Approach to Visual Field Defects and CN2 Palsy

Question 1

CN2 - optic nerve function

Answer 1

1. Visual perception
2. Colour perception
3. Light reflex
4. Accommodation reflex

Question 2

CN2 - optic nerve testing

Answer 2

1. Visual acuity
- Test each eyes individually
- Can you see the numbers? use small numbers (1 to 3) (Do not use 5)

2. Colour
- Big red ball - (usually not done - quite pointless, only testing 1 colour)

3. Visual field
- Close 1 eye with hand, look at my eye (we may or may not need to close our own eye)
- Use big red ball
- From the side of quadrants all the way to the centre, ask:
1. Let me know when it appears
2. Inform me if disappears

4. Pupillary light reflex in dark room
- Shine from far away to look at both pupils - to look for any anisocoria
- Shine from the side then move closer, observe constriction

5. RAPD in dark room
- Affected eye no constriction
- Unaffected eye constriction, with affected eye consensual reflex constriction too

6. Wishlist- fundoscopy

Question 3

The anatomy and cell structures of the eye

Answer 3

1. Rods and cones form bipolar neurons and ganglion cells
   - Rods - rhodopsin - for low light condition
   - Cones - opsin - light adaptation, colour vision
2. Convergence of ganglion cells axons into optic nerve, which leaves via optic canal over sphenoid bone

Question 4

Visual pathway and visual defects

Answer 4

1. Optic nerve leaves optic canal towards optic chiasm (close to pituitary gland)
   –> Unilateral complete vision loss
2. Crossing over at optic chaism
   - Left CN2: left temporal (lateral), right nasal (medial)
   - Right CN2: right temporal, left nasal
   –> Bitemporal hemianopia
3. Travels to lateral geniculate nucleus in thalamus
   –> contralateral homonymous hemianopia
4. Continues at optic radiation (PITS)
   - Upper radiation: travels along parietal lobe to visual cortex (supplying inferior field)
   - Lower radiation: travels along temporal lobe (Meyers’ loop) to visual cortex (supplying superior field)
   –> Optic radiation defect - contralateral homonymous hemianopia
   –> Parietal lesion - contralateral bilateral inferior quadrantanopia (can be complete homonymous)
   –> Temporal lesion - contralateral bilateral superior quadrantanopia
5. Reaches occipital lobe visual cortex
   –> Occipital lobe lesion - contralateral homonymous hemianopia with macula sparing

Question 5

Bitemporal hemianopia
What are possible lesions of optic chiasm?
Additional features to look out?

Answer 5

Lesions of optic chiasm
1. Pituitary tumour or apoplexy
- Compresses from below affecting lower nasal (upper temporal field) first
2. Craniopharyngioma
- Compresses from above affecting upper lasal (lower temporal field) first
3. Suprasellar meningoma
4. Glioma
5. Granuloma
6. Metastasis

Additional features
1. Pituitary tumour: acromegaly, Cushing, hypopituitarism, gynaecomastia
2. Sarcoidosis, tuberculosis
3. Metastasis and primary malignancy

Question 6

Homonymous quadrantanopia
Superior/upper - temporal cortex
Inferior/lower - parietal cortex

What are the possible lesions over the cortex?
Additional features to look out?

Answer 6

Lesions over the cortex
1. Ischaemic or haemorrhagic stroke
2. Intracranial tumour
3. Trauma
4. Surgery

Additional features
1. Craniotomy scars opposite to side of visual defect - previous trauma or surgery
2. Hemiparesis (ipsilateral to visual defect)
3. Dysphasia

Question 7

Unilateral nasal visual loss
Unilateral complete visual loss

Answer 7

Unilateral nasal visual loss
1. Glaucoma
2. Retinal disease
3. Lateral chiasmal compression

Unilateral complete visual loss
1. Optic neuropathy
2. Ischaemia - CRAO, CRVO
3. Retinal detachment
4. Glaucoma
5. Uveitis
6. Maculopathy (often bilateral)
7. Other CN involvement - venous sinus thrombosis, orbital apex, etc

Question 8

Ipsilateral complete loss of vision with contralateral quadrantanipia

Answer 8

Front of optic chiasm lesion
- Affects ipsilateral optic nerve and crossing fibres that loop forwards before turning back into optic tract, usually from upper temporal field

Question 9

Definition of “homonymous”

Answer 9

Identical pattern of visual field defect over both eyes

Question 10

What is Foster-Kennedy syndrome?

Answer 10

Ipsilateral optic atrophy due to compression of optic nerve
Contralateral papilloedema due to raised intracranial pressure

Causes: tumour on inferior surface of frontal lobe
- Olfactory grove meningioma
- Medial third sphenoid wing meningioma

Question 11

What is a scotoma?
What is scintillating scotoma?

Answer 11

Small area of visual loss within visual field
Classified as peripheral or central

Peripheral cause: chorioretinal lesions
Central cause: macular, optic nerve disease

Scintillating scotoma - visual aura preceding migraine, spot of flickering light in centre of visual field expanding into shimmering arcs of white or coloured flashing light