Eye 2

Question 1

Define refraction

Answer 1

bending of light by the lens and cornea

Question 2

Define accommodation

Answer 2

the change in shape of the lens

Question 3

When does refraction occur

Answer 3

the light rays ‘bend’ at the junction between the 2 transparent substances of different densities - light rays refracted at the cornea so they come into exact focus on the retina

Question 4

Describe what happens when an object is 6m or more away from the viewer

Answer 4

the light rays reflected from the object are nearly parallel to one another so the lens must refract these parallel rays just enough so they fall exactly focused on the fovea centralis, where vision is sharpest

Question 5

Describe what happens with objects closer than 6m

Answer 5

the light rays are divergent rather than parallel so the rays must be refracted more if they are to be focused on the retina - this additional refraction = accommodation

Question 6

Describe accommodation

Answer 6

when focusing on a close object = the lens is more curved causing greater refraction of light rays - this increase in curvature of lens for near vision is accommodation

Question 7

How is accommodation achieved

Answer 7

by relaxation or contraction of the ciliary muscle, thus relaxing/contracting the zonular fibres/suspensory ligaments - lens become more spherical or flat accordingly
Oculomotor (CNIII) nerve innervates ciliary muscle and is therefore responsible for accommodation

Question 8

Describe the first part of the visual pathway

Answer 8

visual input travels via the neurons in the retina of one eye, these neurons exit at the optic disc and from the optic nerve on that side, the optic nerves from each eye cross at the optic chiasm

Question 9

Describe what happens to the various neurons at the optic chiasm

Answer 9

neurons from the nasal side of the retina cross to the opposite side of the brain, neurons from the temporal side remain uncrossed, the pathway is now known as the optic tract

Question 10

Describe the second part of the visual pathway

Answer 10

the optic tract enters the brain and terminates in the thalamus, the neurons in the optic tract synapse with neurons that forms the optic radiations - these project to the primary visual areas in the occipital lobes of there cerebral cortex and visual perception begins

Question 11

Describe what is meant by a visual field

Answer 11

refers to the total area in which objects can be seen in the peripheral vision while you focus your eyes on a central point

Question 12

Describe our visual fields

Answer 12

because our eyes are located anteriorly in our eyes, these visual fields overlap, the visual field of each eye is divided into 2 regions = nasal half and the temporal (peripheral) half

Question 13

Describe what happens with the light rays in the visual fields

Answer 13

light rays from an object in the nasal half of the visual field fall on the temporal half of the retina
light rays from an object in the temporal half of the visual field fall on the nasal half of the retina

Question 14

Describe the visual information from each visual field

Answer 14

visual info. from the right half of each individual field is conveyed to the left side of the brain
visual info. form the left half of each visual field is conveyed to the right side of the brain

Question 15

PATHOLOGY - GRAVES DISEASE

Answer 15

most common form of hyperthyroidism - antibodies mimic the action of TSH continually stimulating the thyroid gland to produce thyroid hormones
SYMPTOMS include - oedema behind the eyes - exophthalmos - causes the eyes to protrude

Question 16

PATHOLOGY - ORBITAL CELLULITIS

Answer 16

bacterial orbital cellulitis is a medical emergency - if not treated urgently = may lead to intracranial access, blindness or death
acute sinusitis is most common source infection, trauma and lid infections may also lead to infection
MANAGEMENT - immediate antibiotics, blood cultures and CT scan, surgical drainage may be required

Question 17

ORBITAL IMAGING TECHNIQUES - CT
3 +
2 -

Answer 17

\+ bony detail/calcifications
\+ fast scan times
\+ spatial resolution
- radiation induced cataracts
- beam hardening artefacts from dental fillings

Question 18

ORBITAL IMAGING TECHNIQUES - MRI
1 +
2 -

Answer 18

+ no radiation

• long scan time
• must screen for metallic foreign bodies in orbit before MRI

Question 19

ORBITAL IMAGING TECHNIQUES - US

all +

Answer 19

+ demonstrate the structures of the eye and orbit
+ perform measurements of the eye ensuring they are in normal range
+ can demonstrate haemorrhage, tumour, damaged tissue or injuries in bony orbit, foreign bodies, retinal detachment, swelling (inflammation)