Visual Pathway Lesions

The visual pathway runs from the retina through the optic nerve, optic chiasm (nasal fibres cross), optic tract, lateral geniculate nucleus, optic radiations (temporal Meyer loop and parietal fibres), to the primary visual cortex in the occipital lobe. Lesion location is inferred from the pattern of visual field loss, pupil responses, and associated signs.

• Causes by pathology: vascular (ischaemic stroke, haemorrhage, aneurysm, arteriovenous malformation), neoplastic (pituitary adenoma, meningioma, craniopharyngioma, glioma, metastasis), demyelination (multiple sclerosis, neuromyelitis optica), inflammatory and infectious (sarcoid, abscess, toxoplasmosis, tuberculosis), traumatic, hydrocephalus, congenital anomalies, and degenerative.
• Lesions anterior to the chiasm give monocular loss; chiasmal lesions give bitemporal loss; lesions posterior to the chiasm give homonymous defects, which are the same side of the field in both eyes.

• Optic nerve: monocular visual loss, central scotoma, relative afferent pupillary defect, dyschromatopsia; junctional scotoma (nerve and contralateral superotemporal loss) at the optic nerve-chiasm junction.
• Chiasm: bitemporal hemianopia (upper quadrants first for lesions from below such as pituitary adenomas; lower quadrants first for lesions from above such as craniopharyngioma), see-saw nystagmus, and endocrine and headache features.
• Optic tract: incongruous homonymous hemianopia, contralateral relative afferent pupillary defect (the eye with the temporal field loss), and band atrophy of the contralateral optic disc (late).
• Lateral geniculate nucleus: congruous homonymous defects, sectoranopias.
• Optic radiation: temporal lobe (Meyer loop) lesions give contralateral superior quadrantanopia (pie in the sky) often incongruous, with memory and seizure features; parietal lobe lesions give contralateral inferior quadrantanopia (pie on the floor) with neglect, optokinetic nystagmus asymmetry, and apraxia.
• Occipital cortex: congruous homonymous hemianopia with macular sparing (posterior cerebral artery occlusion), checkerboard defects from bilateral lesions, normal pupil responses, and a normal fundus.
• Pupils react normally in postgeniculate lesions because the pupillary fibres leave the tract before the geniculate nucleus.
• Associated symptoms: headache, hemiparesis, sensory loss, aphasia, alexia, visual hallucinations, and seizures.

• Visual acuity, colour vision, pupils (swinging flashlight test), confrontation fields, dilated fundus examination, and optical coherence tomography (ganglion cell loss in bitemporal and homonymous patterns).
• Formal perimetry: Humphrey 24-2 and 30-2 or Goldmann kinetic perimetry; evaluate congruity (high congruity points to posterior lesions) and macular sparing; repeat for reliability.
• Imaging: MRI brain with contrast and thin cuts through the sella and chiasm, optic tract and occipital regions (diffusion-weighted imaging for stroke); MR or CT angiography for aneurysm and vascular lesions; CT for acute haemorrhage.
• Endocrine tests for sellar lesions; blood tests including ESR, CRP, aquaporin-4 and myelin oligodendrocyte glycoprotein antibodies; lumbar puncture when demyelination or infection suspected; EEG when seizures.
• Visual evoked potentials as an adjunct.

• Treat the cause urgently:
  • Acute ischaemic stroke: intravenous alteplase 0.9 mg/kg (maximum 90 mg) within 4.5 hours, thrombectomy where indicated, aspirin 300 mg, statin, and blood pressure management.
  • Haemorrhage: neurosurgical evaluation; aneurysm coiling or clipping.
  • Pituitary tumour: cabergoline for prolactinoma, transsphenoidal surgery, hydrocortisone for apoplexy (100 mg IV, then 50 mg every 6 hours).
  • Meningioma and glioma: surgery and radiotherapy.
  • Multiple sclerosis: methylprednisolone 1 g IV daily for 3 to 5 days.
  • Abscess: antibiotics and drainage.
• Visual rehabilitation: compensatory scanning training, prismatic field expansion, reading strategies, visual aids, mobility training, and occupational therapy.
• Driving: patients with homonymous or bitemporal field loss generally cannot meet the visual requirements for driving; inform and advise them, and record the advice.
• Follow-up with serial fields and optical coherence tomography to monitor recovery or progression.
• Psychological support and falls prevention in the elderly.