Accommodation

Core Fall 2016

Lisa Ostrin

Accommodation

AccommodationHelmholtz Theory

• The ciliary muscle contracts

• Zonular tension is released

• The lens capsule molds the lens

Adrian Glasser

Accommodation

An increase in dioptric power of the eye that occurs with the attempt to

focus on near objects

RelaxedAccommodated

Distance viewingNear viewing

Goal of Accommodation

• Dynamically change focal length of the eye to bring diverging

rays into focus to maximize retinal image contrast

• Cues to accommodation

– blur

– depth

– chromaticity

– ocular aberrations

– microfluctuations

Candy and Bharadwaj, 2007

Types of Accommodation

• Tonic accommodation

• Proximal accommodation

• Blur drive accommodation

• Convergence accommodation

Near Triad

• Accommodation - Pupil constriction - Convergence

• Three physiological responses are neurologically

coupled in the mid-brain

• Stimulating accommodation also causes pupil

constriction and convergence

• Stimulating convergence also causes accommodation

and pupil constriction

Accommodative Apparatus

• Anterior uveal layer– (Iris – controls amount of light entering eye, not an accommodative

structure)

– Ciliary muscle – controls accommodation and aqueous outflow

• Longitudinal fibers

• Radial fibers

• Circular fibers

– Ciliary processes – controls aqueous secretion

• Zonular fibers– Anterior

– Posterior

• Crystalline lens– Lens Capsule

– Lens Substance

Accommodative Apparatus

Zonule Fibers and Ciliary Processes

Hogan, Alvarado and Weddell, Histology of the Human Eye, 1971

Retzius, From Gullstrand Appendix to Helmholtz Treatise of

Physiological Optics, 1866

Anterior zonule

Ciliaryprocess

Posteriorzonule

Ciliary Muscle

• Multi-unit smooth muscle

• Innervated by CN III, parasympathetic fibers

• 3 parts

– Longitudinal – aka Brucke’s muscle, outer layer, runs from anterior

to posterior along length of ciliary body

– Circular – aka Muller’s muscle, innermost bundles, run

circumferentially, major arterial circle is just anterior

– Radial – few in number, between longitudinal and circular

Ciliary Muscle

Circular

Longitudinal

Radial

Hogan, Alvarado and Weddell, Histology of the Human Eye, 1971

The Crystalline Lens

• Center point of anterior and posterior surfaces are the anterior

and posterior poles, the “axis” is the line joining the poles

• Anterior surface (10mm radius of curvature) is less convex

than posterior surface (6mm radius of curvature)

