Obturator nerve schwannoma presenting as a laterovesical pelvic mass: A case report

A 36-year-old man with no relevant medical history presented with a 2-month history of progressive left inguinal pain, described as a deep ache punctuated by electric shock-like discharges radiating along the medial thigh towards the knee, together with progressive weakness on adduction of the left hip. There were no urinary, digestive, or constitutional symptoms. Examination revealed no palpable mass. Neurological assessment showed mild weakness of the left hip adductors (Medical Research Council grade 4/5) compared with the contralateral side, with preserved patellar reflexes and no sensory deficit. Pain was reproduced by passive extension, abduction, and internal rotation of the hip. Pelvic ultrasound, performed with a high-frequency convex transducer, showed a well-defined, oval, homogeneously hypoechoic mass, measuring 2.39 × 2.43 cm, along the left lateral pelvic wall in contact with the postero-lateral bladder wall, with posterior acoustic enhancement and only mild peripheral vascularity on color Doppler. Continuity with the obturator nerve was not identified on ultrasound ( Fig. 1 ). Fig. 1 Transabdominal pelvic ultrasound demonstrating a well-circumscribed, oval, hypoechoic mass abutting the postero-lateral bladder wall on the left side, measuring 2.39 × 2.43 cm on 2 orthogonal diameters, consistent with the latero-vesical location of the lesion along the course of the obturator nerve (orange arrow). Fig 1 dummy alt text Transabdominal pelvic ultrasound demonstrating a well-circumscribed, oval, hypoechoic mass abutting the postero-lateral bladder wall on the left side, measuring 2.39 × 2.43 cm on 2 orthogonal diameters, consistent with the latero-vesical location of the lesion along the course of the obturator nerve (orange arrow). Pelvic MRI confirmed a well-circumscribed, fusiform, encapsulated mass (3.5 × 2 × 2.4 cm) along the course of the left obturator nerve, hypointense on T1- and markedly hyperintense on T2-weighted images, with a central area of lower T2 signal suggestive of a target sign and a thin peripheral fat rim in keeping with a split-fat sign. Nerve fascicles appearing to enter and exit the mass, in keeping with a tail sign, were also noted, a finding subsequently corroborated intraoperatively. No significant diffusion restriction was appreciated on diffusion-weighted imaging, in keeping with the benign nature of the lesion; a corresponding ADC map was not obtained. Moderate homogeneous enhancement was seen after gadolinium, without bladder invasion or lymphadenopathy ( Fig. 2 ). Taken together, these features were considered suggestive of a benign nerve sheath tumor of the obturator nerve, a diagnosis subsequently confirmed at surgery and histology. Fig. 2 Pelvic MRI showing a well-circumscribed oval mass along the course of the left obturator nerve (Green arrows). (A) Axial T1-weighted image following gadolinium injection, showing homogeneous enhancement of the mass. (B) Axial T1-weighted image following gadolinium injection at an adjacent level, showing homogeneous enhancement of the mass. (C) Coronal T2-weighted image, without fat saturation, showing the mass as well-defined and hyperintense relative to adjacent muscle. (D) Corresponding ADC map showing no significant diffusion restriction within the lesion. (E) Axial diffusion-weighted image. Fig 2 dummy alt text Pelvic MRI showing a well-circumscribed oval mass along the course of the left obturator nerve (Green arrows). (A) Axial T1-weighted image following gadolinium injection, showing homogeneous enhancement of the mass. (B) Axial T1-weighted image following gadolinium injection at an adjacent level, showing homogeneous enhancement of the mass. (C) Coronal T2-weighted image, without fat saturation, showing the mass as well-defined and hyperintense relative to adjacent muscle. (D) Corresponding ADC map showing no significant diffusion restriction within the lesion. (E) Axial diffusion-weighted image. The patient underwent surgical excision via an extraperitoneal pelvic approach. A well-encapsulated, fusiform mass was found arising eccentrically from the posteromedial aspect of the obturator nerve, with identifiable entering and exiting fascicles; enucleation was performed with preservation of uninvolved fascicles ( Fig. 3 ). Fig. 3 Gross pathological specimen showing a well-circumscribed, ovoid, tan-pink mass with a smooth, glistening capsular surface and focal areas of dark discoloration (hemorrhage), weighing 9 g and measuring 3.5 × 2 × 2.4 cm, corresponding to the resected obturator nerve schwannoma. Fig 3 dummy alt text Gross pathological specimen showing a well-circumscribed, ovoid, tan-pink mass with a smooth, glistening capsular surface and focal areas of dark discoloration (hemorrhage), weighing 9 g and measuring 3.5 × 2 × 2.4 cm, corresponding to the resected obturator nerve schwannoma. The specimen was an ovoid, firm mass weighing 9 g and measuring 3.5 × 2 × 2.4 cm ( Fig. 3 ), entirely embedded and examined on multiple H&E-stained section levels ( Fig. 4 ). Microscopy showed a tumor of moderate cellular density composed of spindle cells with small, regular, filiform nuclei and fine chromatin, within a loose stroma containing scattered mast cells; no notable mitotic activity was observed. Immunohistochemistry was not performed, as the diagnosis was considered sufficiently supported by the characteristic morphology in conjunction with the clinical and imaging features, which together were considered consistent with a benign schwannoma of the obturator nerve ( Fig. 5 ). Fig. 4 Photomicrographs of the resected specimen (H&E stain). (A) Low-power view, with a micrometer scale bar, demonstrating a well-defined fibrous capsule delimiting the tumor from the surrounding tissue, with an underlying moderately cellular tumoral proliferation (green arrows). (B-D) Higher-power views showing a spindle-cell proliferation with elongated, wavy nuclei arranged in loosely fascicular bundles, in keeping with the histopathological diagnosis of schwannoma. Fig 4 dummy alt text Fig. 5 Intraoperative laparoscopic view during dissection of the mass. (A) The mass is seen arising posteromedial to the obturator nerve, with stay sutures placed to aid retraction prior to dissection (black arrow). (B) Closer laparoscopic view showing the well-encapsulated mass being separated from the adjacent nerve fascicles, confirming its posteromedial relationship to the obturator nerve (black arrow). Fig 5 dummy alt text Photomicrographs of the resected specimen (H&E stain). (A) Low-power view, with a micrometer scale bar, demonstrating a well-defined fibrous capsule delimiting the tumor from the surrounding tissue, with an underlying moderately cellular tumoral proliferation (green arrows). (B-D) Higher-power views showing a spindle-cell proliferation with elongated, wavy nuclei arranged in loosely fascicular bundles, in keeping with the histopathological diagnosis of schwannoma. Intraoperative laparoscopic view during dissection of the mass. (A) The mass is seen arising posteromedial to the obturator nerve, with stay sutures placed to aid retraction prior to dissection (black arrow). (B) Closer laparoscopic view showing the well-encapsulated mass being separated from the adjacent nerve fascicles, confirming its posteromedial relationship to the obturator nerve (black arrow). The postoperative course was uneventful. At 1 month, the patient reported complete resolution of pain and full recovery of adductor strength (grade 5/5), with only a mild initial sensation of heaviness in the operated limb, which resolved completely. At 3- and 6-month follow-up, the patient remained entirely asymptomatic, with no recurrence, no residual sensory symptoms, and sustained adductor strength.

