Microsurgical Anatomy

Mastering the relevant ventricular anatomy is essential in performing microsurgical approaches to the lateral ventricles. A more detailed review of the anatomy can be found in Professor Albert L. Rhoton’s outstanding monograph on the subject, which was produced after years of study in his laboratory and operating room.⁹

The lateral ventricles are paired C-shaped structures that are anchored by each thalamus in the middle of the brain. Each ventricle is divided into five sections. Starting anterior and superior and moving backward they are the (a) anterior horn, (b) body, (c) temporal horn, (d) atrium, and (e) occipital horn (Fig. 42.2). Each of these five sections has a floor, roof, medial wall, and lateral wall and these sections contain structures that are critical to the surgeon for navigation in the ventricles. Specifically, they are structures that either tolerate manipulation to facilitate visual access or, conversely, should not be manipulated. These relationships are important when considering either the transcortical or the transcallosal approach.

FIGURE 42.2 Each C-shaped lateral ventricle is anchored on the ipsilateral thalamus. The parts of the ventricle include the frontal horn, body, temporal horn, atrium, and occipital horn. The septum pellucidum is in the medial border of the frontal horn and body of the lateral ventricle. The hippocampal formation sits on the floor of the temporal horn. The fornix surrounds the thalamus and runs in the medial part of the temporal horn, atrium, and body.

(Adapted from Rhoton Jr AL. The lateral and third ventricles. Neurosurgery. 2003;53:S235-S299.)

If one visualizes the ventricles in a coronal plane, the anatomical relationships are as follows. At the level of the frontal horn of the lateral ventricle, the lateral wall is composed of the oval caudate nucleus, and the medial wall is the diaphanous septum pellucidum that separates the two frontal horns. The roof is made up of the roof of the genu of the corpus callosum, and the floor is composed of the rostrum of the corpus callosum, which wraps around and tucks under the ventricles until it reaches the anterior commissural fibers. At the level of the body of the ventricle, some of the same relationships exist. The corpus callosum makes up the roof, the caudate nucleus the lateral wall, and the medial wall is again separated by the septum pellucidum. Moving posteriorly within the lateral ventricles, the floor is composed of the thalamus. In the midline of the floor lie the columns of the fornix. In the floor of the body of the lateral ventricle, sitting between the fornix and the thalamus, is the choroid plexus, which attaches to the surgically crucial structure called the choroidal fissure. Each column of the fornix also forms the curved superior and anterior borders of the foramen of Monro. Moving to the temporal horn, each column of the fornix starts as the fimbria of the hippocampi and starts in the medial wall of the temporal horn. After giving off commissural fibers, these nerve bundles continue in the inferior medial wall of the body as the fornix.¹⁰ The roof of the temporal horn is the tapetum of the corpus callosum, and the floor comprises the folded gyri of the hippocampus. Moving within the ventricle to its posterior limits we find the atrium and the occipital horn. The atrium and occipital horn form a triangular CSF cavity in the occipital lobe. Again, the tapetum of the corpus callosum covers the lateral wall and roof of the atrium. The forceps major, which is a fiber bundle that connects the two occipital lobes through the splenium of the corpus callosum, runs in the superior part of the atrium. The floor is composed of the collateral trigone and the medial wall is the calcar avis of the calcarine cortex.⁷

The primary arteries within the ventricles are the anterior and posterior choroidal arteries,^7,10 branches of which provide the vascular supply to tumors in this region. Understanding the course of the arteries helps the surgeon choose an approach for each lesion and thus permits early proximal control, when possible, of the feeding vessels.

The anterior choroidal artery arises from the internal carotid artery, just a millimeter or so distal to the posterior communicating artery. It exits the anterior incisural space and enters the lateral ventricle through the choroidal fissure, heading posteriorly to lie near the lateral posterior choroidal artery.^7,10 The anterior choroidal artery generally supplies the choroid plexus in the temporal horn and atrium. Because the choroidal arteries pass through the choroidal fissure, opening this fissure early also facilitates proximal control of the feeding vessels.

