acommodation & its anamolies final
TRANSCRIPT
ACCOMMODATION & ITS ACCOMMODATION & ITS ANOMALIESANOMALIES
MODERATOR:DR ARVIND TENAGIMODERATOR:DR ARVIND TENAGI
PRESENTER:DR SHARANABASAMMAPRESENTER:DR SHARANABASAMMA
Accommodation
In an emmetropic eye, parallel rays of light
coming from infinity are brought to focus on the
retina, with accommodation at rest. Eyes have
been provided with a unique mechanism where
diverging rays coming from near object are
focused. This mechanism is called
accommodation.
Theories of mechanism of accommodation in humans
Theory of increased tension (Tschering’s theory)
• This theory attributes to the increased curvature of capsule to increasing tension on zonules.
• Tension in zonules causes compression of capsule at the equator of lens so that poles bulge.
• The relaxation theory: This theory was proposed by Thomas Young and elaborated by Helmholtz in 1885. Helmholtz considered that the lens was elastic.
– In normal state it was kept stretched and flattened by the tension of suspensory ligament.
– In the act of accommodation the contraction of the ciliary muscle lessened the circle
formed by the ciliary processes.
– Suspensory ligament was relaxed, the lens
assumed a more spherical form.
– It increases thickness and decreases the
diameter.
– There is increase in convexity of the anterior
capsule.
– Normally radius of curvature is 10 mm.In
accommodation, it decreases to 6 mm.
Role of capsule
• Initially Helmholtz regarded the lens as a whole
elastic body.
• But there was fallacy in his theory.
• Lens is a deformable semisolid mass which is
not elastic.
• Later Helmholtz attributed to the elastic
properties to the lens capsule.
• Fincham showed more unaccomodated flatter
shape of lens.
• Fischer showed that interplay of the elasticity of
the capsule and the lens substance determines
the shape of whole lens.
• Fincham suggested that variation in thickness of lens capsule accounts for the local variation in curvature.
• The modern version of the Helmholtz theory is that during accommodation, the ciliary muscle contracts, the suspensory ligament relaxes and the elastic capsule of the lens acts unrestrained to deform the lens substance into the more spherical ,perhaps conoidal accommodation shape.
Physical and physiological accommodation
Physical accommodation is an expression of the
actual physical deformation of lens and it is
measured in diopters.
Physiological accommodation has a unit the
myodiopter which is taken as the contractile
power of the ciliary muscle required to raise the
refractive power lens by 1D.
Stimulus for accommodation• Blur image • Apparent size and distance of object• Chromatic aberration • Oscillation of accommodation • Scanning movements of the eye
Reaction time
Reaction time refers to the time lapse between
the presentation of an accommodation stimulus
and occurrence of the accommodation
response.
• Average reaction time for far to near
accommodation is 0.64 seconds.
• Average reaction time for near to far is 0.56
second.
• Contraction of pupil to light (0.26 to 0.30
seconds)
• Reaction time of convergence response is 0.20
seconds
Occular changes during accommodation
Changes in lens
• Lens becomes more spherical.
• Thickness of lens is increased
• Anterior chamber is shallower
• There is decrease in the equatorial diameter by
0.4 to 0.45 mm
• The radius of curvature of anterior surface
becomes 6 mm in periphery and 3 mm in central
part
• The central part of the anterior surface bulges
more because capsule is thin in this region.
• The curvature of the posterior surface remains
the same.
Changes in the ciliary body
• Contraction of the ciliary muscle causes ciliary
ring
• Choroid is pulled forward
• The ciliary process are bulged towards the
equator of lens
Changes in the zonules
• Relaxation of the zonules occur during
accommodation.
• Lens is displaced in the direction of gravity by
some 0.3 to 0.35 mm
Miosis
• There is contraction of pupil on accommodation
• It lessens the optical aberration
• It cuts down the relative increase of light
entering the eyes.
• Increase the depth of focus.
Convergence
• When the eyes regard a near object the visual
axis must be directed upon it.
• The unit of convergence is conventionally taken
as meter angle (Ma), the amount normally
required to converge upon an object 1 meter
away.
