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Endoscopic Third Ventriculostomy, Cerebral Aqueductoplasty, and Septum Pellucidotomy
Introduction
Neuroendoscopy for cerebrospinal fluid (CSF) pathology has experienced a resurgence in popularity over the past 30 years. It is an alternative to open transcortical approaches to access intraventricular and periventricular pathology. In cases of hydrocephalus, it can be an excellent alternative to shunt placement. In fact, endoscopic third ventriculostomy (ETV) is a commonly performed neuroendoscopic procedure, and it is a safe and effective procedure that can avoid long-term shunt dependence for select patients.
Cerebral aqueductoplasty is a procedure for the treatment of focal aqueductal stenosis and can be performed with or without stent placement to improve long-term durability of the treatment. The most common complications following the procedure are transient ophthalmoparesis and hemorrhage.
For cases of unilateral ventricular obstruction, septum pellucidotomy is a technique that creates a connection between the lateral ventricles by fenestrating the septum pellucidum. This can be an effective technique either alone or in combination with foraminoplasty or ventricular shunting. Safely performing the procedure requires an understanding of septal vein anatomy and being able to correctly identify the target area for the septostomy.
Historical Background of Neuroendoscopy
Endoscopy was present and used since the turn of the 20th century, when surgical interventions for hydrocephalus were first being developed. The first neuroendoscopic procedure was performed in 1910 by Vincent Darwin L’Espinasse, a Chicago urologist, who used a cystoscope to fulgurate the choroid plexus in two infants with hydrocephalus. One of the patients died; however, the other lived for 5 years. Walter Dandy performed several procedures for hydrocephalus. In 1918, he performed the first open ventriculostomy by fenestrating the lamina terminalis through a transfrontal approach. In 1922, he unsuccessfully attempted a choroid plexectomy using the endoscope and at that time coined the terms “ventriculoscope” and “ventriculoscopy.” Fay and Grant were the first to photograph the inside of the ventricular system in 1923. Interestingly, the exposure time for their images ranged from 30 to 90 seconds, accentuating the extremely poor lighting and visualization of neuroendoscopy at that time.
The first successful ETV was performed in 1923 by William J. Mixter in a 9-month-old hydrocephalic patient. Using a urethroscope, he was able to pass through the foramen of Monro, visualize the third ventricle and cerebral aqueduct, and make a hole in the floor of the third ventricle, connecting it to the interpeduncular cistern. He subsequently enlarged the hole with side-to-side mechanical motion up to a diameter of 4 mm.
Despite the advances made by a few pioneering surgeons, neuroendoscopic procedures were not widely adopted during the first half of the 20th century. Technology was a significant limiting factor because lighting and visualization were very poor and the setup of the endoscopic lenses was complicated and cumbersome. , In 1952, Nulsen and Spitz first reported creation of a shunt diverting CSF from the ventricular system to the jugular vein to successfully treat communicating hydrocephalus. This began a monumental shift in the hydrocephalus treatment paradigm toward shunt diversion. Although a select few surgeons would continue performing neuroendoscopic procedures to treat hydrocephalus, the majority of neurosurgeons adopted CSF shunt diversion as standard practice due to the low complexity and safety of the procedure.
