Accommodation (prob 9 Qs) Flashcards
Why is accommodation important?
It’s a dynamic process to produce and maintain a focused retinal image
What changes in accommodation to maintain the image?
The power of the lens
What about the lens changes to accommodate?
- lens curvature
- lens power
- focusing
What does the change in lens shape allow in accommodation?
Objects are various distances to be focused on the retina
What is the only active element of accommodation?
The ciliary muscle
All other parts are passive
Biomechanics of accommodation
- innervation of the ciliary muscle
- ciliary muscle contracts
- ciliary muscle moves inward an anteriorly/forward
- ciliary ring advances appx 0.5mm along with the ciliary muscle
- choroid and posterior zonules stretch appx 0.5mm
- anterior zonular tension decreases, and the zonules relax
- lens capsule molds the lens, becomes more spherical
- lens power increases, focal length decreases
- eye changes focus from distance to near
Changes to the lens
- equatorial diameter decreases from 10 to 9.6mm
- the anterior lens surface moves anteriorly while posterior surface moves posteriorly
- central anterior radius of curvature changes from 11 to 5.5mm (becomes more steep)
- central posterior radius of curvature decreases from 5.18 to 5.08mm
- central thickness increases by 0.36 to 0.58mm (at the nucleus)
- lens sinks 0.3mm as a result of gravity (denser, heavier)
What causes the lens to change shape in accommodation?
Ciliary muscle
What happens to equatorial diameter of lens during accommodation?
Decreases from 10 to 9.6mm
Shortens to become more round
What happens to the anterior lens surface during accommodation?
Moves anteriorly
What happens to the posterior surface of the lens during accommodation?
Moves posteriorly
What happens to the central anterior radius of curvature of lens during accommodation?
Becomes steeper
11 to 5.5mm
What happens to the central posterior radius of curvature of the lens during accommodation?
Decreases (5.18 to 5.05mm)
What happens to the central thickness of the lens during accommodation?
Increases at nucleus (0.36 to 0.58mm)
What happens to the lens in terms of gravity during accommodation?
The lens sinks 0.3mm (denser, Heavier)
What kind of innervation does the ciliary muscle have?
Parasympathetic
During accommodation, what is the general thing that happens to the ciliary muscle??
The ciliary muscle in the ciliary body contracts and moves forward
What does the contraction of the ciliary muscles cause on other parts of the eye?
Releases the resting tensions on the zonules around the lens equator
What molds the lens?
The lens capsule, to become more spherical
Parasympathetic pathways to ciliary muscle
- unfocused image on retina
- blur signals transmitted to visual cortex
- cortical cell produce sensory blur signals
- signal goes to Edinger-Westphalia nucleus (parasympathetic pathway starts here)
- oculomoter nerve (CN3)—ciliary ganglion—short ciliary nerve
- ciliary muscle contraction
- crystalline lens deforms to produce an in-focus retinal image
This pathway is coupled with pupillary function
Where do blur signals go?
Visual cortex
What produce sensory blur signals?
Cortical cells
Where does the blur signal go to?
Edinger-Westphal nucleus
Edinger-Westphal nucleus
Parasympathetic pathways starts here
What is the motor command transmitted to ciliary muscle?
Oculomotor nerve (CN3)—ciliary ganglion—short ciliary nerve
What is accommodation coupled with?
Pupillary function
-also need to converge eyes to accommodate
The lens during no accommodation
- taught and flat
- ciliary muscle relaxed
- convergence demand is 0
When an object is closer than infinity
- object has divergent rays that focus behind the eye
- lead to unfocused image on the retina
- optical power of the eye has to increase to add positive convergent rays
In the accommodative eye, the image is in focus because…
Of the accommodating lens
How to calculate the accommodative demand
50cm
Divide 100 by the distance (100/50)…2D of accommodation is used
-make sure you use cm!
How many diopter needed for 40cm?
2.5D
Accommodation triad/near reflex
- eye accommodation
- pupil restriction
- eyes converge
What is the accommodation triad /near reflex coupled with?