Nervous pathway
• Accommodation is subserved by each of two
antagonistic constitutes of autonomic system i.e.
parasympathetic and sympathetic.
• The fibers of both are distributed in syncytial
network.
• The parasympathetic is the main supply.
• It starts in paired Edinger westphal nuclei lying in
the central grey matter of tegmentum
• Peripheral station is ciliary ganglion.
• These fibers enter the globe by short ciliary
nerves
• It causes contraction of ciliary muscle.
• Sympathetic supply mainly helps in relaxation of
accommodation.
Far point, near point, range and amplitude of accommodation
• The nearest point at which small objects can be
seen is called near point or punctum proximum.
• The further distance at which an object can be
seen clearly is called far point or punctum
remoter.
• The distance between the far point and near
point i.e. the distance over which
accommodation is effective is called range of
accommodation.
• The difference between the refractivity of the
eyes in two conditions is called the amplitude of
accommodation.
• In hypermetropic eye far point is virtual and lies
behind the eyes.
• In myopic it is real and lies in front of eye.
• In emmetropic ,far point is at infinity, near point
varies with age.
Assessment of accommodation
Measurement of near point accommodation
• The near point of accommodation is the closest point at which small objects can be seen clearly.
• The near point accommodation is measured
using RAF rule or Prince’s rule.
• To determine the NPA, a sliding target with 6/9
letters, numbers or fine lines is moved towards
the eyes.
• It is determined for each eye separately and
then for both the eyes.
Anomalies of accommodation
1) Diminished or deficient accommodation
a.Physiological (presbyopia)
b.Pharmacological (Cycloplegic)
c.Pathological
• Insufficiency of accommodation
• Ill sustained accommodation
• Inertia of accommodation
• Paralysis of accommodation
2)Increased accommodation
– Excessive accommodation
– Spasm of accommodation
Presbyopia
• It is not an error of refraction
• It is physiological insufficiency of
accommodation
• Near point of accommodation recedes beyond
the normal reading range.
It is a condition of failing near vision due to age related decrease in the amplitude of accommodation or increase in punctum proximum.
Pathophysiology
• Decrease in the accommodation power of
crystalline lens leading to presbyopia.
– Decrease in the elasticity and plasticity of the
crystalline lens.
– Age related decrease in the power of ciliary
muscles.
• During early years of life the amplitude of
accommodation is about 14D, the near point is
at 7 cm.
• At the age of 36 years, accommodation is 7D
and near point is 14 cm.
• At the age of 45 near point is 25 cm and
amplitude of accommodation is only 4D.
• At the age of 60 years only 1D of
accommodation is left.
• In the emmetropic presbyopia starts between 40
to 45 years of age.
• It is earlier in people living in tropics.
• In hypermetrope, presbyopia starts early in life.
Causes of premature presbyopia
• Uncorrected hypermetropia
• Premature sclerosis of crystalline lens.
• General debility causing presenile weakness.
• Chronic simple glaucoma.
Symptoms
Difficulty in near vision
– Inadequacy of vision for small print and finer
objects at the usual reading distance.
– To start it is present usually in the evening
– Later it become obvious in the day light
• Asthenopic symptoms due to fatigue of ciliary
muscle.
• Intermittent diplopia can occur.
Treatment
• It is done by supplementing accommodation with
the convex lens.
• The difference between the distance correction
and the strength needed for near vision is called
the add.
A rough estimate for the presbyopic add is
45 Years: + 1.0 D to +1.25 D
50 Years: +1.5 D to +1.75 D
55 Years: + 2.0 D to +2.25 D
60 Years: + 2.5 D to +3.0 D
The power of the presbyopic add should be
adjusted according to working distance.
• To give presbyopic correction
– Working distance should be known
– Refraction should be done.
– Amplitude of accommodation should be
estimated.
eg-if the patient is emmetropic and wishes to work
at 25 cm. accommodation required is 4D. But his
near point is receded to 50 cm. 2D of
accommodation is left.
• To work comfortably 1/3rd of accommodation
should be in reserve.
