Cranial Nerve 2: Optic Nerve

1.The optic nerve leaves via the optic foramen and forms the optic nerve where it passes posteriorly to form the optic chiasm.

2.Post the optic chiasm all lesions will be contralateral, and form the optic tract (consists ipsilateral temporal and contra-lateral nasal fibres) and synapse at the lateral geniculate body in the thalamus.

3.The optic radiation passes deep in the parietal lobe and temporal lobe (via Meyer’s loop) and ends in the calcarine cortex in the occipital lobe.

 

Clinical Lesion: Central Scotoma

Classically with on optic nerve lesion typically present with this pathology:

1. Pain on ocular movements
2. Reduced visual acuity especially colour vision
3. RAPD- Relative Afferent Pupillary Defect
4. Central or Para-Central Scotoma

Causes of Optic Nerve Lesions

1. Multiple Sclerosis
2. Aneurysm- opthalmic artery or A.Comm Aneurysm
3. Optic Canal Meningioma’s e.g skull base lesions

 

Clinical Lesion: Bitemporal Hemianopia

Involvement of the Optic Chiasm. Involvements of the upper quadrants first indicate chiasmal compression from below, and vice versa

Compression Below:

1. Pituitary Adenoma
2. Nasopharyngeal carcinoma eroding superiorly
3. Sphenoid sinus mucocele
4. Internal Carotid Artery Aneurysm

Compression Above:

1. Craniopharyngioma’s
2. 3rd Ventricular Tumour

Clinical Lesion: Homonymous Hemianopia

Occurrs with lesions within the optic tract. Note lesions here will be provide loss of vision on the contralateral field of vision as the tracts have crossed the optic chiasm. For example the right optic tract defects produces loss of field of vision on the left side of both eyes as highlighted in D.

Vascular Causes being the most predominant cause of this lesion.

Clinical Lesion: Superior and Inferior Quadrantinopia’s

This diagram represents this beautifully. Fibres conveying vision in the superior field pass via Meyer’s loop deep in the temporal lobe before synapsing on the inferior aspect of the occipital lobe. The opposite is that of the parietal lobe

Therefore lesions in the temporal lobe may produce superior quadrantinopia’s (TOP in Temporal) and parietal lobe lesions (inferior quadrantinopia’s)

Intrinsic lesions namely tumours, abscesses may produce lesions as of this sort, or vascular events

Clinical Lesion: Homonymous Hemianopia with Macula Sparing

Lesions in the calcarine cortex produce an homonymous hemianopia with macular sparing if the lesion is due to a vascular cause. This is due to the fact that nerve fibres supplying the macula have a dual blood supply from the middle cerebral artery and the posterior cerebral artery.

 

Examining the visual fields via confrontation is an important part of any medical school OSCE therefore learning this properly is essential to passing any neurological station in the exam.