Health & Medical Neurological Conditions

Diseases of the Anterior Upper Third of the Posterior Fossa

´╗┐Diseases of the Anterior Upper Third of the Posterior Fossa
Object: The region in the upper anterior third of the posterior fossa is a surgically hidden, narrow corridor between the petroclival surface anteriorly and the surface of the brainstem posteriorly. Although several approaches have been described to help surgeons reach this region, few of them enable practitioners to reach the different corners of the area and provide as wide a view as the one achieved using the transcavernous route.
Methods: A transcavernous approach was used in 91 cases (50 complex upper basilar artery [BA] aneurysms, 30 upper petroclival junction meningiomas, five trigeminal nerve schwannomas, three upper clival chordomas, and three anterior pontine lesions) involving the anterior upper third of the posterior fossa. The approach uses the pretemporal route with exposure of the lateral wall of the cavernous sinus. It entails removal of the anterior clinoid process. The posterior clinoid process is also removed when necessary. The approach leads to the upper basilar region. It is widened inferiorly to expose the anterior aspect by removal of the posterior clinoid process and the petroclival osseous and dural elements. Its lateral extension exposes the region of the Meckel cave and it can be widened by removal of the petrous apex. Seventy patients experienced new transient mild cranial neuropathies, 67 of whom recovered fully. Surgically related ischemic morbidities occurred in three patients with BA aneurysms (one small medial thalamic infarct, ataxia due to superior cerebellar artery ischemia, and distal middle cerebral artery embolus in a patient with atrial fibrillation in whom anticoagulation therapy was stopped). All the neuropathies in patients with BA aneurysms were oculomotor and recovery was the rule in all of them. Three new permanent cranial neuropathies occurred in the patients with meningiomas. In seven patients with preoperative neuropathy, two had partial improvement. Five patients with atypical meningiomas were treated with postoperative radiation therapy. Progression occurred later in four patients who were treated with gamma knife surgery. There were no surgery-related deaths. More than 1 year of follow-up data were available in 85 patients, and 94% of those patients were in an active and functional state (Glasgow Outcome Scale scores of 4 and 5).
Conclusions: The safety achieved with the transcavernous route allows surgeons to achieve wide exposures to lesions involving the anterior upper third of the posterior fossa. It is an approach that should be mastered by every neurosurgeon dealing with cranial lesions.

The upper third of the anterior aspect of the posterior fossa is a hidden region that is difficult to reach surgically. This is partly due to its narrow corridor between the petroclival surface anteriorly and the surface of the brainstem posteriorly, and partly due to the location of the different neurovascular structures crossing this corridor and limiting the windows through which different pathological processes can be reached. As a result, several approaches were pioneered to overcome the difficulties encountered when trying to obtain access to this difficult region. They all aim at providing a closer and wider access to the lesion, and at achieving a better visualization of the different vascular and neural structures.

The upper third of the anterior aspect of the posterior fossa includes the following structures: 1) the interpeduncular fossa superomedially; 2) the prepontine cistern inferomedially; and 3) the region of the Meckel cave laterally. In our experience, access to all three regions can be achieved safely and effectively by using the transcavernous route. Several pathological entities involve this region, including vascular and neoplastic disease entities. In this paper I review our group┬┤s experience with the transcavernous approach in 91 cases involving the anterior upper third of the posterior fossa and the modifications in the approach needed to provide better access to its different pathological entities.



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