Download MBBS Ophthalmology PPT 25 Convergence And Accommodation Lecture Notes

Download MBBS (Bachelor of Medicine, Bachelor of Surgery) Ophthalmology PPT 25 Convergence And Accommodation Lecture Notes

ACCOMMODATION

AND CONVERGENCE

1

Revision Question

? You have gone fishing and see a fish in the water. You

do not have a fishing rod. The only equipment that you

have is a spear to catch the fish.

? Where do you throw the spear?
a. in front of the fish.
? b. behind the fish.
? c. directly at the fish.
? d. It is not possible to hit the fish as it is a virtual image.

2


Learning Objectives

At the end of the class, students shall be able to
? Understand the basic mechanism of

accommodation and clinical importance of
anomalies of accommodation

? Understand the pathway for near reflex and

importance of convergence insufficiency.

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Looking at the pupils

4


Looking at the pupils

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ACCOMMODATION

? Definition: Accommodation is the mechanism by which the eye

changes its refractive power by altering shape of lens in order
to focus objects at variable distances.

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Mechanism of accommodation

? Increase in the curvature of lens affects mainly anterior

surface.

? Radius of curvature of anterior surface :10 mm
During accommodation



6 mm
This alteration in shape increases the converging power of

lens.

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RELAXATION THEORY OF HELMHOLTZ

? He considered that lens was elastic and in normal state is

stretched and flattened by the tension of suspensory ligament.

? During accommodation: Ciliary muscle contracts causing lens

zonules to slacken, lens assumes more spherical form increasing
thickness and decreasing diameter, forward protrusion at centre
and relative flattening at periphery.

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NEAR REFLEX

? It has 2 components :
? Convergence reflex comprising convergence of visual

axes of eyes and associated constriction of pupil.

? Accommodation reflex includes increased

accommodation and associated constriction of pupil.

? The near reflex comprises :

Accommodation , convergence and miosis of pupils.

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ACCOMODATION REFLEX

? Af erent impulses-from

the retina to the

parastriate cortex

? Internuncial fibres relay

impulses from parastriate

cortex to Edinger

westphal nucleus of both

sides

? Ef erent fibres ?from

Edinger westphal

nucleus the ef erent

impulses travel along the

3rd nerve and reach the

sphincter pupil ae and

ciliary muscle

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Physical and physiological accommodation

Two factors in accommodation

q Ability of lens to alter its shape

q Power of ciliary muscle

1.Physical accommodation- Expression of the actual physical
deformation of lens, measured in dioptres.
2. Physiological accommodation- Contractile power of the ciliary
muscle required to raise the refractive power of lens , measured in
myodioptres.

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qThe far point of distinct vision is the position of an object

such that its image falls on the retina in the relaxed eye, i.e.

in the absence of accommodation. The far point of

emmetropic eye is at infinity.

qThe near point of distinct vision is the nearest point at

which an object can be clearly seen when maximum

accommodation is used.

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qThe range of accommodation is the distance between far point

and near point.

qThe amplitude of accommodation is the difference in dioptric

power between the eye at rest and fully accommodated eye.

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? The amplitude of accommodation is given by the

formula

? A = P - R
? where A is amplitude of accommodation in dioptres
? P is dioptric value of near point distance
? R is dioptric value of far point distance.

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? Applying this formula to the case of an emmetropic eye with a

near point of 10 cm,

? P = 10 D ( the reciprocal of 0.10 m )

? R = 0 ( the reciprocal of infinity is zero)

? A = 10 D

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? Far point and near point of the eye vary with the static

refraction of the eye

? In a hypermetrope eye far point is virtual and lies behind

the eye

? In a myopic eye far point is real and lies in front of the

eye.

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In an emmetropic eye
? Far point is at infinity
? Near point varies with age
? About 7 cm at age of 10 years
? About 25 cm at age of 40 years
? 33 cm at age of 45 years

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ANOMALIES OF ACCOMMODATION

DIMINISHED

INCREASED

ACCOMMODATION

ACCOMMODATION

1. PHYSIOLOGICAL

1. EXCESSIVE

(PRESBYOPIA )

ACCOMMODATION

2. PHARMACOLOGICAL

(Cycloplegia)

2. SPASM OF

3. PATHOLOGICAL

ACCOMMODATION

? Insuf iciency of accommodation
? Il sustained accommodation
? Paralysis of accommodation

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PRESBYOPIA

? Presbyopia is not an error of refraction but a condition of

physiological insufficiency of accommodation due to

reduced amplitude of accommodation, leading to a

progressive fall in near vision.

