• Mudathir S Bakhit, Masazumi Fujii, Shinya Jinguji, Taku Sato, Jun Sakuma, and Kiyoshi Saito.
• Department of Neurosurgery, Fukushima Medical University, Fukushima, Japan. Electronic address: m-bakhit@fmu.ac.jp.
• World Neurosurg. 2017 Jun 1; 102: 694.e9-694.e13.

BackgroundLower cranial nerve sheath tumors are relatively rare. Cases of schwannoma collision tumors have rarely been reported, with most of the reported cases describing schwannoma and meningioma collision tumors.Case DescriptionWe report a very rare case of a cerebellopontine angle collision tumor of the ninth cranial nerve schwannoma with an IgG4 plasma cell pseudotumor. IgG4 plasma cell pseudotumors comprise a group of diseases called IgG4-related diseases (IgG4-RDs). These diseases usually affect organs such as the pancreas and salivary gland. Few cases of nervous system IgG4-RDs have been reported. Under intraoperative microscopy, the tumor in our case did not appear different from usual cases of schwannoma, but histopathology showed significant infiltration of IgG4 plasma cells.ConclusionIgG4-RDs have a distinctive histopathologic pattern; however, their pathophysiology remains unclear. Special attention must be paid to the diagnosis of such diseases because they mimic other diseases and can be missed.Copyright © 2017 Elsevier Inc. All rights reserved.

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