During this hiatus in the clinical use of neuroendoscopy, critical advances in technology would eventually lead to revisiting the role of neuroendoscopy in neurosurgery. First-generation endoscopes used conventional lenses, which had a fixed refractive index. In 1966, Hopkins and Storz revolutionized lens technology by developing the SELFOC lens. These lenses used gradient-index glass, which allowed adjustment based on the lens radius. Compared with conventional lenses, a wider field of vision could be viewed while preserving the illumination. , , In 1969, Smith and Boyle developed the charge-coupled device, which allowed optical data to be converted into electrical current. This precluded the need for large relay stations and allowed for the development of smaller endoscopes. Perhaps most important was the development and improvement of fiberoptic cables that occurred between the 1950s to 1970s. Using fiberoptic cable technology, light could be separated from the rest of the endoscope and emitted from the tip without emitting significant amounts of heat or losing luminescence. ,
With improved technology, use of the endoscope was revisited for hydrocephalus treatment. In 1978, Vries reported a series of five patients who he had treated with ETV using a fiberoptic endoscope. Although all of his patients eventually required a shunt diversion procedure, the patients did well postoperatively, demonstrating that ETV was technically feasible with current technology. In 1990, Jones et al. reported 50% successful outcome from ETV in 24 patients with noncommunicating hydrocephalus with 8% morbidity and no mortality. In 1994, the same group reported an even better success rate (61%) in a series of 103 patients. Given the technological advances in endoscopic illumination and optics, the demonstrated safety of the procedure with second-generation endoscopes, and the clinical efficacy reported in the literature, ETV was widely adopted in the 1990s by the neurosurgical community to treat hydrocephalus. ,
Endoscopic Third Ventriculostomy
Patient Selection
Selection of the appropriate patient for ETV requires an understanding of CSF physiology, detailed study of radiographic anatomy, an appreciation for the location of CSF obstruction, and knowledge of results for various age groups and etiologies of hydrocephalus. , Pathologic obstruction can occur at any point in the CSF flow circuit. ETV treats hydrocephalus by creating an internal shunt from the third ventricle to the interpeduncular cistern. If the pathologic obstruction lies between the third ventricle and the subarachnoid space proximal to the interpeduncular cistern, then ETV will likely be an effective treatment. If the pathologic obstruction lies downstream from the subarachnoid space of the interpeduncular cistern, then ETV will likely be ineffective. In a review published by Madsen et al. of 130 publications, totaling 11,952 patients, a pooled average success rate of 65.3% was reported. They also demonstrated the highest ETV failure rate to be within the first 20 months of the procedure.
The etiology of the pathologic obstruction should be determined when considering the most appropriate treatment for hydrocephalus. The most responsive etiologies of hydrocephalus to ETV are tumors of the tectal region and congenital aqueductal stenosis, which can present in infancy or in a delayed or arrested fashion. , Tectal gliomas or pineal region tumors can compress the cerebral aqueduct, obstructing CSF flow from the third ventricle to the fourth ventricle. Likewise, congenital aqueductal stenosis also results in impaired flow from the third ventricle to the fourth ventricle. It can present in infants, children, adolescents, or adults. , Care must be taken not to confuse congenital aqueductal stenosis with narrowing of the cerebral aqueduct that occurs secondary to inward displacement of the brainstem. This scenario can be present in infants or young children with increased pressure of the superior sagittal sinus leading to decreased terminal absorption of CSF. When the sutures are open, this can lead to global increases in CSF volume, including in the ventricular system and the basal cisterns. An increase in the CSF volume in the prepontine and interpeduncular cisterns can produce compression of the brainstem, leading to secondary compression of the cerebral aqueduct, mimicking aqueductal stenosis. In these cases the site of obstruction lies distal to the interpeduncular cistern and placement of a ventricular shunt, not ETV, would be the indicated intervention. ,
Congenital abnormalities can be associated with hydrocephalus. Approximately 80% to 90% of patients with myelomeningocele have hydrocephalus requiring intervention. , The localization of CSF obstruction in these patients is often more complex than in patients with obstruction secondary to a tumor. If there are multiple sites of obstruction, ETV may not effectively bypass the entire site and hence be unsuccessful. ETV has been shown to be successful in a small cohort of adult Chiari II patients. Thus the role of ETV in cases of congenital malformations associated with hydrocephalus is not clear, and consideration of intervention must be done on a case-by-case basis.
