V.E. Olyushin, D.A. Gulyaev, T.N. Fadeeva, P.G. Goman, A.V. Shcherbinin
Polenov Research Neurosurgical Institute Saint Petersburg, Russia
Meningiomas of the posterior cranial fossa account for about 12% (7.7-17.1%) of all intracranial meningiomas . However, surgery of subtentorial tumors in general and meningiomas in particular is one of the most complicated areas of neurooncology. Even today postoperative mortality in such tumors, treated in specialized neurosurgical clinics, is 5-10%. Neoplasms of this localization are diagnosed mainly in patients, aged 31-60 (more than 70% of cases). Their rate in females is 3 times higher in comparison with males. At present there are more than 20 classifications of subtentorial meningiomas. As for our Department, we stick to classification of G.B. Bradac, R. Ferst, B.E. Kendall , which distinguishes meningiomas of the petrous pyramid apex, a lateral surface of the pyramid posterior side, a convexital surface of the cerebellum, margins of the great foramen, the clivus, the jugular foramen. An operation presents no technical difficulties in convexital localization of neoplasm and can be performed in many neurosurgical clinics. However, conventional approaches and methods cannot be used in its basal localization. It is conditioned by complex topographic-and-anatomic relationship between a tumor and major arteries, cranial nerves, brain stem. Such tumors include meningiomas of the petrous pyramid apex, accounting for about 10-15% of all neoplasms of the lateral cistern of the pons and at the same time forming the largest group among meningiomas of the posterior cranial fossa (43.6% according to our observations). Before 1970 postoperative mortality in such neoplasms reached 50%; their total removal was casuistry. The last decades were characterized by appearance of up-to-date methods of neuroimaging, microsurgical technique, anesthesiologic support, multifunctional neurophysiologic monitoring. It allowed to perform operations in neoplasms, which were considered to be inoperable.
The goal of the present study is obtaining better results in surgery of meningiomas of the petrous pyramid apex, reducing mortality, improving a level of social adaptation of patients in more radical surgical interventions.
The study is based on analysis of data on 95 patients with meningiomas of the petrous pyramid apex, who were examined and treated in the Polenov Neurosurgical Institute in 1993-2003. This group included all cases, in whom a tumor matrix was localized more orally in relation to the inner acoustic meatus. Surgical interventions were performed in all of them. There were 20 male and 75 female patients. A mean age was equal to 51.3; 70% of cases were admitted with signs of moderate or marked decompensation. A clinical picture was characterized by symptoms of brain stem irritation (65%), conduction disorders (38%), lesions of craniocerebral nerves on the side of tumor localization (28%), hypertensive-hydrocephalic syndrome (20%). A degree, to which clinical manifestations were marked, and, as a result, a level of social adaptation were strongly dependent on a tumor size and direction of its growth. In case of small neoplasms, the main cause of hospitalization was persistent neuralgia of the trigeminal nerve, which was resistant to drug therapy.
In our opinion, choosing surgical tactics for treatment of meningiomas of the pons lateral cistern demands more accurate definition of some important aspects. Development of radiosurgery has made it quite clear, that neoplasm presence is not an absolute indication for operation. It is possible to use dynamic monitoring or radiotherapy in minor tumors (up to 1.5-2 cm in diameter), a compensated state of a patient and minimum compression of the brain stem, confirmed by MRI examination. However, the overwhelming majority of cases is admitted in a state of subcompensation or moderate decompensation and with large or giant neoplasms. Thus, a surgeon has no other choice but direct surgical intervention, whose main purpose is compensation of a patient's state.
Shunting operations, used in meningiomas of the petrous pyramid apex as the first stage of treatment, have both merits and demerits. Positive effects of such operations are as follows: arresting hypertensive syndrome, when radical intervention is impossible; prevention of complications, caused by sharp reduction of intracranial pressure (especially if a patient is operated on in a sitting position). There are well-known or non-specific negative effects of shunting operations (dependence on a shunt, a risk of oral dislocation, additional surgical intervention). Besides, they hamper arachnoid dissection during tumor removal. Sometimes it results in smaller efficacy of an operation and a higher risk of damage of neural structures. Thus, shunting operations are inexpedient, when radical surgical intervention is possible. The only justified situation is aged decompensated patients with predominant hypertensive-hydrocephalic symptoms.
