Theories of Accommodation and it’s Anomalies

Presenter: Dr. Rujuta GoreModerator: Dr. Atul Seth

A dioptric change in the power of the eye to see clearly

Relaxation Theory of Helmholtz

Proposed by Thomas YoungElaborated by Hermann von HelmholtzMost widely accepted

Relaxation Theory of Helmholtzeye is at rest and focused for distance

ciliary muscle is relaxed

eye makes an effort to focus on a near object

ciliary muscle contracts

bulk of the anterior ciliary body moves forward

release in tension on the zonular fibres

elastic capsule moulds the lens into a spherical form

Relaxation Theory of Helmholtz

Increase in surface curvatures causes an increase in optical power of the lens and therefore an increase in power of the eye

Helmholtz’s Theory: Disaccommodationciliary muscle contraction ceases

posterior zonular fibres pull the ciliary muscle backward

increases tension on the zonular fibres

increase in lens diameter, decrease in lens thickness and a flattening of the anterior and posterior lens surface curvatures

decrease in optical power

Shortcomings of Helmholtz’s Theory

Since the equatorial diameter increases with age, zonules should relax, and power of the crystalline lens should increase.

Lens should become unstable

Schachar’s Theory

Proposed by Ronald SchacharAlternative theoryContradicts the classical Helmholtz’s mechanism

Schachar’s Theory

ciliary muscle contracts

equatorial zonular tension is increased

anterior and posterior zonules are simultaneously relaxed

central surfaces of the lens steepen

peripheral surfaces of the lens flatten

Helmholtz’s and Schachar’s Theory

Shortcomings of Schachar’s Theory

Based on his theory, Schachar introduced a new surgery in 1992 i.e. the use of scleral expansion bands to increase the distance between the lens equator and ciliary muscle.

Poor results of this surgery challenged the validity of his theory

Catenary (hydraulic suspension) Theory

Proposed by Coleman DJ in 1970Demonstrated in 2001Explains the precise anatomical reproducible

shape of the lens in accommodated state

Assumption : the lens, zonule and anterior vitreous comprise a diaphragm between the anterior and vitreous chambers of the eye

Catenary (hydraulic suspension) Theory

What is “catenary”?

Catenary (hydraulic suspension) Theory

ciliary muscle contracts

initiates a pressure gradient between the vitreous and aqueous compartments

anterior capsule and the zonule form a trampoline shape or hammock shaped surface

steep radius of curvature in the center of the lens with slight flattening of the peripheral anterior lens

Clinical Assessment

Reading progressively smaller letters at nearNPA using RAF ruleRelative positive accommodation using minus

lensesAccommodative flipper test using paired +/–

lensesDynamic RetinoscopyDynamic Distant Direct Ophthalmoscopy

TERMS TO REMEMBER:Range of AccommodationAmplitude of AccommodationRelative Amplitude of AccommodationLeadLagFacility of Accommodation

Range of Accommodation: The distance between the far point and near point ie the distance over which accommodation is effective

Amplitude of Accommodation: The difference between dioptric power needed to focus at far point (at rest) and at near point (fully accommodated)

Relative amplitude of accommodation: The total amount of accommodation which the eye can exert while the convergence of the eyes is fixed

It can be positive (using concave lenses until the image blurs). This is called positive relative accommodation (PRA).

It can be negative (using convex lenses until the image blurs). This is negative relative accommodation (NRA)

Lead of Accommodation: The amount by which the accommodative response of the eye is greater than the dioptric stimulus to accommodation

Lag of Accommodation: The amount by which the accommodative response of the eye is less than the dioptric stimulus to accommodation

Clinical Assessment

Measurement of NPA: It is the closest point at which an object can be

seen clearlyAlso called “near point” or “punctum proximum”Measured with the RAF rule

DDDO

An emmetropic eye has “with” movement on retinoscopy and “superior” crescent on DDDO while 1D myopia (due to accommodation) shows “no movement” on retinoscopy and disappearance of superior crescent on DDDO

DDDO is an easier test than DRLocation of bright crescent moving from top to

the bottom of the pupil is probably easier to recognize[Fig. 5] than change in the movement of the retinoscopy reflex (“with” movement to the “against” movement), more so when the pupils are dilated

Anomalies of Accommodation

General symptoms:Problems are longstanding Intermittently blurred vision Eyestrain and/or headache with visual tasks Fatigue/sleepiness with visual tasks Inattentiveness over time

