Download MBBS (Bachelor of Medicine, Bachelor of Surgery) Ophthalmology PPT 15 Convergence Accommodation Lecture Notes
ACCOMMODATION
AND CONVERGENCE
Department of Ophthalmology
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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.
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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.
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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.
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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.
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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.
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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 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.
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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
? 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.
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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 the 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 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.
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? 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
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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.
? 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
? 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.
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? 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
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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 the eyes when they are completely at
rest, usually at infinity.
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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.
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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.
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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