Accommodation Flashcards
Why is accommodation important?
A dynamic process to produce and maintain a focused retinal image.
The power of the lens changes to maintain the image.
The lens curvature changes, the lens power changes and focusing changes.
The Accommodative Process:
There is a change in the shape which leads to a change in the _____ of the lens. This allows objects at various distance to be focused at the retina.
power
The only ACTIVE element is the _______, while other parts are passive.
ciliary muscle
What are the step to reach a rounded lens and capsule?
Ciliary muscle contracts → pulls the ciliary ring forward & inward → which stretches the choroid & posterior zonules → this yields a rounded lens and capsule.
What are the steps to reduce the pull on the lens?
Ciliary muscle contracts → anterior zonules reduce their tension and relax → reducing the pull on the lens
When the ciliary muscle relaxes, the passive restoring force of the spring-like 1._____ and 2._______ return to their original position.
- choroid
2. posterior zonules
Ciliary muscle is a smooth muscle with __________ innervation
parasympathetic
In accommodation, the ciliary muscle in the ciliary body contracts and moves 1. _____. This contraction releases the 2._______ tension on the zonules around the lens equator. The lens capsule is able to mold the lens to become more 3. ______.
- forward
- resting
- spherical
What is the parasympathetic pathway to the ciliary muscle?
- Unfocused image on the retina
- Blur signals transmitted to visual cortex
- Cortical cell produce sensory blur signals
- Signal goes to midbrain/oculomotor nucleus/Edinger-Westphal nucleus (parasympathetic pathway starts here)
- Motor command transmitted to ciliary muscle:
oculomotor nerve (CN 3) → ciliary ganglion → short ciliary nerve - Ciliary muscle contraction
- Crystalline lens deforms to produce an in-focus retinal image
Parasympathetic Pathway to the ciliary muscle:
Note: This pathway is coupled with pupillary function
- _____ image on the retina
- Unfocused image on the retina
Parasympathetic Pathway to the ciliary muscle:
Note: This pathway is coupled with pupillary function
- Blur signals transmitted to _____
- Blur signals transmitted to visual cortex
Parasympathetic Pathway to the ciliary muscle:
Note: This pathway is coupled with pupillary function
- ______ produce sensory blur signals.
- Cortical cell produce sensory blur signals.
Parasympathetic Pathway to the ciliary muscle:
Note: This pathway is coupled with pupillary function
- Signal goes to _______ (parasympathetic pathway starts here)
- Signal goes to midbrain/oculomotor nucleus/Edinger-Westphal nucleus (parasympathetic pathway starts here)
Parasympathetic Pathway to the ciliary muscle:
Note: This pathway is coupled with pupillary function
- Motor command transmitted to ciliary muscle:
oculomotor nerve (CN 3) → ciliary ganglion → short ciliary nerve
Parasympathetic Pathway to the ciliary muscle:
Note: This pathway is coupled with pupillary function
- Ciliary muscle contraction
- Ciliary muscle contraction
Biomechanics of the accommodative process
1. Innervation to the ciliary muscle
2. Ciliary muscle ______
3. Ciliary muscle moves inward and ____
4. Ciliary ring advances approximately ____ mm along with the ciliary muscle
5. Choroid and posterior zonules stretch approximately ___ mm
6. Anterior zonular tension _____, and the zonules relax
7. The lens capsule molds the crystalline lens;
lens becomes more _______
8. Lens power ______ and the focal length ______
9. The eye changes focus from ______ to _____.
Biomechanics of the accommodative process
1. Innervation to the ciliary muscle
2. Ciliary muscle contracts
3. Ciliary muscle moves inward and anteriorly/forward
4. Ciliary ring advances approximately 0.5 mm along with the ciliary muscle
5. Choroid and posterior zonules stretch approximately 0.5 mm
6. Anterior zonular tension decreases, and the zonules relax
7. The lens capsule molds the crystalline lens;
lens becomes more round/spherical
8. Lens power increased and the focal length decreases
9. The eye changes focus from distance to near
Accommodation occurs when the overall power of the lens increases. The following are the changes in the lenses that lead to the increase in power:
- equatorial diameter decreases from __ to __ mm
- the anterior lens surface moves anteriorly while posterior surface moves posteriorly
- central anterior radius of curvature changes from __ to ___ mm (becomes more ____)
- central posterior radius of curvature decreases from ____ to ____ mm
- central thickness increases by ____ to ____ mm (at the nucleus)
- lens sinks ___ mm as a result of gravity
- equatorial diameter decreases from 10 to 9.6 mm
- the anterior lens surface moves anteriorly while posterior surface moves posteriorly
- central anterior radius of curvature changes from 11 to 5.5 mm (becomes more steep)
- central posterior radius of curvature decreases from 5.18 to 5.05 mm
- central thickness increases by 0.36 to 0.58 mm (at the nucleus)
- lens sinks 0.3 mm as a result of gravity
Anterior and posterior capsule is _____ and ____ when there is no accommodation
Anterior and posterior capsule is taught and flat when there is no accommodation
Light enters the eye and is focused at the retina to give a clear shape image, the ciliary muscle is ____ when there is no accommodation.
