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Endoscopic Third Ventriculostomy
What is endoscopic third ventriculostomy?
Endoscopic third ventriculostomy is a procedure in which a small perforation is made in the thinned floor of the third ventricle, allowing movement of cerebrospinal fluid of the blocked ventricular system and into the interpenduncular cistern (a normal CSF space). This procedure, called an intracranial CSF diversion, is a process whereby cerebrospinal fluid within the ventricle is diverted elsewhere in an attempt to bypass an obstruction in the aqueduct of Sylvius and thereby relieve pressure. The objective of this procedure is to normalize pressure on the brain without using a shunt. Endoscopic third ventriculostomy is "not" a cure for hydrocephalus, but rather an alternate treatment.
Although open ventriculostomies were performed as early as 1922, the advent of shunt systems in the 1960ís displaced the ventriculostomy as the most common method of treating hydrocephalus. Even today, however, we are well aware that despite recent advances in shunt technology and surgical technique, there are many cases in which shunts are inadequate. Extracranial shunts are subject to complications such as blockage, infection, and over-drainage, necessitating surgical revisions for the shunt. For this reason, in selected cases, a growing number of neurosurgeons are recommending endoscopic third ventriculostomy in place of shunting.
The ultimate goal of endoscopic third ventriculostomy is to render a shunt unnecessary or, if previously shunted, to allow itís removal. Although in the best-case scenario endoscopic third ventriculostomy is a one-time procedure, in practice evidence suggests that some patients will require more than one procedure to maintain an adequate opening and drainage.
The development of a new technology called neuroendoscopy, or simply endoscopy, has lead to a renewed interest in third ventriculostomy as a viable alternate procedure for treating hydrocephalus. Neuroendoscopy involves passing a tiny viewing scope into the ventricle that allows images to be projected onto a monitor located next to the operating table, thereby allowing the surgeon to view the inside of the ventricular system during surgery.
Typically, the surgeon places the endoscopic catheter through a small hole drilled in the skull. In people who already have a shunt, the surgeon may be able to use the original bone defect made when the shunt catheter was first placed. Endoscopic third ventriculostomy requires only a small hole in the skull; no additional exposure of the brain is necessary.
How is success defined?
Success is an elusive concept, especially given that patients (and their doctors) approach surgical procedures with varying expectations. "Success" in terms of this procedure is usually considered (by patients and doctors alike) to be the avoidance of a shunt in a patient who otherwise require one. Most doctors would categorize the procedure as successful if patients have clinical evidence of normal intracranial pressure (ICP) and structural evidence of stable or decreased ventricular size. If previously shunted, the shunt has to be either removed or proved to be nonfunctional to assure the procedure is functioning as expected.
Most doctors would describe as failure cases in which patients show no change in their clinical symptoms or ventricular size, or require the placement of a shunt within days or months of the procedure.
Results of endoscopic third ventriculostomy are determined by assessing clinical signs of raised intracranial pressure, head circumference measurements, and fontanelle tension, as well as by MRI/CT and post-operative CSF flow studies. It is important to note that, in some cases, ventricles may remain large after the procedure, despite the return to normal intracranial pressure.
While achieving shunt independence is a critical element of "success", currently published statistics of "success" may not tell the whole story. Several doctors have raised the question of whether a "failed" ventriculostomy may make the resultant shunt easier to manage, though this is an issue requiring further study and consideration. Furthermore, because CSF reabsorption pathways require some time to accommodate the increased amounts of CSF following the ventriculostomy, it may not be possible to determine success for some time after the operation.
Most failed ventriculostomy patients will remain shunt dependent. Doctors attribute this to the inability of arachnoid villi to absorb the excess CSF or, alternatively, to a block in the subarachnoid pathways carrying CSF to the arachnoid villi.
What are the potential complications?
With new technologies, such as magnetic resonance imaging (MRI), stereotactic guided endoscopy, flexible fiber optics scopes, and improved tools for manipulation and hemostasis, the risks of endoscopic third ventriculostomy have been minimized. High resolution MRI allows doctors to see the stenosed or occluded aqueduct and the absence of flow through it, while neuroendoscopic procedures offer unprecedented views within the ventricular system.
The most common complications form endoscopic third ventriculostomy are fever and bleeding. The use of a cold light source and a monopolar coagulation in the confined volume of the third ventricle can increase the temperature of the CSF to high levels, which in turn can turn to fever.
Attempts to perforate the ventricular floor can cause bleeding, as can damage to ventricular walls or perforation of the basilar artery. Large bleeds due to vessel injury under the third ventricle can be catastrophic-but are rare. Damage to the perforating arteries coming from the basilar artery can result in hypothalmic injury.
Short-term memory loss is another potential complication. The hypothalamus and the areas of the mamillary body, which are responsible for memory, may be affected by the procedure. However, given time, any short-term memory loss following endoscopic third ventriculostomy usually recovers.
Because the area where the opening in the floor of the third ventricle is made is responsible for some hormonal function, there is a possibility of endocrinologic dysfunction after the procedure. This is often short-lived as well. Diabetes insipidus is another transient complication.
Who is a candidate?
According to one doctor, the only thing we know for sure about the indications for endoscopic third ventriculostomy is that we do not know what they are. However, most physicians and recent studies indicate that the three factors most responsible for successful ventriculostomies are:
Most studies indicate a higher success for previously shunted patients. Doctors hypothesize that the existence of a previous shunt may allow the subarachnoid spaces around the brain to develop and the presence of a functioning shunt may also buy time while the patient develops absorption abilities. It is important to note, however, that for patients who have been shunted for a number of years, one often cannot determine whether the hydrocephalus is communicating or noncommunicating without undergoing invasive testing.
Additionally, some doctors report higher success rates in patients with aqueductal stenosis. Aqueductal stenosis, the most common cause of congenital hydrocephalus, is the obstruction of the long, narrow passageway between the third and fourth ventricle, causing fluid to accumulate upstream from the obstruction.
Other factors influencing the success of endoscopic third ventriculostomy are ventricular width (7mm or more) no previous radiation treatment, and a thinned third ventricular floor.
Although in the best case scenario, endoscopic third ventriculostomy can produce the much-desired result of treating hydrocephalus without a shunt, doctors caution that this procedure is not appropriate for everyone. For those who meet the criteria, endoscopic third ventriculostomy is a promising procedure that may offer freedom from shunt dependency.
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