CASE REPORT
22 year old woman was seen in the Emergency Department complaining of sudden onset of discomfort in the head, nausea as well as a feeling of weakness or fainting. In the past days, she had experienced a feeling as though her head was too heavy for her neck. She also complained of pain in the back of the head and neck, getting worse during the night and exaggerated by sneezing and coughing. She was unable to turn her neck and she noticed difficulty swallowing; her symptoms were all worsened by moment. She denied any major illness or injury in the past, except a vague headache for the past year.

Neurological examination was normal. Blood tests obtained in the Emergency Room revealed no abnormality.

The CT scan performed with and without contrast showed an oval mass, isodense to brain, at the level of the foramen magnum, with some coarse calcifications (Fig. 1A). There was no abnormality in the bone, nor contrast enhancement of the lesion.

The MRI showed an intradural extra-medullary lesion, isointense in T1 (Fig. 1B), hyperintense in T2 and GR sequences (Fig. 2A, 2B), located in the anterior and left paramedian subarachnoid space, indenting slightly the medulla. A very thin tail arising from the cyst and going anteriorly was noted in all sequences. After gadolinium there was no real enhancement of the lesion but a iso-hyperintense fluid level pattern was seen inside the lesion in axial and sagittal T1 sequences.(Fig. 3A, 3B)

The patient underwent a left posterior fossa craniotomy to remove the lesion thought to be most likely a dermoid cyst. The operative findings described a cystic lesion, ventral to the medulla, indenting it and displacing the 11^th and 10^th cranial nerves posteriorly. There was a very diaphanous cyst wall and the contents varied: some parts were semisolid and others contained a mucoid liquid. The cyst wall was adherent to cranial nerves and brainstem. There was no clear attachment to the dura. Once the cyst was collapsed and most of its wall removed, the cranial nerves and brainstem structures appeared unremarkable. The cyst measured approximately 1.5-2.0 cm in diameter.

The post-operatory course was uncomplicated and she was subsequently discharged home. Microscopic examination showed fragments of a single layer or pseudostratified epithelium, columnar in appearance and non ciliated. Most of the epithelial cells contained a secretory product strongly positive for PAS and mucin stains (Fig. 4). This type of epithelium resembling gastro-intestinal or respiratory epithelium is characteristic of a neurenteric cyst.