Introduction
The optic nerve runs from the back of the orbit and runs postero-medially through the optic canal to the optic chiasma. At the chiasm, partial decussation (crossing) of fibres takes place from the nasal retinal fibres of each eye. Fibres then travel from the chiasm to the lateral geniculate nucleus in the thalamus and then to the visual cortex in the occipital lobe.
The optic nerve (CN II) is a sensory nerve, which carries visual information from the retina to the visual cortex.
Signs of an optic nerve palsy depend on the site of the lesion:
- Lesions distal to the optic chiasma result in loss of vision in the ipsilateral eye (Fig 1)
- Lesions at the optic chiasm(a) cause bitemporal field loss or bitemporal hemianopia (Fig 2)
- Lesions proximal to the optic chiasma result in loss of vision on one side but affecting both eyes (Fig 3)
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| Fig 1: Vision loss in the right eye | Fig 2: Bitemporal haemianopia | Fig 3: Homonymous haemianopia occurring on the right |
Testing
Ask the patient if they have any problems with their vision.
Check visual acuity with a Snellen or LogMAR chart
Test pupillary response by:
- Testing direct
- Consensual pupillary response
- Swinging light test – a test for RAPD (relative afferent pupillary defect)
- Testing accommodation
Test visual fields with confrontation and look for a scotoma.
Assess the optic disc using fundoscopy.
CT head should be requested if an orbital fracture is suspected, or if there is a penetrating injury to the eye.
If there is no history of trauma, CT head is used to look for pituitary tumours, cerebrovascular events or space-occupying lesions.
When optic neuritis is suspected, screening for infection (e.g. mumps and measles) and inflammatory markers (FBC, CRP) should be performed.
Enquire about toxins (methanol, ethylene glycol) where optic neuritis is suspected.
MRI scanning is preferable to CT scanning in optic neuritis as it is better able to demonstrate demyelination.
The optic nerve is the nerve most commonly damaged in ocular injuries associated with major trauma [2]. Even so, it is relatively rare.
Two-thirds of optic nerve injuries are associated with facial fractures. Damage is usually caused by fractures passing through the orbit, but can sometimes be due to sustained raised intracranial pressure. The image shows a supraorbital fracture (click on the image to enlarge).
The incidence of optic nerve injury is 2.1% in patients with facial fractures (TARN database).
The most common cause of painful loss of vision is optic neuritis associated with multiple sclerosis. Infections such as measles and mumps can, rarely, cause optic neuritis.
Optic nerve compression following orbital cellulitis has been described by Armstrong and Nichol [3].
Ethambutol, methanol, and ethylene glycol can cause optic nerve toxicity.
Other causes are glaucoma and ocular tumours compressing the optic nerve.
Compression of the optic chiasm can be caused by:
- Tumour (pituitary adenoma, craniopharyngioma)
- Aneurysm of the internal carotid artery
- Ischaemic neuropathy (diabetes, hypertension, vasculitis)
Causes proximal to the chiasm include:
- Cerebrovascular events
- Brain tumours