31.1 Introduction

Neuroendoscopy has revolutionized the management of hydrocephalus since the mid-1990s. Endoscopic procedures like endoscopic third ventriculostomy (ETV) are the treatment of choice for most cases of obstructive hydrocephalus.1,2,3,4,5,6 Neuroendoscopy, as a diagnostic tool, has allowed a better understanding of the causes and mechanisms that result in hydrocephalus.4,5 However, neuroendoscopic exploration should not be limited to the ventricles as it is possible to access the subarachnoid space of the basal cisterns transventricularly, particularly with a flexible neuroendoscope.7,8,9,10,11 The interpeduncular, prepontine, and prebulbar cisterns can be reached through an ETV,4 and the cisterna magna through the foramen of Magendie of the fourth ventricle.12 This method has enabled neurosurgeons to explore, diagnose, and conduct procedures inside the basal cisterns safely. These procedures include biopsies,4,13 parasitic extraction5 (see Chapter 28), and biopsy of exophytic brainstem cerebral gliomas,14 among others. Basal cisterns can be affected by infectious, auto-immune, granulomatous, and other disorders that can result in basal meningitis with adhesive arachnoiditis and subsequent hydrocephalus.15,16,17 Chronic meningitis and other granulomatous diseases affect the basal cisterns more than the rest of the arachnoid spaces, so diagnosis can be a challenge especially when biopsies are not taken from affected areas (cortex).4 Simultaneously, secondary hydrocephalus can be treated with endoscopic procedures.4,10,11,18 This chapter will review in detail the indications, operative technique, results, clinical outcome, and possible complications of endoscopic transventricular basal cistern exploration and biopsy.

31.2 Indications/Contraindications

Transventricular arachnoid biopsy of the basal cisterns is indicated on patients with chronic meningitis of uncertain diagnosis that despite empiric treatment has had no clinical improvement. Diagnostic work-up includes magnetic resonance imaging (MRI) demonstrating enhancement of the basal cisterns, with or without hydrocephalus, and negative cerebrospinal fluid (CSF) from repeated lumbar punctures (gram, culture, immunologic, and molecular tests).4,9 Thoughtful exploration of the basal cisterns and ventricular system should be performed if necessary. Exploration should include at a minimum the interpenducular and prepontine cisterns, Liliequist’s membrane, both lateral ventricles, and the third ventricle. Exploration of the cerebral aqueduct, fourth ventricle, and cisterna magna is done only when feasible and by an experienced neuroendoscopist4,10,11,12 (see Chapters 27 and 28). Ventriculomegaly is not a requisite for transventricular basal cistern exploration. Endoscopic exploration in pediatric and adult patients with normal ventricular size has been demonstrated to be safe and feasible.1,19,20

Exploration of the basal cisterns is also indicated for patients with intraventricular neurocysticercosis. The presence of parasites may not be evident on neuro imaging, and exploration of the basal cisterns for parasites is necessary5,9 (see Chapter 28).

It is important to be familiar with the term frozen subarachnoid space, a direct endoscopic finding mentioned in the literature. It refers to severe adhesive arachnoid bands attached to the basilar artery and clivus, diminishing the average visible pulsations of the basilar artery; this phenomenon was named “frozen” subarachnoid space due to the immobile appearance of the vasculature.4 This endoscopic finding is determined during the neuroendoscopy posterior to endoscopic third ventriculostomy and no specific measuring method is used to make this differentiation.4 The frozen subarachnoid space plays an important role in predicting the efficacy of ETV, and it can prevent further exploration down into the basal cisterns.21 This is common for patients with chronic meningitis.4,21

Contraindications for basal cistern exploration are patients with aneurysms of the arteries of the skull base and severe anatomical abnormalities that do not allow differentiating the third floor ventricle structures.9,11 Taking an arachnoid biopsy in the pier of the cisterna magna is contraindicated in patients with complete agenesis of the cerebral aqueduct and the foramen of Magendie.9,11

31.3 Operative Technique

31.3.2 Description of Procedure

Once the endoscope is in the lateral ventricle, the foramen of Monro is identified and the third ventricle is accessed. The ETV is performed in a standard fashion (see Chapter 21). It is important to observe the characteristics of the ependyma covering the ventricles and the premammillary membrane, since these have been suggested as a prognostic factor of ETV in infectious diseases.21 The ETV size has to be sufficiently extended to permit the passage of the endoscope; if the Liliequist’s membrane is closed, it must also be opened.4,10 Introduce the endoscope through the fenestration carefully into the interpeduncular cistern in front of the basilar artery, preventing direct contact of the tip with the basilar artery and avoiding tearing of the perforating arteries located in the cistern (Video 31.1). Carefully navigate through the interpeduncular cistern to the prepontine cistern (between the basilar artery and clivus) and the prebulbar cistern, following the ventral part of the basilar artery and the junction of the vertebral arteries (Video 31.1).4,11 During this exploration of the characteristics of the arachnoid, blood vessels, and nerves observe for signs of inflammation, permeability of the subarachnoid space, CSF flow, and the presence of exudates or granulations. Arachnoid biopsy is performed by gentle traction on arachnoid, preferably arachnoid membranes and trabeculae, with a grasping forceps4 (Fig. 31.1). If any particular area is seen to be more inflamed or to have more exudates, it is important to take a biopsy at that site if possible (respecting the neurovascular structures of the skull base). To take a biopsy of the cisterna magna, the fourth ventricle is approached by the endoscope through the foramen of Magendie and then navigated. The technique is described in Chapters 27 and 28, and the biopsy is taken in the same way as described there. In this procedure, it is important not to damage the posterior inferior cerebellar arteries subsequent to passing through the foramen of Magendie.4,12

Related posts:

30 Principles of Endoscope-Assisted Microneurosurgery 35 Endoscopy in Aneurysm Surgery 32 Endoscope-Assisted and Fully Endoscopic Microvascular Decompression 33 Endoscopic-Assisted Resection of Intra-Axial Brain Tumors 34 Endoscopic Resection of Extra-Axial Tumors 36 Use of Fluorescence in Neuroendoscopy

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