumours, central serous retinopathy, macula odema inducing hyperopic shift
Describe the structure of the crystalline lens
zonular fibres attach ciliary body to ant./post. areas of elastic lens capsule with specialist ball-socket joints stopping individual fibres slipping past each other during accommodation
Describe 3 features of accommodation
voluntary reflex, retinal image ‘blur’ is the primary stimulus but sudden changes in object size/distance are also factors
it’s a cone function so works best in photopic conditions
Explain the defects in accommodation
some conditions cause reduced amplitude, accommodative spasm or poor-sustained accommodation response e.g. neurodegenerative disease (MS, DiabetesMellitus, Adie’s pupil)
Explain the ‘near triad’
Accommodation, Convergence, Pupillary Miosis
Accommodation stimulates convergence v.v but accommodative pupillary constriction drives neither
Describe the stability, speed and accuracy of the accommodative response
Viewing a near object at a fixed distance: response isn’t steady ~ tiny 0.10D fluctuations at 1-2Hz
for a new accommodative stimulus there’s ~up to 30sec latency (short reaction time) before response commences (actual response takes .5-1 sec to complete (age-dependent)
typical lag in response for near vision a achieved accommodation is lower in magnitude than stimulus
Explain ‘dark-field’ myopia and ‘Empty-field’ (Ganzfield) myopia and why they might manifest
Dark-field: Young eyes view distant objects (>6m) in complete darkness, their accommodation sytem stabilises to a constant state of ‘over-accommodation’ (myopia) typically ~1.50D in young adults (night driving)
Empty-field: eye presented with ‘empty’ field lacking contrast/structure(s) to e focussed upon (~1.50D in young adults - pilots)
both are manifestations of ‘resting state’/’tonic levels’ of innervation sent to ciliary body in absence of adequate visual stimulus
Explain instrument myopia
young microscope user adjusts instrument focus preferring a setting of myopic defocus as it’s appropriate for accom. viewing rather than relaxed as they find it more comfortable
