Major nerve schwannomas vs intramuscular schwannomas

Background: A schwannoma is a benign peripheral nerve tumor. Predicting the involvement of a nerve on symptoms or MR findings is crucial to the diagnostic process. Purpose: To compare symptoms, MR findings, and histological findings between major nerve schwannomas and intramuscular schwannomas. Material and Methods: Thirty-four patients with 36 schwannomas (29 major nerve schwannomas and 7 intramuscular schwannomas) surgically excised and proven histologically were retrospectively reviewed. Results: Frequencies of the Tinel like sign, split-fat sign, entering and exiting nerve, and low-signal margin indicate the presence of the nerve and were significantly higher in major nerve schwannomas than in intramuscular schwannomas. In tumor morphological patterns (the target sign, inhomogeneous pattern, and homogeneous pattern), there were no significant differences between major nerve schwannomas and intramuscular schwannomas. Schwannomas showing the target sign histologically tended to be less degenerative. All major nerve schwannomas and 5 intramuscular schwannomas produce some characteristic symptoms and/or MR findings, but two intramuscular schwannomas didn’t have any characteristic symptoms and findings. Conclusion: In major nerve schwannomas, the Tinel like sign, split-fat sign, entering and exiting nerve, and low-signal margin are commonly observed and useful for diagnosis. In intramuscular schwannomas, these characteristic findings are less common, which makes diagnosis difficult. A schwannoma is a benign peripheral nerve tumor, usually formed in a peripheral nerve. The diagnosis of a schwannoma is often defined at surgery by its characteristic macroscopic appearance. A schwannoma should be separated from the affected nerve intraoperatively after incision of the epineurium to preserve the native nerve function. Partial resection can be performed to spare the nerve function, and recurrence is unusual. A preoperative correct diagnosis of a schwannoma is important for atraumatic management of the tumor. Holdsworth and Kehoe reported that diagnosis of a peripheral nerve tumor is difficult when based on clinical assessment alone (7, 10). However, Hems and Ogose reported that if a major nerve is involved, it is suspected on clinical grounds alone and the Tinel like sign (paresthesia produced by percussing the tumor) is often observed (4, 6, 16). On the other hand, Kwon reported that clinical symptoms are rare in intramuscular schwannomas (12). On MR images of schwannomas, the frequencies of characteristic findings such as the split-fat sign (fat tissue surrounding the involved nerve), entering and exiting nerve, low-signal margin (low signal rim surrounding the schwannoma), thin hyperintense rim (thin peripheral hyperintense rim on T2-weighted images), and the target sign (peripheral hyperintense rim and central low intensity on T2-weighted