Second Cranial Nerve Lesions

This chapter refers specifically to pathology of the peripheral and central optic nerve pathways that result in visual field defects.

Anatomy

Course of the Optic Nerve

• The optic nerve leaves the orbit via the optic canal of the bony orbit, accompanied by the ophthalmic artery.
• The nerve is surrounded by the three meningeal layers, including the subarachnoid space.
• The two optic nerves meet at the optic chiasma, which sits on the diaphragma sellae, anterior to the pituitary stalk.

At the chiasma:

• Nasal fibres (carrying temporal field information) cross over.
• Temporal fibres (carrying nasal field information) stay lateral and pass back in the optic tract on the same side.

Beyond the chiasma:

• The optic tracts curve around the lateral side of the midbrain and divide into medial and lateral roots.
• The lateral roots synapse in the lateral geniculate bodies.
• Second-order neurones from the lateral geniculate bodies pass back in the optic radiations to the visual cortex:
  • Lower temporal lobe fibres → upper visual fields.
  • Upper parietal lobe fibres → lower visual fields.

Other fibres bypass the lateral geniculate bodies, entering the superior colliculus and relaying to the oculomotor and Edinger-Westphal nuclei — these are involved in ocular and pupillary reflexes.

Optic Nerve Innervations

• The optic nerve carries purely afferent special sensory neurones conveying impulses caused by photon capture by the retina.

Pathology

Lesions affecting the optic neural pathways will produce visual field defects.

Visual Defect	Cause / Site
Concentric diminution of visual field (tunnel vision)	Glaucoma, Papilloedema
Central scotoma	Demyelination, Nutritional, Microvascular, Toxic
Irregular scotoma	Retinal tears, Intraocular lesions (e.g. vitreous haemorrhage)
Total unilateral visual loss	Unilateral eye disease, Optic nerve disease
Bitemporal hemianopia	Lesions of the optic chiasm (e.g. pituitary tumour, sella meningiomas)
Binasal hemianopia	Rare. Bilateral lesions affecting uncrossed fibres around chiasma (e.g. carotid siphon atheroma)
Homonymous hemianopia	Lesions of optic tracts or optic radiations. Defect nature depends on site: – Optic tract lesion → complete hemianopia without macular sparing. – Posterior optic radiation lesion → macular sparing if ischaemic cause.
Homonymous quadrantanopia	Lesions of optic tracts or radiations. – Temporal lobe lesion → upper quadrantanopia. – Parietal lobe lesion → lower quadrantanopia.

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Hayesian Visual Field Defect Chart

A view of Paris demonstrating visual field defect changes

Key	Visual Field Defect
A	…with a right sided complete optic nerve lesion
B	…with tunnel vision of the right eye
C	…with a lesion of the central optic chiasm, (bilateral hemianopia)
D	…with a right homonymous hemianopia
E	…with an upper right quadrantic hemianopia
F	…with a lower right quadrantic hemianopia
G	…with a scotoma of the right eye
H	…with an enlarged blind spot of the right eye

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Investigations

Ultrasound

B-scan ultrasound of the eye is useful when direct fundoscopic visualisation is difficult or impossible (e.g. hyphemas, corneal changes, cataracts).

• Detects vitreous haemorrhages or retinal detachments.

CT Scan / CT Angiogram

• CT: Good screening test for intracranial mass lesions.
• CT Angiogram: Done for suspected aneurysmal disease.

MRI

MRI is the best imaging modality for:

• Intracranial, ocular, and orbital space-occupying lesions.
• More sensitive and specific than CT.
• Distinguishes intrinsic soft tissue lesions of the eye and neural pathways not seen on CT.

Management

Management depends on the underlying cause of the lesion.

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References

Publications

• Brazis PW, Masdeu JC, Biller J. Localization in Clinical Neurology. 8e 2021
• Fuller G. Neurological Examination Made Easy. 6e 2019
• O’Brien M. Aids to the Examination of the Peripheral Nervous System. 6e 2023

FOAMed

• Coni R. Neuro 101: Cranial Nerves. LITFL
• Nickson C. Cranial nerve lesions DDx. LITFL