Visual Field Defects

Question 1

Describe the neurological pathway for vision

Answer 1

1. The photoreceptors (rods and cones) in the retina are stimulated by photos of light entering the eye
2. The light sensitive surface membrane proteins are stimulated to convert light energy into electrical signals
3. Photoreceptors synapse with retinal bipolar cells that transmit these signals to retinal ganglion cells
4. The retinal ganglion cells converge at the optic disc to form the optic nerve
5. The optic nerve exists the eye and travels through the lamina cribrosa of the sclera
6. The optic nerve enters the middle cranial fossa through the optic canal
7. The left and right optic nerves converge at the optic chiasm
8. At the chiasm, fibres from the nasal aspect of each retina cross over (decussate) to the contralateral optic tract. Fibres from the temporal retina remain on their respective sides
9. The optic tracts synapse at the lateral geniculate nucleus in the thalamus
10. Optic radiations loop through either the parietal or temporal lobe
    
    1. Radiations travelling through the parietal lobe correspond to the upper half of the retina/lower visual field, while the radiations travelling through Meyer’s loop in the temporal lobe correspond to the bottom half of the retina/upper visual field.
       Optic radiations terminate in the calcarine sulcus of the occipital lobe, where the cortical visual centre is situated

Question 2

What is the blood supply to the visual pathway

Answer 2

Optic nerve: Ophthalmic artery
Optic chiasm: internal carotid, anterior/posterior communicating, ACA
Optic tract: MCA
Lateral geniculate nucleus: MCA
Optic radiation: MCA
Calcarine sulcus: PCA

Question 3

What are the causes of a unilateral monocular field loss

Answer 3

Lesion in the optic nerve (pre-chiasmal):
Glaucoma
Optic neuritis
Amaurosis fugax
Optic atrophy
Retrobulbar optic neuropathy
Trauma

Question 4

What are the causes of a bitemporal hemianopia

Answer 4

Compression/lesion at the optic chiasm:
Pituitary masses
Craniopharyngiomas
Internal carotid aneurysms

Question 5

What are the causes of homonymous hemianopia

Answer 5

Compression or lesion at the optic tracts (post-chiasmal):
SOL (cancer, abscess, cyst)
Stroke,
MS

Question 6

What are the causes of scotoma

Answer 6

optic nerve inflammation:
Optic nerve compression (neuritis)
MS (+ red colour blindness)
Alcohol, B12 deficiency

Question 7

What causes macular-sparing homonymous hemianopia

Answer 7

Occipital lesions

macula sparing occurs due to the dual blood supply to the anterior portion of the visual centre

Question 8

Wha are the causes of quadrantanopias

Answer 8

Upper: Optic radiation lesions at the Meyers loop in the temporal lobe

Lower: Optic radiation lesions at the parietal lobe

PITS - P - parietal I - inferior; T - temporal S - superior

Question 9

What are the causes of an Ipsilateral monocular nasal hemianopia

Answer 9

Optic chiasm (lateral)

Distension of the 3rd ventricle
Internal carotid/posterior communicating artery atheroma