Endoscopic Treatment of a Midbrain Cyst : Case Report and Review of the Literature

Background and Importance: Midbrain arachnoid cysts are rare and usually present with symptoms associated with the mass effect or secondary hydrocephalus and require treatment. Because of their location, minimally invasive approaches using the endoscope are ideal since the open approach is difficult. Clinical Presentation: A 52-Years-old woman presented in the hospital for an acute confusional state and headaches. On examination, she had a partial Parinaud’s syndrome, and on history, she had been complaining of progressive diplopia for over two years. Imaging showed a large non-enhancing cystic lesion in the right mesencephalic-thalamic junction and descending to the cerebral peduncle, compressing the posterior aspect of the third ventricle and the aqueduct of Sylvius and responsible for secondary ventriculomegaly. She was treated with an endoscopic fenestration of the cyst, which leads to resolution of her symptoms. Conclusion: A contralateral endoscopic approach is a good minimally invasive technique for midbrain cysts, but such cases haven’t been widely reported. We discuss our case and provide a review of the literature showing good long-term outcomes when fenestrated into the ventricular system.


Introduction
Midbrain arachnoid or ependymal cysts are rare, and usually present with symptoms associated with the secondary hydrocephalus, or from the mass effect of the lesion itself.Because of their location, the open approach is difficult, but it is amenable to minimally invasive treatment by endoscopy.Neuroendoscopy is increasingly used to fenestrate arachnoid cysts into the cysternal or ventricular system, as it is less invasive than an open approach.Whether or not to leave a stent behind is a controversial question, as some argue that only fenestrating the cyst might lead to reaccumulation; while other argue that leaving foreign material behind increases the risk of infection.We describe a case of a midbrain cyst presenting with a partial Parinaud's syndrome followed by acute confusion; which was treated by endoscopic fenestration and remained stable for at least 4 years.

Case Report History and Examination
A 52-Years-old woman was brought to the emergency department after a morning at work where she was found confused by her collaborators.She had been complaining of progressive diplopia over the last 2 years, as well as in the last month, worsening headaches and pain in the right arm.When she presented in the hospital, she was confused to date and location, had a limitation in the elevation of both eyes, had a horizontal binocular diplopia, a convergence retraction nystagmus as well as an upbeat nystagmus on upgaze, compatible with a partial Parinaud's syndrome.

Imaging
A CT, followed by an MRI showed a large cystic lesion (2.1 × 2.1 × 2.3 cm) in the right mesencephalic-thalamic junction and descending to the cerebral peduncle, compressing the posterior aspect of the third ventricle and the aqueduct of Sylvius and responsible for secondary ventriculomegaly.The cystic lesion was not enhancing and was thought to be an arachnoid or ependymal cyst (Figures 1-4).

Treatment
A minimally invasive approach with the endoscope was chosen, The patient was operated through a left frontal burr hole, with the assistance of the neuronavigation.We used a rigid zero degree Storz ventriculoscope.Once the third ventricle was entered, the mamillary bodies and the mass intermedia were visualized.The cyst was identified as a bulging structure in the posterior contralateral wall of the ventricle.It was confirmed with neuronavigation, and a sharp endoscopic monopolar was used to mechanically pierce the wall of the ventricle.CSF like fluid was identified in the cyst, and some flow was seen entering the ventricle from the cyst.Then alligator forceps were used to enlarge the hole.An EVD was left in place, clamped, as a safety in case of acute hydrocephalus.It was removed after 24h after having remained clamped.

Postoperative Course
The patient progressively became less confused and normalized her neurological exam over a couple of days.Two months postprocedure, she was asymptomatic and her MRI showed a residual cyst measuring 9mm in its largest axis, no mass effect, and resolution of the ventriculomegaly.After 4 years, the cyst size remained the same and the patient continued to be asymptomatic.
With the technological advancement of neuroendoscopy [22], endoscopic procedures have been increasingly used for intraventricular lesions, as well as arachnoid cysts in general [23,24].Better lenses have enabled cleaner visualization and therefore safer procedures and an increasing number of endoscopic tools have made the interventions easier.While mesencephalic or midbrain ependymal cysts are not directly intraventricular, most have only a small lining of parenchyma between the ependymal wall of the ventricle and their cavity.They are therefore easily accessible with an endoscope, which is an elegant technique with minimal impact on the brain.The advantage of the endoscope over an open technique is the fact that it allows a maximal visualization for a minimal opening.Endoscopes have been used to treat a different kind of intracranial and paraventricular cysts, such as arachnoid cysts [25][26][27], colloid cysts [27,28], Rathke's cleft cysts and cystic craniopharyngiomas [27,29], as well as biopsies of intraventricular and paraventricular tumors [27,30].Lindert E et al. [31] reported a series consisting of five cases, three being treated solely with the endoscope, one as an endoscopicallyassisted case and the last one with the microscope.Conrad j, et al. [6] also reported some ependymal cysts in the mesencephalic region treated endoscopically with good results and Fiorindi A et al. [32] reported four cases treated endoscopically, one of them requiring the insertion of a Ommaya reservoir with another approach.As in our case, in most cases the patients had an improvement of the symptoms after surgery, with most commonly a reduction in size or disappearance of the cyst, however, some cysts remained the same with a reduction of the symptomatology [31], with no recurrence after more than 1y [26,31].

Previous case reports have been reported but remain rare. van
No major complication arising from endoscopic treatment have been reported; the main problem being probably the inability to finish surgery or fenestrate the cyst adequately.In those cases, the procedure can be extended to an endoscopically-assisted surgery (inserting instruments through a second small corticectomy) [31] or converted to a microscopic approach.
Another controversy is whether or not to leave a stent behind.Some argue in favor of a stent or grommet as they experienced a closure of a stoma earlier [14], but our case shows that it is not necessarily the case, even at 4 years.The introduction of a stent or grommet makes the procedure slightly more difficult, with a risk of migration or infection of the device.Other teams have shown that the introduction of a stent after fenestrating an arachnoid cyst is not required [33].When using a rigid endoscope, the use of neuronavigation in planning and performing the surgery allows selection of the best angle without having to retract the fornix or cortex too much.However, once the ventricular system has been entered, the accuracy of the system decreases with the leakage of CSF and the subsequent brain shift; and therefore at that point, one can only rely on visual landmarks.The disadvantage of current endoscopes compared to a microscopic approach is the 2D image, which decreases the cues for the surgeon.The bulging area due to the local mass effect from the cyst can be better appreciated with a binocular view such as seen under a microscope.A solution would be the use of a 3D endoscope.

Conclusion
Endoscopic treatment of midbrain ependymal or arachnoid cyst is an effective and elegant method with minimal comorbidities for the patient.The fenestration into the ventricular system seems to provide good long-term results, and while some cases have required a reoperation, most cases stayed stable without the insertion of a stent.

Figure 1 :
Figure 1: Preoperative MRI a. Postgadolinium axial T1-MRI showing a right midbrain cyst descending into the right cerebral peduncle with local mass effect and ventriculomegaly (enlargement of the right occipital horn).b.Coronal T1-MRI showing the right midbrain cyst with ventriculomegaly.c.Sagittal FLAIR MRI showing the midbrain cyst, the mass effect on the tectal plate and the obstruction of the Aqueduct of Sylvius.

Figure 2 :Figure 3 :Figure 4 :
Figure 2:The postoperative CT (day 1) shows a bit of blood in the cyst as well as in the ventricular system, the size of the cyst is similar to the preoperative imaging