Download MBBS Ophthalmology PPT 15 Convergence Accommodation Lecture Notes

ACCOMMODATION

AND CONVERGENCE

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Department of Ophthalmology

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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 the near reflex and

importance of convergence insufficiency.

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ACCOMMODATION

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? 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.

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Mechanism of accommodation

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? Increase in the curvature of the lens affects mainly the

anterior surface.

? Radius of curvature of anterior surface :10 mm

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During accommodation



6 mm

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This alteration in shape increases the converging power of the

lens.

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

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stretched and flattened by the tension of the suspensory
ligament.

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

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lens assumes more spherical form increasing thickness and

decreasing diameter,

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protrusion forwards at the centre and a relative flattening at the

periphery.

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NEAR REFLEX

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

axes of the eyes and associated constriction of pupil.

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? Accommodation reflex includes increased

accommodation and associated constriction of pupil.

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? The near reflex comprises :

Accommodation , convergence and miosis.

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ACCOMODATION REFLEX

? Af erent impulses-from

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the retina to the

parastriate cortex

? Internuncial fibres relay

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impulses from parastriate

cortex to Edinger

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westphal nucleus of both

sides

? Ef erent fibres ?from

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Edinger westphal

nucleus the ef erent

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impulses travel along the

3rd nerve and reach the

sphincter pupil ae and

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ciliary muscle

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Physical and physiological

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accommodation

Two factors in accommodation
q Ability of lens to alter its shape

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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

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ciliary muscle required to raise the refractive power of the
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.

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The far point of the emmetropic eye is at infinity.

qThe near point of distinct vision is the nearest point

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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

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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

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

dioptres

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

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? 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

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? 33 cm at age of 45 years

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ANOMALIES OF ACCOMMODATION

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DIMINISHED

INCREASED

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ACCOMMODATION

ACCOMMODATION

1. PHYSIOLOGICAL

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1. EXCESSIVE

(PRESBYOPIA )

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ACCOMMODATION

2. PHARMACOLOGICAL

(Cycloplegia)

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2. SPASM OF

3. PATHOLOGICAL

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ACCOMMODATION

? Insufficiency of

accommodation

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? 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

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accommodation due to reduced amplitude of
accommodation, leading to a progressive fall in near
vision.

? This begins between 40 years and 45 years.

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

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the normal reading distance.

? This condition of falling near vision due to age

related decrease in the amplitude of accommodation

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or increase in punctum proximum is presbyopia.

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? Causes of presbyopia :

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qAge related changes in lens which include

? Decrease in elasticity of lens capsule

? Progressive increase in size and hardness (sclerosis)

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of lens substance.

qAge related decline in ciliary muscle power.

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? Causes of premature presbyopia include

? Uncorrected hypermetropia

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? Premature sclerosis of the crystalline lens

? General debility causing presenile weakness of ciliary

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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 the

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usual reading distance. Such problems start occurring in the

evening, and in dim light.

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? 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.

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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

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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

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READING ADDITION

45-50 YEARS

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+1.00 D

50-55 YEARS

+1.50 D

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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 below the normal physiological limits

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for the 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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Clinical features :

Treatment :

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Headache

1.Treatment of the

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Fatigue

systemic cause

Blurring of vision for near

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2.Near vision spectacles

work

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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

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accommodation.

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Causes

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

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of 3rd nerve palsy.

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DRUG

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MAXIMUM

MAXIMUM

DURATION OF

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DURATION OF

MYDRIASIS

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CYCLOPLEGIA

MYDRIASIS

CYCLOPLEGIA

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ATROPINE 1% TID 30-40 MIN

1 DAY

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7-10 DAYS

2 WEEKS

CYCLOPENTOLAT 15 MIN

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15-30 MIN

1 DAY

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24 HRS

E 0.5%-1%

HOMATROPINE

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30-60 MIN

30-60 MIN

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1-2 DAYS

1-2 DAYS

2%

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TROPICAMIDE

15-30 MIN

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20-25 MIN

4-6 HRS

5-6 HRS

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0.5%-1%

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? Clinical features

? Treatment

? Blurring of near vision

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? Self recovery-drug induced

? Photophobia (due to

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paralysis and in cases when

mydriasis )

systemic cause is treated.

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? Micropsia

? Dark glasses ? reduce glare

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? abnormal receding of near

? Convex lenses ?for near

point

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vision if paralysis is

? Signs of 3rd nerve palsy

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permanent.

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Excessive accommodation

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? A situation in which an individual exerts more than the

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

? Near vision difficulty

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

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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 the lines of sight
intersect in front of the eyes.

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? Allows bifoveal single vision to be maintained at any

fixation distance.

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

exercises.

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Convergence reflex

? Afferent pathway ?the afferents from medial recti travel

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

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? Internuncial fibres : from the convergence centre go to

the Edinger Westphal nucleus .

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? Efferent pathway-

along the 3rd nerve.

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From the 3rd nerve
efferent fibres of
convergence reflex
relay in the
accessory ganglion,

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before reaching
sphincter pupillae.

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? Angle of convergence

?It refers to the angle that is formed between the

primary lines of sight during convergence

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?Its size depends on

? the fixation distance

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and

interpupillary distance ( IPD )

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Metre angle

? One metre angle

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convergence is
exerted by each
eye when the eyes
are directed to

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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

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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.

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? 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.

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qThe far point of convergence refers to relative

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

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qThe range of convergence is the distance between

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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

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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 insufficiency
Inability to maintain adequate binocular

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convergence for any length of time
without undue effort.

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? Aetiology

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A. Primary or idiopathic ? wide IPD,

general debility, overwork.

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B. Refractive errors- uncorrected high hypermetropia

and myopia

C. Presbyopia

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D. Muscular imbalances- exophoria,

intermittent exotropia and vertical muscle

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imbalances.

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Clinical features

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1. Symptoms of

2. Symptoms due to

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muscular fatigue

failure to maintain

binocular vision

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? Eyestrain

? Blurred near vision

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? Headache and eye

? Intermittent crossed

ache

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diplopia

? Difficulty in changing

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the focus from

distant to near

? Itching, burning and

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soreness of eyes

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Diagnosis

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

insufficiency is said to exist.

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2. Decreased fusional convergence for near-when

measured on synoptophore, if there is difficulty in

attaining 30? of convergence.

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3. Exophoria

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Treatment
1. Optical treatment- Myopes are given full correction

and hypermetropes undercorrection to stimulate
their accommodation and simultaneously

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convergence.

2. Orthoptic treatment- exercises to increase the near

point of convergence (NPC) and also to increase

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amplitude of fusional convergence.

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

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in the lower segment are useful.

4. Surgical treatment

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? Last resort

? Medial rectus muscle resection can be performed.

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Convergence paralysis

CAUSES
? Head injury

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? Encephalitis
? Tabes dorsalis
? Narcolepsy
? Tumours

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

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near vision

? Adduction is normal

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? TREATMENT

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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

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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

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front of the eyes.

? The near reflex comprises : Accommodation ,

convergence and miosis.

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