Oweye Flashcards
Why is accommodation important?
- A dynamic process to produce and maintain a focused RETINAL image
- The power of the lens changes to maintain the image
- Lens curvature changes, lens power changes, focusing changes
What is the active muscle that leads the accommodative process?
Ciliary muscle
Ciliary muscle contraction process
- ciliary muscle contracts
- pulls ciliary ring forward and inward
- stretches the choroid and posterior zonules
- anterior zonules relax
- rounds the lens and capsule
- lens power increases
- focal length decreases
- eye focuses near
Changes to the lens in accommodation
- Equatorial diameter decreases from 10 to 9.6mm
- Anterior lens moves anteriorly and posterior lens moves posteriorly
- Central anterior radius of curvature STEEPENS (from 11mm to 5.5mm)
- Central posterior radius of curvature decreases (5.18-5.05)
- Central thickness increases AT THE NUCLEUS
- Lens sinks 0.3mm from gravity
What nervous system pathway innervates the ciliary muscle?
Parasympathetic pathway!
What kind of muscle is the ciliary muscle?
Smooth muscle
-in accommodation, ciliary muscle in ciliary body contracts and moves forward
Parasympathetic pathway
- Unfocused image on the retina
- Blur signals to visual cortex
- Cortical cell produces sensory blur signals
- Signal goes to MIDBRAIN/OCULOMOTOR NUCLEUS
- Motor command to ciliary muscle
- Ciliary muscle contracts
- Lens changes shape to focus image on retina
What starts the parasympathetic pathway?
Midbrain/ oculomotor nucleus ALSO KNOWN AS THE EDINGER-WESTPHAL NUCLEUS
Motor command for ciliary muscle
- oculomotor nerve (CN 3)
- ciliary ganglion
- short ciliary nerve
What does accommodation need sensory and motor?
Sensory to tell brain there is a blurry image and motor from oculomotor nerve to contract ciliary muscle
-you need convergence to accommodate
What is the state of the eye when there is no accommodation?
- anterior and posterior capsule is taught and flat
- light enters eye and is focused at the retina
- ciliary muscle is relaxed
Un accommodated emmetropic eye
- focuses on a distant target and there is no need for accommodation
- convergence demand is zero
- object is at optical infinity
Accommodating eye
- near object has DIVERGENT rays that focus behind the eye
- image is blurry on retina
- optical power of the eye has to increase to add POSITIVES convergent rays
- focuses light on the retina
Measure accommodation
- distance in cm where object is
- 100/cm
- measured in diopters
In the accommodating eye, what focuses the image in front?
The the accommodating lens
Accommodation triad/ near reflex
- 3 physiological changes seen in accommodation
- all changes are coupled through parasympathetic innervation from EW nucleus
What are the physiological changes seen in accommodation?
- Eye accommodation
- Pupil constriction
- Eyes converge
If the accommodative stimulus is presented to one eye, which eye shows accommodation changes?
BOTH eyes experience convergence, accommodation, and pupil constriction
What does the change in pupil size do?
- Controls light
- Modifies depth of focus
- Varies any optical aberration(blur)
How could you tell someone is accommodating?
- Red reflex is more dim than non accommodating
- Corneal reflex is more temporal to show convergence
- Pupils are constricted
Name the components of accommodation
Reflex accommodation
Vergence accommodation
Tonic accommodation
Proximal accommodation
Reflex accommodation
- automatic adjustment of refractive status to maintain a focused retinal image
- occurs in responding to A small amount of BLUR
- helpful in small scanning eye movements
- fine changes under MONOCULAR and BINOCULAR vision
Vergence accommodation
- change in accommodation induced during fusion all vergences
- vergences drives/leads to accommodation
Proximal accommodation
- refocusing that occurs due to the apparent nearness of a target
- can be real or perceived
- activated by perceptual cues
- stimulated by targets within 6-3 feet
What is another name for Tonic accommodation
LEAD OF ACCOMMODATION
Tonic accommodation
- residual/ resting level of accommodation
- baseline innervation from midbrain
- present even when there is no blur, disparity, or proximal cues
- first accommodation to kick in
What is a typical tonic accommodation?
- 5D-1.50D
- decreases with age
General factors that affect accommodation
- blur
- convergence(accommodation should be better with convergence and vise versa)
- pharmacology
- minus lens(increase accommodation)
- diseases
More specific factors that affect accommodation
- retinal image factors(contrast, spatial, retinal image motion)
- non retinal factors(mood, voluntary effort, target illumination, training)
- optical cues( provide info about directionality, astigmatism, aberrations)
- non optical cues( size, proximity, apparent distance, depth cues)
Retinal image factors that minimally affect accommodation
Contrast
Spatial frequency
Luminance
Trend of contrast
-more contrast means more accommodation
Trend in spatial frequency
Too low and too high means lower accommodation
-hits an ideal level and plateaus
Trend in illumination
More light means more accommodation
Retinal image factors that have a HUGE impact on accommodation
- retinal eccentricity
- retinal image velocity
Retinal eccentricity
- when you have a strabismus, you don’t use your fovea to focus light, you use an eccentric part of the retina to do it.
