Swelling of the optic nerve due to increased intracranial pressure

Aetiology

• Caused by raised intracranial pressure
• The ICP is the sum of the brain, blood and CSF
• The sum must remain constant - increase in one variable will result in a decrease of one/both other variables (Monro-Kellie hypothesis) as cranium is rigid and cannot expand

Space-occupying lesion

• When a mass expands within the skull, compensatory mechanisms initially maintain a normal intracranial pressure
• Eventually further small increments in volume produce larger and larger increments in intracranial pressure
• Further increases in volume cause blood vessels to be compressed, ultimately causing global brain ischaemia/swelling with herniation through foramen magnum, brainstem compression and death

Problems with cerebral blood flow

• e.g. malignant hypertension - mechanism behind disc swelling poorly understood but may involve leakage and ischaemia of arterioles supplying optic disc, leading to swelling/haemorrhage around disc

Problems with CSF

• Obstruction to CSF circulation e.g. due to congenital malformation
• Overproduction of CSF e.g. due to a tumour
• Inadequate absorption e.g. due to haemorrhage

Idiopathic intracranial hypertension

• Common cause of bilateral disc swelling in young females
• Mechanism of disc swelling not fully understood but theories suggest obstruction of CSF circulation or impairment of absorption

Pathophysiology

• Optic nerve is extension of the brain (with meningeal sheaths)