Compression of the optic nerves with or without decreased visual function may result from increased intracranial pressure (ICP). There may be an identifiable cause of the increased ICP (i.e. brain tumor) or it may be idiopathic pseudo-tumor cerebri. The optic nerves may not actually be swollen but appear swollen due to an anatomic variant-pseudopapilledema. The optic nerve may also appear swollen because the eye is hypotonous, allowing the ICP to exceed the intraocular pressure (IOP) (Fig. 5.1). Inflammation of the optic nerve itself may result in optic disc swelling, most always accompanied by decreased visual function (papillitis: see Chapter 4).