accomodation apparatus and mechanism for optometrist

Definition:

Accommodation is the ability of the eye to change its optical power to focus clearly on objects at different distances — achieved by altering the curvature of the crystalline lens.

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1. Mechanism (How Accommodation Works):

When viewing a near object:
• Ciliary muscles contract.
• Zonular fibers (suspensory ligaments) relax.
• Lens becomes more convex (increased curvature) → increases its refractive power.
• Light rays from near object are focused on the retina.

When viewing a distant object:
• Ciliary muscles relax.
• Zonular fibers become tense.
• Lens flattens (decreased curvature) → decreases its power.
• Light rays from distant object focus on the retina.

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2. Theories of Accommodation:

A. Helmholtz Theory (Accepted one):
• Ciliary muscle contraction reduces tension on zonules.
• Lens capsule’s elasticity allows it to become more convex.
• Explains accommodation well in humans.

B. Schachar Theory:
• Proposes that ciliary muscle contraction increases equatorial zonular tension, making the central lens surface more curved while the periphery flattens.

C. Fincham’s Theory:
• Emphasizes lens capsule elasticity and gradient refractive index changes within the lens during accommodation.
Structure
Function
Ciliary muscle
Main motor of accommodation; its contraction reduces zonular tension, allowing the lens to become more convex for near vision.
Ciliary body
Supports the ciliary muscle and anchors the zonules; provides attachment and nourishment to the muscle and lens.
Zonular fibers (Suspensory ligaments of Zinn)
Transmit the pulling force between ciliary muscle and lens capsule; relax or tighten depending on ciliary muscle action.
Crystalline lens
Elastic structure that changes its curvature — becomes more convex for near vision and flatter for distance vision.
Lens capsule
Elastic covering of the lens that helps reshape the lens when zonular tension changes.
Iris
Regulates pupil size (miosis during near vision) to increase depth of focus and reduce aberrations.
Choroid
Provides mechanical support to the ciliary body and maintains ocular tone.
Parasympathetic pathway (Edinger–Westphal nucleus → Oculomotor nerve → Ciliary ganglion → Short ciliary nerves)
Neural control pathway that initiates accommodation reflex.

1. PRESENTOR: Prabin gautam Suraj Sahani BOVS 6 th Batch Moderator Dikshya Khadka Optometrist LEIRC

2. Introduction Terminologies Anatomical structure Accommodation reflex Mechanism Development Components Theories of accommodation Changes in ocular dimension contents

3. Far point Position of an object when its image falls on to the retina clearly with accommodation fully relaxed Near point Nearest position of object where it seen clearly with maximum accommodation. Range of accommodation Distance between far point and near point Amplitude of accommodation(A) The difference between the dioptric power needed to focus at near point(P) and to focus at far point(R) is called amplitude of accommodation A=P-R

4.   Maximum amount by which the eye can change its power is known as the amplitude of accommodation if the far point(punctum remotum ) is 50 cm in front of the spectacle plane and the punctum proximum is 8 cm in front of it the amplitude is: AA = (1/ kpr )-(1/ kpp ) = (1/-0.5m)-(1/-0.08m) = +10.5D Amplitude of Accommodation

5. Ciliary muscles The ciliary muscle is a straited smooth muscle that is shaped like a ring located in the stromal part of ciliary body It holds the lens with the suspensory ligaments and also adjusts the optical power or shape of the lens during accommodation.

6. 2. Zonules Suspensory ligament made up of collagen Connect ciliary body to lens, acting as the tension regulating link that changes the lens shape arising from ciliary process

7. 3. Lens Transparent and biconvex structure of the eye The lens is held by the suspensory ligament Diameter of 10 mm and a height of 4 mm in an adult R.I: 1.39 These measurements vary due to change in the lens structure during accommodation and aging.

8. 4. Pupil Located in the middle of the eyes, it is black in color and constricts to prevent light rays that have diverged from touching the retina and causing blurred vision.

9. By placing the test object closer than infinity (closer than 6m in practices) By the use of minus lens

10. Blur image ( Ref. status of eye) Apparent size and distance of the object Chromatic aberration Oscillation of accommodation

11. Time lapse between the presentation of an accommodative stimuli and occurrence of accommodative response Average reaction time for far to near accommodation is 0.64 sec Near to far is 0.56 sec Reaction time for miosis is 0.30 sec and convergence is 0.20sec 13

12. ACCOMmODATION REFLEX

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15. Accommodation during the first month appeared to be relatively fixed at approximately 5.00 D During 3 rd month it approached up to adult level 17

16. As age grows the accommodative ability of the eye decreases gradually. At birth , it is 18.5D , at 25 yrs , 7-8D, At 50 yrs , 1-2D This linear relationship was derived by Hoffstetter using his and Donders data for ages of 8 to 30 yrs. The regression equation describing this was calculated by Hoffstetter as Amplitude= 18.5 – (age /3)

