Endoscopic Anatomy of Lateral and Third Ventricles: A must Know for Performing Endoscopic Third Ventriculostomy
Correspondence Address: Source of Support: None, Conflict of Interest: None DOI: 10.4103/0028-3886.310075
Source of Support: None, Conflict of Interest: None
Keywords: Aqueductal stenosis, endoscopic anatomy, ETV, lateral ventricle, third ventricle
The first successful neuro-endoscopic procedure was performed by L' Espinasse in 1910. In 1923, Mixter successfully performed endoscopic third ventriculostomy (ETV) for obstructive hydrocephalus in a 9-month-old girl child. In 1935, Scarff described the use of novel endoscope with mobile cauterizing electrode and irrigation system with movable operating tip. Fay and Grant were the first to photograph the insides of ventricles using a cystoscope. The success rates of ETV may vary with age and etiology with much inferior outcomes in children less than two years of age. The ETV procedure seems more physiological for treatment of hydrocephalus as CSF route is diverted from ventricle to subarachnoid space.
The current video article demonstrates the endoscopic anatomy of lateral and third ventricles, and describes the procedure of ETV, for educational purpose.
A 6-month-old girl child was brought to our hospital with progressive enlargement of head since birth. On examination head circumference of child was 46 centimeters and sunset sign was present. The anterior fontanelle of child was tense with frontal bossing. Child was able to move all four limbs normally. Overall delayed developmental milestones were present.
Computed tomography (CT scan) of brain showed tri-ventriculomegaly with obstructive hydrocephalus. Fourth ventricle was normal. No mass lesion was identified. Diagnosis of aqueductal stenosis was provisionally made. Patient was planned for ETV.
Operative procedure was done under general anesthesia; position was supine with head end elevated to 15 degree. A 'U' shapedskin incision was marked on right Kochers' point. The endoscope used was Karl Storz neuroendoscope (LOTTA system, Zero degrees). The details of procedure are mentioned in the video article.
Video link: https://youtu.be/GkrYr9wAIXs
Video timeline with audio transcript:
0.14–0.29 min: Patient was positioned supine, with head slightly elevated. A 'U' shaped skin incision was made over the right Kochers' point. Burrhole was made, dura coagulated and cut. Ventricle was tapped. Through the tract so formed, ventriculoscope was introduced.
0.30–1.19 min: On entering the right lateral ventricle the first thing that's striking is the absence of septum pellucidum, because of which we are able to visualize both the ventricles together. The septum is often absent in cases of long-standing hydrocephalus.
Reddish fronds like choroid plexuses are seen on either side. The thalamostriate veins are seen. Normally, the septal vein and the thalamostriate veins along with the anterior caudate vein join to form the internal cerebral veins on each side at the venous angle, which traverses in the roof of third ventricle. This anatomy is not appreciated in this patient.
We are in the right ventricle and the way to identify it is by inspecting the relative position of choroid plexus and thalamostriate veins. The thalamostriate vein is on the right of the choroid plexus on the right side and on the left of the choroid plexus on the left side.
1.21–1.44 min: The columns of fornices form the anterior boundaries of foramen of Monro and posterior -inferior part of medial walls of frontal horn.
The body of fornix is seen medial to choroid plexus. It wraps around the thalamus and forms the medial part of floor of lateral ventricle. It also forms one of the layers of roof of the third ventricle.
The two thalami can be seen on either side, joined by interthalamic adhesion/massa intermedia. They are lateralto the choroid plexus in the lateral ventricle and form the floor of its body.
1.53–2.09 min: The area under vision now is the opposite frontal horn and the lateral wall of the body of lateral ventricle. The rostrum of corpus callosum forms the floor of the frontal horn. The head and body of caudate form the lateral wall of the frontal horn and body of lateral ventricle.
2.10–2.17 min: It was possible to turn the scope posteriorly and visualize the rest of the ventricular cavity, as the ventricles were roomy.
2.18–2.45 min: We are able to appreciate, the triangular cavity formed by the atrium and occipital horn together. The glomus of choroid plexus is seen, it serves as a landmark for atrium. The medial wall is formed by bulb of corpus callosum and calcar avis, which is the eminence overlying calcarine sulcus. Floor is formed by the collateral trigone. Roof is formed by body, splenium and tapetum of corpus callosum. The lateral wall is formed by pulvinar and tapetum.
2.47 – 2.53 min: The ventriculoscope is now being negotiated through the foramen of Monro into the third ventricle, which is quite distended here.
2.54–3.04 min: The floor of third ventricle is seen formed by two mammillary bodies along with the premammillary membrane, anterior to it. The impression of dorsum sellae is seen anterior to the premammillary membrane.
3.05–3.21 min: On directing the scope slightly anteriorly, we are able to appreciate, the optic chiasm, the infundibular recess, which form the anterior part of the floor of third ventricle.