• So patient is left with 1.3D of accommodation
and required is 4D.
• 2.7 D of power should be added theoretically.
• In all cases it is better to under correct than to
over correct.
• In any case, a lens which brings the near point
closer than 28 cm is rarely tolerated.
Modes of prescribing presbyopic add
• Single vision reading glasses.
• Bifocal glasses
• Multifocal glasses
Basic principles for presbyopic correction
• Refractive error for distance should be corrected
first.
• Presbyopic correction should be done for each
eye separately and add it to distant vision.
• Presbyopic add should leave atleast 1/3rd of
accommodation in reserve.
• Near point should be fixed.
• The weakest convex lens with which an
individual can see clearly and comfortably with
both the eyes at near point should be
prescribed.
Insufficiency of accommodation
Here the accommodation power is significantly
less than normal physiological limits for the
patients age.
Causes
• Premature and sclerosis of lens.
• Weakness of ciliary muscle due to systemic
causes.
• Weakness of ciliary muscle due to local cause
Clinical features
• Asthenopic features are more prominent.
• Headache, fatigue and irritability of the eye
• Near work is blurred and becomes difficult or
impossible.
• Intermittent diplopia due to associated
disturbance of convergence.
• The above symptoms are stable in
accommodation insufficiency of lenticular origin.
Treatment
• Treatment of the causes
• Near vision spectacles in the form of weakest
convex lens.
• In convergence insufficiency, base-in prism
should be added.
• In convergence excess, full spherical
correction should be prescribed.
• Accommodation exercise – if the underlying
debility has passed.
Illsustained accommodation
It is a condition of accommodation fatigue and
refers to a situation in which range of
accommodation is normal but it cannot be
sustained for sufficient time period.
Aetiology
• Stage of convalescence from debilitating illness.
• Stage of general tiredness
• When patient is relaxed in bed
Clinical features
• Tired early while doing near work.
• Near point gradually recedes.
• Near vision blurred.
Treatment
• General tonic measures.
• Improve visual hygiene with reference to
illumination and posture.
Accommodation inertia
It is a condition in which there is difficulty in
adjusting the accommodation according to the
distance of the object of regard so as to gain
clear vision.
Treatment
• Correction of refraction error.
• Accommodation exercise.
Paralysis of accommodation
Paralysis of accommodation is known as
cycloplegia
Causes
• Drug induced due to atropine, homatropine or
other parasympatholytic drugs.
• Internal ophthalmoplegia due to neuritis
associated with diphtheria, syphilis, diabetes,
chronic alcoholism, cerebral or meningeal
disease. • Paralysis of accommodation as a component of
complete third nerve paralysis.
Clinical features
• Blurring of near vision
• Photophobia
• Abnormal receding of near point
• Decrease in range of accommodation
Increased accommodation
Excessive accommodation
It is used to describe a situation in which
individual exerts more than normal
accommodation for performing near work.
Causes
• Young hypermetropes
• Young myopes
• Astigmatic error
• Presbyopes
• Use of improper spectacles
Clinical features
1)Blurred vision due to induced pseudomyopia
2)Symptoms of accommodative asthenopia
3)Both far &near points are brought nearer to eye
Treatment
• Optical treatment :refractive error should be
corrected after performing cycloplegic refraction
• General treatment: near work should be
forbidden
• Improve general health of patient
Spasm of accommodation:
It refers to exertion of abnormally excessive
accommodation which is out of voluntary control
of the individual
Causes• Drug induced spasm
• Excessive near work in unfavourable circumstance
• Iridocyclitis
• In disturbed individuals
• Lesion of brain stem in their irritative phase
• Toxic reaction: exogenous poison
Clinical featuresClinical features
• Blurred vision due to induced myopia
• Asthenopic symptoms
• Near point is abnormally close
• Macropsia due to optical illusion
TREATMENTTREATMENT
• Relaxation of ciliary muscle:Most effective treatment is production of complete ciliary paralysis with atropine,for 4 weeks or more.
• Optical treatment:Correcting spectacles should be worn immediately when eyes are used again.
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