- more peripheral means less accommodation
Retinal image velocity
- how fast an image passes on your retina
- speeding car goes by, accommodation won’t kick in
- if car going 5 mph, accommodation will set in
Do factors of accommodation work together?
- they work together/simultaneously
- makes it easy to detect fine details
Aberration
- peripheral rays don’t coincide with the central/axis rays
- astigmatism has this and it causes blur
Depth of focus
- variation/small range in the image distance that is tolerable without a profound focus.
- small range will not kick in accommodation
- outside of range, increase accommodation
Muscarinic blockers
- antagonize parasympathetic system
- prevent acetylcholine from binding
- stops signals for ciliary muscles to contract
Common muscarinic blockers used in practice
Tropicamide and cyclopentolate
Contrast tropicamide and cyclopentolate
Tropicamide has a really short half life so it cannot determine cyclo refraction
Cyclopentolate has a long half life so it can be used and is typically used in children
Atropine
- used to block parasympathetic and keep eyes dilated (super long half life)
- used for iritis which the iris is flamed and constancy constriction and dilation is painful
Phenylephrine
- causes mydriasis(dilation) but no effect on accommodation
- used with tropicamide to keep dilation longer
Rando drugs that affect accommodation
- alcohol
- ganglion blockers
- antidepressants
- marijuana
- antihistamines
Systemic problems that effect accommodation
Diabetes TBI MS downs Eye trauma
Presbyopia
Gradual age related irreversible loss of accommodative amplitude
- 45-50 years old onset (can be earlier)
- complete loss by 50-55
- 2.5D loss per year
Complaints that would make you think presbyopia
- receded near point of accommodation
- blurred vision
- discomfort and asthenopia at near
Contributing factors that lead to decrease in accommodation
- lens thickens and size increases
- capsule thickens and loses elasticity
- anterior surface increases curvature
- lens and cortex stiffen
- lose number of zonules
What factors DO NOT contribute to presbyopia
- zonules still have their elasticity
- ciliary muscle still functions
- motor neuronal pathway still functions.
Treatment for presbyopia
- bifocals
- reading glasses
- monovision
- bifocal contacts
Oweyes treatment chart
40:+1.00/1.25
45:+1.50
50:+2.00
55:+2.25
60 and up: +2.50
what should you consider when prescribing presbyopia correction?
-OCCUPATION AND FUNCTION
Accommodative excess
- in children and adults
- too much accommodation
- a plus lens WILL NOT help
- treated with proper distance correction and vision therapy
Accommodative infacility
- they have the ability, but they cant use it.
- children and adults
- PLUS LENS WILL NOT HELP
- treat with distance correction and vision therapy
Accommodative insufficiency
- children and adults
- not enough accommodation
- correct distance first, then use plus lens
Accommodative convergence/ accommodation ratio
AC/A
- the amount of convergence induced by a change in accommodation
- a change in accommodation is accompanied by a change in vergence
- allow stable, single, and clear over all distances
accommodation and vergence relationship
accommodation=convergence
no accommodation=divergence
Abnormal AC/A ratios are commonly seen in…
binocular vision problems
what are the two ways to measure AC/A ratio?
- gradient determination
2. near-far (calculated) determination
Gradient determination of AC?A ratio
- phoria is measured at the same near distance (40cm), but with different lenses to change accommodative demand
- use phoropter or modified thorington, through subjective refraction
gradient advantage
since accommodation and NPC are measured at the same distance, there is no proximal accommodation used.
-no other accommodations are induced
Prisms in a phoropter
dissociate to provide an OPEN feedback loop so accommodation is not influenced by other stimulus
Why should you have your patients read letters out loud?
-you can make sure they are actually using accommodation and keeping letters clear
How to determine the gradient AC/A
- measure phoria
- then measure phoria through -1.00D lens added to the refraction
- the difference between subjective and with the lens is the change in convergence
What is the AC/A ratio determining
-how convergence responds to accommodative stimulus
gradient equation
(phoria with lens-baseline phoria)/absolute power of the lens
What is a normal/expected AC/A
3:1 to 5:1
some say from 2:1 to 6:1
what if you go from exo to eso?
- it is the absolute change in phoria
- picture a number line
For near-far AC/A, what is PD measured in and what is near fixation distance measured in?
PD: cm
NFD: m
Near-far AC/A equation
PD+NFD(near phoria-distance phoria)
Why is near-far ratio larger than gradient typically?
-because near-far allows other accommodation/ proxima vergences influence near phoria
What does high AC/A ratio indicate?
- excess convergence with accommodation
- overconverging
What does low AC/A ratio indicate?
- LOW convergence with accommodation
- underconverging