The posterior choroidal arteries are grouped into lateral and medial divisions. The lateral posterior choroidal artery is composed of one to six branches, which arise in the ambient and quadrigeminal cisterns, typically from the posterior cerebral aratery (PCA). These branches then pierce the ventricle and pass around the pulvinar and enter through the choroidal fissure at the level of the fimbria/body of the fornix to supply the choroid plexus in the posterior temporal horn, atrium, and body of the ventricles.⁷ The medial posterior choroidal arteries arise as one to three branches from the PCA in the interpeduncular and crural cisterns. These arteries circumnavigate the midbrain and move to the pineal gland to enter the roof of the third ventricle and reside in the tela choroidea called the velum interpositum, adjacent to the internal cerebral veins. The medial posterior choroidal artery supplies the choroid plexus in the roof of the third ventricle and sometimes the choroid plexus of the lateral ventricle.¹⁰

The veins are useful as landmarks to help navigate in the body of the ventricle, especially in cases in which hydrocephalus is present (Fig. 42.3). There are many important veins composing the lateral and medial groups, but perhaps the best known for surgical and angiographic orientation is the thalamostriate vein, which helps orient the surgeon toward the foramen of Monro. The thalamostriate vein traverses the lateral wall of the body of the ventricle adjacent to the choroidal fissue between the caudate and thalamus. It then forms the venous angle with an acute posterior turn into the foramen of Monro to empty into the internal cerebral veins traversing the velum interpositum. The veins in the temporal horn drain into the basal vein of Rosenthal (basal vein) as it passes through the ambient cistern on each side. Veins from the atrium and occipital horn drain into the basal internal cerebral veins as well, and finally into the vein of Galen which empties into the straight sinus and torcular herophili.⁷

FIGURE 42.3 A drawing with venous relationships of the lateral ventricles in an axial view of the ventricular system. The ventricular veins can be used as navigational guides, once in the ventricle. They are divided into a deep/medial paired system and a paired lateral group of vessels. The medial ventricular veins drain into the internal cerebral vein, which run in the velum interpositum on the roof of the third ventricle. They are joined by the paired basal vein of Rosenthal, as they drain into the great vein of Galen followed by the straight sinus and finally to the torcula. Starting at the frontal horn, the lateral group consists of the anterior caudate vein and anterior septal vein in the frontal horn, which join the thalamostriate vein adjacent to the choroidal fissure. The medial and lateral atrial veins are in the atrium and occipital horn, and the inferior ventricular and amygdalar veins lie in the temporal horn.

(Adapted from Rhoton Jr AL. The lateral and third vertricles. Neurosurgery. 2003;53:S235-S299.)

Pathological Entities

The differential diagnosis of tumors in the lateral ventricle depends on several factors: the age of the patient, location of the tumor, and specific radiological characteristics as described by computed tomography (CT), magnetic resonance imaging (MRI), and cerebral angiography.^11,12 Tumors found in the lateral ventricle of children younger than 5 years of age are often choroid plexus tumors, whereas in older children they tend to be astrocytoma or ependymoma.¹¹ Choroid plexus tumors are rare lesions with a prevalence of 0.3 per million. They comprise only about 1% of all brain tumors and occur in children with a median age of 3.5 years at the time of presentation.^13,14 Choroid plexus tumors are often quite vascular and demonstrate a tumor blush on CT, MRI, and cerebral angiography. On MRI, they may possess heterogeneous signal characteristics caused by necrosis and calcification and are iso- to hypodense on T1-weighted images relative to the white matter.