Anomalies of Accommodation

Classification

Decreased Accommodation

Insufficiency

Ill-Sustained Accommoda

tionInertia Paralys

is

Increased Accommodation

Excess Spasm

Accommodation Insufficiency

The accommodative amplitude is distinctly below the lower limit of the expected amplitude in relation to the age of the individual

Similar to presbyopiaCan result from systemic conditions such as

diabetes mellitus, multiple sclerosis, anemia, general physical fatigue, myasthenia gravis, trauma, malnutrition, convalescence from debilitating illnesses and chronic alcoholism

Accommodation Insufficiency

Specific symptoms: Blurred vision/eyestrain with NEAR visual tasks Intermittent diplopia due to associated

disturbances of convergenceExamination findings

Reduced amplitude of accommodation Higher than normal lag of accommodation Difficulty clearing -2.00 D lenses on monocular and

binocular accommodative facility testing PRA (positive relative accommodation) lower than -

1.50

Causes of Unilateral Accommodation Failure:Congenital unilateral third nerve palsyTransient, post traumatic, accommodation failure

associated with traumatic mydriasis

Causes of Bilateral Accommodation Failure:Cortical vision impairment Foveal hypoplasia (albinism, aniridia)Down syndrome Iso-ametropic amblyopiaEctopia lentis Macular degeneration NanophthalmosNear vision palsy

Rule out…

Treatment: Accommodation Insufficiency

Spectacle correctionFor near- weakest convex lenses should be

prescribedIf there is associated convergence insufficiency

base out prism may be added to patient comfortIn cases with convergence excess full spherical

correction should be prescribedACCOMMODATION TEST-CARD EXERCISE

Vision Therapy: To stimulate accommodation mono-ocularlySmall print targets that are slowly moved CLOSER

to the eye Reading print through MINUS lenses (gradually

increasing the power) using “Monocular minus lens rock”

Monocular lens flippers Monocular minus lens clear/blur/clear (for fine

voluntary control) Binocular lens flippers

Treatment: Accommodation Insufficiency

Ill-sustained Accommodation

Initial stage of true insufficiencyRange is normalDuring prolonged near work, accommodative

power weakens, the near point gradually recedes and vision becomes blurred

Inertia of Accommodation

Rare conditionDifficulty in altering the range of accommodationRequires time and effort to focus a near object

after looking into distance

Treatment:Correction of refractive errorAccommodative Exercises

Paralysis of Accommodation

Causes:Drug induced cycloplegia –atropine ,homatropine Internal opthalmoplegia [paralysis of cilliary muscle

& sphincter pupillae]Neuritis associated with chronic alcoholism,

diabetesCNS infectionsHead Injury

Specific Symptoms:Blurring of near vision Photophobia [glare]

Treatment: Paralysis of Accommodation

Self recovery occurs in drug induced paralysisDark glasses are effective in reducing the glareConvex lenses for near vision may be prescribed

Accommodative Excess

Treatment: Accommodative Excess

Prescribing lenses Distance lens prescription Added plus lenses are not usually accepted for near

work

Vision Therapy: To relax accommodation monocularlySmall print targets slowly moved AWAY from the

eye Reading print through PLUS lenses (gradually

increasing the power)

Spasm of Accommodation

Abnormally excessive accommodation which is out of voluntary control of the individual

Causes:Drug induced spasm after use of strong mioticsSpasm of near reflex

Spasm of Accommodation

Specific symptoms: Blurred vision at DISTANCE after performing near

visual tasks Examination findings:

Lead of accommodation Difficulty clearing +2.00 D. lenses on monocular

and binocular accommodative facility testing NRA lower than +1.50

Treatment: Spasm of Accommodation

Relaxation of ciliary muscle: the most effective method of treatment is complete ciliary paralysis with atropine

Accommodative Infacility

Specific symptoms:Blurred vision when CHANGING focus far → near

and near → far Examination findings:

Difficulty clearing both +2.00 and -2.00 D. lenses on monocular and binocular accommodative facility testing

PRA lower than -1.50 and NRA lower than +1.50

Treatment: Accommodative Infacility

Vision Therapy: to stimulate/relax accommodation monocularly Alternately focusing on small print targets at near

and far (with the near target slowly moved closer to the eye).

Reading near print through alternating PLUS and MINUS lenses (gradually increasing the power)