Structure & Function of The Eye PART 2

Question 1

What is the idea of refraction?

Answer 1

LIGHT is passing through one MEDIUM into another

Question 2

Index of refraction?

Answer 2

Measured by:

n = speed of light in a vacuum (c)
———————————————
speed of light in a substance (v)

As c is a set limit AND the denominator can NOT be greater than it, value will ALWAYS be > = 1

Question 3

What happens when light enters a new medium?

Answer 3

Light bends TOWARDS the normal when slowing down (entering a new medium)

Some light:
o reflects off the boundary
OR
o refracts through the boundary

Question 4

What are the 2 basic types of lenses?

Answer 4

o Converging (convex)

• focuses light rays onto a SINGLE point
• used for long-sighted glasses & cameras

o Diverging (concave)
 - diverge light rays

Question 5

Emmetropia?

Answer 5

Adequate correlation between axial length & refractive power

This is NORMAL - parallel light rays will always fall on the retina with NO accommodation required

Question 6

Ametropia?

Answer 6

MISMATCH between axial length & refractive power

i.e. Parallel light rays do NOT fall on retina with NO accommodation

Question 7

Accommodation?

Answer 7

The ability of the eye to change its focus from distant to near objects (and vice versa).
o this process is achieved by the lens changing its shape.

Accommodation is the adjustment of the optics of the eye to keep an object in focus on the retina as its distance from the eye varies

Question 8

Forms of Ametropia?

Answer 8

1. Myopia (near-sightedness)
2. Hyperopia (far-sightedness)
3. Astigmatism
4. Presbyopia

Question 9

Explain Myopia

Answer 9

Parallel rays converge as a focal point ANTERIOR to the retina

Aetiology is unclear BUT may be genetic

THIS ME!!!!!!!!!

Question 10

Causes of Myopia?

Answer 10

o Axial myopia (long eyeball/globe)
- MORE common

o Refractive myopia (excessive refractive power)

Question 11

Symptoms of Myopia?

Answer 11

Blurred DISTANCE vision

Squint in an attempt to improve uncorrected visual acuity
o this is as the CENTRAL LIGHT RAY does NOT refract so can be seen

Headache!

Question 12

Treatments for Myopia?

Answer 12

o Concave lens (ONENOTE!!)

o Contact lens

o Laser treatment to remove part of the lens

Question 13

Explain Hyperopia

Answer 13

Parallel rays converge at a focal point POSTERIOR to the retina

Aeitology unclear BUT may be inherited

Question 14

Causes of Hyperopia?

Answer 14

Axial hyperopia (short eyeball/globe)
 - more common

Refractive hyperopia (insufficient refractive power)

Question 15

Symptoms of Hyperopia?

Answer 15

o Visual acuity at near tends to blur relatively early

o Asthenopic symptoms
- e.g. eye pain, headache in frontal region, burning sensation in the eyes, blepharoconjunctivitis

Question 16

Lazy eyes?

Answer 16

Lazy eye - if one eye is HYPEROPIC in childhood and goes unnoticed, the brain starts to ignore this eye and concentrates on the good eye so leads onto lazy eye if not identified

Question 17

Amblyopia?

Answer 17

Uncorrected hyperopia > 5 dioptres

lens = 20 dioptres

Question 18

Treatments for Hyperopia?

Answer 18

o CONVEX lens

o REMOVE lens with laser treatment and have a greater power lens

o CONTACT lens

o Intra-ocular lens

Question 19

Astigmatism?

Answer 19

Parallel rays come to focus in TWO focal lines rather than a single focal point

Aetiology is hereditary

Question 20

Cause of astigmatism?

Answer 20

Refractive media is NOT spherical
SO
rays refract differently along one meridian perpendicular to it 
HENCE
producing more than one focal point

ONENOTE!!

Question 21

Symptoms of astigmatism?

Answer 21

o Asthenopic symptoms
- e.g. headsche, eyepain etc.

o Blurred & distorted vision

o Head tilting & turning

Question 22

Treatment for astigmatism?

Answer 22

Regular astigmatism
o cylindrical lens +/- spherical lens

Irregular astigmatism
o rigid contact lens +/- surgery

Question 23

What is the Near Response Triad and what does it consist of?

Answer 23

Mediates the response for near-vision

Triad consists of:
1. Sphincter pupillae - pupillary miosis to increase depth of field

2. Circular ciliary muscle - accommodation to increase refractive power of the lens
3. Medial rectus - align BOTH eyes towards a near object

Question 24

Presbyopia?

Answer 24

Naturally occurring loss of accommodation with age

Onset from 40years of age and DISTANT VISION remains intact

Question 25

How can Presbyopia be treated/corrected?

Answer 25

Corrected by reading glasses (convex lens) to increase refractive power of the eyes

Can also treat with bifocal, trifocal and progressive power glasses

Question 26

4 types of Optical correction?

Answer 26

Spectacle lenses

Contact lenses

Intraocular lenses

Surgical correction

Question 27

Spectacle lenses?

Answer 27

Monofocal lenses - spherical OR cylindrical lenses

Multifocal lenses

Question 28

Contact lenses?

Answer 28

Higher quality optical image (less influence on size of the retinal image than spectacle lenses)

Can be worn for sports/aesthetic reasons BUT require careful daily cleaning and are £££

Complications include:
o infectious keratitis
o giant papillary conjunctivitis
o severe chronic conjunctivitis

Question 29

Intra-ocular lenses?

Answer 29

REPLACEMENT of cataract crystalline lens

Give the best optical correction for aphakia (do NOT have lens)
AND
avoid the magnification/distortion caused by spectacles

Question 30

Surgical correction?

Answer 30

1. Keratorefractive surgery
2. Intra-ocular surgery (clear lens extraction with or without intra-ocular lens)

o small flap of eye made = laser changes refractive power of cornea = epithelial flap re-sewn onto eye
o extra intra-ocular lens can be placed on top OR old lens can be removed

LOSE accommodation so patients will need reading glasses

Question 31

Mechanism of accommodation?

Answer 31

1. Circular ciliary muscle CONTRACT
2. RELAXES the ZONULES that are usually taught
3. In absence of zonular tension, the LENS RETURNS to the natural CONVEX SHAPE
4. This INCREASES natural power of lens

Mediated by CN III (oculomotor)

Zonules - are passive elastic band with NO active contractile activity