What work-up do you recommend for optic nerve edema in a patient who is immunocompromised?

Symptoms (is visual loss present or not?) and time of onset/pace help to direct the workup. For unilateral optic nerve edema with vision loss, optic neuritis and NAION are always high on the differential. In immunocompromised patients, an infectious cause such as herpes zoster, syphilis, TB, and art...