Download MBBS Ophthalmology PPT 25 Convergence And Accommodation Lecture Notes

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Revision Question

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

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

have is a spear to catch the fish.

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? 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.

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Learning Objectives

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

accommodation and clinical importance of

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anomalies of accommodation

? Understand the pathway for near reflex and

importance of convergence insufficiency.

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Looking at the pupils

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Looking at the pupils

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ACCOMMODATION

? Definition: Accommodation is the mechanism by which the eye

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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.

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

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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.

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? During accommodation: Ciliary muscle contracts causing lens

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

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and relative flattening at periphery.

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NEAR REFLEX

? It has 2 components :

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? Convergence reflex comprising convergence of visual

axes of eyes and associated constriction of pupil.

? Accommodation reflex includes increased

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accommodation and associated constriction of pupil.

? The near reflex comprises :

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Accommodation , convergence and miosis of pupils.

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ACCOMODATION REFLEX

? Af erent impulses-from

the retina to the

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parastriate cortex

? Internuncial fibres relay

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impulses from parastriate

cortex to Edinger

westphal nucleus of both

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sides

? Ef erent fibres ?from

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Edinger westphal

nucleus the ef erent

impulses travel along the

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3rd nerve and reach the

sphincter pupil ae and

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ciliary muscle

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Physical and physiological accommodation

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Two factors in accommodation

q Ability of lens to alter its shape

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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

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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

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

in the absence of accommodation. The far point of

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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

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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

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power between the eye at rest and fully accommodated eye.

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? The amplitude of accommodation is given by the

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formula

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

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? 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 )

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? 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

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? 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

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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

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? 33 cm at age of 45 years

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ANOMALIES OF ACCOMMODATION

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DIMINISHED

INCREASED

ACCOMMODATION

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ACCOMMODATION

1. PHYSIOLOGICAL

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1. EXCESSIVE

(PRESBYOPIA )

ACCOMMODATION

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2. PHARMACOLOGICAL

(Cycloplegia)

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2. SPASM OF

3. PATHOLOGICAL

ACCOMMODATION

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? 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

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physiological insufficiency of accommodation due to

reduced amplitude of accommodation, leading to a

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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

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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

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? Decrease in elasticity of lens capsule

? Progressive increase in size and hardness (sclerosis) of

lens substance.

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qAge related decline in ciliary muscle power.

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Causes of premature presbyopia include

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? Uncorrected hypermetropia

? Premature sclerosis of crystalline lens

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? General debility causing presenile weakness of ciliary

muscle

? Chronic simple glaucoma

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Symptoms

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? 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,

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and in dim light.

? Asthenopic symptoms due to fatigue of ciliary muscle

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? Intermittent diplopia at near may develop.

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Treatment

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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

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? 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.

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AGE RANGE

READING ADDITION

45-50 YEARS

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+1.00 D

50-55 YEARS

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+1.50 D

55-60 YEARS

+2.00D

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OVER 6O YEARS

+2.50 D

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MODES OF PRESCRIBING PRESBYOPIC ADD

o SPECTACLES
o CONTACT LENSES FOR PRESBYOPIA

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2.SURGICAL TREATMENT OF PRESBYOPIA

o Refractive surgeries

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Insufficiency of accommodation

Accommodative power is significantly and persistently

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below normal physiological limits for patient's age
? Causes

?Premature sclerosis of lens

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?Weakness of ciliary muscle due to systemic causes

such as diabetes mellitus.

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Insufficiency of accommodation

Clinical features :

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Treatment :

Headache

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1.Treatment of systemic

Fatigue

cause

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Blurring of vision for near

2.Near vision spectacles

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work

3.Accomodation exercises

Intermittent diplopia

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Paralysis of accommodation

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? Paralysis of accommodation ,also known as

cycloplegia, refers to complete absence of
accommodation.

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Causes

? Drug induced
? Internal ophthalmoplegia

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? Paralysis of accommodation as a component of 3rd

nerve palsy.

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DRUG

MAXIMUM

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MAXIMUM

DURATION OF

DURATION OF

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MYDRIASIS

CYCLOPLEGIA

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MYDRIASIS

CYCLOPLEGIA

ATROPINE 1% TID 30-40 MIN

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1 DAY

7-10 DAYS

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2 WEEKS

CYCLOPENTOLAT 15 MIN

15-30 MIN

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1 DAY

24 HRS

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E 0.5%-1%

HOMATROPINE

30-60 MIN

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30-60 MIN

1-2 DAYS

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1-2 DAYS

2%

TROPICAMIDE

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15-30 MIN

20-25 MIN

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4-6 HRS

5-6 HRS

0.5%-1%

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? Clinical features

? Treatment

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? Blur ing of near vision

? Self recovery-drug induced

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? Photophobia (due to mydriasis )

paralysis and in cases when

? Micropsia

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systemic cause is treated.

? Abnormal receding of near point

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? Dark glasses ? reduce glare

? Signs of 3rd nerve palsy

? Convex lenses ?for near vision

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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.

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? 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

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? 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

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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.

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? Allows bifoveal single vision to be maintained at any fixation

distance.

? Convergence does not deteriorate with increasing age.

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? 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

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5th nerve, to a presumptive convergence centre in tectal or
pretectal region.

? Internuncial fibres : from the convergence centre go to the

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Edinger Westphal nucleus .

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? Ef erent pathway-

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along the 3rd nerve.
From the 3rd nerve
ef erent fibres of
convergence reflex

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relay in the accessory
ganglion, before
reaching sphincter
pupil ae.

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? Angle of convergence

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?It refers to the angle that is formed between primary

lines of sight during convergence

?Its size depends on

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? the fixation distance

and
interpupillary distance ( IPD )

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Metre angle

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? One metre angle

convergence is
exerted by each eye
when the eyes are

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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

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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

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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.

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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

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qThe amplitude of convergence is the difference in

convergence power exerted to maintain the eye in a

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position at rest and in a position of maximum convergence.

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Measurement of amplitude of convergence

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1. Prism bar

2. Synoptophore

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Anomalies of convergence

1) Convergence insuf iciency
Inability to maintain adequate binocular

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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

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C. Presbyopia
D. Muscular imbalances-
Exophoria, intermittent exotropia and vertical muscle
imbalances.

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Clinical features

1. Symptoms of

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2. Symptoms due to

muscular fatigue

failure to maintain

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binocular vision

? Eyestrain
? Headache and eye

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? Blurred near vision

ache

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? Intermittent crossed

? Difficulty in changing

diplopia

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the focus from distant

to near

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? Itching, burning and

soreness of eyes

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Diagnosis

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

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insufficiency is said to exist.

2. Decreased fusional convergence for near-when measured

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

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convergence.

3. Exophoria

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

hypermetropes undercorrection to stimulate their

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accommodation and simultaneously convergence.

2. Orthoptic treatment- exercises to increase the near point

of convergence (NPC) and also to increase amplitude of

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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.

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4. Surgical treatment

? Last resort

? Medial rectus muscle resection may be performed.

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Convergence paralysis

CAUSES

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? Head injury
? Encephalitis
? Tabes dorsalis
? Narcolepsy

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? Tumours

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Convergence paralysis

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? Clinical features

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

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? Adduction is normal



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Convergence paralysis

? TREATMENT

qBase In (BI) prisms

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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

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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.

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? Near reflex comprises : Accommodation , convergence and

miosis.

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Question

? Which of the following is true regarding accommodation and

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contact lenses?

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

accommodative demand compared to spectacles.

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

accommodative demand compared to spectacles.

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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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