L8 Accommodation and Presbyopia

Question 1

Accommodation provides the ability to…

Answer 1

Change the point of focus of the eye from distant to near objects

Question 2

What is the formula which decides how powerful the lens must be to focus at a certain distance?

Answer 2

Dioptric power = 1/distance

Question 3

In humans, contraction of the ciliary muscles … zonular tension and causes … of the lens

Answer 3

Releases zonular tension and acuses rounding of the lens

Question 4

The ciliary muscle is attached to:

Answer 4

• Scleral spur
• Inner surface of anterior sclera
• Stroma of choroid

Question 5

Ciliary muscles consist of:

Answer 5

Radial, circular, and longitudinal fibres

Question 6

Zonules are secreted by … and consists of…

Answer 6

Ciliary epithelium, non-collagenous carbohydrate-protein complexes

Question 7

Zonules are ___-based fibrils, __nm in diameter. Fibres are grouped in fibre bundles, some are 4-50um wide

Answer 7

elastin-based fibrils, 70-80nm wide

Question 8

Zonules are attached to…

Answer 8

Pars plana and equatorial lens capsule

Question 9

The anterior and posterior lens curvatures ___ with age because…

Answer 9

Increases, because lens grows throughout life, adding layers of fibre cells.

Question 10

What is the Helmholtz theory of accommodation?

Answer 10

• Ciliary muscle contraction releases zonular tension, causing lens to become spherical. This allows accommodation to close.
• When accommodation ceases, the elastic choroid and posterior zonular fibres return the ciliary muscle to the dis-accommodated (relaxed) state.
• The reconfiguration of ciliary miuscles increases tension of zonular fibres at equatorial edge o lens. This pulls the edges of lens towards the sclera, flattening it out and dis-accommodating.

Question 11

What type of innervation to the ciliary muscle causes it to contract?

Answer 11

Postganglionic parasympathetic innervation to ciliary muscle releases Ach, causing lens to accommodate.

Question 12

Accommodation can be stimulated by administration of muscarinic agonists like ___

Answer 12

pilocarpine (mimics Ach)

Question 13

Accommodation can be inhibited by administration of antagonists like … This process is called ____

Answer 13

Atropine, cyclopegia

Question 14

Range of accommodation =

Answer 14

Distance between near point and far point

far point = infinity when refractive error of eye is fully corrected

Question 15

What is presbyopia?

Answer 15

Age related loss of accommodative amplitude

Accommodative ability declines throughout life and is usually gone by 50 y/o

Question 16

What are the two theories to explain presbyopia?

Answer 16

• Hardening or “sclerosis” of the lens “substance”
• Loss of ciliary muscle function

Question 17

What is the Lenticular theory of presbyopia?

Answer 17

Based on age-related changes in the lens fibres, lens capsule, or zonular fibres. Could be due to:
- Alteration in the tissue elasticity
- Continued growth (i.e. change in thickness and curvature of the lens with age)

Question 18

What is the Extralenticular theories of presbyopia?

Answer 18

Includes anatomical, morphological and physiological changes in the ciliary muscle, CT, and choroid

Question 19

What is a cataract?

Answer 19

Any opacification of the lens - leading cause of blindess in the world. No light can pass through lens. Cataracts can be classified on the basis of region of the lens that is affected.

Question 20

How can cataracts be treated?

Answer 20

• Can be treated in surgery to replace the cataractic lens with a clear plastic intraocular lens (IOL) implant.

Question 21

What is a nuclear cataract?

Answer 21

When the opaque part is at the middle of the lens, often associated with a shift in refraction (“second sight”) - may be getting better, but still a sign. Characterised by brunescence (discoloration of nucleus - sclerotic). Opacification results from oxidative damage, rather than fibre morphology

Question 22

What is a cortical cataract?

Answer 22

Opacification forms at the edges of lens, seen as wedges or “spokes” in the outer cortex. Rarely threaten vision (located out of optical axis). Often start in inferior nasal lens - most exposed to sunlight. Generally associated with structural changes to fibres (cell swelling) as well as oxidative stress.

Question 23

What are the risk factors of cataracts?

Answer 23

• Age
• Sun
• Metabolism (i.e. diabetes)
• Trauma
• prior diseases
• Drugs (steroids)