Intracranial neurenteric cyst: A rare cause of chemical meningitis.

Intracranial neurenteric cysts are exceedingly rare congenital intracranial lesions that result from disorder of gastrulation. Still, more rarely, the cyst contents may leak into the CSF and give rise to recurrent episodes of chemical meningitis. We present a case of chemical meningitis due to a leaking posterior fossa neurenteric cyst in a young female, with emphasis on its imaging features. The final diagnosis was achieved by sufficiently characteristic imaging features; histopathologic documentation could not be achieved as the patient denied surgery.

Introduction

Neurenteric cysts are benign endodermal lesions that occur three times more commonly in spine. Intracranial neurenteric cysts are mostly seen in posterior fossa (most commonly in the prepontine, cerebello-medullary, and cerebello-pontine angle cisterns); rarely, they can be seen in supratentorial locations. Embryologically, they result from persistent endodermal ectodermal adhesions. These cysts may be discovered incidentally on imaging studies; patients may also present with headache, cranial neuropathies, or recurrent aseptic meningitis.[1-3] We present a case of neurenteric cyst presenting as chemical meningitis with emphasis on its imaging features.

Case Report

A 23-year-old unmarried female presented with 4 days history of headache and multiple episodes of vomiting, which was treated with analgesics prior to her admission. Physical signs revealed an afebrile patient with signs of meningeal irritation with neck rigidity and positive Kernig’s sign. The patient had no significant past history and did not recall any such episode in the past. CSF analysis showed a grossly clear CSF with white cell count of 880/sq mm with predominant lymphocytes (65%). The CSF protein was 62 mg/dl and glucose 32 mg/dl. The CSF staining and culture was negative for mycobacteria or other bacterial pathogens. The patient had been empirically put on broad-spectrum antibiotics (Ceftrioxone, vancomycin, and levofloxacin) without any clinical improvement. A CT scan done 2 days after admission revealed lobulated extra-axial hyperdense lesions in the prepontine and cerebello-medullary cisterns [Figure 1]. The neuroparenchyma, ventricular system, and other basal cisterns were normal. Contrast-enhanced MRI was done subsequently, which showed a lobulated extra-axial lesion in the prepontine cistern extending into both cerebello-medullary cisterns. The lesion showed hyperintense signal on T1 [Figures 2 and 3] and FLAIR [Figure 4] images, with mildly hypointense signal on T2-weighted images [Figure 5]. There was no evidence of restricted diffusion on DWI [Figure 6]. Susceptibility-weighted images did not reveal any evidence of blooming within the lesion [Figure 7]. There was no evidence of signal suppression on T1-weighted fat-suppressed images. Post-gadalonium images did not show any intralesional or peripheral rim enhancement. In view of the characteristic location and signal intensity of the lesion, a diagnosis of leaking neuroenteric cyst was made. The patient was put on steroids (inj. dexamethasone 8 mg 8 hrly) with dramatic improvement in the next 24 hours. Neurosurgical consultation was sought; the patient refused operative treatment and is presently on close follow-up.

Figure 1

Axial non-contrast CT shows lobulated extra-axial hyperdense lesions anterolateral to the lower brainstem

Figure 2

Sagittal T1 wt unenhanced images show a lobulated extraparenchymal lesion in the prepontine and cerebello-medullary cisterns with hyperintense signal. No signal suppression was seen on fat-suppressed sequences

Figure 3

Axial T1 wt unenhanced images show a lobulated extraparenchymal lesion in the prepontine and cerebello-medullary cisterns with hyperintense signal. No signal suppression was seen on fat-suppressed sequences

Figure 4

Axial FLAIR image shows that the lesion shows hyperintense signal

Figure 5

T2 wt axial image shows relatively hypointense signal extra-axial lesion anterolateral to lower brainstem

Figure 6

ADC map does not show any diffusion restriction within the cyst

Figure 7

Susceptibility-weighted image shows the lesion with hyperintense signal; no hypointense blooming is observed, ruling out hematoma or a thrombosed aneurysm

Discussion

Neuroenteric cysts (also called as enteric cysts, enterogenous or endodermal cysts) are endodermally-derived rare lesions of CNS. In spine, these are found ventral to cord (most commonly in cervical region). Vertetbral anomalies occur in 50% of cases of spinal neuroenteric cysts. Intracranial neuroenteric cysts are uncommon and occur predominantly in posterior fossa in midline location (anterior to brain stem, in CP angle). Bone abnormalities are distinctly rare in intracranial neurenteric cysts.[4] The generally accepted mechanism of formation of neurentric cysts is failure of separation of foregut and notochord during the process of exclation, causing incorporation of primitive endodermal cells into notochord. The endodermal cell nests ultimately give rise to a cyst.[3] On histopathological examination of excised cysts, 2 major patterns are seen. The more common pattern is the cyst wall depicting pseudostratified highly ciliated columnar epithelium with sparse mucin-producing cells. The second histological pattern shows the cyst wall being composed of simple columnar epithelium and rich in mucin-producing cells. The symptomatology of neuroenteric cysts varies and includes headache (app 47%), cranial nerve palsies (24%), motor and sensory deficits (app 24%) and recurrent meningitis (12%).[256] The imaging spectrum of neuroenteric cysts is broad. Typically, the cysts are well-demarcated, extra-axial, infratentorial, midline with hypointense signal on T1 and hyperintense signal on T2 and FLAIR images with no contrast enhancement and no or mild diffusion restriction. Occasionally, the cysts may be bright on T1-weighted images and relatively hypointense on T2-weighted images as a result of increased protein content or hemorrhage within the cysts (as in the case described).[478] Intracystic hemorrhage could be ruled out by susceptibility-weighted imaging, which shows profound hypointense signal in case of blood products or calcification. The chief differential diagnosis in this case is an atypical epidermoid tumor (called as white epidermoid). Even though typical epidermoids are CSF isointense lesions, white epidermoids show T1 hyperintense signal and are hyperdense on CT. However, they characteristically show evidence of diffusion restriction on diffusion-weighted images, which was not observed in this case. The other differentials include arachnoid cysts (that are isointense to CSF on all sequences) and dermoid cysts (that show suppression of signal on fat-suppressed sequences). Rathke cleft and colloid cysts chiefly occur in supratentorial location.[14] Chemical meningitis as a result of leaking neurenteric cyst is a rare entity. Unlike in case of a ruptured dermoid cyst where the disseminated fat globules in sub-arachnoid space can be easily demonstrated by fat-sensitive MR sequences, the diagnosis of chemical meningitis due to a leaking neurenteric cyst or an epidermoid can be difficult on imaging studies alone and requires a close correlation with the clinical and laboratory data. Neurenteric cysts may also predispose to recurrent bacterial meningitis, in which case a persistent fistulous communication with aerodigestive tract should be suspected and sought on imaging studies.[56] The definitive treatment for a symptomatic neurenteric cyst is surgical excision. The possibility of future similar episodes of meningitis was discussed with the patient; however, as the patient was reluctant for surgery, she is being kept on a close follow-up. Even though this case report is limited by lack of histopathological confirmation, the imaging and clinical findings were characteristic for a leaking neurenteric cyst. The MRI was repeated after 2 months, which showed persistence of imaging findings.