ACCOMMODATION FC1 Flashcards
Accommodation is a dynamic process to produce and maintain a
focused retinal image
The power of the lens changes to
maintain the image
What are the three components of the lens that change?
The lens curvature changes, the lens power changes and focusing changes.
There is a change in the shape which leads to a change in
the power of the lens
Note: This allows objects at various distance to be focused at the retina
The only active element is
the ciliary muscle, while other parts are
passive
1-Ciliary muscle contracts →
pulls the ciliary ring forward & inward → which stretches the choroid & posterior zonules → this yields a rounded lens and
capsule.
2- 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
choroid and posterior zonules return to their original position
Ciliary muscle is a ________ muscle with__________
smooth, parasympathetic
In accommodation, the ciliary muscle in the
ciliary body
contracts and moves forward
Note: This contraction releases the resting tension on the zonules around the lens equator. The lens capsule is able to mold the lens to become more spherical.
Anterior and posterior capsule is taught and flat when
there is no accommodation
Light enters the eye and is focused at________ to give a clear shape______
retina, 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
NOTE: Convergence demand is zero
With a near object (object closer than infinity), the object has_________ that focus behind the eye, leading to an unfocused image on the_______________
Divergent, 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 accommodating lens
The 3 physiological changes seen in accommodation:
Eye accommodation
Pupil constriction
Eyes converge
NOTE: Also known as the near reflex. All of these are coupled neuronally through the parasympathetic innervation from the EW nucleus.
The convergence, accommodation and pupil constriction occur in_____________
eye/eyes - even if accommodative stimulus is presented to__________ eye/eyes
both eyes, one eye
Change in pupil size
controls light
modifies depth of focus
varies any optical aberration
Components of Accommodation
Reflex accommodation
Vergence accommodation
Tonic accommodation
Proximal accommodation
Reflex accommodation
An automatic adjustment of the refractive state to maintain a focused
retinal image.Occurs when responding to blur, or a reduction in contrast.
It is a response to a small amount of blur.
This is particularly important for small scanning eye movements or
micro saccades.Very important because it makes the fine change under binocular and
monocular conditions.
Vergence accommodation
This is the change in accommodation induced during fusional
vergences. This leads to the convergence accommodation/convergence ratio
(CA/C)
Proximal accommodation
The refocusing that occurs due to the apparent (or perceived) nearness
(proximity) of a target. Activated by perceptual cues. Stimulated by targets located within 3 meters of an individual.
Tonic accommodation
“Lead of accommodation”
The residual/resting level of accommodation
due to baseline stable innervation input from the midbrain. The accommodation at rest (0.5D to 1.5D). Present even in the absence of blur, disparity (that requires fusional
vergences) and proximal cues. Reduces with age
Factors that affect accommodation
Blur Convergence Proximal issues Pharmacology Minus lens Diseases Retinal image factors: contrast, spatial frequency, retinal image motion Non retinal factors: mood, voluntary effort, target luminance, training Optical cues: offer information about directionality, astigmatism, aberrations Non optical cues: size, proximity, apparent distance, depth cues
*Aberrations (an optical cues) occur when
the peripheral rays don’t coincide with
the central and on axis rays
*Depth of focus is the
variation/small range in the image distance that is tolerable without a profound defocus. This influences accommodation as it increase.
Accommodation is stimulated by
mediated parasympathetic system
NOTE: Antagonized best with muscarinic blockers – they prevent acetylcholine from
binding.
Muscarinic antagonists commonly used in practice:
Tropicamide (Mydriacyl)
Cyclopentolate
Atropine (iritis)
Homatropine
Scopolamine
They produce mydriasis and loss of accommodation
Phenylephrine (adrenaline), a sympathomimetic, causes mydriasis but
has no significant effect on accommodation
Tropicamide (Mydriacyl)
has a very short half-life and should not be used to determine the cycloplegic refraction.
Cyclopentolate
is effective with sufficient half-life, used frequently in peds. USUALLY USED IN CHILDREN
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 40-45 years of age
Complete loss of ability to accommodate by about 50-55 years.About a 2.5D loss of accommodation per year.
Complaints of Presbyopia are:
receded near point of accommodation
blurred vision
discomfort and asthenopia at near.
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
Accommodative Excess (dont need a plus lens)
treated with proper distance correction and VT
Accommodative Infacility (have accommodation, but do not know how to use it, need teraphy)
proper correction and vision therapy
Accommodative insufficiency
proper distance correction and plus lenses
Accommodation Convergence/Accommodation Ratio
Also known as AC/A
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_________
With no accommodation, there is ________
convergence, divergence
AC/A ratio helps evaluate the strength between the
e accommodative and vergences systems
Abnormal AC/A ratios are seen
in binocular problems
2 ways of measuring the AC/A ratio
- Gradient determination
- Near-far (or calculated) determination
Calculated near-far AC/A ratio is usually_________ the gradient AC/A because of the proximal vergences that influences the near phoria
larger than
A high AC/A ratio means there is___________ with accommodation
excess convergence
A low AC/A ratio means there is_______ with accommodation
low convergence
