A Case of Trigeminal Neuralgia Due to Cerebellopontine Epidermoid Cyst: Discrepancy between Intraoperative and Radiological Findings of Constructive Interference in Steady State (CISS)

Trigeminal neuralgia (TN) is characterized by lightning pain paroxysms in the somatosensory distribution of the trigeminal nerve accompanied by hypersensitivity to non-nociceptive stimuli. Epidermoid cysts sometimes cause TN. To plan the surgery, constructive interference in steady state (CISS) image is useful for understanding the tumor's location, extent, and relationship against the cranial nerves, and epidermoid cysts are shown as hypointense compared to cerebrospinal fluid (CSF). However, we herein describe a case with TN due to epidermoid cysts, whose intraoperative findings are different from the preoperative and postoperative CISS image. A 49-year-old woman has suffered from TN. CISS images revealed the prolonged trigeminal nerve and the hypointense tumor compared to the CSF at the right cerebellopontine angle. CISS image suggested that the tumor would surround the trigeminal nerve, reach into the Meckel cavity, and offend and compress the trigeminal nerve's root entry zone (REZ). However, contrary to our expectation, the trigeminal nerve was not surrounded by the tumor. Neuroendoscope revealed that the tumor compressed the REZ, but the tumor was not present in the Meckel cavity. We performed partial tumor removal around the trigeminal nerve, and her symptoms improved. However, the postoperative CISS image was similar to the preoperative one, and so we could not evaluate the remaining tumor. The pathological diagnosis was epidermoid cysts. Intraoperative findings are sometimes different from the pre- and postoperative CISS images, making it difficult to follow up the remaining part of the epidermoid cyst.