The results of ETV in patients affected by myelomeningocele are not satisfactory in very young children [8], while they are good in older children and adolescents (>70 % success).

The presence of a neural tube defect with the consequent CSF leak before birth and its rapid closure immediately after birth may explain impaired development of the subarachnoid spaces and Pacchioni granulations. This could add the factor of poor resorption to the CSF pathways anatomic obstructions (aqueductal stenosis, Chiari II malformation). Later in life, subarachnoid spaces and Pacchioni granulations could develop and allow normal circulation and resorption, thus leaving only the anatomic, obstructive component of hydrocephalus and explaining the good results of ETV performed during shunt malfunction in older meningomyelocele patients.

At the beginning of the experience with endoscopic surgery, patients who suffered CSF infection or intraventricular hemorrhage were considered poor candidates for ETV because of the fear for impairment of mechanisms of CSF absorption. Actually some forms of CNS infection, such as congenital toxoplasmosis, mumps meningoencephalitis, and tuberculous meningitis, can be complicated by obstructive hydrocephalus that can be successfully treated by ETV [1]. Also some cases of hydrocephalus secondary to intraventricular hemorrhage may be attributable to organization of the clot in the aqueduct with secondary aqueductal stenosis. Recent studies report that endoscopic third ventriculostomy is effective in approximately two-thirds of the patients who suffered meningitis, shunt infection, and subarachnoid and/or intraventricular hemorrhage [1, 27]. These data were first shown by the multicenter study of Siomin and colleagues that evaluated the safety, efficacy, and indications of ETV in patients with a history of subarachnoid hemorrhage or intraventricular hemorrhage and/or CSF infection [7]. These authors underlined the concept that patients with obstructive hydrocephalus with a history of either hemorrhage or infection may be good candidates for ETV, while patients who suffered both hemorrhage and infection should be poor candidates.

Hydrocephalus may complicate posterior fossa tumors during all phases of the disease. Most often hydrocephalus is already present at diagnosis, but it may also occur following surgical removal of the tumor, in case of tumor recurrence and in case of spreading of malignant tumors in the cerebrospinal fluid (CSF) [28].

Surgical options to manage this kind of hydrocephalus include steroids and early surgery, external ventricular drainage (EVD), placement of ventriculoperitoneal (VP) shunt, and ETV. Theoretically ETV offers the same advantage of implantation of ventriculoperitoneal shunting, such as rapid normalization of raised ICP, improvement of the patient’s general condition, prevention of postoperative ICP elevation, and long-term control of hydrocephalus, without the shunt-related complications and the complication related to external ventricular drainage. At the beginning of the 2000s, since the publications of the group of Hôpital Necker-Enfants Malades of Paris [29], ETV obtained great popularity in this context, because it was considered able not only to control hydrocephalus in emergency but also to reduce incidence of postoperative hydrocephalus and to improve postoperative course following tumor removal reducing complications such as cerebellar swelling, pseudomeningocele, and CSF leak. However, subsequent studies reproduced only partially the initial good results of Sainte-Rose et al. [29], questioning, above all, the possibility of ETV to provide long-term control of hydrocephalus [30–32]. Moreover, some concern was raised about the systematic preoperative use of ETV in posterior fossa tumors, because a percentage of patients would be submitted to an unnecessary procedure [30, 31, 33], considering that ETV is not without risk [34] and that in most patients hydrocephalus can be resolved with surgical removal of posterior fossa tumor alone [35]. Therefore, the role of preoperative ETV still remains controversial [30, 31]. Moreover, it should be considered that preoperative ETV is at risk of secondary closure by blood products and tumor debris following operation for tumor removal. The risk of postoperative ETV failure might also depend on an impaired CSF absorption within the peripheral subarachnoid spaces because of inflammatory reactions secondary to the operation [34].

Recent orientation is to reserve preoperative ETV to those tumors with higher risk of postoperative hydrocephalus and only in cases where urgent management of hydrocephalus is required. Tumors of the midline, involving the fourth ventricle, especially if malignant and obstructing the foramina of Luschka and Magendie are at high risk of postoperative hydrocephalus and usually present with acute intracranial hypertension. The presence of papilledema at diagnosis is also considered to be significant in a recent study carried out to evaluate the risk of persisting hydrocephalus [36]. This study proposes a risk score where under the age two, the presence of cerebral metastases and the presence of initial hydrocephalus and its severity are confirmed to be the most important features to predict the persistence of postoperative hydrocephalus. The presumed histology of medulloblastoma, ependymoma, or dorsally exophytic brainstem glioma as well as the presence of papilledema seem also to have a role, even if less important.

