Vision 4: Refractive Media of the Eye

Question 1

Optical Power of the eye

Answer 1

67 Diopters

Question 2

Optical Power of the Cornea

Answer 2

45 Diopters

Question 3

Optical Power of the lens

Answer 3

• Rest: 20 Diopters
• Accomodated: 34 Diopters

Question 4

Importances of Cornea

Answer 4

Fixed Powerful convexity:
* high curvature makes it responsible for 2/3 of optical power of the eye

Formation of Sharp retinal Image: Perfectly regular curvature

Protective function:
* Superficial reflex: Afferent= Ophthalmic div. of Trigemnal N, Efferent=Facial (bilateral), blinking of both eyes
* Absorption of some UV rays

Question 5

Corneal Opacity treated by

Answer 5

Corneal graft

Question 6

Properties of the lens

Answer 6

• Avascular
• Biconvex
• Transparent

Question 7

Describe structure of the lens

Answer 7

General:
* Highly condensed proteins
* Cortex is made up of young fibers
* center (nucleus) made up of old fibers
* old fibers have higher power than young fibers

At Rest:
* It’s more flat, for farsightedness (20 D)
* The posterior is more convex than the anterior

during Accomodation:
* More convex with higher power (34 D)

Question 8

Describe metabolism of the lens

Answer 8

1. Low metabolism
2. Mainly Anaerobic
3. Aerobic at some level, with the presence of Glutathione (anti-oxidant)

Question 9

Causes of transparency of the lens

Answer 9

• Avascular
• Non-innervated
• Uniform arrangement of fibers that are densly packed together

Question 10

Definition and causes of Cataract

Answer 10

Loss of the lens transparency by:
1. Glutathione deficiency
2. Denaturation/Coagulation of lens proteins
3. Senile Cataract (most common): DM, frequent exposure to UV rays, increase temperature

Question 11

Treatement for Cataract

Answer 11

Surgical treatement

Question 12

Definiton of Accomodation

Answer 12

Increasing Dioptric Power of The eye Lens by increasing its Curvature (convexity), for Near vison

Question 13

Mechanism of Radical Muscle fibers of Cillary muscles in accomodation

Answer 13

1. Extend from Corneo-scleral Junction all the way to the choroids
2. Their insertion is behind the suspensory ligament’s insertion
3. So their contraction causes pulling of the Ligament’s Insertion foreward
4. Causing the ligaments to relax, and the lens to be more convex

Question 14

Mechanism of Circular Muscle fibers of Cillary muscles in accomodation

Answer 14

1. Surround the lens in ring fashion, thus acting as a Sphincter
2. when contracting, they create a smaller ring , which release the tension in the ligaments, causing the lens to be more convex

Question 15

Accomodation mainly affects …… surface of the lens

Answer 15

Anterior surface

Question 16

State changes that occur during Near response

Answer 16

1. Accommodation: to increases Dioptric power of the lens
2. Converegence of both eyes: by Medial recti muscles to focus the light waves onto fovea centralis of the retina
3. Myosis (pupil constriction): by constrictor pupillae Muscle, to guarantee that light waves pass through Nucleus (old fibers) of the lens

Question 17

Define Power of Accommodation

Answer 17

differences between the 2 powers of lens during Resting and Accommodation , it decreases by age due to decrease in elasticity

Question 18

Define the Near point

Answer 18

It’s the Shortest distance from the eye, where person can see clearly using maximum accommodation
* Young adult= 10cm
* 60 year old= 80 cm

Question 19

Define far point

Answer 19

the furthest point from the eye, which can be seen clearly without any accommodation
=6m (unchanged)

Question 20

Range of Accommodation

Answer 20

Distance between far point and near point, which decreases by age

Question 21

Enumerate types of errors of Refraction of the eye

Answer 21

1. Myopia (Near sightedness)
2. Hypermetropia (far sightedness)
3. prespyopia
4. Astigmatism

Question 22

Causes, optical changes & correction for Myopia

Answer 22

Cause
* Longer eyeball than usual
* abnormally High curvature of cornea or lens

Optical changes:
* parallel rays: Convergence of light in front of retina
* Near Point: less than 10 cm
* far point: less than 6m
* Accommodation: Aggravate (worsens) the myopia

Correction: concave lens

Question 23

Causes, Optical Changes & Correction for Hypermetropia (farsightedness)

Answer 23

Cause:
* Shorter Eyeball
* Low curvature (convexity) of lens or cornea

Optical Changes
* Parallel rays: light converges behind the retina
* Near point: Increases (more than 10cm)
* Far point: Normal (but under continuous accommodation/ continuous headache)
* Accommodation: Essential non stop

Correction: convex lens

Question 24

Causes, Optical Changes & Correction for prespyopia

Answer 24

Cause: decrease in power of lens due to decrease in elasticity by age

Optical Changes:
* Near point: increases (moves away)
* Far point: unchanged

Correction: convex lens

Question 25

Causes, Optical Changes & Correction for Astigmatism

Answer 25

Causes: Inequality of Corneal curvature

Optical changes:
* Power of lens: different at different axis, causing the person to try and focus vison on either horizontal plane or vertical plane

Correction: Cylindrical lens