Parasympathetic innervation from the EW nucleus
What happens if accommodative stimulus is presented to one eye?
The convergence, accommodation, and pupil contrisction occur in both eyes
Change in pupil size in accommodation
- controls light
- modifies depth of focus
- varies any optical aberration
The triad: distance fixation
- no accommodation
- absence of the triad
The triad: near fixation
- accommodation (the red reflex change show change in power, dimmer because the lens changed shape)
- eyes converge (the corneal reflex more temporally shows convergence
- pupil constriction
Components of accommodation
- reflex accommodation
- vergence accommodation
- tonic accommodation
- proximal accommodation
Reflex accommodation
-automatic adjustment of the refractive state to maintain a focused retinal image
When does reflex accommodation occur?
- responding to blur
- reduction in contrast
What is reflex accommodation important for?
Small scanning eye movements or micro saccades
Vergence accommodation
Change in accommodation induced during fusional Vergence
-leads to the convergence accommodation/convergence ratio (CA/C)
PRoximal accommodation
Refocusing that occurs due to the apparent (or perceived) nearness (proximity) of a target
What is proximal accommodation activated by?
Perceptual cues
- phoropter coming closer to face
- stimulated by targets located within 3m (within optical infinity)
Tonic accommodation
- lead of accommodation (first start of accommodation)
- residual/resting level of accommodation due to baseline stable innervation input from the midbrain
What is accommodation at rest?
0.5D to 1.5D
When is tonic accommodation present?
- present even in the absence of blue, disparity and proximal cues
- even if nothing else is happening
When does tonic accommodation reduce?
With age
Kids have a ton of it
Factors that affect accommodation
- blur
- Convergence
- proximal issues (close to me)
- pharmacology (meds)
- minus lens
- diseases
What type of lens will help EVERYONE accommodate?
Minus lens
Retinal image factors that affect accommodation
- contrast
- spatial frequency
- retinal image motion
non retinal factors that affect accommodation
- mood
- voluntary efffort
- target luminance
- training
Optical cues that affect accommodation
Offer information about directionality (of image on the retina), astigmatism, aberrations
Non optical cues that affect accommodation
- size
- proximity
- apparent distance
- depth cues
When do accommodative errors increase significantly?
With changes in eccentricity and retinal image velocity
Retinal eccentricity
Not on fovea, due to strabismus
Are all these factors that affect accommodation happening in isolation?
No
Accommodation has a very robust and healthy way of detecting optimally and discriminating fine details
Aberrations
(On optical cues)
-occur when the peripheral rays don’t coincide with the central and on axis rays
Depth of focus
Variation/small range in the image distance that is tolerable without a profound defocus
-this influences accommodation as it increases
What is accommodation stimulated by in the CNS?
Parasympathetic system
What is accommodation best antagonized with?
Muscarinic blockers
-they block acetylcholine from binding
What are some common muscarinic blockers used in practice?
- tropicamide (mydriacly)
- cyclopentolate
Trpicamide
- muscarinic antagonist
- very short half life and should not be used to determine the cycloplegic refraction
Cyclopentolate
- muscarinic antagonists
- effective with sufficient half life, used frequently in peds
What are some other muscarinics?
- atropine
- homatropine
- scopolamine
Produce mydriasis and loss of accommodation
Atropine
- muscarinic antagonist
- used for Iritis
Iritis
- jolted iris
- inflamed
- light sensitivity due to focusing back and forth
Phenylephrine
- adrenaline
- sympathomimetic
- causes mydriasis but has not significant effect on accommodation
- not muscarinic
Other drugs that affect accommodation
- alcohol
- ganglion blockers
- phenothiazides and antidepressants
- CNS stimulants
- marijuana
- carbonic anhydrase inhibitors
- antihistamines
- morphine
Some conditions that affect accommodation
- DM
- TBI
- MS
- myasthenia gravis
- botulism
- down syndrome
- glaucoma
- iritis
- iris tear
- eye trauma
- aides tonic pupil
- syphilis
- neuro-ophthalmic lesions
Presbyopia
Gradual age-related irreversible loss of accommodative amplitude
When is presbyopia typically reported
40-45 years old
Some are earlier like little kids who accommodate so much, they will grow up to report presbyopia earlier
When do you typically have complete loss of accommodation?
