Is primary optic nerve sheath schwannoma a misnomer? Report of two cases and literature review.

Aims: To report clinicopathological characteristics of two patients with optic nerve sheath schwannoma (ONSS) and review the literature. Method: The first patient (22-year-old man) presented with left eye proptosis and decreased vision in 2012 whose orbital imaging showed a large cystic lesion around the optic nerve. The second patient (52-year-old man) presented with decreased vision in the left eye (without proptosis) in the 2006. His imaging showed a small orbital apex lesion between the medial rectus and optic nerve. Both lesions were histopathologically consistent with ONSS. ONSS has previously been reported in 12 patients. Results: Orbital biopsy and subsequently external beam radiotherapy were performed for the first patient who showed a temporary improvement of vision. However, his proptosis progressed and vision decreased to light perception (LP) a few months after radiotherapy when the lesion was resected in April 2013. Second patient declined any procedure until his vision gradually decreased to LP in October 2007. It was then removed through a combined medial and lateral orbitotomy procedure. Both patients ended up with visual acuity of no LP and no recurrence in their last follow up visits in April 2018. No histopathological evidence was found to show that optic nerve sheath could be the origin for presenting and previously reported cases in the literature. Conclusion: Presenting cases and literature review imply that ONSS is a misnomer and all the lesions are different types of orbital schwannomas with optic nerve sheath attachment.