Download MBBS Ophthalmology PPT 15 Convergence Accommodation Lecture Notes

Download MBBS (Bachelor of Medicine, Bachelor of Surgery) Ophthalmology PPT 15 Convergence Accommodation Lecture Notes


ACCOMMODATION

AND CONVERGENCE

Department of Ophthalmology

1

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 the near reflex and

importance of convergence insufficiency.

2


ACCOMMODATION

? Definition: Accommodation is the mechanism by which the

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

3

Mechanism of accommodation

? Increase in the curvature of the lens affects mainly the

anterior surface.

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



6 mm
This alteration in shape increases the converging power of the

lens.

4


RELAXATION THEORY OF HELMONTZ
? He considered that lens was elastic and in normal state is

stretched and flattened by the tension of the suspensory
ligament.

? During accommodation:
Ciliary muscle contracts causing the lens zonules to slacken,

lens assumes more spherical form increasing thickness and

decreasing diameter,

protrusion forwards at the centre and a relative flattening at the

periphery.

5

6


NEAR REFLEX

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

axes of the eyes and associated constriction of pupil.

? Accommodation reflex includes increased

accommodation and associated constriction of pupil.

? The near reflex comprises :

Accommodation , convergence and miosis.

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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 the ciliary muscle

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

9

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 the 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 the

far point and the near point.

qThe amplitude of accommodation is the difference in

dioptric power between the eye at rest and the fully

accommodated eye.

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? The amplitude of accommodation is given by

the formula

? A = P - R
? where A is the amplitude of accommodation in

dioptres

? P is the dioptric value of the near point distance
? R is the dioptric value of the far point distance.

12
? 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

? Insufficiency of

accommodation

? Ill 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 begins between 40 years and 45 years.

17

? 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.

19

? Causes of premature presbyopia include

? Uncorrected hypermetropia

? Premature sclerosis of the crystalline lens

? General debility causing presenile weakness of ciliary

muscle

? Chronic simple glaucoma

20
? Symptoms

? Difficulty in near vision : patients start complaining of

inadequacy of vision for small print and finer objects at the

usual reading distance. Such problems start occurring in the

evening, and in dim light.

? Asthenopic symptoms due to fatigue of ciliary muscle

? Intermittent diplopia at near may develop.

21

? Treatment
Optical correction of presbyopia
Done by supplementing accommodation with
convex lens of appropriate power.

The difference between the distance correction
and the strength needed for near vision is called
the add.

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PRESBYOPIC ADD

? If the 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 the 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 the normal physiological limits
for the patient's age

? Causes

?Premature sclerosis of lens

?Weakness of ciliary muscle due to systemic

causes such as diabetes mellitus.

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Clinical features :

Treatment :

Headache

1.Treatment of the

Fatigue

systemic 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

? Blurring of near vision

? Self recovery-drug induced

? Photophobia (due to

paralysis and in cases when

mydriasis )

systemic cause is treated.

? Micropsia

? Dark glasses ? reduce glare

? abnormal receding of near

? Convex lenses ?for near

point

vision if paralysis is

? Signs of 3rd nerve palsy

permanent.

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Excessive accommodation

? A situation in which an individual exerts more than the

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

? Near vision difficulty

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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

34
? Definition: Convergence is a disconjugate movement in

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

35

Convergence reflex

? Afferent 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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? Efferent pathway-

along the 3rd nerve.
From the 3rd nerve
efferent fibres of
convergence reflex
relay in the
accessory ganglion,
before reaching
sphincter pupillae.

37

? Angle of convergence

?It refers to the angle that is formed between the

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

40
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 the eyes when they are completely at
rest, usually at infinity.

41

qThe range of convergence is the distance between

the far point and the 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.

42


Measurement of amplitude of convergence

1. Prism bar

2. Synoptophore

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Anomalies of convergence

1) Convergence insufficiency
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

? Blurred near vision

? Headache and eye

? Intermittent crossed

ache

diplopia

? Difficulty in changing

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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3. Prism Therapy
Base in ( BI ) prisms reading glasses or bifocals with prism

in the lower segment are useful.

4. Surgical treatment

? Last resort

? Medial rectus muscle resection can be performed.

49

Convergence paralysis

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

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? Clinical features
? Convergence is completely absent
? Exotropia and crossed diplopia occurs on attempted

near vision

? Adduction is normal



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? TREATMENT

qBase In (BI) prisms

qPlus lenses with BI prisms

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Summary

? Accommodation is the mechanism by which the 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 the eyes.

? The near reflex comprises : Accommodation ,

convergence and miosis.

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This post was last modified on 07 April 2022