Postinfectious hydrocephalus can also require CSF diversion. Obstruction resulting from infection can be localized to the basal cisterns or to the cortical subarachnoid space. If the obstruction is in the basal cisterns, then ETV can be considered. If the obstruction is distal, then ETV would not be beneficial. However, determination of the exact site of obstruction in these patients can be challenging and is not always possible. Another consideration in postinfectious patients is that there can be significant meningeal thickening of the leptomeninges and third ventricular floor. Thus creation of a hole in the floor of the third ventricle can be more challenging and have a higher risk of closure. History of infection has been shown to be a negative predictor of ETV success. , However, in properly selected patients, ETV can be successful in treating postinfectious hydrocephalus. ,
Although ETV has previously been used as a treatment for idiopathic normal pressure hydrocephalus (iNPH), it has been the subject of much debate. However, patients with arrested congenital aqueductal stenosis, or long-standing overt ventriculomegaly (LOVA), can successfully be treated with ETV. , ETV has previously been reported as a successful treatment for iNPH, but many of these studies were performed prior to the advent of high-definition magnetic resonance imaging (MRI) scans and likely identified patients with LOVA rather than true iNPH. Gangemi and colleagues reported the largest series to date of normal pressure hydrocephalus patients treated with ETV. Out of 110 patients, improvement was seen in 69.1% of patients, no change in 21.8%, and continued deterioration in 9.1%, with low rates of overdrainage; however, given the fact that the diagnosis was made on computed tomography (CT) findings alone, it is not clear that this was a true iNPH population.
There are factors other than pathology that have been shown to impact success of ETV for hydrocephalus. Age is one of the most important factors in predicting success of ETV in the pediatric population. Kulkarni et al. performed a multicenter, retrospective analysis on 618 pediatric patients who had undergone ETV. A training set of 455 patients was used to identify predictors of ETV success, and the remaining 163 patients were used to internally validate those predictors. In their analysis, age was the most significant predictor of ETV success. Younger patients (<1 year old) had a higher chance of failure compared with older patients, and infants (<1 month old) in particular had the highest likelihood of failure. , This confirmed previous reports and added weight to those that discourage ETV in infant patients. , In adult patients, there does not appear to be an association between age and risk of failure.
The presence of a previous shunt has different implications when considering treatment with ETV (sometimes called secondary ETV when it is performed in the setting of shunt malfunction, compared with primary ETV where it is performed before any shunting or other hydrocephalus procedures ). In the prior analysis of 618 pediatric patients by Kulkarni et al., presence of a shunt was shown to increase risk of ETV failure, although to a lower degree than certain hydrocephalus etiologies (e.g., postinfectious and very young patient age). However, the effect of treatment with shunting prior to ETV likely is different depending on the pathology. Sankey et al. published a retrospective review of 103 adult patients, comparing primary versus secondary ETV. They found patients with secondary ETV had a higher rate of symptom recurrence and surgical revision rate. In the series reported by Siomin et al., treatment with shunting prior to ETV was a positive predictor of ETV success. This difference was especially pronounced in the subset of patients with a history of premature birth. A review of secondary ETV by Waqar et al. included 519 patients from 15 observational studies had an overall 68.2% success rate of secondary ETV over a mean follow-up period of 37 months. When they compared primary versus secondary ETV, the percentage of patients remaining shunt free was similar (and excellent) for patients with aqueductal stenosis or tumors but was much better for secondary ETV compared with primary ETV in patients with infection, hemorrhage, Chiari malformations, and spina bifida. This demonstrates that patients with infection, hemorrhage, Chiari malformations, and spina bifida who have failed shunt placement may be candidates for ETV with a better success rate than if an ETV was performed as initial therapy.
In chronically shunted patients with associated symptoms, conversion to an ETV from a shunt can be done by following an algorithm. The shunt is first removed and replaced with an external ventricular drain (EVD). The patient is placed in the intensive care unit for monitoring and the EVD subsequently clamped. If the ventricles enlarge and the intracranial pressure (ICP) increases, ETV is attempted. If there is subsequently no ventricular enlargement after ETV, patients are either observed for 48 hours (no change in ICP) or shunted (significant increase in ICP).
Endoscopic Third Ventriculostomy Versus Ventriculoperitoneal Shunt Placement
Determining whether to perform ventriculoperitoneal shunt (VPS) placement or ETV is a complicated decision that must be made on a case-by-case basis. However, the current data demonstrate similar outcomes between the two procedures, and families may opt for a trial of ETV prior to VPS to potentially avoid long-term shunt dependence. If there is equipoise between the two procedures, this is a reasonable approach.
