US diagnosis of sciatic nerve tumor proximal to gluteal fold

High-resolution ultrasound (US) is a helpful technique for the evaluation of peripheral nerves. It is now well accepted that especially in nerve mononeuropaties the combination of morphological data, obtained through US, and functional data, obtained through electrophysiology, is the appropriate way to reach the best diagnosis including information for therapeutical decision. Literature data show that US may crucially influence diagnosis and clinical care in nerve tumors [1, 2]. However, nerve US has some limitations. In particular the visualization of deep nerves is difficult, especially in overweight people. This is the case of proximal part of sciatic nerve that is commonly detectable only distal to the gluteal fold. We report on a 48-year-old woman complaining of pain and electric shocks radiated to sciatic course and trigger point in the right gluteal region. Clinical examination showed only mild right extensor hallucis longus weakness. Needle electromyography of tibialis anterior, gastrocnemius, peroneus longus was normal and mild chronic neurogenic recruitment was observed in extensor hallucis longus. It was suspected a right L5 radiculopathy but magnetic resonance (MR) showed only a mild L5–S1 bulging that was not considered the cause of symptoms. US of sciatic nerve was then performed although the trigger point was in a region where usually sciatic nerve is not detectable, proximally to the gluteal fold. US was performed using a linear 6–10 and 10–18 MHz transducer and sciatic nerve was bilaterally evaluated in distal–proximal direction starting from the popliteal fossa. Right sciatic nerve was detectable along all its course, even proximally to the gluteal fold although with no optimal visualization. At the middle-third of the gluteus a fusiform hypoechoic increase of nerve volume was found. The maximum longitudinal diameter was 2.5 cm and maximum antero-posterior diameter was 2.0 cm (max cross-sectional area 4.5 cm2). An accurate evaluation of the US video clip suggested that the mass raised from lateral fascicles sparing and dislocating remaining medial fascicles (Fig. 1a, b). US and clinical data suggested a sciatic nerve tumor, likely a schwannoma. MR confirmed the lesion showing a partially preserved fascicular structure, suggesting a schwannoma (Fig. 1c–f). Surgical excision was refused by the patient and US monitoring planned. Our case report confirms the usefulness of US in the diagnosis of nerve tumors. US is not able to clearly differentiate the type of nerve tumor but can provide useful information to discriminate between the two main types, schwannoma or neurofibroma [3–5]. Schwannoma typically displaces the nerve fascicles and generally is positioned eccentrically to the affected nerve segment. Another typical feature especially of large lesions is the presence of hyperechoic calcifications and internal degenerative cystic foci. On the other side neurofibroma presents as a concentric lesion that does not displace the fascicular elements of the nerve but interferes with them (usually fascicles are not detectable). Another typical sonographic feature of this kind of tumor is the so-called ‘‘target sign’’ (layered aspect, with a hyperechoic centre and a peripheral hypoechoic rim, best seen on transverse scans) [6]. Usually sciatic nerve is not detectable with US proximally to gluteal fold but in our case the visualization of proximal part of the nerve was possible because the patient was athletic, skinny and the tumor was big. In general our case suggests to perform US also in district where usually for anatomical reasons nerves are not clearly or totally viewable, because sometimes macroscopic nerve enlargement may be detected.