• Outer edges are the equator

Layers of the Lens

• Lens capsule – outermost

elastic layer

• Epithelium – only on the

anterior surface

• Lens fibers

– Cortex

– Nucleus – innermost

Lens Capsule

• Consists of about 40 lamella made up of reticular fibers

(netlike or entangled) embedded in sulfated glycoaminoglycan

• Composed of collagen type IV

• Forms a barrier to bacteria and inflammatory cells

• Will allow diffusion of smaller molecules

• Acellular basement membrane that

completely envelopes lens

• Thinnest at poles and equator

• Thickest in midregions

• Formed by lens epithelium anteriorly

and lens fibers posteriorly

Lens Position

• Lens is suspended and under tension during distance viewing

• During accommodation the ciliary muscle contract, releases

zonular tension, and allows lens to fatten

• Space between ciliary processes and lens equator decreases

throughout life as the lens grows

Mechanism of Accommodation

Mechanism of Accommodation

Innervation to the Anterior Uvea

• Peripheral nerve supply

– Sensory innervation

• From the trigeminal ganglion

• Convey afferent responses to mechanical, thermal and

chemical stimuli

• Autonomic system – regulation of cardiac muscle, smooth

muscle and glands

– Sympathetic innervation - from the superior cervical ganglion

– Parasympathetic innervation – EW nucleus to ciliary ganglion to

short ciliary nerves

Innervation to the Anterior Uvea

• Sensory innervation

– Long ciliary nerves from the nasociliary branch of the ophthalmic

division of CN V from the trigeminal ganglion

– Convey afferent responses to mechanical, thermal and chemical

stimulation

Parasympathetic Pathway

Edinger-Westphal nucleus

ciliary ganglion

short ciliary nerves

ciliary body, sphincter muscle

acetylcholine

acetylcholine

pre-ganglionic

post-ganglionic

CN III

remember: SLUD

Sympathetic Pathway

Superior cervical ganglion

Long ciliary nerves

Dilator muscle

post-ganglionic

through carotid plexus

joins nasociliary branch

Q: what happens in the eye with the fight or flight response?

para-

Autonomic Neurotransmitters

• Sympathetic

– Noradrenaline, dopamine, seratonin, purines, neuropeptide Y,

epinephrine, norepinephrine

– Pharmacological agonists: phenylephrine

– Pharmacological antagonist: timolol maleate

• Parasympathetic

– Acetylcholine, vasoactive intestinal polypeptide

– Pharmacological Agonists: pilocarpine, carbachol, echothiophate

– Pharmacological Antagonists: atropine, scopolamine, cyclopentolate,

tropicamide

Autonomic Innervation – Iris Sphincter and CB

• Parasympathetic innervation to ciliary body and iris sphincter

– Muscarinic receptor types 1-5

– Neurotransmitter acetylcholine

– Agonists: pilocarpine, carbachol, echothiophate

– Antagonists: atropine, scopolomine, pirenzepine

M3 (2)

Autonomic Innervation – Iris Dilator

• Sympathetic innervation to iris dilator

– Adrenergic receptors α1, α2, β1, β2, β3

– Neurotransmitter noradrenaline

– Agonists: phenylephrine

– Antagonists: prazosin (α), timolol (β), propranolol (β)

adrenergic α1

Chen, Schmid, Brown. The autonomic control of accommodation and implications for human myopia

Development: a review. OPO; 23(5): 401-422.

EW nucleus

CN V, ophthalmic division,

nasociliary branch, Long ciliary nerves

Ciliary ganglion Short ciliary nerves

cervical sympathetic trunkSuperior Cervical

ganglion

Iris, cornea, cb

Ciliary Body/SphincterIII

Long ciliary nerves Dilator/BV

Parasympathetic

Sympathetic

Sensory

M3

α1

How to Measure Accommodation

Subjective Push Up Method

• The distance corrected subject moves a near letter chart

towards her eyes until she reports first sustained blur.

• The reciprocal of the distance from the letter chart to the eyes

in meters is expressed as “accommodative amplitude” in

diopters.

21.5 cm: 10 cm:

-1/0.215 m = 4.65 D -1/0.10 m = 10D

Push-up Accommodative Amplitude

Duane, 1912

How to Measure Accommodation

Subjective Push Up Method

• Relies on subject’s ability to perceive blur

• Blur perception varies with each individual subject

• Includes a measure of the depth of field of the eye (which is

not accommodation)

• In eyes without accommodation (presbyopes, multifocal

pseudopkhakic eyes), this only measures depth of field of the

eye

• Measurements depend on individual subject’s interpretation of

blur

• Measurements can vary depending on instructions given to

subjects (first blur vs sustained blur)

How to Measure Accommodation

Objective Trial Lens Method

• Minus powered trial lenses placed in front of the eye to

defocus a distant letter chart. The subject accommodates to

overcome the imposed blur and the response is measured with

a Hartinger coincidence refractometer (static response)

The Stimulus Response Function

How to Measure Accommodation

Grand-Seiko Autorefractor (Static)

Win-Hall, Ostrin, Kasthurirangan & Glasser, OVS, 2007

Stimulus-responsefunction

Power Refracter (Dynamic)

