Giant pseudomeningocele

A 23-year-old woman with biopsy proven 7cm benign left-pelvic schwannoma. The patient underwent surgical excision and represented 8 months later with a large fluctuant swelling in the left hemipelvis. Further imaging was arranged with a view to drainage.

An abdominal MRI study performed several months following surgery reveals a huge collection within the left side of the pelvis, measuring 20 cm in maximum diameter. It is thin-walled, containing predominantly high T2 and low T1 signal, consistent with fluid, but no solid enhancing components. It displaces the left-iliac vessels and bowel loops to the right and the left psoas posterolaterally. It extends into a left rectus incisional hernia. Arcs of mixed T1 signal within the collection suggest internal swirling of fluid. A linear T2 hypointensity within the collection resembles jet artefact and originates from the left L5/S1 intervertebral foramen. Turbulent flow artefact is also seen in the vertebral canal from L5 downwards, which implies a communication between the vertebral canal and the collection. A diagnosis was made of a giant post-operative CSF leak. The findings were conveyed urgently to the surgical team and conventional drainage was averted.

Background: A pseudomeningocele is a collection of cerebral spinal fluid (CSF) that is not lined by dura [1]. This is in contrast to a meningocele, which is lined by dura.  A pseudomeningecele occurs due to a breach in the intracranial or spinal dura and subsequent CSF leak. The main causes are post-traumatic or iatrogenic, either secondary to direct surgical instrumentation of the dura or extradural procedure in close proximity [1,2], as in this case.
Exiting nerve roots are enclosed in a sleeve of dura containing CSF. At the junction between the nerve root ganglion and the spinal nerve, the dura adheres to the nerve and becomes the epineurium, no longer containing CSF [3]. Damage proximal to this junction can cause a CSF leak.

Clinical Perspective: Pseudomeningoceles are often asymptomatic. Common clinical features include postural headaches and a fluctuant swelling [4].  In this case, further questioning confirmed low pressure headaches since surgery. This emphasises the importance of correlating the clinical picture with the radiological impression.
Imaging Perspective: We produce on average 500ml of CSF per day [5,6]. CSF flow artefacts are often regarded as a pitfall to imaging. In this case, however, knowledge and exploitation of this artefact enabled the diagnosis to be made. This highlights the advantages of MRI in being able to provide some functional information from anatomical sequences.
In spin echo sequences, protons in a selected volume (corresponding to a ‘slice’ on imaging) are subjected to an excitation pulse. Protons which then move (flow) out of this volume before the refocusing pulse are replaced by non-excited protons which produce no signal and thus return a flow related signal void [6,7].

Outcome: Initially the collection was thought to represent a post-operative seroma which was planned for drainage by the surgical team. Prompt communication of the imaging interpretation averted potentially harmful conventional drainage. Drainage would likely have exacerbated the low-pressure headache and could have resulted in meningitis [2].
Small asymptomatic pseudomeningoceles are preferentially managed conservatively. Indications for active treatment include size, symptoms, infection and fistulas [2,4]. In this case, the patient was transferred to a neurosurgical centre where the defect was repaired.
Teaching Points:
- Knowledge of MRI artefacts can help establish certain radiological diagnoses.
- Correlation with the clinical picture (low pressure headache) added to the diagnostic confidence.
- Prompt communication averted the potentially harmful effects of conventional drainage.

Written informed patient consent for publication has been obtained.