A prospective study by Kulkarni et al. demonstrated a higher failure rate of ETV with choroid plexus cauterization (CPC) compared with VPS in the short term (<6 months) in the treatment of congenital hydrocephalus, but long-term follow-up data were not available. In a randomized trial by Kulkarni et al. analyzing cognitive outcomes in infants with postinfectious hydrocephalus following ETV-CPC versus VPS, they did not demonstrate a significant difference between the two groups in terms of cognitive outcomes, rates of treatment failure, or brain volume. In a large review article published by Li et al. consisting of a total of 6995 pediatric patients, ETV and shunting had similar 1-year success rates.
When comparing the outcomes of patients who have had clinical success in treatment with VPS placement and ETV, a difference in hydrocephalus metrics has been demonstrated. In a study by Dewan et al. on infants, a decreased ventricular size in those treated with VPS was demonstrated, whereas the ventricles in patients treated with ETV was unchanged. The head circumference growth curve control was similar between both groups of patients. Regardless of treatment modality, almost all patients treated in the study with an initially bulging fontanelle had a flat or sunken fontanelle at the time of last follow-up. However, shunting with a gravitational regulated shunt valve often leads to unchanged ventricular caliber as well.
Endoscopic Third Ventriculostomy With Choroid Plexus Cauterization
ETV has been successfully performed in conjunction with CPC for the treatment of congenital hydrocephalus and may be easier with a flexible endoscope, although many surgeons are successful with a rigid endoscope. A large prospective study by Warf studied 550 children in Uganda who underwent ETV, half with CPC and half without. Overall, the success rate of ETV with CPC was 66%, higher than that of ETV alone (47%). Further analysis revealed the greatest benefit of the addition of CPC in patients who had hydrocephalus secondary to myelomeningocele (76% success rate compared with 35% without CPC) and other non-postinfectious etiologies (70% compared with 38% success rate). For patients with postinfectious hydrocephalus, the benefit of CPC was not statistically significant (62% success rate with CPC compared with 52% without). Notably, in patients who were older than 1 year of age, there was no difference in patients treated with CPC and those without (80% success rate in both groups). Later, Warf and Stone published a North American prospective series in 2014 of 91 children who underwent ETV with CPC. Their success rate (57% requiring no further hydrocephalus procedure, 65% shunt free at last follow-up) was similar to the larger series out of Uganda.
On the other hand, a study by Kulkarni et al. demonstrated no statistically significant difference in success rate in patients treated with ETV and CPC compared with ETV alone. However, those patients who underwent a greater extent of CPC had a higher success rate compared with those who underwent less CPC, suggesting some level of CPC may be of benefit in select patients. In addition, this study was limited by a small sample size.
Based on the available evidence to date, the decision to perform CPC should be made on a case-by-case basis. It may be indicated in patients younger than 1 year of age across all underlying etiologies, especially those with noninfectious pathology. In patients older than 1 year of age, the addition of CPC is less likely to be of benefit.
Contraindications to Endoscopic Third Ventriculostomy
There are certain situations in which an ETV is contraindicated. Anatomically, the lateral and third ventricles can be too small to safely pass an endoscope. This occurs in situations of “normal volume hydrocephalus,” when little to no change is observed in ventricular size when ICP increases, such as in idiopathic intracranial hypertension or slit ventricle syndrome. In other cases, one of the anatomic structures can block the path to the floor of the third ventricle. For example, the massa intermedia in Chiari II patients can be so large as to prevent safe passage of the endoscope. A tumor or other mass lesion can also block the trajectory to the floor of the third ventricle. The basilar artery or a major perforator can block the exit point through the floor of the third ventricle. Physiologically, the CSF flow should be examined on preoperative imaging. If the obstruction is above the level of the third ventricle, distal to the interpeduncular cistern, or if no clear obstruction can be identified, then the utility of ETV is limited.
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