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.

THANK YOU