5 & 6. Accommodation, convergence and pupil responses Flashcards
Define what is meant by Near Vision Complex?
Changing fixation from distance to near requires an increase in both accommodation and vergence and these 2 changes are accompanied by pupillary constriction. The combination of these 3 forms the near vision complex.
What is accommodation?
Change in the curvature & thickness of the crystalline lens due to the constriction of the ciliary muscle.
Describe what happens to zonnule of zinn and lens when ciliary muscles constrict?
When ciliary muscle constricts, it pulls forward and into the lens, relaxing tension of zonnules of zinn and allowing the lens to expand.
What stimulates accommodation?
Blurred retinal images
What is far point of accommodation in an emmetrope? (FPA)
Infinity, where no accommodation is required
What is the near point of accommodation in an emmetrope? (NPA)
Closest point to eye where image is focused on the retina with max accommodation
FPA to NPA =?
Amplitude of accommodation
Give 1 way in which you can obtain true measurements of accommodation- FPA & NPA?
Uncorrected refractive errors- patients should be wearing glasses when accommodation is measured.
Relationship of accommodation and age
Accommodation declines with age
What are the other factors that can affect accommodation rather then age and uncorrected refractive error?
- Systemic diseases
- Patients with disabilities or learning difficulties
- Medications
- Myopes have more accommodation compared to emmetropes and emmetropes have more accommodation then hyperopes
- Race
- Diurnal variation
4 components of accommodation
- Tonic
- Reflex
- Vergence/ convergence
- Proximal
Define tonic accommodation? and how can it be measured?
This is accommodation present in resting state, when there is no stimulus.
Measured by comparing ret at distance then repeating ret in complete darkness.
Define reflex accommodation and how is it measured?
What is its stimulus?
Automatic response of accommodation to maintain a clear retinal image.
How it is measured: measured by monocular ret using negative lenses at a fixed distance.
Stimulus: blurred image
What is vergence/ convergence as a component of accommodation?
Stimulus is convergent and this is only present when both eyes are open. Contributes to increase in amplitude of accommodation when binocular cf. monoc
Measured by ret at fixed distances with prisms to change vergence
What is proximal accommodation?
Stimulus is the perception of the proximity of the fixation target.
Measured by ret at 2 distances, adding plus to correct for distance between 2
What is the relation between accommodative response and stimulus? and what is the theatrically the same?
Accommodative response: how much is actually executed.
Accommodative stimulus: how much accommodation is theoretically required.
Theoretically they should be the same
What are the 2 problems with accommodation?
- Accommodative lag: less accommodation is exerted then required.
Accommodative lead: More accommodation is exerted than required
How is accommodative stimulus measured?
Using RAF rule
How is lag or lead measured?
Dynamic ret
What are the 4 problems with accommodation?
- Accommodation infacility: Delay in accommodative response- patients report with blurred vision when looking up from near tasks that clear slowly.
- Accommodative fatigue: Amplitude declines or reaction time increases with repeated measures.
- Accommodative insufficiency: low amplitude of accommodation.
- Accommodative spasm: over accommodation- causes blurry vision in the distance.
How are accommodative problems managed?
- Management of underlying health issues- GP
- Correct refractive error
- Orthoptic exercises
- Reading addition
What is meant by convergence?
Equal adduction of each eye to a point closer than infinity
What is symmetrical and asymmetrical convergence?
Symmetrical: if on saggital plane
Asymmetrical: if not on saggital plane
What is NPC (Near point convergence) and when is it considered to be adequate?
Nearest point which can be converged upon.
Adequate when: closer than 10cm
What is tonic convergence?
Baseline convergence- leads to physiologic position of rest.
Anatomic position of rest: where there is no tonus of extraocular muscles.
No stimulus
What is accommodative convergence?
Vergence change due to change in accommodation (the stimulus).
It varies from person to person.
Measured in prism diopters
What is proximal convergence?
This is vergence introduced by the perception of proximity. The closer the object, the more proximal convergence is elicted.
How is proximal convergence measured?
By changing distance of fixation object and measuring change in vergence
What is fusional convergence? And what is its stimulus?
Any error (resultant phoria), is compensated for by this. It is an involuntary movement and the stimulus is retinal disparity.
What happens if fusional vergence can not compensate for a phoria?
It will result in a tropia
What is voluntary convergence?
The ability to converge beyond tonic position with no stimulus
What is convergence insufficiency?
Reduced near point of convergence, causes difficulties at near (blur, diplopia, asthenopia) and exophoria at near.
4 managements of convergence insufficiency?
- Orthoptic exercises
- Manipulation of Rx
- Prism
- Surgery- for extreme cases
How is convergence insufficiency managed in presbyopic patients using Rx manipulation?
Reduce hyperopic Rx increase myopic Rx (this stimulates accom and increases convergence)
What is AC/A Ratio?
Measures how much an individual convergences for each diopter of accommodation- horizontal movements only.
How is AC/A ratio associated with esotropia?
For every diopter of accommodation patient inserts, the eye will turn more, higher the ratio greater the change in vergence gained per change in accommodation.
High AC/A Ratio can be corrected using max plus, at distance why?
Corrected for distance however when patient looks at near target more convergence is required hence esotropia will still be present at near.
Use of +/- lenses in relation to manipulate accommodative demand and deviations?
NEGATIVE LENSES: increase accommodative demand, decrease exo deviation.
POSITIVE LENSES: Reduce accommodative demand, decrease eso deviation. (Less useful at distance as image will be blurred)
AC/A can be used in presbyopic px?
No- Px needs to have adequate accommodation to use this test.
3 ways to measure AC/A ratios?
- Clinically: Cover Test
- Maddox wing- measures phoria
- Gradient method
Why are only horizontal movements considered with AC/A ratio?
because vertical movements are not affected by convergence and accommodation
Importance of AC/A Ratio?
- Manipulate Px Rx to help them overcome problems with vergence
AC/A ratio trend with age
As Px moves through presbyopia, the ratio drops then eventually becomes stable. Accommodative effort is measured in this case as part of AC/A ratio
Can you change AC/A ratio?
No- it is inbuilt- can not be changed, orthoptic exercises doesn’t change the ratio but helps
How are AC/A ratios used?
Can be used to solve Px problems:
Predict the change in vergence that change in accommodation will elicit. Then adjust bilateral spherical component of Rx to change vergence position.
How are -ve and +ve sphere’s related to stimulus and changes they can cause to deviations?
-ve sphere: + stimulus- decrease exo
+ve sphere: - stimulus- decrease eso
How is distance eso managed?
Can’t increase +ve sphere for distance problem since it will blur distance vision. Distance eso can not be corrected.
Difference between fully accommodative esotropia and partially accommodative esotropia?
Fully accommodative esotropia: Deviation fully corrected when glasses are worn.
Partially accommodative esotropia: Deviation not fully corrected when glasses are worn. In those with high AC/A Ratios, bifocals may help straighten eyes at near
Pupils react to 3 numbers of stimulus, list this?
- Increase/ decrease of light
- Distance of target
- Sympathetic/ parasympathetic stimulation (Flight/ fight response)
Abnormal pupil response caused by what issues?
Eye disease, systemic disease & medication
Direct pupil response?
Increased illumination in RE– constriction of Right pupil
Consensual pupil response?
Increased illumination in RE– constriction of left pupil (equal, simultaneous)
