The anatomical characteristics of the IIIVT and cisterns are well-known, and several descriptions exist, dating from the XVII century. Initially, these studies focused on descriptive anatomy, using fixed anatomical specimens. The earliest documented LM description was in 1875, by Key and Retzius, but the structure remained unelucidated for many years , until it was “rediscovered” in 1957 by Liliequist, through pneumoencephalography observations in human cadavers . Between the 1970s and 1990s, many studies of cisterns and the LM were performed, especially after Yasargil's work . Several authors have described their anatomical findings, and numerous in-depth reports have been published [5,31,32,36,38,44,45]. However, most of these studies used the microscopic surgical approach [48,31,45,44].
In the 2000s, Inoue et al.  studied the ventricular system and cisterns, including angles viewed during neuroendoscopy. Since then, many other studies have examined the LM neuroendoscopically [1,14,37,38,48], which has clarified that the LM varies widely in morphology, behavior, and consistency. The wide range in characteristics of LMs leads to differences in what the ETV technique can achieve. However, despite this acquired knowledge, little progress has been made toward in vivo LM characterization to determine its relationship with the TC.
Impact of TC/LM anatomy on ETV
ETV requires opening the IIIVT floor at the TC epicenter. The TC is a thin layer of gray substance, which is associated with the ependyma, starting anteriorly from the infundibular recess to the mammillary body posteriorly and limited laterally by the hypothalamus. LM forms a band of arachnoid tissue and is inferior to TC, which originates from the posterior sela turcica and splits. In most cases, there are two portions, including an upper diencephalic leaf, which connects to the diencephalon along the posterior edge of mammillary bodies, and a lower mesencephalic leaf, which attaches along the midbrain junction and the pons. The space between the two leaves is the IC (44) (Figure 4). Generally, the mesencephalic leaf has an open border at its lower limit, which allows natural ICs to communicate with the pre-pontine cistern below it. (Figure 5). However, this border is naturally closed sometimes, so communication between cisterns does not always occur, or the LM may be absent .
ETV must facilitate communication between the IIIVT and IC through the surgical opening of the TC and LM (diencephalic portion). If this does not occur, communication will not complete and the surgery will be ineffective [1,8,16,22,26,33,41,49]. When the mesencephalic portion of the LM is not naturally continuous with the pre-pontine cistern, surgical opening of the mesencephalic part will be required .
ETV success and technical aspects of the TC/LM overture
Currently, ETV is widely performed [49,42,41,27,25,20,40,26,7,39]. However, discrepancies in its efficacy can be found throughout the literature [3,7,4,13,25,26,40,42,46]. The technical details of the ETV procedure may vary among providers, although incomplete opening of the TC or LM could also contributed to these discrepancies [22,43]. Misunderstanding the anatomy can lead to technical failure . It remains unclear whether the LM has the same characteristics in vivo as those described during anatomical dissections and whether the TC/LM relationship in vivo differs from that described in previous anatomical [8,1,48,31]. Additionally, the direct and superior surgical view that results from neuroendoscopy could result in structures being viewed from different perspectives than those presented in anatomical studies, which may be confusing for the surgeon [33,35,37]. After opening the TC, the LM characteristics must be noted and may require additional maneuvers for successful opening. In the present study, we observed a thick LM in one-third of cases, and the overture was hard in 39% of procedures. Despite the fact that a “naked basilar artery” was achieved in all cases of this sample, the difficulty during the procedure resulted in a lower ETV success rate in a long-term analysis. Furthermore, an anomalous IIIVT floor is correlated with a higher rate of ETV failure in the first six months after the procedure and was a valid intraoperative parameter impacting the ETV success rate. This impression aligns with several reports that have considered the anatomic aspects of TC and LM and could assist surgeons in identifying procedures that may be difficult or require extra care [3,9,10,20,21,23,24,28,35,39,43,47,46,51].
