Teaching Case

A 54-year-old gentleman presented to the Urology outpatient department of our hospital with acute urinary retention. The patient admitted to having intermittent constipation over the past 6 months. There was no history of abdominal pain or hematuria or bleeding per rectum. As the patient refused a digital rectal examination, he was catheterised and referred for ultrasound examination to look for prostatomegaly.

Ultrasound showed a large heterogeneous solid cystic lesion in the pelvis which was displacing the urinary bladder and prostate anteriorly (Figure 1).

Contrast-enhanced Computed Tomography (CT) performed for further assessment showed the presacral location of the solid cystic lesion with enhancing solid component on venous phase. Few specks of calcification were noted with no intralesional fat component. The mass was seen compressing and displacing the rectum right laterally. Intervening fat plane with the lower third of the rectum was not delineated on CT. Posteriorly, there was mild scalloping of the anterior sacral surface, however, there was no sacral destruction or neural foraminal widening (Figure 2).

Magnetic resonance imaging (MRI) was then performed to delineate the rectal fat plane and to look for evidence of foraminal extension. The lesion was well-encapsulated and showed thick irregular diffusion restricting T2 hyperintense wall and central T2 hyperintense cystic component. Fat planes with the mid and lower rectum were well maintained with a normal rectal wall appearance, thus ruling out the possibility of Gastrointestinal stromal tumour (GIST). Posteriorly, the lesion was abutting the S4 vertebra but there was no neural foraminal extension (Figures 3 and 4).

The presacral space is an extra-peritoneal potential space between the upper two-thirds of the rectum anteriorly and the sacrum posteriorly. Its superior and inferior boundaries are marked by the peritoneal layer of the rectum and Waldeyer’s fascia respectively. It is bounded laterally by the iliac vessels and ureters. Sacral nerve roots course through the lateral aspect of the space [1]. This space may be the site of a heterogeneous group of masses that pose a diagnostic challenge [2]. A simplified diagnostic algorithm based on imaging features is given in Figure 7.

Schwannomas are tumours that arise from the surface of neural elements in the peripheral or central nervous system [3]. They are a rare entity in the presacral space constituting only about 1-5% of all schwannomas. They usually present with nonspecific symptoms due to mass effect upon adjacent structures.

Ultrasound may be the first radiological investigation but CT and MRI are the mainstay. On CT, heterogeneous presacral soft tissue mass with or without calcifications and cystic or necrotic central areas can be seen. A subset of schwannomas, known as “ancient” schwannomas have cystic changes with areas of haemorrhage and necrosis [4]. But, the major limitation of CT is that it cannot distinguish schwannomas from other presacral neoplasms and therefore MRI remains the imaging gold standard.

MRI helps in determining the size, exact relationship with the neighbouring organs, and also highlights the presence of a capsule. Neurogenic lesions have particular signs on MRI, such as the entering or exiting nerve sign, target sign, fascicular sign and split fat sign [5]. These signs when used in combination may help identify a neurogenic lesion. They exhibit iso- to hypointense signals on T1-weighted imaging and hyperintense signals on T2 sequences. Schwannomas avidly enhance on postcontrast sequences [6].

The gold standard in diagnosis is a preoperative biopsy. Histologically, we can demonstrate areas of high and low cellularity (Antoni A and Antoni B) respectively. By immunohistochemistry, schwannomas typically show strong expression of S100 protein and abundant pericellular collagen type IV [7].

The surgical procedure of choice is enucleation. When the schwannoma is voluminous, intratumoral piece-meal excision can be proposed and even preferred to radical procedures, which may entail nerve sacrifice, a disproportionate procedure in view of the benign character [8].

The patient underwent surgical excision of the mass. Intra-operatively the mass was well-encapsulated and was resected enbloc (Figure 5). The specimen was subjected to histopathological and immunohistochemistry profiles which confirmed the diagnosis of schwannoma with diffuse S-100 positivity (Figure 6).

Written informed patient consent for publication has been obtained.