Fourth Ventricular Tumors
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Fourth ventricular tumors pose a surgical challenge, albeit less than their third ventricular counterparts, because of their relation to the brainstem; this relation can vary from simply displacement to invasion.
This technical challenge is further complicated because the tumor is often concealed by key cerebellar structures, including the cerebellar tonsils and hemispheres or the vermis. Tumors may involve adjacent structures through extension via the foramen of Luschka and by reaching the premedullary, cerebellomedullary, prepontine, and anterior spinal cisterns.
The major structures that compose the borders of the fourth ventricle along the craniocaudal extent are:
- Anteriorly (floor): midbrain, pons, medulla
- Laterally: superior, middle, and inferior cerebellar peduncles
- Superiorly (roof): superior medullary velum, cerebellar lingula, fastigium
- Inferiorly (roof): choroid plexus, tela choroidea, inferior medullary velum, cerebellar uvula, and nodulus
Tumors of the fourth ventricle commonly originate from the following structures composing the floor: the ependyma, choroid plexus, and tela choroidea. Lesions may also arise outside the ventricle and secondarily extend into this chamber, including medullary, tectal, and cerebellar hemispheric masses. These lesions are thus accessible via a fourth ventricular approach.
Importantly, preoperative magnetic resonance imaging (MRI) may be inconclusive with regard to the presence of brainstem invasion, and in fact, it can often be falsely indicative of invasion. Many fourth ventricular tumors, particularly those with an exophytic growth pattern, have a vascularized pedicle and demonstrate compression of the adjacent structures, but lack invasion.
Lateral extension into the fourth ventricle’s outlets typically occurs in medulloblastomas, ependymomas, and gliomas. This extension may progress to involve the foramen of Luschka and even cerebellopontine angle cisterns. This tumor configuration can facilitate cranial nerve involvement via tumor impingement or encasement. Similarly, caudal extension may progress via the obex to affect the superior cervical spinal cord and produce upper cervical myelopathy or even radiculopathy.
Diagnosis and Evaluation
For a general discussion of diagnosis and evaluation for ventricular tumors, see the Principles of Intraventricular Surgery chapter where Table 1 provides a summary of radiologic features consistent with each tumor type.
Hydrocephalus and gait ataxia are common symptoms and signs. Dysmetria and dysdiadochokinesia are possible with laterally located tumors. Less common signs include diplopia, facial weakness, and lower cranial nerve dysfunction. Commonly encountered in the fourth ventricle are ependymomas, medulloblastomas, epidermoid cysts, pilocytic astrocytomas, hemangioblastomas, and cavernous malformations. For a more detailed description of these tumors, see the Ventricular Tumors chapter.
Indications for Surgery and Preoperative Considerations
For a general discussion of the indications for surgery and preoperative considerations for patients with ventricular tumors, see the Principles of Intraventricular Surgery chapter.
Neurophysiologic monitoring is recommended if manipulation of the brainstem or any of the cranial nerves is expected. Brainstem auditory evoked responses (BAERs) are highly sensitive to early signs of brainstem dysfunction. Fluctuation in vital signs may occur during manipulation of the floor and is a serious warning sign to halt retraction and dissection in the area.
The vascular structure most pertinent to a fourth ventricular approach is the posterior inferior cerebellar artery (PICA).
Up to 20% of PICAs originate from the vertebral artery extradurally. This anatomical variant should be considered during the extradural dissection of the vertebral artery at the craniocervical junction. The caudal loop of PICA encompasses the segment of the PICA between the lower cranial nerves and the pole of the tonsil. The cranial loop of the PICA courses between the rostral pole of the tonsil and the inferior medullary velum.
MICROSURGICAL RESECTION OF FOURTH VENTRICULAR TUMORS
Historically, the approach to fourth ventricular tumors involved either cerebellar hemisphere resection or vermian split. Vermian split syndrome is characterized by neurobehavioral abnormalities, imbalance, and cerebellar mutism. In an attempt to avoid these untoward side effects, an alternative approach has been designed.
The telovelar (transcerebellomedullary fissure) approach is flexible and allows resection of most lesions in this area. It facilitates generous exposure of most of the fourth ventricular space, with minimal disruption of the normal structures. For more details, please refer to the chapter on Telovelar Approach in the Cranial Approaches volume.
Initial opening of the arachnoid layers covering the cisterna magna achieves desirable brain relaxation.
The following steps may help simplify tumor resection:
- Microsurgical dissection of the PICA branches from the lateral and superior poles of the tumor protects the relevant vasculature. These arteries provide feeding perforators to the tumor. These perforators must be carefully isolated, coagulated, and shapely cut. Blunt avulsion injury to these vessels places the parent arteries at risk.
- Next, the tumor is internally debulked while avoiding inadvertent penetration of the capsule. The plane of floor of the ventricle may be approximated by gentle elevation of the inferior pole of the tumor to inspect the degree of the floor’s tumor infiltration.
- Tumor enucleation allows the superior pole of the tumor to be rolled inferiorly; the egress of cerebrospinal fluid around this pole confirms entry into the fourth ventricle. This view over the superior pole allows me to approximate the level of the floor with respect to the anterior tumor capsule, further guiding tumor debulking.
- The superior pole and middle of the tumor are further debulked while the lateral poles are mobilized into the resection cavity.
- An ultrasonic aspirator is used to shave tumor off while leaving a thin sheet of tumor over the adherent areas of the floor. The floor is strictly protected and not manipulated.
Most tumors have insertion points along the brachium pontis and potentially the lateral recesses, but rarely along the midline. An ultrasonic aspirator is used to remove the middle section of the tumor, which can usually be lifted off readily from the midline floor.
Closure and Postoperative Considerations
For a detailed discussion of recommendations for postoperative care of patients with ventricular tumors, see the Principles of Intraventricular Surgery chapter.
A watertight dural closure is necessary since the ventricle is entered within the posterior fossa. I recommend the use of a short course of postoperative steroids to minimize the risk of aseptic meningitis. The bone flap is replaced or a cranioplasty attempted.
Pearls and Pitfalls
- Aggressive manipulation of the floor of the ventricle is strictly avoided.
- Strategic tumor debulking and circumdissection based on the steps listed above is advised.
Contributor: Benjamin K. Hendricks, MD
*Redrawn with permission from Tew JM, van Loveren HR, Keller JT. Atlas of Operative Microneurosurgery, WB Saunders, 2001. © Mayfield Clinic
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