1
Q
Why is accommodation important?
A
- 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
2
Q
What is the active muscle that leads the accommodative process?
A
Ciliary muscle
3
Q
Ciliary muscle contraction process
A
- 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
4
Q
Changes to the lens in accommodation
A
- 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
5
Q
What nervous system pathway innervates the ciliary muscle?
A
Parasympathetic pathway!
6
Q
What kind of muscle is the ciliary muscle?
A
Smooth muscle
-in accommodation, ciliary muscle in ciliary body contracts and moves forward
7
Q
Parasympathetic pathway
A
- 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
8
Q
What starts the parasympathetic pathway?
A
Midbrain/ oculomotor nucleus ALSO KNOWN AS THE EDINGER-WESTPHAL NUCLEUS
9
Q
Motor command for ciliary muscle
A
- oculomotor nerve (CN 3)
- ciliary ganglion
- short ciliary nerve
10
Q
What does accommodation need sensory and motor?
A
Sensory to tell brain there is a blurry image and motor from oculomotor nerve to contract ciliary muscle
-you need convergence to accommodate
11
Q
What is the state of the eye when there is no accommodation?
A
- anterior and posterior capsule is taught and flat
- light enters eye and is focused at the retina
- ciliary muscle is relaxed
12
Q
Un accommodated emmetropic eye
A
- focuses on a distant target and there is no need for accommodation
- convergence demand is zero
- object is at optical infinity
13
Q
Accommodating eye
A
- 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
14
Q
Measure accommodation
A
- distance in cm where object is
- 100/cm
- measured in diopters
15
Q
In the accommodating eye, what focuses the image in front?
A
The the accommodating lens
16
Q
Accommodation triad/ near reflex
A
- 3 physiological changes seen in accommodation
- all changes are coupled through parasympathetic innervation from EW nucleus
17
Q
What are the physiological changes seen in accommodation?
A
- Eye accommodation
- Pupil constriction
- Eyes converge
18
Q
If the accommodative stimulus is presented to one eye, which eye shows accommodation changes?
A
BOTH eyes experience convergence, accommodation, and pupil constriction
19
Q
What does the change in pupil size do?
A
- Controls light
- Modifies depth of focus
- Varies any optical aberration(blur)
20
Q
How could you tell someone is accommodating?
A
- Red reflex is more dim than non accommodating
- Corneal reflex is more temporal to show convergence
- Pupils are constricted
21
Q
Name the components of accommodation
A
Reflex accommodation
Vergence accommodation
Tonic accommodation
Proximal accommodation
22
Q
Reflex accommodation
A
- 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
23
Q
Vergence accommodation
A
- change in accommodation induced during fusion all vergences
- vergences drives/leads to accommodation
24
Q
Proximal accommodation
A
- 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
25
What is another name for Tonic accommodation
LEAD OF ACCOMMODATION
26
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
27
What is a typical tonic accommodation?
0. 5D-1.50D
| - decreases with age
28
General factors that affect accommodation
- blur
- convergence(accommodation should be better with convergence and vise versa)
- pharmacology
- minus lens(increase accommodation)
- diseases
29
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)
30
Retinal image factors that minimally affect accommodation
Contrast
Spatial frequency
Luminance
31
Trend of contrast
-more contrast means more accommodation
32
Trend in spatial frequency
Too low and too high means lower accommodation
| -hits an ideal level and plateaus
33
Trend in illumination
More light means more accommodation
34
Retinal image factors that have a HUGE impact on accommodation
- retinal eccentricity
| - retinal image velocity
35
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
36
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
37
Do factors of accommodation work together?
- they work together/simultaneously
| - makes it easy to detect fine details
38
Aberration
- peripheral rays don't coincide with the central/axis rays
| - astigmatism has this and it causes blur
39
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
40
Muscarinic blockers
- antagonize parasympathetic system
- prevent acetylcholine from binding
- stops signals for ciliary muscles to contract
41
Common muscarinic blockers used in practice
Tropicamide and cyclopentolate
42
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
43
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
44
Phenylephrine
- causes mydriasis(dilation) but no effect on accommodation
| - used with tropicamide to keep dilation longer
45
Rando drugs that affect accommodation
- alcohol
- ganglion blockers
- antidepressants
- marijuana
- antihistamines
46
Systemic problems that effect accommodation
```
Diabetes
TBI
MS
downs
Eye trauma
```
47
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
48
Complaints that would make you think presbyopia
- receded near point of accommodation
- blurred vision
- discomfort and asthenopia at near
49
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
50
What factors DO NOT contribute to presbyopia
- zonules still have their elasticity
- ciliary muscle still functions
- motor neuronal pathway still functions.
51
Treatment for presbyopia
- bifocals
- reading glasses
- monovision
- bifocal contacts
52
Oweyes treatment chart
40:+1.00/1.25
45:+1.50
50:+2.00
55:+2.25
60 and up: +2.50
53
what should you consider when prescribing presbyopia correction?
-OCCUPATION AND FUNCTION
54
Accommodative excess
- in children and adults
- too much accommodation
- a plus lens WILL NOT help
- treated with proper distance correction and vision therapy
55
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
56
Accommodative insufficiency
- children and adults
- not enough accommodation
- correct distance first, then use plus lens
57
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
58
accommodation and vergence relationship
accommodation=convergence
| no accommodation=divergence
59
Abnormal AC/A ratios are commonly seen in...
binocular vision problems
60
what are the two ways to measure AC/A ratio?
1. gradient determination
| 2. near-far (calculated) determination
61
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
62
gradient advantage
since accommodation and NPC are measured at the same distance, there is no proximal accommodation used.
-no other accommodations are induced
63
Prisms in a phoropter
dissociate to provide an OPEN feedback loop so accommodation is not influenced by other stimulus
64
Why should you have your patients read letters out loud?
-you can make sure they are actually using accommodation and keeping letters clear
65
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
66
What is the AC/A ratio determining
-how convergence responds to accommodative stimulus
67
gradient equation
(phoria with lens-baseline phoria)/absolute power of the lens
68
What is a normal/expected AC/A
3:1 to 5:1
| some say from 2:1 to 6:1
69
what if you go from exo to eso?
- it is the absolute change in phoria
| - picture a number line
70
For near-far AC/A, what is PD measured in and what is near fixation distance measured in?
PD: cm
NFD: m
71
Near-far AC/A equation
PD+NFD(near phoria-distance phoria)
72
Why is near-far ratio larger than gradient typically?
-because near-far allows other accommodation/ proxima vergences influence near phoria
73
What does high AC/A ratio indicate?
- excess convergence with accommodation
| - overconverging
74
What does low AC/A ratio indicate?
- LOW convergence with accommodation
| - underconverging
Methods Block 3
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Decks in class (5)
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