The most common hypo- or isodense, nonenhancing tumor in the body or foramen of Monro is a subependymoma. In children with tuberous sclerosis (TS), these lesions are often a subependymal giant cell astrocytoma (SEGA). Roughly 6% of patients with TS develop subependymal giant cell tumors.¹⁵ In this location, they can reach a large size before being diagnosed, sometimes by unilateral ventricular obstruction.^16–18 Astrocytomas can be found in any part of the ventricle as they are surrounded by white matter tracts. Astrocytoma will most frequently arise from the thalamus, where they can infiltrate. Ependymoma, when supratentorial, can be intraventricular as well as intraparenchymal.¹¹

In adults older than 30 years of age, tumors in the atrium and trigone are most commonly meningiomas. Intraventricular meningiomas are well-circumscribed, homogeneously enhancing lesions.¹⁹ Meningiomas tend to be isodense to brain on T1-weighted images and brightly enhance with gadolinium administration. Intraventricular masses that engulf the choroid glomus, are calcified, or are demonstrated to have a choroidal artery supply on angiography are usually benign meningiomas.^20,21

Outside the trigone, tumors in older patients are often either a primary or metastatic malignancy.¹¹

Primary or metastatic malignancies present late in life with attachment to the ventricular walls and parenchymal invasion.¹¹ Malignant intraventricular metastases include renal cell carcinoma, pulmonary adenocarcinoma, gastrointestinal carcinoma, transitional cell carcinoma, and adrenocortical carcinoma.

Central neurocytomas, which occur mostly in adults in the second to fourth decades of life, appear to attach or grow from the septum pellucidum and possess a characteristic imaging appearance.²² These lesions may be solid with cystic components and flow voids on MRI, and may contain calcifications visualized on CT.²³ These benign World Health Organization (WHO) grade II lesions derive their name from the original description of a well-differentiated neural lesion that was adherent to the septum and ventricle wall.²⁴ These tumors can be quite large on presentation. Although the natural history and long-term prognosis of these lesions are not completely understood, excision appears to be curative in those patients in whom a gross total resection is achieved. Significant blood loss and a “sticky” connection to the thalamus and third ventricle structures may complicate the attempts at a radical resection.

Cystic lesions include colloid cysts of the third ventricle and infectious lesions such as neurocysticercosis, nocardiosis, and cryptococci. Colloid cysts are covered in another chapter but at our institution they are routinely approached with a single port endoscopic resection. Other non-neoplastic processes such as cysts, sarcoidosis, xanthogranulomas (which appear to be dense on CT scans with flecks of calcification), arteriovenous malformations, and cavernous hemangiomas are also found in the lateral ventricle with geographic variation.²⁵ In some endemic regions of the world, a new seizure or hydrocephalus associated with cystic intraventricular lesion often indicates neurocysticercosis. In our institution, the history, MRI, and serum or CSF markers may secure the diagnosis of neurocysticercosis, and these patients then receive medical therapy. Endoscopic exploration and resection are used to remove obstructive ventricular neurocysticercosis lesions when indicated by symptoms or examination.

Preoperative Planning and General Surgical Considerations

A CT scan is often the first radiological study patients undergo simply because of diagnostic ease in the surveillance workup. The presence of calcifications on the CT image may further narrow the differential diagnosis.²⁶ Calcification may be seen in central neurocytoma, subependymal giant cell tumor, meningioma, and ependymoma.

Subsequent MRI, with and without gadolinium, reveals the precise location and extent of the lesion within the ventricle and, thus, helps guide the surgical approach. At a minimum this should include sagittal, coronal, and axial reconstructions of T1-weighted sequences (with and without gadolinium), T2-weighted images, and fluid-attenuated inversion recovery (FLAIR). These sequences are sufficient to refine the differential diagnosis and begin basic surgical planning. The role of new specialized MRI sequencing is rapidly evolving. Diffusion tensor imaging (DTI) permits reconstruction of large fiber tracts, which may contribute to surgical planning by guiding the surgical trajectory through the least disruptive pathway.²⁷ Functional MRI (fMRI) has a role in cases in which the approach encroaches on motor or language cortex. Some authors advocate magnetic resonance spectroscopy (MRS) to characterize the chemical composition of the lesion in malignant lesions that may require a biopsy.²⁸ However, we have found the DTI and fMRI to be more helpful in surgical planning. It is our practice to also obtain magnetic resonance angiography (MRA)and venography (MRV). These images serve several purposes. First, they permit visualization of the blood supply to the lesion. Second, MRV through the vertex allows the surgeon to tailor the placement of the craniotomy based on draining cortical veins. Finally, MRV is helpful when considering choroidal dissection to gain access to the third ventricle.²⁹

