Optic neuritis is a common feature of inflammatory diseases of the central nervous system such as multiple sclerosis and neuromyelitis optica. It may also reveal systemic inflammatory disorders including sarcoidosis, lupus, Sjögren's syndrome, Behçet's disease, or infections such as neurosyphilis or with Bartonella. In some patients diagnostic workup remains negative leading to the diagnosis of an idiopathic optic neuritis. A painful loss of vision is the usual clinical presentation. The fundoscopy is normal or shows papilloedema. Brain magnetic resonance imaging is a mandatory in all patients.