Kasthurirangan & Glasser, Vis Research, 2005

Subjective vs. Objective Measures

Win-Hall, Ostrin, Kasthurirangan & Glasser, OVS, 2007

Presbyopia

• Age related decrease in

accommodation

• Generally attributed to hardening

of the crystalline lens

• Other factors

– change in geometry of zonular

insertion to capsule

– loss of elasticity of the posterior

attachment of the ciliary muscle to

Bruch’s membrane

Duane, 1912

Presbyopia

Massive Increase in Stiffness of the Lens

A. Glasser, M.C.W. Campbell / Vision Research 39 (1999) 1991–2015

Presbyopic Correction

• Non-surgical

– spectacles

– contact lenses

• Surgical

– LASIK monovision

– IOL monovision or multifocal

Presbyopic Correction

• Surgical, dynamic

– accommodative IOL

– lens refilling

– scleral expansion

Rhesus Monkeys as Model of Human Accommodation

• Similar anterior segment anatomy and accommodative

mechanism (Glasser & Kaufman, 1999)

• Receptor characteristics resemble those of humans (van

Alphen, 1976)

• Develop presbyopia at a similar rate when adjusted for

life span (Bito, DeRousseau, Kaufman, & Bito, 1982)

P. L. Kaufman, L. Z. Bito, and C. J. DeRousseau.

Trans Ophthalmol Soc U K 102:323-326, 1982.

Rhesus Monkeys as Model of Human Accommodation

Edinger-Westphal Stimulation

in Rhesus Monkeys

Edinger-Westphal Stimulation

in Rhesus Monkeys

• Permanent indwelling electrode in the Edinger-Westphal (EW) nucleus

• Neuronally driven accommodation

• Efferent pathway similar in humans

EW Stimulated Accommodation

Hartinger coincidence refractometerMonkey # 38, 7-29-03

Hartinger voltage response function ODUnder Propofol

Current ( amps)

0 100 200 300 400 500

Acc

om

mo

da

tio

n (

D)

0

2

4

6

8

10

12

14

max

supramax

Infrared Photorefraction

Ciliary Process and Lens Edge Movement

reference mark

circumlental space

ciliary processes

lens edge

Swan-Jacob gonioscopy Lens

Ostrin & Glasser, Experimental Eye Res., 2007

Ciliary Process and Lens Edge Movement

Unaccommodated Accommodated

• During EW accommodation, anterior surface moves anteriorly, posterior

surface moves posteriorly

• Pharmacological accommodation may be different than EW driven

accommodation

Findl O, 2001Vilupuru and Glasser, 2005

Lens Position with Accommodation

Lens Position with Accommodation

• EW and pharmacological stimulation

were performed to produce maximum

accommodation

• Biometry measured continuously with

A-scan

anterior and posterior corneal

surfaces

anterior lenssurface

posterior lenssurface

retina

Anterior chamber

depth

Lens thickness Vitreous chamber depth

Anterior segment length

CUB

Computer

Lens Surface Movement

Distance from Cornea (mm)

2 3 4 5 6 7 8

Carbachol Final

Carbachol Initial

Supramax EW

Max EW

At Rest

Posterior lens surface

Anterior lens surface

Lens Thickness

Towards the cornea

In Vitro Scanning Experiments

• Enucleate eye and isolate anterior segment, leaving

accommodative apparatus intact

• Mount preparation in fluid filled chamber

• Direct laser through lens across the diameter of the

lens

• Can be used to test pharmacologically or

mechanically induced accommodative lens changes

Lens Scanning

ScanToxTM, curtousey V Choh

Lens Scanning

Negative Spherical Aberration

lens

Lens Scanning

Negative Spherical Aberration

Back Vertex Distance

Lens Stretching

Conclusions

• During accommodation

– the ciliary muscle contracts, the apex of the ciliary body moves

towards the lens equator, zonular tension is released to allow the

elastic capsule to mold the lens into an accommodated form

– lens thickness increases, lens diameter decreases and lens surface

curvatures become steeper.

– accommodative physical changes in the lens are linearly

correlated with the accommodative refractive change

• Objective measurements are essential for true accommodative

measurements