Light enters the eye and is focused at the retina to give a clear shape image, the ciliary muscle is relaxed when there is no accommodation.
The unaccommodated emmetropic eye focuses on a distant target and there is no need for accommodation
Convergence demand is _____.
zero
With a near object (object closer than infinity), the object has _____ rays that focus behind the eye, leading to an ____ image on the retina.
The optical power in the eye has to _____ to add positive _______ rays.
With a near object (object closer than infinity), the object has divergent rays that focus behind the eye, leading to an unfocused image on the retina.
The optical power in the eye has to increase to add positive convergent rays.
In the accommodative eye, the image is in focus because of the _______ lens.
accommodating lens
Accommodation is measured in Diopters. How do you solve it from cm?
First, convert the distance to cm. Then, do 100/__cm.
Accommodation Triad/Near Reflex
The 3 physiological changes seen in accommodation:
- Eye accommodation
- Pupil constriction
- Eyes converge
All of these are coupled neuronally through the parasympathetic innervation from the EW nucleus.
The convergence, accommodation and pupil constriction occur in both eyes - even if accommodative stimulus is presented to one eye. So, when there is a change in pupil size, these 3 things happen:
Change in pupil size
- controls light
- modifies depth of focus
- varies any optical aberration
Components of Accommodation are:
- Reflex accommodation
- Vergence accommodation
- Tonic accommodation
- Proximal accommodation
An automatic adjustment of the refractive state to maintain a focused retinal image.
Reflex accommodation
Reflex accommodation occurs when responding to ____, or a reduction in contrast. It is a response to a small amount of _____.
blur
blur
Reflex accommodation is particularly important for small scanning eye movements or ________.
Very important because it makes the fine change under binocular and monocular conditions.
micro saccades
Microsaccades are a kind of fixational eye movement. They are small, jerk-like, involuntary eye movements, similar to miniature versions of voluntary saccades.
This is the change in accommodation induced during fusional vergences. This leads to the convergence accommodation/convergence ratio (CA/C).
Vergence accommodation
Vergence accommodation leads to what ratio?
It leads to the convergence accommodation/convergence ratio (CA/C).
The refocusing that occurs due to the apparent (or perceived) nearness (proximity) of a target.
Proximal accommodation
Activated by perceptual cues.
Stimulated by targets located within 3 meters of an individual.
Proximal accommodation
Tonic accommodation is also called
“Lead of accommodation”
The residual/resting level of accommodation due to baseline stable innervation input from the midbrain.
Tonic accommodation
Tonic accommodation at rest is how many diopters?
(0.5D to 1.5D)
Present even in the absence of blur, disparity (that requires fusional vergences) and proximal cues.
Reduces with age
Tonic accommodation
Factors that affect accommodation
Blur Convergence Proximal issues Pharmacology Minus lens Diseases
Retinal Image Factors that affect accommodation
Retinal image factors: contrast, spatial frequency, retinal image motion
Non retinal factors that affect accommodation
Non retinal factors: mood, voluntary effort, target luminance, training
Optical cues that affect accommodation:
offer information about directionality, astigmatism, aberrations
Non optical cues that affect accommodation:
size, proximity, apparent distance, depth cues
Accommodation is sensitive and accommodative errors increase significantly with changes in _____ and ______.
With significant change, accommodative response returns towards tonic (slow) levels.
eccentricity and retinal image velocity
Accommodation is sensitive and accommodative errors increase significantly with changes in _____ and ______. With significant change, accommodative response returns towards tonic (slow) levels. In real life, these factors are not isolated, so the accommodation has a very robust and healthy way of detecting optimally and discriminating fine details.
eccentricity and retinal image velocity
Eccentricity: deviation of a curve or orbit from circularity.
Aberrations (an optical cues) occur when the ______ don’t coincide with the central and on axis rays
peripheral rays
_______ is the variation/small range in the image distance that is tolerable without a profound defocus. This influences accommodation as it increase.
Depth of focus
Accommodation is stimulated by mediated parasympathetic system and antagonized best with _________.
muscarinic blockers – they prevent acetylcholine from binding.
Muscarinic antagonists commonly used in practice are:
Tropicamide (Mydriacyl) and Cyclopentolate
Tropicamide (Mydriacyl) but has a _____ half-life and should not be used to determine the _____ refraction.
very short
cycloplegic
Cyclopentolate is effective with ____ half-life, used frequently in kids.
sufficient
Which one is used in kids: Tropicamide or Cyclopentolate?
