Optic Disc & Visual Field Assessment Flashcards
What is the optic disc?
The site where the optic nerve cell axons accumulate and exit the globe
How many nerve fibres make up the optic nerve?
Does this change depending on disc diameter?
~1.2 million
No
Normal optic disc diameter usually ranges from ~X-Ymm?
1-2.5mm
What is the biggest influencer of optic disc diameter?
Refractive power
How does refractive power influence optic disc diameter?
People with myopia (short-sightedness) tend to have a larger optic disc
People with hyperopia (long-sightedness) tend to have a smaller optic disc
What are the 3 C’s to assess when examining the optic disc/nerve on fundoscopy?
Contour
Colour
Cup
How is the contour of the optic disc assessed?
The border of the optic nerve should be clear and well-defined
What are the two main causes of poor contour when viewing the optic disc?
- Swollen disc e.g., in inflammation, raised IOP or papilloedema
- Drusen deposits in optic nerve obscuring the contour
What is papilloedema?
Disc swelling secondary to raised intracranial pressure
How is the colour of the optic disc assessed?
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
What is the optic cup?
The opening in the sclera (aka. the scleral canal) where the optic nerve exits the globe
How is the optic cup assessed?
The optic cup should take up 1/3rd of the optic disc
What does an increased optic cup size and/or a paler coloured optic disc suggest?
A decrease in the quantity of healthy neuro-retinal tissue
Describe the appearance of an eye with glaucoma when examining the optic disc
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
Describe the appearance of a swollen disc e.g., due to inflammation, increased IOP or papilloedema (raised ICP)
CONTOUR: blurring/loss of disc margins
COLOUR: pale discs with red areas of haemorrhage
CUP: Not visible
Describe the appearance of an eye with disc drusen when examining the optic disc
CONTOUR: indistinct/irregular due to calcified cell deposits
How can optic disc drusen be distinguished from papilloedema?
Optic disc drusen are auto-fluorescent when special camera filters are applied on fundoscopy
What causes the optic disc to become paler than normal?
Death of optic nerve fibres and loss of healthy neuro-retinal tissue
Suggest some pathologies that can cause paling of the optic disc
Ischaemia Direct compression e.g., by a lesion Optic neuritis (inflammation) Toxins Hereditary
Why are optic nerve fibres unmyelinated?
Because myelination would obstruct photoreceptors and result in blind spots in the vision
Myelinated optic nerve fibres are seen in some congenital conditions. How does this appear down the fundoscope?
As grey-white well-demarcated patches with frayed borders, surrounding the contour of the optic disc
How are a patient’s visual fields assessed in the clinic?
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
What is meant by the terms... Homonymous Incongruous Hemianopic Quadrantanopic ...?
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
What visual field defect is caused by optic nerve pathology?
Complete loss of vision in one eye
What visual field defect is caused by pathology at the optic chiasma?
Bitemporal hemianopia
vision loss in both temporal fields
What visual field defect is caused by optic tract pathology?
Right optic tract: left hemianopia
Left optic tract: right hemianopia
(as fibres from the nasal retina cross over at the optic chiasma)
What visual field defect is caused by pathology in the primary visual cortex?
Right PVC: left macula-sparing hemianopia
Left PVC: right macula-sparing hemianopia
What pathologies most commonly cause defects in the... Optic nerve Optic chiasma Optic tract Primary visual cortex
Optic nerve: ischaemia or optic neuritis
Optic chiasma: pituitary tumour
Optic tract: tumours
Primary visual cortex: vascular disease e.g., stroke