In our center, patients with midline solid tumors and severe hydrocephalus are considered candidates for preoperative ETV, while patients with hemispheric cystic tumors undergo steroid therapy and early surgery. Urgent ETV is also important to obtain time to conclude the diagnostic workup and schedule tumor surgery in the first available surgical session, following improvement of signs and symptoms of intracranial hypertension.

This practice is in line with the more recent publication of the group of Hôpital Necker-Enfants Malades of Paris, in which it is underlined that correct indication for preresectional ETV concerns essentially tumors involving the fourth ventricle [34]. Preoperative ETV should also be avoided when CSF metastases are suggested by the neuroimaging investigations.

Recently, El-Ghandour [37] reported the first study specifically addressed to midline posterior fossa tumors (medulloblastomas and ependymomas) in pediatric patients with advanced hydrocephalus. He compared preresectional ETV vs. ventriculoperitoneal (VP) shunt. He concluded that the lower incidence of morbidity, the absence of mortality, the lower incidence of procedure failure of ETV as compared to VP shunt, and the significant advantage of not becoming shunt dependent render endoscopic third ventriculostomy the first choice in the treatment of pediatric patients with marked obstructive hydrocephalus due to midline posterior fossa tumors.

More agreement in the neurosurgical community is present in considering postoperative hydrocephalus obstructive in nature and to offer ETV as an alternative to shunt insertion to such patients [29–31].

Tamburrini et al. [38] recently have proposed a different strategy for management of hydrocephalus in posterior fossa tumors: perioperative external ventricular drainage positioned at time of tumor removal, postoperative ICP monitoring through the external ventricular drainage, ETV in case of persistent ventricular dilation and abnormally high ICP values, and VP shunt implantation in case of ETV failure.

In case of failure of ETV following posterior fossa tumor removal, a redo ETV can allow controlling the hydrocephalus when the failure is due to a closure of the stoma; conversely, in case of permeable ETV, the hydrocephalus must be treated with an extrathecal shunt [34].

At the beginning of the experience with ETV, patients previously shunted were considered as poor candidates. The shunt, diverting CSF away from the site of CSF absorption (Pacchioni granulations), was believed to impair the mechanism of CSF absorption. This belief led to the assumption: “once a shunt, always a shunt.” More recent observations did not confirm this; on the contrary, it is now believed that a shunt, diverting CSF away from the ventricular system, may allow the CSF spaces around the brain to re-expand, increasing the likelihood of CSF resorption following an ETV [39]. Moreover, it was shown that the anatomy of initially communicating hydrocephalus may change over time following shunt insertion, increasing the likelihood of success of an ETV after a shunt malfunction [4, 6, 40]. Following shunting of originally communicating hydrocephalus, an acquired aqueductal stenosis may develop: the continuous CSF diversion from the lateral ventricle may create a pressure gradient between the supratentorial and infratentorial compartment, causing an anterior rotation of the upper vermis, with subsequent aqueductal stenosis. The results of Siomin in 2002 [7] and O’Brien [6] in 2005 support this hypothesis: Siomin reported an increase in ETV efficacy in posthemorrhagic hydrocephalus from 60.9 to 100 % in primary and secondary ETV, and O’Brien reported an increase of success rate from 27 to 71 % in posthemorrhagic hydrocephalus and from 0 to 75 % in post-meningitic hydrocephalus. Also, in patients with myelomeningocele, a better outcome after secondary ETV has been reported by Teo (84 % vs. 24 %) [8].

In summary, candidates for ETV as alternative to shunt revision are those patients who present an obstructive hydrocephalus at the time of shunt malfunction, regardless of the original cause and radiological appearance of the hydrocephalus. Etiologies of hydrocephalus in which secondary ETV may be considered include aqueductal stenosis, meningomyelocele, obstruction caused by tumor, and posthemorrhagic or post-meningitic hydrocephalus with secondary aqueductal stenosis. We do not agree with those authors who perform ETV whatever radiological appearance of hydrocephalus at time of shunt malfunction.

Preoperative evaluation by magnetic resonance imaging is mandatory to assess anatomical suitability and the patency of the aqueduct and fourth ventricle outlets. MRI T2 sagittal images, with CISS or DRIVE sequences (which are particularly useful in defining structures with two CSF interfaces), should be carefully checked in order to verify the patency and morphology of the prepontine cisterns and, above all, the anatomical features of the third ventricle floor, to be sure that there is enough space to perform the perforation of the floor.