If increased vascularity of the tumor is noted, when appropriate, we prefer our neurointerventional team to perform an endovascular embolization procedure to decrease the blood flow to the lesion. In cases of successfully preoperatively embolized tumors, the resection is markedly easier, with far less blood loss. We have performed embolization of choroidal vessels with large intraventricular lesions in children as young as 2 years of age. Seemingly small blood loss constitutes a significant blood volume reduction in young children and can cause shock quickly or cause the surgeon to halt or stage an operation. We favor preoperative embolization whenever it is technically feasible in adults as well as children.

Preoperative visual field testing, a neuro-ophthalmological examination, and neuropsychological evaluations are performed for all patients as part of our baseline examination. These examinations often delineate subtle deficits not appreciated on neurological evaluations. The comprehensive neuropsychological examination consists of a thorough assessment of intellectual, academic, sensorimotor, language, spatial, memory, attention, and executive skills. The examination is repeated postoperatively at various intervals to provide data for a more objective assessment of outcomes and a guide for rehabilitation when necessary.

Ventricular communication after tumor resection is an important goal. Septations need to be removed and CSF spaces need to be in communication both anatomically as well as physiologically. Microscopic or endoscopic examination is performed to confirm the absence of blood clot or residual tumor and ensure that intraventricular communication has been achieved. Fenestration of the septum pellucidum to allow bilateral ventricular communication is a maneuver used prior to closure. A saline warmer filled with saline or lactated Ringer’s solution is used to ensure that physiological conditions are maintained during the intraventricular irrigation and exploration.

Patients in whom a cortical incision is required receive a loading dose of antiepileptic medicine preoperatively. They are maintained on prophylactic anticonvulsant therapy for approximately 1 week after the procedure, if they have not suffered a preoperative seizure.

A standard external ventricular catheter is placed in the ventricle and run outside the dura through one of the bur holes after surgery. This provides external drainage of proteinaceous material and blood products from the surgery and serves as a convenient ICP monitor.

The beautiful anatomy of the lateral ventricle facilitates a wide variety of surgical approaches. The location and size of the lesion, hemispheric dominance, preoperative deficits, associated hydrocephalus, vascularity of the lesion, and experience of the surgeon all contribute to the ultimate selection of surgical approach. There is simply no class I or II data that dictates one approach is superior to another. The optimal approach facilitates the primary goal of surgery, which is gross total resection of the lesion with minimal trauma to surrounding structures and neural function. At our institution, most lateral ventricle lesions are approached through a transcortical route, a transcallosal route, or in rare cases a combination of the two. As detailed in the following sections, these routes provide excellent access to lesions within the third and lateral ventricles (Fig. 42.4).

FIGURE 42.4 Approaches to the ventricular system. A composite of various approaches to the cerebral ventricles is illustrated. The optimal approach provides the surgeon with a perpendicular line of sight to the lesion within short working distance while minimizing functional deficits from manipulating surrounding structures. The site of the skin incision and the bone flap are shown for each approach to the specific part of the ventricle. (The incisions can be curvilinear as shown, linear, serpentine, or zigzag, depending on surgical preference. The cranial bone flap can be shaped to provide appropriate exposure.)

FIGURE 42.5 Adult male patient with no prior neurological history presented with personality changes, memory loss, and headache over several months. The lesion arises from the medial aspect of the floor of the frontal horn of the lateral ventricle, near the septal nuclei. The lesion enhances briskly with gadolinium administration. Fluid-attenuated inversion recovery (FLAIR) imaging reveals asymmetrical transependymal flow, suggesting a trapped left ventricle (arrows). This lesion was approached through the transcortical middle frontal gyrus in order to achieve gross total resection and fenestration of the septum pellucidum. The patient returned to baseline premorbid function and his job as an attorney without deficits or seizures. Pathological examination revealed subependymal giant cell astrocytoma (SEGA).