Another interesting finding of our study was the significant ability of ETVSS to predict the actual ETV success as well as the level of difficulty of the LM overture, which validates the importance of this score in warning neurosurgeons about the significance of the three preoperative parameters and the associated risk of ETV failure. Additionally, difficult cases are more prone to intraoperative complications, such as severe bleeding. According to the findings of this study, the cut-off ETVSS was 50%. A high failure rate of ETV was observed in myelomeningocele patients (60%) and in patients younger than one month of life (75%). This also represents the more difficult level at which ETV must be performed in both situations, as well as the lower probability for success based on the ETVSS [20,35]. This fact could explain the variations in =predictability of ETVSS at difficult levels in our sample.
Limitations of the study and lessons to be learned
This study presented several limitations, including the absence of digital high quality radiologic data needed to compare anatomic details of TC and LM and measure the degree of ventriculomegaly, which could add relevant information about the difficult level of prediction, as well as the late radiologic outcome. Furthermore, the use of other instruments to perform ostomy by in other institutions could cause a bias in comparison with our parameters, since neurosurgeons have performed ETV with forceps instead bipolar or embolectomy catheters. Thus, further studies using different equipment, techniques, and surgeon experiences when opening the TC/LM could add more evidence to expand our findings. In spite of these limitations, the results of our study shed light on some of the factors that contribute to failure of the ETV procedure. For example, very young patients are at a greater risk, certain hydrocephalus etiologies, and the anatomic abnormalities of the IIIVT floor.
Because this was a prospective study, the cohort in this study reflected patients that our medical center treats, which influenced a pediatric patient prevalence. Our experience is likely to differ from most medical institutions where adults and the elderly generally predominate.
Neuroendoscopic in vivo observations showed that the LM anatomical characteristics were variable for a considerable proportion of the time. Anatomical TC changes were associated with different LM configurations. In these situations, TC and LM anatomical characteristics may be challenging to interpret, and the surgical response may be insufficient, which can make ETVs difficult to perform successfully.
Technical difficulty arises when with the TCs are distorted and/or opaque, with thickened LMs and with the existence of a space between the TC and LM, which requires the structures to be opened separately.
Hydrocephalus is associated with congenital malformations and inflammatory processes. Bleeding increased the difficulty of opening the LM. Because these etiologies are more common in childhood, difficulty performing ETVs under these conditions may explain the higher failure rate observed among children who are younger than one month.
A higher incidence of congenital malformations among infants was observed, especially with a myelomeningocele, which generally results in a complex TC/LM relationship, as well as a thickened LM that is more difficult to open. Therefore, advanced technical expertise is necessary, and experience is advised.
Congenital malformations and "infection/parasitic" structural changes are associated with difficulty opening the LM. In these etiologies, TC anatomical changes predominate. As the degree of anatomical distortion increases, the technical difficulty of the ETV also increases.
Therefore, we can made the following assumptions, based on our findings.
- When performing ETVs, neurosurgeons have a 33% chance of encountering anomalies on the IIIVT floor , including three different characteristics: anatomical distortions, tissue alterations, and hemorrhage. In addition, this anomaly is converted with a 50% of chance of failure of the procedure during the first six months after the ETV procedure, even when the LM was successfully opened.
- After opening the IIIVT floor, a neurosurgeon will find a thickened LM in one-third of cases, thereby increasing the difficulty of the surgery. TC anatomical alterations, opacity, and the degree of difficulty required to open the TC are factors that were significantly associated with difficulty opening the Furthermore, in 55.6% of cases, TC and LM could not be opened simultaneously, so they needed to be opened separately. Usually, when the floor anatomy is altered, the TC and LM tend to be separated. In this situation, two distinct technical acts are required to open both structures. Additionally, the LM opening becomes more difficult to open in this circumstance, further increasing the risk of complication. Thus, these types of cases show a trend toward failure after 10 months of ETV was performed.
This procedure has a high risk of complications due to various factors. In this study, the ETVSS demonstrated that it is a valuable indicator for predicting certain difficulties based on preoperative features, such as patient age, hydrocephalus etiology, and the previous shunt, which must be recognized in advance by the neurosurgeon so that preventative techniques or alternate approaches can be implemented.
ETV is a relatively quick procedure due to its reliable anatomical references and the availability of standard techniques, so it is considered by some to be easy to perform. However, morphological difficulties are common, particularly in certain patients, such as children under two years of age. Therefore, LM recognition and correct management must be performed each time to ensure that the ventricular system is fully opened and communicates with the subarachnoid space.