Overall, successful ETV in selected patients who had previously undergone shunt placement has been reported in 42–100 % of patients in different studies [4, 6, 40–44]. In most series the results of secondary ETV are not different from those of primary ETV. It should be remembered that a higher number of complications and abandoned procedures are reported [27, 44–46]. Performing ETV for malfunctioning or infected shunts, in fact, is more difficult than in primary cases because presentation is usually acute, with less severe ventricular dilatation and a third ventricular floor thicker than those seen in chronic hydrocephalus. Moreover, other anatomical abnormalities can be found [39, 47]. The CSF, in case of infection, may be not clear, affecting the surgeon’s view through the optic device. Considering these factors, ETV in shunt malfunction should be performed by expert neuro-endoscopists.

The usual management of shunt infection consists of shunt removal, external ventricular drainage placement, and antibiotics administration, until CSF sterilization. ETV may be performed as alternative of shunt reinsertion in case of compatible anatomy. In case of distal shunt infection (i.e., abdominal pseudocyst or ascites) without ventriculitis (normal CSF sampling), ETV may be performed at time of shunt removal. There are few reports in literature specifically addressing the role of ETV in shunt infection. However, preliminary results are encouraging [4, 48–50]. Recently, Shimizu et al. retrospectively reviewed children with shunted hydrocephalus extracting data on CSF shunt infection. They observed a 30 % risk of CSF reinfection in the patients treated with shunt reinsertion and a 11 % risk in the patients in which ETV was performed as alternative to shunt reinsertion. There were no significant differences between the longevity of the reinserted shunt and of ETV. ETV allowed to completely remove the shunt in only a minority of cases (<20 %). The authors conclude that even if a high number of ETV finally fail, the reinserted VP shunt has significantly better longevity than a VP shunt reinserted without using ETV [51].

In conclusion, shunt infection should not be considered a contraindication to ETV, in case of compatible anatomy. ETV may help in managing those difficult cases of chronic bacterial shunt colonization, avoiding reimplantation of foreign material [4].

ETV is the treatment of choice of obstructive hydrocephalus, with limited role in case of communicating hydrocephalus. However, reports attesting its efficacy also in case of communicating hydrocephalus have been published, with a successful rate of 45–70 % [52, 53].

The rationale of efficacy of ETV in case of communicating hydrocephalus is more difficult to understand. Differently from obstructive hydrocephalus, where the treatment is aimed to bypass the obstruction, the primary goal in chronic communicating hydrocephalus is to restore intracranial compliance: after opening the ventricle into the basal subarachnoid space, there is an increased expulsion of ventricular CSF during systole, thus reducing the intraventricular pulse pressure, the transmantle pulsatile stress, and the ventricular size. As a consequence, the subarachnoid spaces and their contents expand including the cortical veins, thus restoring intracranial compliance, cerebral blood flow, and perfusion pressure. ETV decreases the transmantle pulsatile stress, thereby reducing the resistance to CSF reabsorption [53, 54]. However, these results should be considered with caution, because in these series a number of patients with shunt malfunction or blockage at the level of the fourth ventricle outlet or basal cistern may be included.

In series of ETV performed in case of normal pressure hydrocephalus (NPH), the success rate is surprisingly high, usually exceeding 65 % [53, 55]. This can be explained with the same physiopathologic considerations as for communicating hydrocephalus. In patients with NPH, the brain elasticity is lost due to multiple factors, including an insufficient transcortical subarachnoid space, fibrosis meningitis, and, above all, periventricular ischemic lesions that weaken the cerebral ventricles. Fenestration of the floor of the third ventricle results in a decrease in the intraventricular pressure, consequently increasing cerebral blood flow and perfusion pressure. However, these cases should also be considered with skepticism, because some cases classified as NPH actually are secondary to a condition known as LOVA (long-standing overt ventriculomegaly in adults) [56]. This is a form of chronic obstructive hydrocephalus with its roots in childhood but that manifests in middle-aged adults: the patients had severe triventriculomegaly and macrocephalus and become symptomatic during adulthood, mimicking a normal pressure hydrocephalus syndrome (dementia, gait disturbances, and urinary incontinence). In these cases, ETV usually restores CSF dynamics.

ETV may fail if the stoma becomes blocked by clot, debris, or development of new membranes [57]. A repeat ETV can be proposed in alternative to shunt insertion in well-selected cases: in patients that had experienced excellent clinical response to the first ETV and have favorable radiological presentation (disappearance of the flow artifact on sagittal T2-weighted MRI sequences and presence of typical anatomic deformation of the third ventricle) [58]. In these cases reopening or enlargement of the stoma carries the same success rate as the primary treatment (>65 %) [57–59]. The presence, at surgery, of membranes and arachnoid adhesions in the subarachnoid cisterns and age younger of 2 years at first ETV are important predictors of failure of the second ETV [58].