An interesting location of the arachnoid cyst: cavum velum interpositi

A 46-year-old female patient is referred to our radiology clinic for Magnetic Rezonans Imaging (MRI) due to headache for 2 months. Her neurological examination was unremarkable.

As a result of the MRI examination, a thin-walled,well-circumscribed cystic lesion with a size of 40x20 mm is detected in the Cavum Velum Interpositi (CVI) cystern. The lesion has similar signal characteristics with cerebrospinal fluid (CSF)  in all sequences. No restriction of diffusion is noted in diffusion weighted images (Figure 2). The cystic lesion was located between the posterior part of the lateral ventricles and the roof of the 3rd ventricle. The posterior and inferior walls of the cyst was clearly observed in FLAIR images. Cyst was causing elevation and splaying of the fornices with mild compression of the inferior aspect of the splenium of the corpus callosum and inferolateral displacement of the internal cerebral veins. The quadrigeminal cistern was unremarkable and hydrocephalus is not seen (Figure 1a-d). No change was observed in the patient's follow-up Magnetic Rezonans (MR) images taken 6 months later.

CVI is a potential space that is the extension of the quadrigeminal cystern. It has a triangular structure and its apex extends towards the foromen monro [1]. Possible origin of CVI arachnoid cyst is thought to be the tela choroidea or vascular connective tissue between the lateral ventricles and the third ventricle [2]. Arachnoid cysts defined in this area are quite rare and there are no well-defined clinicopathological definitions [3]. The differential diagnosis of the cyst includes qadrigeminal cystern arachnoid cyst, epidermoid cyst and cystic dilatation of the CVI [4].

  Arachnoid cyst of CVI is a midline cyst that is located between the 3rd ventricle and the posterior part of the lateral ventricles and pushes the internal cerebral veins inferiorly [4]. It may cause similar symptoms with 3rd ventricular masses. Arachnoid cyst may cause a decrease in perfusion and metabolism in the surrounding cortex. These changes may cause mental discomfort. It has been reported in the literature that it can cause symptoms such as headache, loss of consciousness, dizziness, disorientation, confusion and memory disorders [5]. Headache was only symptom in our case.

CVI cystic dilatation is associated with adjacent cystern and does not cause mass effect or hydrocephalus. However, arachnoid cyst tends to obstruct ventricles and cause symptoms similar to 3rd ventricular masses, especially hydrocephalus [6]. Cerebral veins can be used to separate these two entities. It pushes inferiorly and the vessels are located around the cyst. In CVI cystic dilatation, there is no sign of pushing of the vascular structures and the vascular structures are included in the dilatation [7].

Diffusion-weighted images can be used to differentiate between epidermoid cyst and CVI arachnoid cyst. Epidermoid cyst shows a restricted diffusion finding, and FLAIR  images shows a heterogeneous signal compared to CSF. CVI arachnoid cyst has signals similar to CSF in all sequences and restricted diffusion is not observed as in this case [7].

In the differential diagnosis, there is also a quadrigeminal cystern arachnoid cyst if it extends in the superoanterior direction [5]. This lesion pushes the internal cerebral veins superiorly as arachnoid membrane in the quadrigeminal cistern is topologically below the veins, while the CVI arachnoid cyst pushes the internal cerebral veins inferiorly [7].

When these cysts reach large sizes and be symptomatic, the most common treatment method is endoscopic ventricular fenestration. In this way, a connection between the cyst and the ventricular system is provided [7].

Written informed patient consent for publication has been obtained.