The mechanisms by which optic nerve-sheath fenestration is effective remain speculative. Possibilities include surgical production of a cerebrospinal fluid filtration outlet or scarring in the subarachnoid space around the nerve in the vicinity of the fenestration site, with shifting of the pressure gradient from the nerve head to the retrobulbar portion. Two patients who underwent successful optic nerve-sheath fenestration developed a cystlike structure contiguous to the fenestration site, apparently in direct communication with the optic nerve sheaths. This was shown on magnetic resonance imaging (one patient) and orbital echography (both patients). These previously unreported observations may support the hypothesis that fenestration works by creating a filtration apparatus that controls the intravaginal pressure in the subarachnoid space surrounding the orbital segment of the optic nerve.