Departments of 1Anatomy and 3Pathology, Kasturba Medical College Manipal, Basic Sciences Building, Manipal Academy of Higher Education, Manipal, 
Karnataka, India; 2Undergraduate Medical Student, Kasturba Medical College, Manipal Academy of Higher Education, Manipal, Karnataka, India
*Corresponding Author’s e-mail: nandini.bhat@manipal.edu

Serendipitous Discovery of a Benign Obturator 
Nerve Schwannoma

Case report with a brief clinical review
Suhani Sumalatha,1 Nikhila Appala,2 Ashwija Shetty,1 Deepak Nayak,3 Sushma Prabhath,1 *Nandini P. Bhat1

Sultan Qaboos University Med J, August 2021, Vol. 21, Iss. 3, pp. 477–480, Epub. 29 Aug 21
Submitted 19 Jun 20
Revision Req. 16 Aug 20; Revision Recd. 5 Sep 20
Accepted 4 Oct 20

Department of Anatomy, Kasturba Medical College, 
Manipal, India, in 2019. The fusiform swelling was on 
the left obturator nerve, located extraperitoneally in 
the pelvis, at the level of the sacral promontory. On 
the contralateral side, however, the obturator nerve 
was observed to be typical with no visible abnormality. 
On examination, the mass was 2.5 cm in length, 
encapsulated and had a hard consistency. The breadth 
of the swelling at the midpoint of the mass was 3.5 cm. 
However, the obturator nerve breadth was measured 
as 1 cm below as well as above the swelling. On gross 
examination, the swelling was identified as a peripheral 
nerve sheath tumour [Figure 1].

The mass was excised, histologically processed 
and stained with haematoxylin and eosin. On 
microscopic examination of the specimen, all 
three layers of the peripheral nerve (perineurium, 
epineurium and endoneurium) could be identified. 
A proliferation of spindle cells with a fascicular 
architectural configuration and areas of loosely cellular 
corresponding to Antoni B patterns of arrangement 
were observed. The hypocellular tumour was arranged 
in a sweeping fascicle form with patches of myxoid 
degeneration. Tumour cells showed elongated 
buckled nuclei with no mitotic figures. The tumour was 
determined to be a schwannoma. The obturator nerve 
on the right side was also excised and histologically 
processed. It showed features of a typical nerve [Figure 2]. 

Discussion

Schwannomas, also known as neurilemmomas, are 
classified under the peripheral nerve sheath tumours 

The obturator nerve originates from the ventral rami of the second, third and fourth lumbar nerve. The nerve runs down 
into the pelvis along the medial border of the psoas 
major muscle. It enters the anterior compartment of 
the thigh through the obturator foramen to supply 
the muscles of the medial compartment of the thigh. 
Further, it distributes articular branches to the hip, 
knee joints and sensory innervation to the medial side 
of the thigh.1

A schwannoma is a benign, encapsulated, non- 
invasive tumour of Schwann cells, that rarely under- 
goes malignant transformation. There is no gender 
predilection and it presents commonly between the 
ages of 20–50 years.2 Schwannomas are rare tumours 
that can develop at any site in the body and are most 
commonly found in the head and neck region. They 
are rarely located in the lower extremities where 
they can mimic compression neuropathies.3 In the 
current literature, there have been approximately 60 
retroperitoneal schwannomas recorded and among 
them, only about 20 are in the pelvis.4 Schwannomas 
are usually solitary tumours extending from 1–3 cm in 
diameter.5 Here, an incidental finding of an obturator 
nerve schwannoma in a 65-year-old male cadaver 
during a routine undergraduate dissection session and 
its histological findings are reported.

Case Report

In the present case, during a routine undergraduate 
dissection, a mass on the course of the obturator nerve 
was discovered in a 65-year-old male cadaver the 

This work is licensed under a Creative Commons Attribution-NoDerivatives 4.0 International License.

https://doi.org/10.18295/squmj.4.2021.016

CASE REPORT

abstract: Schwannomas are typically benign tumours of the peripheral nerves. However, they seldom arise from 
the obturator nerve. We report a case of an uncommon swelling (2.5 × 3.5 cm) in a 65-year-old male cadaver, 
found during a routine dissection session for first Bachelor of Medicine and Surgery students in the Department of 
Anatomy, Kasturba Medical College, Manipal, India, in 2019. It was seen originating from the left obturator nerve 
in the pelvis at the level of the sacral promontory. Histopathological investigation revealed a schwannoma. The 
hypocellular tumour was arranged in a sweeping fascicle pattern with patches of myxoid degeneration. Obturator 
schwannomas, though rare, can exist in cadavers, as seen in the present case. Hence, it should be considered as a 
differential diagnosis for clinical cases of pelvic masses and eliminated only after thorough radiological examination. 
Knowledge about the existence of such schwannomas is therefore essential. 

