Transcript Accommodation

ACCOMMODATION
Accommodation
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It is the ability to see
the near object clearly
by increasing the
converging power of
the eye.
This is by increasing
the refractivity of the
lens by increasing the
curvature of its
anterior surface.
Accommodation
Mechanism of accommodation
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There is contraction of the ciliary muscle
Reduction of the circle formed by the
ciliary processes
Suspensory ligament relaxes
Elastic capsule of the lens acts
Mechanism of accommodation
Mechanism of accommodation
Types
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Physical accommodation
Physiological accommodation
Range of accommodation
Amplitude of accommodation
Classification
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Insufficiency of accommodation
Paralysis of accommodation
Spasm of accommodation
Insufficiency of accommodation
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In this condition, the accommodative
power is below the lower limit of normal
for the patient’s age
Causes
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Early Presbyopia
Weakness of the ciliary muscle- due to
general debility, anemia or toxemia
Open- angle glaucoma ( due to
impairment of the effectivity of the ciliary
muscle by the increased intraocular
pressure.
Symptoms
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Eye strain
Difficulty with near work
Treatment
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Treatment of the causes
Reading spectacles (near addition)
Accommodation exercises
Paralysis of accommodation
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Mydriasis usually accompanies the
paralysis of accommodation. The
prognosis is good in cases due to drugs or
diphtheria. In traumatic cases the
condition may be permanent
Causes
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1. Unilateral
*Cycloplegics (Ex- Atropine)
*Contusion of the eye
*Paralysis of the third cranial nerve
2. Bilateral: ( Paresis is more common)
*Diphtheria
*Syphilis
*Diabetes
*Alcoholism
Treatment
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Treatment of the causes
If the paralysis is permanent- suitable
convex glasses may be prescribed
Miotics are seldom useful
Spasm of accommodation
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Spasm of accommodation in other name is
Pseudo myopia
Causes
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Found mainly in children, Who attempt to
compensate his refractive error
Myopes are more affected than
hypermetropes
May occur artificially by instillation of
miotics (Ex- with pilocarpine in young
glaucoma patients)
Neurotic individuals who converge
excessively
Symptoms
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Asthenopia
Blurring for distant vision (due to variable
degrees of artificial myopia)
Treatment
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Atropinisation for a few days or weeks
Assurance, and if necessary,
psychotherapy.
Convergence
Convergence
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Convergence is a disjugate movement in
which both eyes rotate inward so that the
lines of sight intersect in front of the eyes.
Types of Convergence
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Voluntary convergence
Involuntary convergence
Voluntary convergence is thus a
separate phenomenon from the reflex
convergence involved in normal visual
activities.
Reflex Convergence
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Tonic convergence
Fusional convergence
Accommodative convergence
Proximal convergence
Tonic Convergence
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It is that part of the convergence which
results from some inherent innervational
tone of the extra ocular muscles when the
patient is awake.
Fusional Convergence
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It is the Convergence that is produced to
ensure that similar retinal images are
projected onto corresponding retinal
areas.
The normal fusional convergence
amplitude for distance is about 18D and
for near it is 35D.
Accommodative Convergence
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It is that component of convergence which
occurs when the eyes accommodate, or
when a nerve impulse to accommodate is
discharged to the eyes.
Proximal Convergence
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This component of reflex convergence is
induced by the proximity of the object of
regard or the awareness of the proximity
of a near object.
Angle of convergence
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It refers to the angle that is formed between the
primary lines of sight during convergence. Its
size depends on the fixation distance, becoming
smaller with increasing distance (IPD), becoming
larger with increasing IPD
The effect of IPD on the angle of convergence is
usually negligible and so practically not taken
into consideration while measuring the
convergence angle. Convergence angle can be
measured in metre angle or in prism dioptres
Near point, Range and Amplitude
of convergence
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Near point of Convergence
Far point of Convergence
Range of Convergence
Amplitude of Convergence
Near point
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NPC is the closest point at which an object
can be seen singly during bifoveal vision
Normal NPC – less than 8cm
Far point
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FPC refers to relative position of the eyes
when they are completely at rest. It is
usually infinity.
Range of convergence
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RC is the distance between far point of
convergence and near point of convergence.
The part of the range of convergence between
the eye and infinity is called Positive convergence,
and the part beyond infinity
When eyes are in slight divergence is
called negative convergence or
divergence.
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