Patients diagnosed with hydrocephalus related to myelomeningocele face a higher risk of procedure failure, whether with VPS or ETV, compared to patients with hydrocephalus due to other etiologies [5, 9]. In this study, the incidence of MMC-associated hydrocephalus (83.4%) was similar to that reported in the general literature (80%) [3, 4]. The overall ETV success rate was 57.1%, with a six-month success rate of 61.9%.
The influence of demographic, radiological, and clinical factors on ETV outcomes in myelomeningocele (MMC) patients remains uncertain [10]. Our study highlighted age as a critical clinical determinant of ETV success. Infants ≤ 6 months old had lower success rates (25%) compared to older patients (76.9%) [RR: 0.1; 95% CI 0.01–0.78; p = 0.0318]. These findings are consistent with prior research. Zaben et al. [11] found lower success rates in infants under six months (44.4%) compared to older infants (66.7%, p = 0.0007). Similarly, Teo and Jones [12] reported a success rate of 12.5% in infants under six months, contrasting with 80% success in older infants.
The prevalent view suggests that in younger patients, the immaturity of arachnoid villi leads to reduced cerebrospinal fluid (CSF) absorption. This results in a non-communicating state, potentially reducing the effectiveness of ETV. However, recent research indicates that arachnoid villi are less critical in CSF dynamics among younger patients. As such, it is imperative to explore alternative explanations for the observed correlation between younger age and decreased ETV success rates [13, 14].
A contributing factor could be the cranial volume expansibility in infants due to incomplete skull ossification. In younger patients, a CSF volume increase would be accommodated by an increase in head diameter, maintaining intracranial pressure. However, it could lead to an increased venous pressure gradient, inhibiting CSF absorption, especially in patients with MMC due to the significantly smaller posterior fossa volume. Epstein and Hochwal (1975) advocated compressive cranial bandages to limit cranial growth in infants with hydrocephalus, promoting CSF absorption and avoiding shunting [25].
In our study, with an average patient age of 234 days, the applicability of the Monroe-Kellie doctrine may be questioned due to potential ventricular volume increase and subsequent head circumference change. However, our analysis of head circumference, an indirect measure of CSF compartment volume, showed no significant difference between successful and failed ETV groups, indicating no apparent impact.
Linear indices, offering more accurate ventricular volume estimation than head circumference [15], were similar in both successful and failed ETV groups, aligning with Börcek et al.'s findings [16] that preoperative lateral ventricle measurements did not correlate with ETV success.
Another plausible explanation for the high failure rates in younger patients may be a greater predisposition towards stoma closure produced during ETV [17]. In a study involving eleven infants under one year who underwent ETV revision following previous procedure failure, stoma occlusion was evident, ranging from partial closure to complete occlusion or blockage [18]. The increased stoma occlusion rates in infants might stem from a higher tendency for the formation of new arachnoid membranes, gliotic tissue, and ependymal tissue. This stoma obstruction likely contributed more significantly to ETV failure than the prevalence of disabsorptive components [17, 18].
The absence of a pressure gradient between the III ventricle and the subarachnoid space could also exacerbate stoma occlusion. Börcek et al. [16] identified changes in the morphology of the third ventricle as having the strongest correlation with the success or failure of ETV [16, 19]. A meta-analysis showed an 85% success rate in patients exhibiting third ventricle bulging, suggesting an increased success rate associated with this finding [20]. Consequently, the bulging of the third ventricle's floor may indirectly indicate a pressure gradient between the ventricular system and the pre-pontine cistern, potentially tripling the chance of ETV success [16, 21]. However, our study lacks sufficient postoperative MRI images to demonstrate these associations conclusively with ETV failures.
The average success rate, as determined by the ETV Success Score (ETVSS), is closely aligned with the actual success rate when considering the margin of error (46.7 ± 11.7% and 61.9%, respectively). Although ETVSS is commonly used in contemporary practice, its limitations should be acknowledged, particularly in patients with myelomeningocele. For populations under two, ETVSS tends to underestimate the success rate [22]. This discrepancy can be attributed to the ETVSS's foundation in multivariate regression models of multicentric variables, which disproportionately associate age with ETV failure [23]. Further caution should be exercised due to the ETVSS's underrepresentation of MMC patients in its development studies [23].
From a radiological perspective, particular attention was given to the posterior fossa due to its disproportionate size-to-content relationship inherent to myelomeningocele's (MMC) pathophysiology [4]. The posterior fossa volume was indirectly assessed via the clivus-occipital angle and directly through planimetric measurements proposed by Krogness et al. [24].
A h/H ratio less than 0.2 was associated with a lower success rate of ETV, albeit not statistically significant (RR 0.38; 95% CI 0.04–3.11, p = 0.61). Similarly, an angle less than 76º corresponded to a 2.5 times higher ETV success rate, yet also not statistically significant (p = 0.36). Other measurements did not reveal associations with ETV success.
The h/H measurement, indicative of infratentorial to supratentorial disproportion, does not necessarily imply a small posterior fossa. If there is significant supratentorial disproportion, the value can be altered despite a normal-sized posterior fossa. This criticism extends to the posterior fossa ratio. Although Krogness's (1978) measurements are easy to apply, cranial compartment volume can be more effectively calculated using specific software despite requiring additional training [25].
This study linked a smaller posterior fossa dimension to ETV success. An angle less than 76º corresponded to a 2.5 times higher ETV success rate, yet not statistically significant (p = 0.3575). This could be seen as paradoxical, as a restricted fossa would intuitively cause increased venous congestion and hinder cerebrospinal fluid absorption. Therefore, we might hypothesize that in cases with severely restricted posterior fossa volumes, the disproportion between the recipient compartment and its content could cause significant anatomic distortions and obstructions to fluid flow, outweighing the effects of venous congestion.
In assessing cases where ETV failed, two patient groups emerged. In the first group, ETV failed around 15 days post-procedure, while in the second group, failure was only recorded 100 days after ETV. Preliminary analysis revealed normal h/H, h/Tw, and posterior fossa ratios (0.31, 0.32, and 14.4%) in patients with early failure. Conversely, those who experienced late failure showed values typical of a narrow posterior fossa (0.17, 0.23, and 7.8%), supporting the structural modification theory leading to hydrocephalus. However, no statistical relationship could be evaluated due to limited available imaging data.
This study acknowledges several limitations, including challenges in assembling a large sample group due to its retrospective nature and incomplete historical medical records. Restricting the evaluation to DICOM-available imaging studies limited the availability of images, primarily from after 2015, and the low prevalence of pre-ETV MRIs hindered comprehensive radiological factor analysis. Identifying patients who are more likely to achieve ETV success is crucial for reducing valve dependency among MMC patients, emphasizing the need to establish preoperative factors predictive of ETV outcomes tailored to MMC patients' unique characteristics. Future research should prioritize prospective, multicenter studies assessing correlations between clinical and radiological factors and short- and long-term ETV outcomes. Nevertheless, developing a new classification system or score to evaluate ETV success in MMC patients requires extensive development, sensitivity testing, reliability assessment, and validation.