Teaching Case

The patient had no specific complaints.

37-year-old-man was examined routinely by ultrasonography (US) and a large fluid-filled lesion was discovered incidentally in pelvis. The lesion was 10x14x16 cm in size and the bladder was pushed upwards. The patient was further searched by using plain films, computed tomography (CT) and magnetic resonance imaging (MRI). Plain film demonstrated sacral bony defect in the midline (Figure 1). On MRI, the cystic lesion had signal intensities in all sequences similar to cerebro-spinal fluid (CSF). The continuity between cystic lesion and dural sac was clearly seen in the sagittal plane so that the lesion was diagnosed as an anterior sacral meningocele (ASM) (Figure 2). There was a mural mass probably representing (epi)dermoid which was hyperintense on T1 and isointense on T2 and PD weighted images. The meningocele had narrow neck and was surrounded with a lipoma (Figure 3). Medulla spinalis was low-lying and ended at L3-4 level. The meningocele contained no nerve roots. On CT examination, horseshoe kidney was detected in addition to the meningocele (Figure 4). Sacral bony defect and mural mass on the left side were seen definitely (Figure 5).

Anterior sacral meningocele is a unilocular or multilocular extension of the dura mater and arachnoid out of the sacral spinal canal into the retroperitoneal and infraperitoneal space, either anteriorly through the body of the sacrum or anterolaterally through an enlarged intervertebral foramen or coalesced foramina. The embryogenesis of ASM is not determined. A relatively simple form arises in patients with neurofibromatosis type I and patients with Marfan syndrome (1,2). A more complex form is associated with the Currarino triad of anorectal malformations, sacral defects and presacral masses. A familial form with concurrent ASM, tethered cord, lipomas, teratomas and dermoids may be inherited in families as an autosomal dominant trait with incomplete penetrance (3). They are less common than posterior lumbosacral meningoceles, which are easily diagnosed at birth. ASM accounts for 3-5% of retrorectal masses (1). They may be detected at any age. In adults, females appear to be affected more commonly (approximately 10:6). However, the lesion is equally frequent in boys and girls below age 15, suggesting that increased detection of ASMs during pregnancy accounts for the female predominance in adult patients (3). The meningocele can enlarge with time, probably as the result of hydrostatic pressure created by the CSF within it. When it enlarges, it does so slowly, and the meningocele may not give rise to symptoms until adult life. Clinical symptoms in ASM are produced as a result of pressure on the pelvic viscera, causing constipation, urinary frequency and incontinence, dysmenorrhea, dyspareunia, or pain in the lower back or pelvis. Furthermore, pressure may be exerted on nerve roots, resulting in sciatica, diminished rectal and detrusor tone, or numbness and paresthesia in the lower sacral dermatomes. Finally, fluid shifts between the sac and the spinal subarachnoid space can cause intermittent high-pressure headache (when supine), nausea, vomiting (especially during defecation), or low-pressure headache (while in the erect position) (1,4). In imaging studies, plain radiographs may demonstrate sacral bony anomalies: the coccyx and one side of the lower sacrum at one or more levels are absent giving a suggestive “scimitar” aspect of sacrum. In approximately 20% of cases, the sacral defect is in the midline (5). US of the pelvis demonstrates the fluid-filled lesion, which can be misinterpreted as an ovarian cyst in females and as a bladder in males. Associated uterine malformations may be present. CT and MRI clearly depict the anterior sacral bony defect and the fluid-containing presacral lesion, if CT is used, contrast should be injected intrathecally to show optimally the continuity of the meningocele with the subarachnoid space. On MR studies, the continuity is best demonstrated dynamically in the sagittal plane. Whatever imaging technique is used, the most important features to demonstrate are (a) tethering of cord and the associated tethering lipoma or dermoid, if present, and (b) whether nerve roots traverse the sacral defect (1,5).