Craniotomy Flap and Cortical Incisions

A variety of incisions can be used for ventricular operations, including linear, “zigzag,” bicoronal, horseshoe, question mark, and serpentine incisions. Incisions are all performed behind the hairline. “Zigzag” and linear incisions, which heal well, seem to cause slightly less cosmetic hair issues and do not compromise surgical exposure. Regardless of the type of incision, the goal is to provide adequate exposure for a craniotomy that will minimize cortical retraction. For example, for an anterior transcortical approach, the bone flap will have its medial border adjacent to but not over the midline and its anterior border will be above the floor of the anterior fossa. A unicoronal or modified bicoronal incision will best achieve this goal. For the posterior transcortical approaches, the border of the craniotomy should be adjacent to the sagittal and transverse sinuses; linear, serpentine, and horseshoe-shaped incisions all work well. For middle fossa approaches, the craniotomy should be based on the floor of the middle fossa and extend high enough so that all three temporal gyri can be visualized; a standard question mark– or T-shaped incision is sufficient. Interhemispheric incisions can be linear, “zigzag,” or box-shaped. The craniotomy is taken across the midline by placing contralateral bur holes and stripping the sagittal sinus prior to passing the craniotome. If the patient does not have hydrocephalus and needs a ventriculostomy, a lumbar drain may be helpful in interhemispheric approaches.

The size and location of the cortical incision are important. The site is chosen based on the topography of the sulci and gyri or location of eloquent cortex based on mapping findings, DTI maps, or directed by frameless navigation. The choice of traversing the sulcus versus gyrus for the cortisectomy in the transcortical approach remains a subject of discussion and primarily is based on surgeon preference. The sulcus incision for the removal of subcortical tumors or arteriovenous malformations works well, although the transcortical gyrus incision, which can be extended, is ideal for the removal of large intraventricular tumors.

To enter the ventricle directly, we employ a simple technique of placing a ventricular catheter by frameless stereotactic guidance, or through a freehand pass to enter the ventricle. Once the trajectory to the ventricle is determined, a small cortical incision is made and followed down to the catheter in the ventricle. The ependyma is opened with nonstick or irrigating bipolar electrocautery, and the ventricle is entered. A cortical aperture is made using flat microinstruments or microsuction to develop a subcortical path to the lesion. Very little of the subcortical white matter is removed, and flat malleable brain retractors or a microsurgical speculum is placed to increase visibility. The goal is not necessarily to make the smallest cortical incision but to make the most appropriate one. Transcortical surgery has the advantages of wide avenues and a maneuverable microsurgical field.³⁰

The anterior transcortical approach is well suited for tumors arising within the lateral ventricle. The principal advantage of the transcortical approach is a direct line of sight to lesions within the lateral ventricle. Isolated lesions in the frontal horn, body, and atrium may be accessed easily. This approach strategy is particularly attractive for lesions that derive their blood supply from the lateral wall of the ventricle, or lesions that are firmly adherent to the structures that constitute the lateral wall of the ventricle. The caudate nucleus, thalamostriate vein, thalamus, and genu of the internal capsule may be identified early in transcortical approaches.

There are limitations to the transcortical approach. This approach may not provide optimal visualization of the contralateral ventricle due to the linear trajectory, which may obscure lesions sitting in a location out of the line of sight of the microscope. The transcortical approach may also predispose the patient to postoperative seizures. The incidence of postoperative seizures ranges in the literature from 5% to 70%,^2,3,31–34 which is higher than in transcallosal approaches when no cortisectomy is performed (0-10%). However, our experience with seizure activity after the transcortical approach parallels that of the transcallosal approach. Specifically, the postoperative seizure rate with very small corticectomy (<15 mm) and prophylactic anticonvulsant treatment for 1 week has successfully lowered the immediate postoperative seizure incidence to the 5% to 7% range.

