Q1 LONG ANSWER 10M ★ High-yield

Describe the anatomy of the optic nerve and visual pathway with the localisation of field defects.

Ophthalmology · 3rd Professional

Anatomy of the Optic Nerve and Visual Pathway

Visual pathway and the localisation of field defects

The visual pathway runs from the retina through the optic nerve, chiasma, tract, lateral geniculate body and optic radiation to the occipital (striate) cortex.
It is the only part of the central nervous system that can be seen directly.
The optic nerve is not a peripheral nerve but a tract of the brain: it is myelinated by oligodendrocytes, is invested in meninges with a subarachnoid space continuous with that of the brain, and cannot regenerate. Two consequences follow — it is affected by demyelinating disease, and raised intracranial pressure is transmitted directly to it, producing papilloedema
It carries 1.2 million axons, the axons of the retinal ganglion cells
The four parts:
Intraocular (1 mm) — the optic disc and the lamina cribrosa; the fibres are unmyelinated in front of the lamina and myelinated behind it.
Intraorbital (25 mm) — deliberately sinuous and longer than the 18 mm it must span, giving slack that permits free ocular movement and allows considerable proptosis before the nerve is stretched
Intracanalicular (6 mm) — within the optic canal, where the dura is fused to the periosteum. The nerve is tethered here, which makes it uniquely vulnerable to shearing in traumatic optic neuropathy and to compression by any swelling in a rigid canal
Intracranial (10 mm) — to the chiasma
Blood supply — the disc and prelaminar nerve by the short posterior ciliary arteries and the circle of ZINN-HALLER, which is why ischaemic optic neuropathy and giant cell arteritis affect the disc; and the intraorbital nerve by pial vessels and the central retinal artery

Structure	Features
Chiasma	The nasal (crossed) fibres decussate, 53% of the total; the temporal fibres remain uncrossed. Lies above the pituitary fossa, below the third ventricle and floor of the hypothalamus, with the internal carotid arteries laterally — and these relations determine the field defects of each tumour
Chiasmal position	Normal over the sella in 79%; prefixed (anterior, over the tuberculum) in 17%, so a pituitary tumour affects the tracts; and postfixed in 4%, so it affects the optic nerves
Optic tract	Carries fibres from the contralateral half-field. Lesions give an incongruous homonymous hemianopia, a contralateral relative afferent pupillary defect, and band (bow-tie) optic atrophy in the contralateral eye
Lateral geniculate body	Six layers: 1, 4 and 6 receive contralateral (crossed) fibres and 2, 3 and 5 ipsilateral. Layers 1 and 2 are magnocellular (motion, contrast) and 3 to 6 parvocellular (detail, colour)
Optic radiation — temporal	The inferior fibres sweep forward around the temporal horn as MEYER’S loop, carrying the superior visual field. A temporal lobe lesion therefore gives a superior quadrantanopia — "pie IN the sky"
Optic radiation — parietal	The superior fibres carry the inferior field, giving "pie ON the floor"; with asymmetrical optokinetic nystagmus
Occipital cortex	The macula is represented at the occipital pole and has a dual supply from the middle and posterior cerebral arteries — which is why posterior cerebral occlusion characteristically spares the macula

Localisation of Field Defects

Two rules govern all localisation, and together they answer most questions:
Rule 1 — the defect respects the vertical midline once the fibres have reached the chiasma, because the decussation divides the field into halves. A defect respecting the horizontal midline is retinal or optic nerve in origin, because retinal nerve fibres arch around the macula and do not cross the temporal raphe
Rule 2 — the more posterior the lesion, the more congruous the defect. In the tract the corresponding fibres are still far apart.

Site of lesion	Field defect
Retina or optic nerve	Monocular — a central or centrocaecal scotoma, an altitudinal defect (respecting the horizontal midline), or an arcuate scotoma
Junction of nerve and chiasma	A junctional scotoma — an ipsilateral central scotoma with a contralateral superotemporal defect, from involvement of the looping inferonasal fibres. It is of great value.
Chiasma (from below — pituitary adenoma)	Bitemporal hemianopia respecting the vertical midline, beginning in the superior quadrants because the tumour rises from below
Chiasma (from above — craniopharyngioma)	Bitemporal hemianopia beginning in the inferior quadrants
Optic tract	Incongruous homonymous hemianopia, with a contralateral RAPD and band atrophy
Temporal lobe (MEYER loop)	Superior homonymous quadrantanopia — "pie in the sky"
Parietal lobe	Inferior homonymous quadrantanopia — "pie on the floor"; with asymmetrical optokinetic nystagmus and inattention
Occipital cortex	Congruous homonymous hemianopia with macular sparing
Occipital pole	A congruous homonymous scotoma confined to the central field
Bilateral occipital	Cortical blindness with normal pupils and a normal fundus — the combination that identifies it

All questions in Optic Nerve and Neuro-ophthalmology

Every Ophthalmology chapter →