Keywords: Schwannoma; Obturator Nerves; Neurilemmoma; Nerve Sheath Neoplasms; Case Report; India.

https://creativecommons.org/licenses/by-nd/4.0/


Serendipitous Discovery of a Benign Obturator Nerve Schwannoma 
Case report with a brief clinical review

478 | SQU Medical Journal, August 2021, Volume 21, Issue 2

(PNSTs) and arise from Schwann cells of the outer 
nerve sheath, i.e. the epineurium.2 They are rare 
tumours that can develop at any site in the body. Only 
13% of PNSTs arise in the trunk; approximately 3% are 
retroperitoneal and the incidence of retroperitoneal 
location of a schwannoma is 0.5–5%.4 Schwannomas 
are benign, well-circumscribed lesions with 30–40 
years being the peak age at diagnosis. The World 
Health Organization classifies schwannomas as 
grade 1 tumours that are generally benign and whose 
malignant transformation is extremely rare.6 They may 
be present with neurological manifestations such as 
pain in the limbs, paraesthesia and motor weakness 
due to compression of the adjacent structures or 
may be found incidentally as asymptomatic masses.7 
The low incidence is due to the fact that there are no 
specific signs and symptoms as the tumours develop 
in the broad area of the retroperitoneum.8 The clinical 
manifestations are produced only after the tumours 
have grown to a substantial size; hence, there is a long 
latency period. The tumours, when large enough, 
cause pressure symptoms through compression 
or displacement of adjacent structures.9 As the 
tumours are deep-seated in the retroperitoneum, 
often on ultrasound they are erroneously seen as 
gynaecological tumours, only to be proven wrong 
during surgery.10,11 Therefore, as confirmed in most 
of the clinical cases after surgery and definitive 
histopathological examination, presurgical diagnosis 
is very challenging.12,13 The computed tomography 
(CT) and magnetic resonance imaging (MRI) narrow 

the diagnosis but are not able to assess it as imaging 
features are not specific to any type of tumour.4 
Laparoscopic resection is the treatment of choice with 
favourable post-operative recovery.14 

Benign peripheral nerve sheath tumours are 
classified into two main groups: neurofibromas and 
schwannomas. The distinction of schwannomas from 
neurofibromas is of importance to surgeons as schwa- 
nnomas can be easily enucleated using laparoscopic 
surgery while preserving nerve contiguity. However, 
neurofibromas are intraneural; hence, resection without 
nerve deficits is difficult.15 The characteristic of a 
peripheral nerve sheath tumour is the identification 
of a nerve that is proximal and distal to the mass. 
However, this may be difficult to visualise as the nerve 
itself may be compressed or distorted by the tumour.16

Histologically, schwannomas can be differentiated 
from other peripheral nerve tumours by the presence 
of capsule and fascicular growth patterns, increase 
in nuclear size and large nuclear hyperchromasia.9 
Areas of elongated cells that are densely packed, 
arranged in fascicles and showing an Antoni type A 
pattern may be seen, forming Verocay bodies when 
they are prominent. Cells are less compact and prone 
to cystic degeneration in the Antoni type B pattern.4 
Large schwannomas generally go through progressive 
degenerating alterations, comprising of cyst formation 
and hyalinisation of vessels. Benign schwannomas 
have large expanses of eosinophilic atypical round 
cells, whereas their epithelioid appearance is seen 
when they undergo malignant transformation.17 In 
the current case, histopathology showed a distinct 
capsule with an Antoni B pattern of fascicular cells. 
Degenerative changes were not observed.18

Obturator nerve schwannomas should be cons- 
idered a differential diagnosis when dealing with cases 

Figure 1: Showing schwannoma (*) in the retroperi- 
toneal area of the pelvis originating from the left 
obturator nerve in a 65-year-old male cadaver in the 
Department of Anatomy, Kasturba Medical College, 
Manipal, India, in December 2019.
LST = lumbosacral trunk; U = ureter; PM = psoas major; SP 
= sacral promontory; IAA = internal iliac artery.

Figure 2: A: Histology of schwannoma using 
haematoxylin and eosin (H&E) staining at ×40 
magnification in a 65-year-old male cadaver showing 
a flattened nerve fascicle in the periphery (arrow). 
In addition, it shows the capsule with the three layers 
(epineurium, perineurium and endoneurium) enclosing 
the tumour (arrowhead) and fascicle pattern with 
patches of mixed degeneration (asterisk). B: Histology 
of normal obturator nerve of the right-side using H&E 
staining at ×40 magnification showing the bundles of 
nerve fascicles enclosed in the perineurium (arrows).



