2. Contents
• Accomodation
• Types of accomodation
• Anomalies of accomodation
• Convergence
• Types of convergence
• Anomlies of convergence
• AC/A ratio
• Methods of claculation of AC/A ratio
3. Accomodation
• Accommodation is the mechanism by which the
eye changes refractive power by altering the shape
of lens in order to focus objects at variable
distances.
• Ability of eye to focus on near objects.
4. Far point
• Position of an object when its image clearly falls on
retina with no accommodation
Near point:
• Near point: Nearest point clearly seen with
maximum accommodation. •
Range of accommodation:
• Distance between far point and near point.
5. Amplitude of accomodation
Amplitude of accommodation:
• Dioptric power difference between rest and fully
accommodated eye.
• A=P-R. Where
• A: amplitude of accommodation;
• P:dioptric value of near point; and
• R: dioptric value of far point
6. Types of accomodation:
• Tonic accommodation – It is due to tonus of ciliary
muscle and is active in absence of a stimulus. The
resting state of accommodation is not at infinity but
rather at an intermediate distance. •
• Proximal accommodation – Is induced by the
awareness of the nearness of a target. This is
independent of the actual dioptric stimulus.
• .
7. • Reflex accommodation – Is an automatic
adjustment response to blur which is made to
maintain a clear and sharp retinal image.
•Convergence-accommodation – Amount of
accommodation stimulated or relaxed associated
with convergence.
• The link between accommodation and
convergence is known as accommodative
convergence and is expressed clinically as AC/A
ratio.
8. Assessment of accomodation
• Dynamic retinoscopy
• Subjective measurement of accommodation
amplitudes with e.g., RAF rule
• Facility of accommodation with "lens flippers"
11. Presbyopia
• Presbyopia is a condition of physiological
insufficiency of accommodation leading to a
progressive fall in near vision.
• In emmetropic eye far point is infinity and near
point varies with age (being about 7 cm at 10 years,
25 cm at 40 years and 33 cm at 45 years).
• Failing near vision due to age-related decrease in
amplitude of accommodation is called presbyopia.
•
12. • Causes
• Decrease in accommodative power of lens with
increasing age, leads to presbyopia, occurs due to:
–
• Age-related changes in lens:
• Decrease in elasticity of lens capsule,
• Progressive, increase in size and hardness
(sclerosis) of lens substance
• Age related decline in ciliary muscle power.
13. Cycloplegia
• Cycloplegia, refers to complete absence of
accommodation.
• Causes
• Atropine, homatropine or other parasympatholytic
drugs.
15. Accomodation paralysis
• Complete third nerve paralysis due to intracranial
or orbital causes. – Systemic medications such as
antihypertensive, antidepressants.
16. Illsustained accommodation
• Accommodation fatigue.
• It is a situation in which though range of
accommodation is in normal range but it cannot
sustain it for a sufficient period of time.
17. Accomodation inertia
• It is a condition in which patient faces difficulty in
altering the range of accommodation
• Amplitude of accommodation is normal
• Ability to make use of this amplitude quickly and
for long periods of time is inadequate.
18. • Clinical features •
• Difficulty changing focus from one distance to
another
• Headaches
• Eyestrain
• Fatigue
• Difficulty sustaining near tasks
• Blurred vision
• Treatment:
• correcting any refractive error and accommodative
19. Pre-presbyopia
• Uncorrected hypermetropia.
• Premature sclerosis of the crystalline lens. •
• General debility causing pre-senile weakness of
ciliary muscle.
• Chronic simple glaucoma.
• Symptoms
• Difficulty in near vision.
• Patients complaint of difficulty in reading small
prints
• Asthenopic symptoms after reading or doing any
20. Excessive accomodation
• Accommodative response is greater than the
accommodative stimulus.
• There is functional increase in tonus of ciliary
muscle, results in a constant accommodative effect.
21. • Causes
• Young hypermetropes frequently uses excessive
accommodation as a physiological adaptation •
• Young myopes performing excessive near work,
associated with excessive convergence.
• Astigmatic error in young patients
• Presbyopes in the beginning
• Use of improper and ill fitting spectacles
22. Accomodation spasm
• Spasm of accommodation refers to exertion of
abnormally excessive accommodation.
• . Causes
• Drug induced spasm of accommodation is known
to occur after use of strong miotics.
• Spontaneous spasm of accommodation: attempt to
compensate for a refractive anomaly.
• Occurs when excessive near work is done with bad
illumination, bad reading position, state of
neurosis, mental stress or anxiety.
23. • . Clinical features
• Defective vision: due to induced myopia. •
• Asthenopic symptoms
• Precipitating factors like marked degree of
muscular imbalance.
24. Convergence
• Connvergence is the simultaneous inward
movement of both eyes toward each other, usually
in an effort to maintain single binocular vision
when viewing an object.
