Binocular Vision 1 - Week 2 Flashcards
By what age is accommodation fully developed?
6 months
What are the 4 major factors that stimulate accommodation?
- Blur - primary driver
- Proximal Awareness - drives accommodation based on where the object is in space or where you think it is
- Vergence - movements are accompanied by corresponding accommodation change
- Tonic Accommodation - baseline neural activity without any stimuli
What visual symptom can be caused by tonic accommodation?
Night Myopia
Describe the neurological pathway for blur driven accommodation
Blur triggers afferent pathway
- Blur signals in LGN into cortical area 17 + parietal/temporal areas
- signal transformed to motor Efferent response at EWN
- Efferent: innervate cil. muscle to cause accommodation
Describe the Efferent pathway for accommodation
via CIII nerve – cil. ganglion – short cil. nerve – innervate cil. muscle – cause accommodation
What mediates the parasympathetic and sympathetic innervation for accommodation? [2]
Parasympathetic: acetylcholine mediated
Sympathetic: noradrenaline mediated
Which type of innervation might play a role in assisting the re-establishment of distance focusing after prolonged near work?
Sympathetic
Name 6 factors that influence accommodation
Blur Luminance Proximity Chromatic aberration Convergence Age
Average amplitude of accommodation at 45 years?
About 4 diopters
Expected amplitude of accommodation at 20 years?
About 10 diopters (10cm) or more
Name 5 features of accommodation that we can measure clinically
Posture (lag/lead) Amplitude (NPA) Accommodative Facility Relative Accommodation (NRA/PRA) Convergence Accommodation (AC/A)
Rank the amplitude of accommodation levels for different ametropia
Myopes > Emmetropes > Hyperopes
How do glasses affect amplitude of accommodation? Is this change significant?
Hyperopes need to accommodate more with their glasses while Myopes need to accommodate less.
This is only significant if the correction is greater than +/- 5D
How does luminance affect amplitude of accommodation?
Accommodative response (relative to demand) is reduced in dim light
What type of neurological deficit might result in changed amplitude of accommodation?
Third nerve problems
What two tests are used to measure posture of accommodation?
- MEM retinoscopy (monocular estimate method)
2. Binocular X cyl
What is the advantage of MEM retinoscopy?
Is objective and is undertaken in free space (no phoropter needed)
What does it mean when a px has a ‘lag’ of accommodation?
It means they focus just behind the near task they are performing (i.e. focus just slightly further away)
Which is more common: accommodative lag or accommodative lead?
Lag. Most people have lag.
They should get better gaming computers then
In MEM retinoscopy, why must you scan both eyes first with the retinoscope?
Because the accommodative posture in the 2nd eye can change while doing ret on the 1st. You want to know what it was originally also
In MEM retinoscopy, what movements represent a lag or lead in accommodation?
With movement = LAG
Against movement = LEAD
- way to remember: so just like with regular ret, the most common condition is the one with the ‘with’ movement *
In MEM retinoscopy, for how long should you place the trial lens in front of the spectacle plane?
For 1 second each time
also, it’s good to have the lenses on standby so you don’t faff about
What is the expected value for MEM retinoscopy?
+0.50DS +/- 0.25DS
Is Binocular Cross-Cyl objective or subjective?
Subjective
List disadvantages of binocular cross-cyl
- is NOT in free space and requires a phoropter head
- is NOT close enough to a real life near task and therefore may not be a true representation of accommodative response at near (b/c lack of proximal cues)
Explain the binocular cross-cyl technique
Px is placed behind phoropter with NEAR PD and metal rod is inserted into hole. Chart cross target is attached to rod at 40cm.
Add a +/-0.50 auxillary lens to the eyes, with negative cyl axis oriented vertically
Add +ve lenses binocularly until px reports both horizontal and vertical lines are equal
In binocular cross-cyl: What represents a LAG in accommodation, and what represents a LEAD in accommodation
LAG: horizontal focal lines appear sharper/clearer (b/c they are closer to the retina)
LEAD: vertical focal lines appear sharper/clearer
When doing binocular cross-cyl, why do you add a +/- 0.50D auxillary lens with negative cyl axis oriented vertically?
This will add minus to the vertical axis and plus to the horizontal axis. That way, assuming posture is perfect, both focal lines should be equally blurry
(we don’t add just + or - to both because we want to distinguish/separate the axes)
Expected value for binocular cross-cyl?
+0.50 +/- 0.50DS
What tests can you use to measure amplitude of accommodation? Are these tests performed binocularly or monocularly?
Near Point of Accommodation (NPA)
Refractor Head Near Point Rule
Minus lens to blur
All these tests are performed monocularly
For NPA, what is the expected: average amplitude and minimum amplitude?
Average: 18 - 1/3(age)
Minimum: 15 - 1/4(age)
Is the ‘reverse push up method’ liable to over-estimate or under-estimate accommodation? Why is this?
Over-estimate. Due to the magnification effect caused by bringing the target closer and improving acuity
Why might being in front of a phoropter provide different or inaccurate results for amplitude of accommodation?
lack of proximal cues. Different to real life near vision
Define Positive Relative Accommodation (PRA)
The amount of accommodation that can be EXERTED to maintain clear single binocular vision without changing vergence or proximal
Define Negative Relative Accommodation (NRA)
The amount of accommodation that can be RELAXED to maintain clear single binocular vision without changing vergence or proximal
How do you perform PRA and NRA?
Do this at distance and near:
- Start with best corrected VA behind either trial frames or phoropter
- do NRA: add +ve lenses in +0.25DS steps until first sustained blur (ask px to blink eyes to see if sustained)
- do PRA starting from value obtained from NRA: add -ve lenses in -0.25DS steps until first sustained blur
Expected values for accommodative range? (PRA/NRA)
+/-2D at near
-2D at distance
What is Accommodative Facility?
Is the flexibility and sustainability of accommodation while vergence and proximal stay the same
- i.e. “how quickly and efficiently can accommodation change on demand”
What test is used to measure Accommodative Facility?
Near accommodative facility/flippers
Explain the procedure for measuring Accommadative Facility
use flippers
- target at preferred WD (but 33-40cm)
- flip lenses continuously for 1 minute
- first test +/-2D flippers: ask which sign is preferred if both +/- cleared
- if failed to clear/struggled: repeat with +/-1D flippers
Count the cycles per minute for each set of flippers
What is the expected cycles per minute for +/-2D near accommodative flippers?
8cpm at near with +/-2D flipper
*If using 1D flipper, record preference for + or -. (do this for 2D flipper as well I think)
What factors affect our clinical tests of accommodation?
Age Ocular conditions Systemic conditions Medications Lighting levels Tests used Testing conditions Patient engagement/cognition
Name 5 common accommodative dysfunctions
Insufficiency Excess Spasm Ill-sustained in-facility
What are the clinical characteristics of accommodative:
- insufficiency
- excess
Insufficiency: High lag, low amplitude, poor facility
Excess: Variable VA, NO lag or lead, fails +ve facility
What are the clinical characteristics of accommodative spasm
Spasm: Reduced VA, Lead, fails +ve facility
What are the clinical characteristics of:
- ill-sustained accommodation
- accommodative infacility
ill-sustained: High variable lag, slow facility
in-facility: Slow facility for both + and -