Suhani Sumalatha, Nikhila Appala, Ashwija Shetty, Deepak Nayak, Sushma Prabhath and Nandini P. Bhat

Case Report | 479

of a pelvic mass. As the obturator nerve is in close 
proximity to vital pelvic structures, the symptoms 
arising from these growing masses usually mimic 
gynaecological or urological tumours.19,20 Literature 
cites schwannoma cases misdiagnosed as lymph node 
metastasis and ovarian malignancies before surgery.21–23 
Most of the cases of obturator schwannomas that 
have been reported in the literature were diagnosed 
postoperatively, only after laparoscopic resection and 
pathological examination [Table 1].10,13

However, a case reported by Takahashi et al. 
diagnosed the obturator nerve tumour preoperatively, 
as the CT and MRI showed clear continuity with the 
obturator nerve.24 Being in an anatomically complex 
and surgically inaccessible site with surrounding vital 
structures, pelvic tumour enucleation necessitates 
familiarity and knowledge of the pelvic retroperitoneal 
anatomy to avoid damage to the adjacent vascular and 
urinary structures.25

Conclusion

There is a varied range of benign and malignant 
tumours in the pelvic retroperitoneum. The patients 
commonly present with vague pain at a very late 
stage when the tumour is large. CTs and MRIs help 
significantly in diagnosis; however, the imaging 
features are non-specific and diagnosis is confirmed 
only by postoperative histopathology. This report 
concludes that obturator schwannomas, though very 
rare, can occur in unusual locations, as seen in this 
cadaveric dissection. Hence, it should be considered as 
a differential diagnosis for cases of pelvic masses and 
should be ruled out only after careful investigation. 

a u t h o r s’ c o n t r i b u t i o n
SS contributed to the concept and design of the 
report. NA and AS contributed in the acquisition 
and compilation of data. DN contributed with data 
analysis and interpretation. SP contributed with the 

Table 1: Cases of pelvic retroperitoneal schwannoma in the current literature

Author and year of 
publication

Number 
of cases

Presenting complaint Radio diagnosis Laparoscopic resection and 
postoperative pathological 

diagnosis

Di Furia et al.20 (2018) One case Dysuria and strangury CT – well circumscribed 
mass

Pelvic schwannoma

Chopra et al.13 (2017) One case Left pelvic pain of 
unknown origin

CT – multi-loculated 
cystic mass on lateral 
pelvic wall

Obturator schwannoma

Gleason et al.11 (2017) One case Pelvic pain diagnosed as 
ovarian malignancy

MRI – 2.6 × 2.1 × 2.7 cm 
mass adjacent to the left 
pelvic sidewall

Benign obturator nerve 
schwannoma

Yamada et al.23 (2015) One case Lymph node metastasis 
of rectal cancer

CT and MRI – 15-mm 
tumour

Benign obturator nerve 
schwannoma

Coskun et al.21 (2016) One case Initially diagnosed as 
pelvic metastasis of right 
kidney mass

CT – a well demarcated 
left iliac mass of 30*29 cm

Benign obturator nerve 
schwannoma

Takahashi et al.24 (2016) One case Left lower abdominal 
pain

CT and MRI – a mass of 
30 mm 

Benign obturator nerve 
schwannoma

Okuyama et al.22 (2014) One case Diagnosed as a 
mesenteric tumour

CT and MRI – 
heterogeneous tumour, 
5 cm in diameter, in the 
pelvic cavity

Pelvic schwannoma

Takaaki et al.25 (2013) One case Anal pain CT and MRI – prominent 
cystic degeneration and 
calcification.

Pelvic schwannoma

Ningshu et al.14 (2012) Six cases 3 incidental, 3 vague 
pelvic pain

One case preoperatively 
diagnosed

Pelvic schwannoma

Aubert et al.19 (2000) One case Urological manifestations CT – mass in paravesical 
position

Pelvic schwannoma

Scotto et al.10 (1998) One case Not specified No Obturator schwannoma

Hunter et al.12 (1988) Two 
cases

Asymptomatic No Retroperitoneal schwannoma

CT = computed tomography; MRI = magnetic resonance imaging.



Serendipitous Discovery of a Benign Obturator Nerve Schwannoma 
Case report with a brief clinical review

480 | SQU Medical Journal, August 2021, Volume 21, Issue 2

critical review of the manuscript. NPB contributed 
to the preparation and drafting of the manuscript. All 
authors approved the final version of this manuscript.

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