Cranial Nerve Palsy - II : Optic Nerve

Question 1

Anatomical Course of Optic Nerve.

Answer 1

1. Convergence of Retinal Ganglion Cells (receive impulses from photoreceptors).
2. Leaves bony orbit via Optic Canal through Sphenoid Bone.
3. Enters Cranial Cavity.
4. Runs along surface of Middle Cranial Fossa close to the Pituitary.
5. Unite to form Optic Chiasm.
6. Optic Tract - Corresponding Cerebral Hemisphere (Lateral Geniculate Nucleus).
7. Optic Radiation.

Question 2

What important event happens at the Optic Chiasm?

Answer 2

1. The nasal fibres of each retina cross over to the contralateral optic tract.
2. The temporal fibres remain ipsilateral.

Question 3

What are the Divisions of the Optic Radiation? (2)

Answer 3

1. Upper Optic Radiation : Superior Retinal Quadrants - Inferior Visual Field Quadrants to PARIETAL LOBE to VISUAL CORTEX.
2. Lower Optic Radiation : Inferior Retinal Quadrants - Superior Visual Field Quadrants to TEMPORAL LOBE to MEYER’s LOOP to VISUAL CORTEX.

Question 4

What is the significance of the anatomical course in the Middle Cranial Fossa?

Answer 4

A pituitary adenoma can cause compression to the optic chiasm, particularly affecting the fibres that are crossing over from the nasal half of each retina : BITEMPORAL HEMIANOPIA.

Question 5

Origin of Optic Nerve.

Answer 5

Cerebrum.

Question 6

Causes of Homonymous Hemianopia (3).

Answer 6

1. Incongruous Defect - Lesion of Optic Tract.
2. Congruous Defect - Lesion of Optic Radiation or Occipital Cortex.
3. Macular Sparing - Lesion of Occipital Cortex.

Question 7

Causes of Homonymous Quadrantanopia (2).

Answer 7

1. Superior : Lesion of Inferior Optic Radiation in Temporal Lobe (Meyer’s Loop).
2. Inferior : Lesion of Superior Optic Radiation in Parietal Lobe.

PITS.

Question 8

Causes of Bitemporal Hemianopia.

Answer 8

Lesion of Optic Chiasm :

1. Upper Defect > Lower Defect = Inferior Chiasm e.g. Pituitary Tumour.
2. Lower Defect > Upper Defect = Superior Chiasm e.g. Craniopharyngoma.