Written informed consent was obtained from the patient for the anonymized information to be published in this article.

In conclusion, obturator nerve schwannoma is a rare but important entity to consider in the differential diagnosis of a latero-vesical pelvic mass associated with obturator neuropathy, particularly when groin or medial thigh pain is accompanied by hip adductor weakness. Its deep pelvic location and close relationship to the bladder, ureter, ovary and iliac vessels mean that it is frequently mistaken preoperatively for gynecological, urological, nodal or other soft-tissue pelvic pathology, and a high index of clinical suspicion, informed by the characteristic neurological findings, remains essential to reaching the correct diagnosis before surgery. Suggestive ultrasound and MRI features—a well-defined, eccentrically situated fusiform mass exhibiting the target, split-fat and fascicular/tail signs, with clearly identifiable entering and exiting nerve fascicles—can substantially raise preoperative suspicion, help distinguish schwannoma from neurofibroma and other pelvic sidewall masses, and guide surgical planning towards a nerve-sparing approach. As illustrated by our patient, prompt recognition and complete, nerve-sparing surgical excision offer an excellent prognosis, with a low risk of recurrence and the potential for full functional recovery.

Schwannomas are the most common peripheral nerve sheath tumor, yet retroperitoneal and pelvic locations account for only a small proportion of cases, and an obturator nerve origin is exceptionally rare, reported mainly as isolated cases and a single surgical series of 6 patients [ 2 , 5 ]. The obturator nerve arises from the anterior divisions of the L2-L4 ventral rami within the psoas major muscle, emerges from its medial border near the pelvic brim, and descends posterior to the common iliac vessels along the lateral pelvic wall, running close to the internal iliac vessels, ureter, ovary and obturator vessels before dividing, typically within the obturator canal, into anterior and posterior branches. The anterior branch supplies the adductor longus, gracilis and adductor brevis, together with a cutaneous branch to the medial thigh, while the posterior branch innervates the obturator externus, adductor magnus and part of the adductor brevis; both branches also contribute articular fibers to the hip and knee joints. This deep, sheltered course along the lateral pelvic wall, immediately adjacent to the bladder, ureter, ovary and iliac vessels, explains the frequent preoperative misdiagnosis of obturator nerve tumors as gynecological, urological or nodal masses [ [2] , [3] , [4] ]. Existing reports do not point to any particular age group or sex being more susceptible, with cases occurring fairly evenly across both. Most lesions are incidental, but when symptomatic, compression of nerve fascicles typically produces medial thigh pain radiating to the knee, together with adductor weakness, as in our patient [ 2 ]. Gleason et al. described a case mimicking an ovarian malignancy on imaging [ 4 ], and Ningshu et al., in the largest surgical series to date, found that none of 6 obturator schwannomas were correctly diagnosed preoperatively [ 2 ]. Imaging, while not pathognomonic, raises strong preoperative suspicion. On ultrasound these tumors appear as well-defined, homogeneously hypoechoic masses. MRI is the modality of choice: the target sign, split-fat sign, and fascicular/tail sign considerably strengthen the diagnosis of a nerve sheath tumor when present together, as in our case [ 1 , 6 ]. Beyond confirming a neurogenic origin, these features can help distinguish schwannoma from neurofibroma, the other principal benign nerve sheath tumor: schwannomas typically arise eccentrically from the parent nerve, are well encapsulated, and show a clearly defined entering and exiting fascicle (tail sign), whereas neurofibromas tend to be centered symmetrically within the nerve, are nonencapsulated, and more often display a pronounced target sign on T2-weighted images; considerable overlap exists, however, and no single imaging feature is entirely reliable for this distinction [ 1 , 6 ]. In our patient, the combination of an eccentric, well-encapsulated fusiform mass with clearly identifiable entering and exiting fascicles was more in keeping with schwannoma than neurofibroma, a