Optic Disc & Visual Field Assessment

Question 1

What is the optic disc?

Answer 1

The site where the optic nerve cell axons accumulate and exit the globe

Question 2

How many nerve fibres make up the optic nerve?

Does this change depending on disc diameter?

Answer 2

~1.2 million

No

Question 3

Normal optic disc diameter usually ranges from ~X-Ymm?

Answer 3

1-2.5mm

Question 4

What is the biggest influencer of optic disc diameter?

Answer 4

Refractive power

Question 5

How does refractive power influence optic disc diameter?

Answer 5

People with myopia (short-sightedness) tend to have a larger optic disc
People with hyperopia (long-sightedness) tend to have a smaller optic disc

Question 6

What are the 3 C’s to assess when examining the optic disc/nerve on fundoscopy?

Answer 6

Contour
Colour
Cup

Question 7

How is the contour of the optic disc assessed?

Answer 7

The border of the optic nerve should be clear and well-defined

Question 8

What are the two main causes of poor contour when viewing the optic disc?

Answer 8

• Swollen disc e.g., in inflammation, raised IOP or papilloedema
• Drusen deposits in optic nerve obscuring the contour

Question 9

What is papilloedema?

Answer 9

Disc swelling secondary to raised intracranial pressure

Question 10

How is the colour of the optic disc assessed?

Answer 10

The optic disc should look like an orange-pink donut with a pale centre

The orange-pink part represents healthy, well-perfused neuro-retinal tissue
The pale centre is called the optic cup and it is pale because it is devoid of neuro-retinal tissue

Question 11

What is the optic cup?

Answer 11

The opening in the sclera (aka. the scleral canal) where the optic nerve exits the globe

Question 12

How is the optic cup assessed?

Answer 12

The optic cup should take up 1/3rd of the optic disc

Question 13

What does an increased optic cup size and/or a paler coloured optic disc suggest?

Answer 13

A decrease in the quantity of healthy neuro-retinal tissue

Question 14

Describe the appearance of an eye with glaucoma when examining the optic disc

Answer 14

Contour: usually normal, may find peripapillary atrophy (paler area) in later disease

Colour: usually normal (unless enlarged cup takes up majority of space, making the whole optic disc look pale)

CUP: enlarged cup, indicates death of nerve fibres and loss of healthy neuro-retinal tissue due to the increased pressure around/lack of blood supply to the optic nerve

Question 15

Describe the appearance of a swollen disc e.g., due to inflammation, increased IOP or papilloedema (raised ICP)

Answer 15

CONTOUR: blurring/loss of disc margins

COLOUR: pale discs with red areas of haemorrhage

CUP: Not visible

Question 16

Describe the appearance of an eye with disc drusen when examining the optic disc

Answer 16

CONTOUR: indistinct/irregular due to calcified cell deposits

Question 17

How can optic disc drusen be distinguished from papilloedema?

Answer 17

Optic disc drusen are auto-fluorescent when special camera filters are applied on fundoscopy

Question 18

What causes the optic disc to become paler than normal?

Answer 18

Death of optic nerve fibres and loss of healthy neuro-retinal tissue

Question 19

Suggest some pathologies that can cause paling of the optic disc

Answer 19

Ischaemia
Direct compression e.g., by a lesion
Optic neuritis (inflammation)
Toxins
Hereditary

Question 20

Why are optic nerve fibres unmyelinated?

Answer 20

Because myelination would obstruct photoreceptors and result in blind spots in the vision

Question 21

Myelinated optic nerve fibres are seen in some congenital conditions. How does this appear down the fundoscope?

Answer 21

As grey-white well-demarcated patches with frayed borders, surrounding the contour of the optic disc

Question 22

How are a patient’s visual fields assessed in the clinic?

Answer 22

In each of the below, compare patient’s answers to when you can see the objects

Assess for…
- Gross visual defects: ask if any part of your face is missing when patient looks at you, hold up two hands and ask if patient can see them to assess for hemianopia

• Quadrant visual defects: ask patient to cover one eye, close your eye opposite theirs, ask patient to look into your open eye and present 1 or 2 fingers in each of the 4 quadrants, ask them to report back the number without looking away from your eye
• Subtle field vision loss: ask patient to cover one eye, close your eye opposite theirs, bring a fine white pinhead at a diagonal from the outer periphery of each quadrant towards the centre, ask the patient to report when the pin comes into their line of vision
• Examining blind spot: ask patient to cover one eye, close your eye opposite theirs, hold a fine red pinhead centrally, then move it slowly temporally until the patient’s blind spot is found, move it superiorly until the patient can see it again, then inferiorly until they can’t

Question 23

What is meant by the terms...
Homonymous
Incongruous
Hemianopic
Quadrantanopic
...?

Answer 23

Homonymous: same part of the field is affected in each eye

Incongruous: different sized defects are seen between eyes, but defects are in the same field

Hemianopic: half the field is affected

Quadrantanopic: a quarter of the field is affected

Question 24

What visual field defect is caused by optic nerve pathology?

Answer 24

Complete loss of vision in one eye

Question 25

What visual field defect is caused by pathology at the optic chiasma?

Answer 25

Bitemporal hemianopia

vision loss in both temporal fields

Question 26

What visual field defect is caused by optic tract pathology?

Answer 26

Right optic tract: left hemianopia
Left optic tract: right hemianopia

(as fibres from the nasal retina cross over at the optic chiasma)

Question 27

What visual field defect is caused by pathology in the primary visual cortex?

Answer 27

Right PVC: left macula-sparing hemianopia

Left PVC: right macula-sparing hemianopia

Question 28

What pathologies most commonly cause defects in the...
Optic nerve
Optic chiasma
Optic tract
Primary visual cortex

Answer 28

Optic nerve: ischaemia or optic neuritis
Optic chiasma: pituitary tumour
Optic tract: tumours
Primary visual cortex: vascular disease e.g., stroke