? This usually begins between 40 years and 45 years of age.

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? After the age of 40 years ,the NPA recedes beyond the

normal reading distance.

? This condition of falling near vision due to age related

decrease in the amplitude of accommodation or increase in
punctum proximum is presbyopia.

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Causes of presbyopia

qAge related changes in lens which include

? Decrease in elasticity of lens capsule

? Progressive increase in size and hardness (sclerosis) of

lens substance.

qAge related decline in ciliary muscle power.

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Causes of premature presbyopia include

? Uncorrected hypermetropia

? Premature sclerosis of crystalline lens

? General debility causing presenile weakness of ciliary

muscle

? Chronic simple glaucoma

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Symptoms

? Difficulty in near vision : patients start complaining of

inadequacy of vision for small print and finer objects at usual

reading distance. Such problems start occurring in evening,

and in dim light.

? Asthenopic symptoms due to fatigue of ciliary muscle

? Intermittent diplopia at near may develop.

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Treatment

Optical correction of presbyopia
Done by supplementing accommodation with convex
lens of appropriate power.
The difference between distance correction and
strength needed for near vision is called the add.

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PRESBYOPIC ADD

? If patient is presbyopic, calculate the likely reading addition

and add this to the distance lenses in the trial frame. In
practice the reading addition is estimated from patient's age.

AGE RANGE

READING ADDITION

45-50 YEARS

+1.00 D

50-55 YEARS

+1.50 D

55-60 YEARS

+2.00D

OVER 6O YEARS

+2.50 D

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MODES OF PRESCRIBING PRESBYOPIC ADD

o SPECTACLES
o CONTACT LENSES FOR PRESBYOPIA

2.SURGICAL TREATMENT OF PRESBYOPIA

o Refractive surgeries

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Insufficiency of accommodation

Accommodative power is significantly and persistently
below normal physiological limits for patient's age
? Causes

?Premature sclerosis of lens

?Weakness of ciliary muscle due to systemic causes

such as diabetes mellitus.

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Insufficiency of accommodation

Clinical features :

Treatment :

Headache

1.Treatment of systemic

Fatigue

cause

Blurring of vision for near

2.Near vision spectacles

work

3.Accomodation exercises

Intermittent diplopia

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Paralysis of accommodation

? Paralysis of accommodation ,also known as

cycloplegia, refers to complete absence of
accommodation.

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Causes

? Drug induced
? Internal ophthalmoplegia
? Paralysis of accommodation as a component of 3rd

nerve palsy.

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DRUG

MAXIMUM

MAXIMUM

DURATION OF

DURATION OF

MYDRIASIS

CYCLOPLEGIA

MYDRIASIS

CYCLOPLEGIA

ATROPINE 1% TID 30-40 MIN

1 DAY

7-10 DAYS

2 WEEKS

CYCLOPENTOLAT 15 MIN

15-30 MIN

1 DAY

24 HRS

E 0.5%-1%

HOMATROPINE

30-60 MIN

30-60 MIN

1-2 DAYS

1-2 DAYS

2%

TROPICAMIDE

15-30 MIN

20-25 MIN

4-6 HRS

5-6 HRS

0.5%-1%

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? Clinical features

? Treatment

? Blur ing of near vision

? Self recovery-drug induced

? Photophobia (due to mydriasis )

paralysis and in cases when

? Micropsia

systemic cause is treated.

? Abnormal receding of near point

? Dark glasses ? reduce glare

? Signs of 3rd nerve palsy

? Convex lenses ?for near vision

if paralysis is permanent.

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Excessive accommodation

? A situation in which an individual exerts more than normal

required accommodation for performing a certain near work.

? Excessive near work is an important precipitating factor

especially when done in inadequate illumination.

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Clinical features

? Varying degrees of blurred vision

? Symptoms of accommodative asthenopia

? Difficulty in performing near vision tasks

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Treatment

1. Optical treatment : refractive error to be corrected
2. General treatment : Near work should be minimised and

when done should be in proper illumination.

3. The general health of the patient should be improved.

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CONVERGENCE

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? Definition: Convergence is a disconjugate movement in which

both eyes rotate inward so that lines of sight intersect in front
of eyes.

