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.