accommodation
DESCRIPTION
AccommodationTRANSCRIPT
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)