Anterior Transcortical Approaches

Tumors in the frontal region are primarily astrocytoma, subependymal giant cell astrocytoma, ependymoma, and central neurocytoma. Tumors of the frontal horn can become very large and cause obstruction of the foramen of Monro with ventricular dilation. The transcortical middle frontal gyrus approach is an excellent route for the excision of tumors in the ipsilateral anterior horn of the lateral ventricle, the anterior body of the lateral ventricle, and the anterior third ventricle. Tumors that extend inferiorly from the lateral ventricle into the third ventricle and require an interforniceal or subchoroidal exposure for removal, can be approached using either a transcortical or a transcallosal route.¹⁰ In patients with small ventricles, tumor in both lateral ventricles, or tumor in the body of the lateral ventricle, the transcallosal approach is employed.

The patient is placed in the supine position with the head elevated 10 to 30 degrees. A free 3 × 4-cm bone flap is placed over the central portion of the middle frontal gyrus. The flap is based on the coronal suture, with the medial border off the midline and the anterior border at least 2 cm anterior to the coronal suture and the posterior border about 2 cm behind. The dura is opened in a cruciate or box-shaped fashion and a ventricular catheter is placed into the frontal horn. This is followed by microsurgical cleavage of the white matter until the ependymal lining is broached. A small ⅜-inch retractor or speculum retractor is used. After the anatomy of the lateral ventricle is visualized through the microscope, regardless of the approach, the anatomy is similar. Once inside the ventricle, surgical landmarks provide orientation. The most important landmarks are the foramen of Monro, the thalamostriate vein, the choroid plexus, and the fornix. Cotton paddies are placed over the foramen and around the lesion to minimize the circulation of blood products and debris during tumor resection.

Lesions within the third ventricle may be accessed from the transcortical approach. If the foramen is enlarged from hydrocephalus or the lesion itself, no additional dissection is necessary. If not, additional access to the third ventricle is made possible by a transchoroidal or suprachoroidal, trans–velum interpositum dissection.³⁵ To achieve this exposure, the choroidal fissure is opened between the fornix and the thalamus.³⁶ The suprachoroidal or transchoroidal approach divides the taenia of the fornix between the fornix and choroid plexus. The subchoroidal approach divides the taenia thalami leaf of the tela choroidea between the choroid plexus and the thalamus. The latter approach is more commonly associated with direct injury to the thalamostriate vein and thalamus. Rhoton’s elegant lifetime work has been captured in his collected works in Neurosurgery and is arguably the most complete work on the subject.^9,37 In his ventricular surgery experience, he has demonstrated that the transchoroidal or suprachoroidal approach, opening the fissure through the taenia fornix, is both safe and effective.¹⁸ Although we have safely opened the fissure through the “subchoroidal” approach on the taenia thalami side, the risk of thalamic damage is quite real and can be devastating. In contrast, opening on the taenia fornix side in which a unilateral forniceal injury occurs often results in no permanent memory loss (Fig. 42.6). Regardless, these aforementioned considerations are important but must be applied to the specific anatomy at the time of the intraventricular dissection. Complications from injury to the surrounding structures result in hemiparesis, mutism, amnestic syndromes, and confusion.³⁸ In general, these risks are minimized by a suprachoroidal approach. However, maintaining the integrity of both fornices is important for preservation of memory function. The fornix, which has been reported to carry as many as five times the number of axons as the optic tract,³⁹ links the hippocampal formation (including the hippocampus proper, dentate gyrus, parahippocampal gyrus, and subiculum) with the septal nuclei, mammillary bodies, hypothalamus, and thalamic nuclei. Visualization of the fornix may be obscured by lesions based on the septum pellucidum, such as central neurocytomas. Patient dissection and meticulous technique here will be rewarded with a favorable postoperative outcome.

FIGURE 42.6 The third ventricle has been exposed by opening the choroidal fissure along the site of the attachment of the choroid plexus to the fornix. The choroidal fissure can be split on the taenia thalami or taenia fornix side but thalamic injury is avoided if it is opened on the fornix side. This exposes the internal cerebral veins and medial posterior choroidal arteries in the velum interpositum on the roof of the third ventricle.