Cyclopentolate
Others muscarinic include:
Homatropine
Atropine (iritis)
Scopolamine
Think: HAS
They produce mydriasis (dilation of the pupil) and loss of accommodation.
Phenylephrine (adrenaline), a _______, causes mydriasis (dilation of pupil) but has no significant effect on accommodation.
sympathomimetic
sympathomimetic (of a drug) producing physiological effects characteristic of the sympathetic nervous system by promoting the stimulation of sympathetic nerves.
Other drugs that affect accommodation
Alcohol Ganglion blockers Phenothiazides & antidepressants CNS stimulants Marijuana Carbonic Anhydrase Inhibitors Antihistamines Morphine
Some conditions that affect accommodation
Diabetes Traumatic Brain Injury (TBI) Multiple Sclerosis Myasthenia Gravis Botulism Down syndrome Glaucoma Iritis Iris Sphincter tear Eye trauma Adie’s tonic pupil Encephalitis Syphilis Neuro-ophthalmic lesions
Presbyopia: Gradual age-related irreversible loss of accommodative amplitude. Reported clinically about 1.____ years of age. Complete loss of ability to accommodate by about 2.____ years.
About a 3.___ D loss of accommodation per year.
- 40-45
- 50-55
- 2.5 D
Presbyopia complaints are:
- receded near point of accommodation
- blurred vision
- discomfort
- asthenopia (eye strain) at near
Presbyopia: Contributing factors and biochemical changes that lead to the decrease of accommodation:
- lens thickness and size increases
- springiness of capsule decreases (thickens)
- anterior surface curvature increases
- stiffer lens
- ciliary muscle remains stable
- cortex stiffens
- zonules become less dense (number of them)
The following factors do not change in presbyopia:
- zonules still have their elasticity
- ciliary muscle still functions
- motor neuronal pathway still functions
How is presbyopia treated?
Plus lenses are used to treat presbyopia.
It could be form of bifocals, reading glasses, monovision or bifocal contact lenses or even surgical correction.
Other accommodative conditions:
These occur in adults and children. As a result of medication, illness or even an accommodative anomaly. Name them.
Accommodative Excess
Accommodative Infacility
Accommodative insufficiency
Treated with proper distance correction and VT
Accommodative Excess
Proper correction and vision therapy
Accommodative Infacility
Proper distance correction and additional plus lenses
Accommodative insufficiency
What is the Accommodation Convergence/Accommodation Ratio?
The amount of convergence induced by a change in accommodation.
A change in accommodation is accompanied by a change in vergence
Accommodation and vergence permit clear stable single binocular vision across a range of viewing distances.
In accommodation, there is convergence With no accommodation, there is divergence
AC/A ratio helps evaluate the strength between the accommodative and vergences systems
Abnormal AC/A ratios are seen in _____ problems
binocular
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.
Gradient advantage: At the same distance, ________ accommodation is controlled
This can be done in the phoropter or with Modified Thorington, through the _____ refraction (with prisms in phoropter to dissociate to provide an open loop so accommodation is not influenced by any other stimulus)
Remind the patient to keep the near point target clear to maintain accommodation. Have the patient read the letters.
Measure the phoria.
Then measure the phoria again through ___ D lenses added to the refraction.
The difference in prism diopters between the phoria with the _______ and the phoria with the ____ is the change in convergence.
That is – this is how convergence responses to the accommodative stimulus.
Phoria is measured at the same near distance (40cm), but with different lenses to change the accommodative demand.
Gradient advantage: At the same distance, proximal accommodation is controlled
This can be done in the phoropter or with Modified Thorington, through the subjective refraction (with prisms in phoropter to dissociate to provide an open loop so accommodation is not influenced by any other stimulus)
Remind the patient to keep the near point target clear to maintain accommodation. Have the patient read the letters.
Measure the phoria.
Then measure the phoria again through -1.00D lenses added to the refraction.
The difference in prism diopters between the phoria with the subjective and the phoria with the -1.00 is the change in convergence.
That is – this is how convergence responses to the accommodative stimulus.
What does the formula of the AC/A ratio mean?
AC/A = (phoria with minus lenses – baseline phoria) divided by (absolute power of additional minus lenses)
Calculated near-far AC/A ratio is usually larger than the gradient AC/A because of the proximal vergences that influences the near phoria.
Near-far/calculated AC/A ratio is:
AC/A = PD (cm) + NFD (P’near – P distance)
PD =interpupillary distance in cm
NFD = near fixation distance in meter
P’near = near phoria (eso is plus and exo is minus)
P distance = distance phoria
A high AC/A ratio means there is ____ convergence with accommodation A low AC/A ratio means there is _____ convergence with accommodation
A high AC/A ratio means there is excess convergence with accommodation A low AC/A ratio means there is low convergence with accommodation