Background: A schwannoma is a benign peripheral nerve tumor. Predicting the involvement of a nerve on symptoms or magnetic resonance (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 palpable schwannomas (29 major-nerve schwannomas and seven 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 a nerve, and were significantly higher in major-nerve schwannomas than in intramuscular schwannomas. In tumor morphological patterns (target sign, inhomogeneous 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 five of the intramuscular schwannomas produced some characteristic symptoms and/or MR findings, but two intramuscular schwannomas did not 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.
Target sign: An applanation epitheliopathy