17. Changes in accommodative amplitude 19

18. Reflex accommodation is the automatic adjustment of the eye's refractive state to maintain a clear and focused retinal image in response to blur. Triggered by blur input . Effective for small amount of blur (2.00 diopters ) beyond that, voluntary effort is needed. Microsaccades help extract blur information by creating multiple luminance gradients around the fovea. It is the largest and most important of accommodation in both monocular and binocular vision. 21

19. Vergence accommodation is the accommodation induced by the innate neurological linking and action of disparity (fusional) vergence. This gives rise to the convergence accommodation/convergence (CA/C) ratio. For young child the CA/C ratio is about 0.5 D per meter angle MA MA is determined by dividing ipd in mm by 10 and expressing value in prism diopter 22

20. It is the passive state of accommodation in the absence of a stimulus, that is when the eye is either in complete darkness, or when it is looking at a bright empty field. Ciliary tonicity is responsible for this

21. Proximal accommodation is also known as psychic accommodation and is induced by the awareness of the nearness of a target. This is independent of the actual dioptric stimulus.

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23. It is also known as capsular theory First proposed by Thomas young and elaborated by Helmholtz in 1885 In unaccommodated eye, ciliary muscle is relaxed and the ciliary ring expanded that keeps the zonular fibers tight helps in flattening the lens which decreases the accommodative power of lens During accommodation, contraction of the ciliary muscle causes the ciliary ring to shorten and moves the forward the equator of lens. It also pulls the choroid forward as a result, zonular fibres are relaxed, the tension on the capsule is relieved and convexity of lens increases due to which accommodative power also increases 26

24. Imaging technique(UBM) of accommodated eye showed that ciliary muscle move anteriorly and the equatorial edge of lens move away from sclera Gonio-videography show zonular fibres extending from ciliary process to lens equator are relaxed during accommodation 27

25. It doesn’t tells about declining of accommodative power with age 28

26. Based on Helmholtz hypothesis The cord between the two springs represents the zonules. The upper spring represents the lens, and its contraction represents change of the shape of the lens during accommodation. The lower spring represents the elasticity of choroid and a cord passed over pulley supporting the weight represent the pull of circular fibre of ciliary muscle 29

27. It shows in unaccommodated state elasticity of choroid is stronger than lens. Ciliary muscle relaxed exert no pull causing relaxation of upper string lens and flattening of lens occur When accommodation takes place due to pull exerted by the contraction of ciliary muscle the lower spring is pulled and the zonules are loosen ; this allows the upper string to contract and convexity of lens increases 30

28. Proposed by Ronald schachar in 1992 Associated with increased tension on equatorial zonules and relaxation of anterior and posterior zonules during accommodation During accommodation, ciliary muscle contracts, tension on equatorial zonules increased which helps to pull the equatorial region toward sclera and tension on anterior and posterior zonules decreased which causes steepening of anterior and posterior part of lens which increases the accommodative power of eye 31

29. According to Schachar’s theory, presbyopia results from growth in the equatorial diameter of the lens, such that the perilenticular space is reduced and ciliary muscle contraction can no longer tense the zonules and expand the lens coronally . Schachar introduced a new surgery for presbyopia, i.e. the use of scleral expansion bands (SEB). The aim of these segments is to increase the working distance between the ciliary muscle and the lens equator 32

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31. Marius Tscherning proposed this theory This theory attributes to the increased curvature of the anterior capsule increasing tension on the zonules. It states that, In accommodated eye contraction of the ciliary muscle occurs which pulls the zonules directly and increases the tension on the zonules . This results in a compression of the capsule at the equator of the lens so that the anterior poles bulge and become steeper due to which the accommodative power increases 34

32. Hydraulic suspension theory: Proposed by Coleman et al It state that the lens, zonules and anterior vitreous comprise a diaphragm between the aqueous and vitreous chambers of the eye During accommodation, contraction of the ciliary body produced a rise in vitreous pressure, causing anterior movement of the lens zonule diaphragm and steepening of anterior central lens curvature and slight flattening of the anterior peripheral lens curvature. According to the catenory system of Coleman and Fish, presbyopia occurs due to increasing lens volume with age that results in a reduced response of anterior radius of curvature to the vitreous pressure gradient created by ciliary body contraction 35

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34. Pupil Constriction of pupil by approx. 0.25 to 0.4 mm per diopter for young adults 2. Lens Equatorial diameter decreases by 0.4mm (from 10 mm to 9.6 mm) The ant pole moves forward 0.3 mm and central radius of curvature of ant surface decreases from 11 to 5.5 mm The post. Pole may move back 0.15 mm and central radius of curvature of post Surface decreases from 5.18 to 5.05 mm The central thickness increases by 0.36 to 0.58 mm The lens sinks 0.3 mm as a result of gravity

35. 3. AC depth Depth of ant. Chamber decreases due to forward movement of ant. Pole of lens makes AC shallow slightly 38

36. Theory and practice of optics and refraction 5 Th edition Theory and practice of squint and orthoptics 3 rd edition Textbook of visual science and cilinical optometry 1 st edition 2009 Borish clinical refraction 2 nd edition Anatomy and physiology of eye by Ak khurana 3 rd edition Disorders of lens and cataract surgery by Ak khurana 1 st edition 2022 Internet 39

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