Accommodation Flashcards
Define accommodation. What are the changes associated with positive accommodation ?
Change in refractive power of the lens from contraction of ciliary muscle
Equatorial diameter decreases
Anterior pole moves forward
Central anterior radius of curvature decreases
Central posterior radius of curvature decreases slightly
Central thickness increases (ant to post)
Lens sinks due to gravity
Ciliary muscle contracts (circular)
Zonules relax
Lens bulges forward
Why do accommodate?
Allow near objects to be focused on the retina
Distance objects in turn are focused in front of the retina
How can we calculate stimulus to accommodation? Does this differ between axial and refractive errors?
Required power- unaccommodated power
(Subtract myopic or add refractive hyperopic error to target distance)
Does not carry for axial vs refractive
Define tonic accommodation.
Underlying tonus of the ciliary smooth muscle which never relaxes
Mean value= 0.50D
Define convergent accommodation (CA).
Stimulated with eyes turn inwards towards each other (converge)
1 meter angle (=6prism diopters) of convergence will stimulate 0.40D of accommodation to give CA/A ratio of 0.40D/6prism diopters
Define blur-driven accommodation.
Accommodation to clear retinal blur
Define proximal accommodation.
Caused by awareness of nearness of an object even is the optical image is at infinity
(Ex: microscope or slit lamp)
Describe the accommodative stimulus.
What we should do
Describe the accommodative response. How does it compare to stimulus?
What we actually do
In most cases this does not equal the AS
Does AR ever equal AS?
Yes
When do we have a LEAD of accommodation?
AR greater than AS
For low dioptric stimuli (<1.00D)
When do we have a LAG of accommodation?
AR is less than AS
For high dioptric stimuli (>1.00D)
