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Background: Trochlear nerve schwannoma is a very rare tumor encountered especially in patients without type 2 neurofibromatosis (NF2). Most of the time, this tumor is diagnosed intraoperatively. We describe a rare case of trochlear nerve schwannoma. Clinical Presentation: A 63-year-old male presented with generalized headache from 8 months earlier, without nausea and vomiting. The headache had worsened during the last months. Clinically, he suffered from transient diplopia. Magnetic resonance imaging (MRI) demonstrated a mass in the left ambient and interpeduncular cisterns that was compressing the midbrain and upper pons. The preoperative impression was of trigeminal schwannoma. However, intraoperatively, the tumor originated in the trochlear nerve. The mass was removed entirely via left suboccipital retrosigmoid approach. The histopathology diagnosis confirmed schwannoma. The patient's diplopia did not improve postoperatively. Conclusion: Trochlear nerve schwannoma is a rare type of schwannoma and in almost all of the cases is diagnosed intraoperatively. There are three types of trochlear nerve schwannoma according to the classification proposed for trigeminal schwannoma: cisternal type-confined to the precavernous segment of the trochlear nerve; cistocavernous type-invading the cavernous sinus and the retroclival and retropetrosal cistern; cavernous type-located in the middle cranial fossa on the cavernous or paracavernous segment of the fourth cranial nerves, with or without cavernous sinus invasion. The cisternal type was the most common type encountered in previous studies. The clinical signs and symptoms of trochlear nerve schwannoma are similar to the trigeminal schwannoma and should be considered in the differential diagnosis of trigeminal schwannoma.

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