Accommodation and Presbyopia Flashcards
what happens to the lens in accommodation
the optical power increases (focal length decreases) by increasing the surface curvature and thickness
what is accommodation measured in
diopters (1/meter)- a measure of the vergence of light
what is accommodation
a dynamic change in the power of the eye- change in the focus from distance to near
what mediates accommodation
contraction of the ciliary muscle, release of zonular tension at the equator and “rounding up” of the lens (force exerted by capsule)
can a person with myopia focus at near or distance without accommodation
they can focus clearly on objects closer than optical infinity without accommodation
what has a higher refractive index: the cornea or aqueous humor
the cornea (n= 1.376 and aqueous humor n= 1.336)
can a person with hyperopia focus at optical infinity without accommodation
no, they are only able to focus clearly on objects at optical infinity only through an accommodative increase or with a (+) lens
where does the optical power of the cornea come from
the change in refractive index and the positive radius of curvature
do both the anterior and posterior sides of the lens increase in curvature with accommodation
yes- although historically it was thought the posterior did not move
what is depth of field
the range over which a target can be moved toward or away from the eye without a perceptible change in the blur or focus of the image
what is depth of focus
the focusing error that can be tolerated without an appreciable decrease in acuity or change in blur or focus of the image on the retina
what is depth of focus dependent on
the pupil size
what size pupil is the depth of focus larger
a small pupil has a larger depth of focus but a smaller field of view
what happens to the depth of focus during accommodation
the pupil size decreases during accommodation, which will increase the depth of focus
why would there be a decreased need for presbyopic correction as a person ages
their pupil size decreases with aging (senile miosis) which give a pinhole effect and increases the depth of focus
where is the eye focused when the eye is at rest
for distance
what are the lens and zonule positions when the eye is at rest
the zonules apply an outward tension on the lens and it is in a flattened and unaccommodated state
name the 3 parts of the accommodative triad or near reflex
- pupils constrict
- eyes converge
- eyes accommodate
what is blur-driven accommodation
place a (-) lens in front of an eye and the eyes will accommodate to overcome the blur/defocus
what is tonic accommodation
when the eye is at rest it has some residual or resting level of accommodation- about 1.5 D due to muscle tone
how can you induce blur with a BO prism
it increases the vergence- causing pupil constriction, convergence, and accommodation occurs
how can you induce accommodation with pharmacologic stimulation
use of a topical muscarinic cholinergic agonist (pilocarpine) results in stimulation of the ciliary muscle causing pupil constriction but not convergence
what is accommodative esotropia
occurs in uncorrected hyperopes because they need to accommodate to see distance objects and it over-converges the eyes
what drops can block accommodation
cycloplegia drops- atropine, cyclopentolate, tropicamide
how do cycloplegic drops work
they competitively bind to the same muscarinic receptors as the agonists: preventing the agonist from binding and block accommodation
what is the push-up method
a way to measure accommodation: the patient reports when a near letter chart is no longer in sharp focus as it is brought closer to the eye
what is a downside of the push-up method for measuring accommodation
it is a subjective evaluation of best image focus and can overestimate the amount of accommodation
what can influence the push-up method
depth of focus, visual acuity, contrast sensitivity of the eye and contrast of the image
what is the minus lens to blur method
a subjective measurement of accommodation amplitude by placing a (-) lens in front of one or both eyes to blur a distant letter- stimulating accommodation
what is a problem with the minus lens to blur method
it minifies the letters and can underestimate the amount of accommodation
what does accurate objective measurement of accommodation require
static or dynamic refractometers
when does a person have complete loss of their accommodative ability
usually by age 50 (presbyopia)
how many diopters of accommodation are lost per decade
2.3 diopters
when is 2/3 of our accommodation amplitude lost
between ages 15 and 35
what are 3 surgical options for presbyopia
corneal refractive surgery, cataract removal, and accommodative IOL’s
what is a controversial surgical inventions to restore natural accommodation
scleral expansion surgery
what is being researched as an option to restore natural accommodation
a deformable polymer- an IOL that can bend just like the natural lens
what happens to the ciliary muscle as we age
there is a loss of muscle fibers and an increase in connective tissue, it is less able to hold or pull the flattened and unaccommodated configuration
do the ciliary muscle contractile forces decrease as we age
no it actually increases and is at maximum during presbyopic age
why does the configuration of the older unaccommodated ciliary muscle appear like a young accommodated ciliary muuscle
the inner apex of the unaccommodated ciliary muscle resides further forward and inward toward the anteroposterior axis of the aging eye
how much does the lens capsule increase in thickness from birth to 60 years old
11 microns at birth to 20 microns at 60 (then decreases slightly)
what happens to the lens capsule as we age
it becomes more brittle, gets thicker and less extensible (elasticity)
what is the Fisher theory of the lens capsule
that the force that can be transmitted per unit of thickness of the capsule decreases by half after age 60 but the increased thickness offers some compensation
what increases in the lens as it grows throughout life
thickness, increased surface curvatures and equatorial diameter
what is the linear increase in mass over the human life span
more than 1.5 times
what happens to a young lens if you remove it from the eye (no zonular forces)
it becomes accommodated (older lenses don’t change shape)
how much does the lens increase in hardness during the human life span
more than a 4 fold increase
what happens to the anterior chamber depth as the lens ages
it decreases (due to the increase in lens thickness)
does the anterior segment length change as we age
no it stays constant
what is lenticular sclerosis theory of presbyopia
the most common theory: the crystalline lens gets harder with age
what is the geometric theory of presbyopia
that presbyopia is a consequence of the altered geometry of the lens/zonular relationship
what is the explanation for the geometric theory of presbyopia
the anterior zonular connections are near the lens equator in a young lens and as we age the connections are farther forward so there is not effective relaxation of the anterior zonular/capsular force with an accommodative ciliary muscle contraction
what is the disaccommodation theory of presbyopia
suggests that presbyopia is not caused by a failure of the lens to accommodate, but by a gradual failure of the lens to be held in an unaccommodated form at rest
what does the multifactorial theory of presbyopia suggest
that presbyopia doesn’t result from a single causal factor, but through global deterioration of accommodative function of several aspects of the accommodative apparatus
what are some optical compensations used for presbyopia
spectacle lenses, CL, corneal refractive surgical procedures and IOL’s