? Allows bifoveal single vision to be maintained at any fixation

distance.

? Convergence does not deteriorate with increasing age.
? The power of convergence can be increased by exercises.

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Convergence reflex

? Af erent pathway ?the afferents from medial recti travel

centrally via the 3rd nerve to the mesencephalic nucleus of the
5th nerve, to a presumptive convergence centre in tectal or
pretectal region.

? Internuncial fibres : from the convergence centre go to the

Edinger Westphal nucleus .

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? Ef erent pathway-

along the 3rd nerve.
From the 3rd nerve
ef erent fibres of
convergence reflex
relay in the accessory
ganglion, before
reaching sphincter
pupil ae.

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? Angle of convergence

?It refers to the angle that is formed between primary

lines of sight during convergence

?Its size depends on

? the fixation distance

and
interpupillary distance ( IPD )

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Metre angle

? One metre angle

convergence is
exerted by each eye
when the eyes are
directed to object at a
distance of 1 m of the
meridian line between
the two eyes.

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? In an emmetropic eye, the number of dioptres of

accommodation required to see an object clearly is equal

to the number of metre angles through which each eye

must converge to see the object singly.

? Thus 1D of accommodation is associated with 1 ma of

convergence of each eye

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qThe near point of convergence is the closest point at

which an object can be seen singly during bifoveal vision
when maximum convergence is exerted.

qThe far point of convergence refers to relative position of

eyes when they are completely at rest, usually at infinity.

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qThe range of convergence is the distance between far

point and near point of convergence

qThe amplitude of convergence is the difference in

convergence power exerted to maintain the eye in a

position at rest and in a position of maximum convergence.

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Measurement of amplitude of convergence

1. Prism bar

2. Synoptophore

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Anomalies of convergence

1) Convergence insuf iciency
Inability to maintain adequate binocular
convergence for any length of time without
undue effort.

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? Aetiology

A. Primary or idiopathic ?
Wide IPD, general debility, overwork.
B. Refractive errors- uncorrected high hypermetropia and myopia
C. Presbyopia
D. Muscular imbalances-
Exophoria, intermittent exotropia and vertical muscle
imbalances.

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Clinical features

1. Symptoms of

2. Symptoms due to

muscular fatigue

failure to maintain

binocular vision

? Eyestrain
? Headache and eye

? Blurred near vision

ache

? Intermittent crossed

? Difficulty in changing

diplopia

the focus from distant

to near

? Itching, burning and

soreness of eyes

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Diagnosis

1. Remote NPC ? if NPC > 10 cm, Convergence

insufficiency is said to exist.

2. Decreased fusional convergence for near-when measured

on synoptophore, if there is difficulty in attaining 30? of
convergence.

3. Exophoria

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Treatment
1. Optical treatment- Myopes are given full correction and

hypermetropes undercorrection to stimulate their

accommodation and simultaneously convergence.

2. Orthoptic treatment- exercises to increase the near point

of convergence (NPC) and also to increase amplitude of

fusional convergence.

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Treatment

3. Prism Therapy
Base in ( BI ) prisms reading glasses or bifocals with prism
in lower segment are useful.
4. Surgical treatment

? Last resort

? Medial rectus muscle resection may be performed.

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Convergence paralysis

CAUSES

? Head injury
? Encephalitis
? Tabes dorsalis
? Narcolepsy
? Tumours

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Convergence paralysis

? Clinical features

? Convergence is completely absent
? Exotropia and crossed diplopia occurs on attempted near vision
? Adduction is normal



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Convergence paralysis

? TREATMENT

qBase In (BI) prisms

qPlus lenses with BI prisms

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Summary

? Accommodation is the mechanism by which eye changes its

refractive power by altering the shape of the lens in order to
focus objects at variable distances.

? Convergence is a disconjugate movement in which both eyes

rotate inward so that the lines of sight intersect in front of eyes.

? Near reflex comprises : Accommodation , convergence and

miosis.

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Question

? Which of the following is true regarding accommodation and

contact lenses?

? a. In a myopic patient, contact lenses decrease the

accommodative demand compared to spectacles.

? b. In a hyperopic patient, contact lenses increase the

accommodative demand compared to spectacles.

c. In a myopic patient, contact lenses increase
accommodative demand compared to spectacles.
? d. There is no difference noted in accommodation when

wearing contact lenses.

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