Trochlear Nerve Neurofibroma in a Clinically NF-1-Negative Patient; A Case Report and Review of Literature.

Background: Isolated orbital neurofibroma unassociated with systemic neurofibromatosis is relatively rare and may be difficult to clinically differentiate from other orbital tumors. Sensory branches of the trigeminal nerve-namely lacrimal, nasociliary, and frontal-are the most common nerves of origin for intraorbital neurofibroma, but we discovered a neurofibroma arising out of the right trochlear nerve, in absence of clinical stigmata of neurofibromatosis type 1, which is rare.

Case Description: A 41-year-old adult presented with painless progressive proptosis of the right eye for 10 years without history of visual problems or diplopia. The right eye had axial proptosis with periorbital swelling. On magnetic resonance imaging (MRI), a right orbital extraconal, expansile, lobulated, cystic space-occupying lesion was seen with an enhancing component, pushing the lateral rectus with T1 isointensity and T2 hyperintensity, suggesting a preoperative working diagnosis of pseudotumor or lymphoproliferative tumor. Intraoperatively, a rudimentary slender, white, elongated structure was passing through the length of the tumor. The elongated tumor engulfing the trochlear nerve was traced up to the lateral part of the superior orbital fissure. The tumor was excised completely and was found to be a neurofibroma.

Conclusion: Isolated trochlear nerve neurofibromas, in the absence of clinical stigmata of NF1, are rare. Multiplicity, multilobulation, ring-configured contrast enhancement, and heterogenous MRI signal intensities help in the accurate preoperative imaging diagnosis. A possible cure is thus achievable with complete excision without damaging important adjacent neurovasculo-musculotendinous structures in the orbit. To the best of our knowledge, this is the fourth reported case of isolated trochlear nerve neurofibroma.