Another transversely traversing structure is seen connecting the two thalami- This structure is another massa intermedia, which is rarely seen.
3.25–4.04 min: The third ventricle itself was distended, and therefore it was possible to negotiate the scope posteriorly between the massa intermedia and choroid plexus at the roof of third ventricle. We are now able to appreciate the aqueduct, which is a cavity in the midbrain.
The aqueduct of sylvius is often described as a triangular opening, with its base formed by posterior commissure and the grey matter of midbrain forming the two lateral limbs. It appears more slit-like in the deeper areas due to stenosis in this patient. It forms the posterior most part of the floor of third ventricle. It also forms the inferior-most part of the posterior wall of third ventricle extending downwards from the suprapineal recess, which is usually not visualized through a rigid endoscope.
4.13–4.20 min: ETV is done at the premammillary membrane. The opening in the premammillary membrane is dilated using Fogarty's catheter.
4.28–4.41 min: To check the adequacy of the opening, the scope is introduced into the chiasmatic and interpeduncular cisterns through the ETV stoma. The mesencephalic layer of Liliequist's membrane is seen with multiple perforations. The basilar artery is seen over the pons.
4.42–4.48 min: On directing the scope slightly laterally, occulomotor nerve is seen emerging from the interpeduncular fossa.
Outcome: Patient did well after surgery with significant improvement in sunset sign and bogginess of anterior fontanelle. She was discharged on 7th day of surgery. She has been under regular follow-up for 4 months.
Pearls and Pitfalls: Care should be taken to minimize damage to the fornix while negotiating the foramen of Monro. Adequate hemostasis should be ensured using copious irrigation and judicious coagulation when needed. Posterior angulation of endoscope should be tried only if the ventricles are distended.
ETV is a widely performed procedure for CSF diversion. Careful patient selection is a must. Aqueductal stenosis and pathologies causing CSF outflow obstruction around the posterior third ventricle are best suited for this procedure. The physiology behind working of this procedure is presence of pressure gradient across two compartments where the new opening is made. In children absence of bone overlying the anterior fontanelle, may sometimes lead to failures as the pressure gradient is often less. Laxity of premammillary membrane and adequacy of space between the clivus and basilar artery over pons should be assessed on MRI. The outcomes of ETV depend upon two major factors -age and etiology. The outcomes tend to improve with increasing age with reported one year reliability rates of ETV as low as 31% (14–53%) for less than 1 month, to about 84% (79–89%) after 24 months of age. The reliability rates for 1–6 months and 6 months to 2 years have been reported as 50% (32–68%) and 71% (55–85%), respectively. The outcomes after ETV also varies with the pathology, with best outcomes for aquedutal stenosis (67–93.5%) and obstructive hydrocephalus and less favorable outcomes with communicating hydrocephalus due to infections (60–64%), and intraventricular hemorrhage (43–73%). The outcomes for intraventricular cysts and anatomical aberrations are reported as (21–80%) and (56–81%), respectively. Considering both age and etiology, Baldauf et al. in their study of 21 patients of children less than 2 years of age suffering from obstructive hydrocephalus, found the overall success rate of ETV as 43%.
Various developments in endoscopy equipment have occurred. Rigid zero-degree endoscopes provide the best means of performing this procedure. Thirty-degree scopes may be used to visualize the posterior parts of the third ventricle. Flexible endoscopes provide means of exploring the posterior part of the third ventricle, the aqueduct and even the fourth ventricle in certain cases. They are less preferred, because of better quality of visuals provided by rigid scopes. While positioning the patient and marking the entry point into the ventricle, care should be taken to ensure that the trajectory of endoscope should be in a straight line joining the cortex to the foramen of Monro and third ventricle floor. Some surgeons prefer to use neuro-navigation for this.
Endoscope holders hold the endoscope steady while performing this procedure. We routinely use two surgeons for this procedure––one holds the endoscope for providing mobility when needed, while the other performs the procedure.
While perforating the floor of third ventricle, care should be taken to avoid damage to hypothalamus laterally, infundibulum anteriorly and mammillary bodies posteriorly. Some authors prefer blunt perforation of premammillary membrane, others prefer sharp. While dilating the stoma with Fogarty's catheter, inflating the balloon inferior to floor and pulling it up should be avoided, as injury to the basilar artery or its perforators may lead to devastating consequences. Adequacy of stoma should be assessed by introducing the scope through it, allowing visualization of the Liliequist's membrane. Hemostasis should be achieved with copious irrigation. Arterial bleeding should be controlled with coagulation. The endoscope should remain focused at the site of bleeding during hemostasis. Some prefer to leave an EVD behind. We avoid it as it may change the pressure gradient across the stoma. Complications rates in the order of 8.5% ranging from hemiparesis, gaze paresis, cognition disorders, hormonal disturbances, wound leak and infections have been reported.
Good understanding of anatomy and careful patient selection goes a long way in obtaining good results for ETV.
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