Tumors of the fourth ventricle
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.
Anatomy of tumors of the fourth ventricle
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
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.
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.
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