US diagnosis of sciatic nerve tumor proximal to gluteal fold

High-resolution ultrasound (US) is a helpful technique for\r\nthe evaluation of peripheral nerves. It is now well accepted\r\nthat especially in nerve mononeuropaties the combination\r\nof morphological data, obtained through US, and functional\r\ndata, obtained through electrophysiology, is the\r\nappropriate way to reach the best diagnosis including\r\ninformation for therapeutical decision. Literature data show\r\nthat US may crucially influence diagnosis and clinical care\r\nin nerve tumors [1, 2]. However, nerve US has some limitations.\r\nIn particular the visualization of deep nerves is\r\ndifficult, especially in overweight people. This is the case\r\nof proximal part of sciatic nerve that is commonly\r\ndetectable only distal to the gluteal fold. We report on a\r\n48-year-old woman complaining of pain and electric\r\nshocks radiated to sciatic course and trigger point in the\r\nright gluteal region. Clinical examination showed only\r\nmild right extensor hallucis longus weakness. Needle\r\nelectromyography of tibialis anterior, gastrocnemius, peroneus\r\nlongus was normal and mild chronic neurogenic\r\nrecruitment was observed in extensor hallucis longus. It\r\nwas suspected a right L5 radiculopathy but magnetic resonance\r\n(MR) showed only a mild L5–S1 bulging that was\r\nnot considered the cause of symptoms. US of sciatic nerve\r\nwas then performed although the trigger point was in a\r\nregion where usually sciatic nerve is not detectable, proximally\r\nto the gluteal fold. US was performed using a linear\r\n6–10 and 10–18 MHz transducer and sciatic nerve was\r\nbilaterally evaluated in distal–proximal direction starting\r\nfrom the popliteal fossa. Right sciatic nerve was detectable\r\nalong all its course, even proximally to the gluteal fold\r\nalthough with no optimal visualization. At the middle-third\r\nof the gluteus a fusiform hypoechoic increase of nerve\r\nvolume was found. The maximum longitudinal diameter\r\nwas 2.5 cm and maximum antero-posterior diameter was\r\n2.0 cm (max cross-sectional area 4.5 cm2). An accurate\r\nevaluation of the US video clip suggested that the mass\r\nraised from lateral fascicles sparing and dislocating\r\nremaining medial fascicles (Fig. 1a, b). US and clinical\r\ndata suggested a sciatic nerve tumor, likely a schwannoma.\r\nMR confirmed the lesion showing a partially preserved\r\nfascicular structure, suggesting a schwannoma (Fig. 1c–f).\r\nSurgical excision was refused by the patient and US\r\nmonitoring planned. Our case report confirms the usefulness\r\nof US in the diagnosis of nerve tumors. US is not able\r\nto clearly differentiate the type of nerve tumor but can\r\nprovide useful information to discriminate between the two\r\nmain types, schwannoma or neurofibroma [3–5]. Schwannoma\r\ntypically displaces the nerve fascicles and generally\r\nis positioned eccentrically to the affected nerve segment.\r\nAnother typical feature especially of large lesions is the\r\npresence of hyperechoic calcifications and internal degenerative\r\ncystic foci. On the other side neurofibroma presents\r\nas a concentric lesion that does not displace the fascicular\r\nelements of the nerve but interferes with them (usually\r\nfascicles are not detectable). Another typical sonographic\r\nfeature of this kind of tumor is the so-called ‘‘target sign’’\r\n(layered aspect, with a hyperechoic centre and a peripheral\r\nhypoechoic rim, best seen on transverse scans) [6]. Usually sciatic nerve is not detectable with US proximally to gluteal\r\nfold but in our case the visualization of proximal part\r\nof the nerve was possible because the patient was athletic,\r\nskinny and the tumor was big. In general our case suggests\r\nto perform US also in district where usually for anatomical\r\nreasons nerves are not clearly or totally viewable, because\r\nsometimes macroscopic nerve enlargement may be\r\ndetected.