• This is the only eye movement that is not
conjugate, but instead adducts the eye
25. Types of convergence
• 1.Voluntary convergence
• Convergence of eye at our own will.
• Different entity from reflex convergence.
• Some consider voluntary convergence is attained
by accommodating eye more with out
accommodating stimulus.
• Example :- converging eye to reduce nystagmus ,
some comedian apply voluntary convergence to
obtain crossed eyes.
27. Tonic convergence
• Occurs due to normal muscle tone of EOMs.
• Helps to bring eye from anatomical diverged
position to physiological position.
• Tonic convergence decreases with age passes.
• Emotional energy is found to rise tonic
convergence. .
• Tonic convergence can be eliminated by
patching(30 mins) or deep anesthesia.
28. Proximal convergence
• Proximal convergence
• induced by proximity of object of regard.
• Also seems to be initiated by psychological factor.
• It is also induced when person feels he is looking at
near object although he is not doing so.
• Proximal convergence has linear relationship with
change in fixation distance.
29. Accomdative convergence
• Convergence induced when a person
accommodates
• Induced or stimulated by blurred retinal image.
• Independent of binocular vision ie can even occur
in one eye blind or occluding one eye.
• It has linear relationship with change in fixation
distance
• AC/A will better define accommodative
convergence.
30. Fusional convergence
• Ensures similar images are imaged on
corresponding retinal points.
• It is mainly induced by bitemporal image disparity.
• No refractive changes seen in eye during fusional
convergence.
• Involuntary mechanism to obtain bifoveal fixation
• Fusional convergence can be improved by
orthoptic exercises.
32. Convergence insufficiency
• . Inability to maintain or obtain adequate
convergence over certain period time without
undue effort.
• Commonest cause of asthenopia.
• Causes:
• 1. Idiopathic ( developmental delay ,wide IPD )
• 2. Refractive errors ( High hyperopia , Myopia )
• 3. Presbyopia or pts corrected recently for
Presbyopia
33. Convergence paralysis
• It is defined as total lack of ability to overcome
base out prism.
• Uncommon condition confused with convergence
insufficiency.
• Causes:-
• Occurs secondary to organic disease of brain
especially at corpora quadrigemina and nuclei of
3rd cranial nerve .
34. • Clinical features:-
• 1. Complete absence of convergence
• 2. Exotropia and crossed diplopia on attempted
near fixation
• 3. Adduction remains normal .
• 4. Accommodation is usually normal but reduced
and absent sometimes.
35. Convergence spasm
• Condition characterized by intermittent episode of
maximum convergence usually associated with
accommodative spasm.
• Causes
• 1. Functional causes ( associated with hysteria and
neurosis )
• 2. Organic causes ( organic lesions , head traumas ,
pituitary adenomas )
36. • Clinical features:-
• 1. Extreme convergence ( intermittent )
• 2. Homonymous diplopia
• 3. Blurring of vision due to accommodative spasm (
near triad)
• 4. Miosis ( near triad)
• 5. High induced myopia (> 5D)
37. Ac/A ratio
• The measurement of the convergence induced by
accommodation per diopter of accommodation
• Purpose:
• To determine the change in accommodative
convergence that occurs when the patient
accommodates or relaxes.
39. Hetrophoria method
• Simple method, consists of comparing the
measurement of the latent deviation of eyes
• Using prisms & alternate cover test at a point of
distance fixation ( 6m ) with refractive correction.
• At a point of near fixation ( 33cm ) with refractive
correction.
• IPD should be measured.
• +ve sign for esodeviation,
• -ve sign for exodeviation.
40. • AC/A ratio is calculated from this following
formula:-
• AC/A = IPD (cm) + N (m) (D’-D)
• IPD = interpupillary distance in centimeters
• N = near fixation distance in meters
• D’ = near phoria (eso is plus and exo is minus)
• D = far phoria (eso is plus and exo is minus)
41. Gradient method
• This method is based onthe fact that for agiven
fixation distance , minus lenses placed before the
eyes increase the requirement of accomodation
and plus lenses relax accomodation.
• Gradient AC/A
• Phoria is measured a second time using a -1.00/
+1.00 lens
• The change in phoria with the additional minus or
plus is the AC/A ratio
42. Fixation disparity Method
• In this method AC /A ratio is determined indirectly
from the fixation disparity method either by forced
convergence by use of prism or by altering the
accomodative stimulus by use of optical lenses.
• Because of its complexity ,this test is not
performed in routine clinical practice.
43. Haploscopic method
• In haploscopy , the visual fields ofthe two eyes
aredifferntiated and a separate target is presented
to each eye.
• Hering's original instrument was designed primarily
for studimg AC/A ratio.
• This method is no more in use.
