Response

Abstract

We thank Dr. Vaphiades for his interest in our article and agree that the etiology of optic nerve enhancement may be difficult to distinguish based on magnetic resonance imaging (MRI) findings alone. With an optic nerve sheath meningioma (ONSM), the tumor typically shows gadolinium enhancement rather than the nerve itself, giving rise to the typical tram-track sign, while with radiation optic neuropathy (RON), the optic nerve itself enhances and may therefore be indistinguishable from the enhancement of the surrounding ONSM. In our patient, we noted several features that supported the diagnosis of RON. Radiographically, the area of new optic nerve enhancement corresponded to the exact location of the highest proton beam radiation dose. Although the optic nerve showed normal short T1 inversion recovery signal at the time of the initial ONSM diagnosis, it showed increased signal with proximal enhancement at the time the patient presented with visual loss, which began 27 months after completion of radiation therapy. Diffuse enhancement of the nerve was noted such that it could not be separated from the surrounding sheath, as opposed to clear tram-track enhancement of the remaining distal nerve. Clinically, it would be unusual for an ONSM to grow rapidly, causing progressive visual loss over a period of days to weeks, and then remain stable for the next year, as in our patient. The onset of symptoms in our patient likewise fell within the typical time course for RON. We hope that these details help to further clarify the diagnosis of RON in our patient. The above findings were included in our original submission but were then omitted through the editing process.