50-55 years old
How much accommodation do you lose per year in presbyopia?
2.5D
Complaints of presbyopia
- receded near point of accommodation
- blurred vision
- discomfort and asthenopia at near
Contributing factors and biochemical changes that lead to the decrease of accommodation
- lens thinkens and size increases
- springiness of capsule decreases (thickness)
- anterior surface curvature increases
- stiffer lens (cataracts)
- ciliary muscle remains stable
- cortex stiffens
- zonules become less dense (number of them, cant relax them enough)
What factors do not change in presbyopia?
- zonules still have they elasticity
- ciliary muscle still functions
- motor neuronal pathway still functions
Treatment of presbyopia
- plus lenses
- in form of bifocals, reading glasses, mono vision or bifocal CL or even surgical correction
Take into consideration what distance they like to work at
Accommodative excess
- treated with proper distance correction and VT
- adults and children
- result of medication, illness or even an accommodative anomaly
Accommodative infacility
- adult and children
- result of medication, illness, or even an accommodative anomaly
- proper correction and vision therapy is treatment
- have it, don’t know how to use it
Accommodative insufficiency
- adult and children
- proper distance correction and plus lenses
- result of medication, illness or even an accommodative anomaly
AC/A
Accommodative convergence/accommodation ratio
The amount of convergence induced by a change in accommodation
Change in accommodation is accompanied by…
Change in vergence
What permits clear, stable single binocular vision across a range of viewing distances?
Accommodation and vergence
With accommodation there is…
Convergence
With no accommodation, there is…
Divergence
Abnormal AC/A ratios cause what
Binocular problems
What are 2 ways of measuring the AC/A ratio?
- gradient determination
- near-far (or calculated) determination
Gradient determination of AC/A ratio
Phoria is measured at the same near distance (40cm) but with different lenses to change the accommodative demand
Induce accommodation and measure phoria
What is the advantage of gradient determination of AC/C ratio?
At the same distance, the proximal accommodation is controlled
How can gradient determination of AC/A ratio be done?
In phoropter or with modified thorington, through the subjective refraction
-with prisms in phoropter to dissociate to provide a open loops so accommodation is no influenced by any other stimulus
What to tell patient when doing gradient determination of AC/A ratio
Remind the patient to keep the near point target clear to maintain accommodation. Have patient read the letters, measure the phoria
-measure phoria again through -1.00D lenses added to refraction
What is the change in convergence in gradient determination of AC/A ratio
The difference in prism diopter between the phoria with the subjective and the phoria with the -1.00D lens
-This is how convergence responds to the accommodative stimulus
AC/A=
(Phoria with plus or minus lenses-baseline phoria)/(absolute power of additional minus or plus lenses)
Expected AC/A
3/1 or 5/1 (3:1 To 5:1)
Some state 4:1 with SD+/- 2
If the near phoria is 2EP’ through subjective and then 7EP’ through -1.00D lens, what is the AC/A ratio?
(7-2)/1
5/1
If near phoria is 2XP’ through subjective and then 7EP’ through -1.00D, what is AC/A?
(7-(-2))/1
9/1
Absolute change in the phoria
Near-far/calculated AC/A ratio
AC/A=PD(cm) + NFD(P’near-Pdist)
PD=pupillary distance (cm)
NFD=near fixation distance in meter
P’near=near phoria (eso is plus and exo is minus)
Pdist=distance phoria
What is the AC/A ratio if the PD=60mm, 2XP at distance, 10XP’ at near (40cm)
AC/A=6+0.4(-10-(-2))
=2.8
What does a high AC/A ratio mean?
There is excess convergence with accommodation
What does a low AC/A ratio mean?
There is low convergence with accommodation