distinction later confirmed histologically. Definitive diagnosis nonetheless relies on histology, classically showing alternating compact (Antoni A) and myxoid (Antoni B) areas with diffuse S100 immunoreactivity when performed [ 1 ]; in our patient, microscopy showed a moderately cellular spindle-cell proliferation with regular nuclei, fine chromatin, a loose mast-cell-rich stroma and no mitotic activity, a picture in keeping with this diagnosis given the clinical and radiological context. Given this anatomical setting, the differential diagnosis of a latero-vesical pelvic mass is broad and includes pelvic or iliac lymphadenopathy (reactive, metastatic or lymphomatous), adnexal masses such as an ovarian fibroma or a subserosal uterine leiomyoma, psoas abscess or hematoma, and desmoid tumor, in addition to other peripheral nerve sheath tumors such as neurofibroma [ 7 ]. The potential for confusion with lymphadenopathy is not merely theoretical: a lateral pelvic wall schwannoma has itself been mistaken for a metastatic lymph node on preoperative imaging in a patient with known rectal cancer, only clarified at surgery and histology [ 8 ]. Deep infiltrating endometriosis involving the obturator nerve is a further, albeit rare, consideration in women of reproductive age presenting with cyclical groin pain and adductor weakness; a systematic review identified only 8 such cases in the literature, underscoring how infrequently this diagnosis, like obturator schwannoma, is considered before imaging or surgery [ 9 ]. In male patients such as ours, this diagnosis is excluded by definition, further narrowing the differential towards a primary nerve sheath tumor once a gynecological or nodal origin has been considered and excluded. Complete surgical excision is curative, with an excellent prognosis and negligible risk of recurrence or malignant transformation [ 1 , 2 ]. Open retroperitoneal or transperitoneal approaches have progressively been replaced by laparoscopic and, more recently, robotic-assisted nerve-sparing techniques, which improve visualization and dexterity in this technically demanding location [ 2 , 5 ]. In the largest laparoscopic series, 2 of 6 patients developed a permanent neurological deficit and 2 a transient 1, underscoring the difficulty of preserving uninvolved fascicles [ 2 ]. In our patient, nerve-sparing enucleation allowed complete symptomatic resolution, comparing favorably with previously reported cases ( Table 1 ). Table 1 Comparative summary of reported obturator nerve schwannoma cases, including the present case. Table 1 dummy alt text Study Age/Sex Clinical presentation Lesion size Preoperative diagnosis Management Outcome/ Complications Our case 36/M Groin pain + adductor weakness (MRC 4/5), 2-mo duration 3.5 × 2 × 2.4 cm Suspected (target, split-fat, and tail signs on MRI) Open extraperitoneal excision, nerve-sparing Complete resolution at 1 mo Ningshu et al. [ 2 ] Variable ages Variable, often incidental Not specified (series) 0/6 correctly diagnosed preoperatively Laparoscopic 2 permanent neurological deficits, 2 transients Gleason et al. [ 4 ] 63/F Pelvic pain, mass followed for 5 y, mistaken for ovarian tumour Mixed solid-cystic mass (size not specified) No—mistaken for ovarian malignancy Hysterectomy + bilateral salpingo-oophorectomy (intraoperative discovery) No residual adductor weakness Bayne et al. [ 5 ] 40/F Right lower quadrant pain 4.6 × 3.3 × 3.6 cm Not specified Robotic-assisted laparoscopic excision Transient sensorimotor deficit, resolved at 3-y follow-up Comparative summary of reported obturator nerve schwannoma cases, including the present case.

Schwannomas are benign, encapsulated tumors arising from Schwann cells of the peripheral nerve sheath [ 1 ]. They most commonly involve the head, neck, and flexor surfaces of the limbs; retroperitoneal and pelvic locations are uncommon, and schwannomas originating from the obturator nerve are exceptionally rare, with the literature limited to isolated case reports and a small number of surgical series [ 2 , 3 ]. Their deep pelvic location and proximity to the bladder, ureter, and iliac vessels make them a frequent source of preoperative diagnostic confusion with gynecological, urological, or nodal pathology [ 3 , 4 ]. We report a case of obturator nerve schwannoma presenting with groin pain and adductor weakness, and review the relevant literature.