The history given by you is not complete. Fluid behind the eye do you mean something in orbit or a central nervous problem.it appears that probably the optic nerve
sheath is somewhat showing fluid which can occur in case where the intracranial pressure If increased is seen as fluid in optic nerve sheath , which can be measured as optic nerve sheath diameter on MRI
. CSF tap ie taking the spinal fluid will reduce the pressure and swelling and also give sample to investigate it for any probable cause.