One of the main problems in surgery of parastem tumors is an approach choice. A lot of surgical approaches to different neoplasms of the skull were developed during the last quarter of the century. This fact shows, that each access has its own limitations and does not provide satisfactory visualization of a tumor during its removal. It is especially typical of large neoplasms.
Removal of meningiomas of the petrous pyramid apex is usually performed, using a unilateral suboccipital retrosigmoid approach. If such meningioma penetrates deeply into upper segments of the clivus and at the same time has a supratentorial growth, there appear grounds for subtemporal approaches. As for our study, a transpyramidal supratranstentorial access, subtemporal approach with resection of the petrous pyramid apex and subtemporal transtentorial transmastoid approach were used more frequently.
Mastering all necessary accesses to different segments of the posterior cranial fossa is an indispensable condition of successful operation. It allows a surgeon to choose the most appropriate access for removal of meningioma of this or that localization, as well as to change its direction and character during intervention.
A suboccipital paramedian retrosigmoid access W. Dandy, (Fig.1) provides a rather wide operative field and, as a result, good conditions for identification of acoustic-facial and caudal groups of nerves, vessels and their interrelations with tumor, brain stem, cerebellum tentorium, pachyon and great foramens, petrous pyramid . A considerable advantage of this access is easiness of its performance and adequate exposure of tumors, located behind the internal acoustic meatus. In particular, it concerns cases, when resection of neoplasm's lateral portion results in appearance of a reserve space, necessary for removal of its fragment, localized along the median line (Fig.2). One of the principal drawbacks of the method is insufficient visualization of the 5 and 6th cranial nerves, located behind neoplasm and even in a matrix area, which is not a rare phenomenon. If a retrosigmoid approach is used, visualization of these neural structures is often possible only after neoplasm removal and sometimes after their damage. In our opinion, this approach is indicated in neoplasms, when caudal localization of a matrix in relation to the internal acoustic meatus is combined with a brachycephalic form of a skull and tumor, growing on a dominant hemisphere side.
It is expedient to use supratentorial accesses in more rostral localization of a tumor, i.e. in front of the internal acoustic meatus (preacoustic meningiomas), and in its marked supratentorial growth. They allow to achieve devascularization at an early stage of tumor removal and to provide a "dry" operative field.
A transpyramidal supratranstentorial approach, proposed by Schisano and Tovi in 1962 and modified by G.S.Tigliev and M.F.Chernov [4, 5], is adequate for such localization of meningiomas of the petrous pyramid apex. Removal of a supratentorial segment of neoplasm and wide parallel dissection of the cerebellum tentorium in immediate proximity to the upper petrous sinus permit to visualize the 5 and 6th cranial nerves, an oral pole of a tumor, medial segments of the affected pons at early stages of operation (Fig.3). Demerits of this method include limited visualization of a subtentorial space, risk of damage of Labbe's vein and possible traction injuries of a temporal lobe (Fig.4).
M. Samii  proposed to resect the pyramid apex in tumors of the posterior cranial fossa and upper segments of the clivus, using a subtemporal approach. After extradural resection of the pyramid apex (Fig.5) with the help of a subtemporal approach, dura of the posterior cranial fossa is exposed in the anterior segment of Kawase's triangle. It is performed in a forward direction from the opening of the internal acoustic meatus and backward direction from the trigeminal nerve entry into Mekkel's cavity. This approach is rather narrow, but in contrast to posterior transpyramidal accesses it allows to expose dura of the posterior cranial fossa and upper clivus without destroying the cochlea and labyrinth. The approach has several advantages over other subtemporal accesses. It is characterized by a wider angle of operative activity, a reduced risk of traction injuries of a temporal lobe due to an extradural access, which is of particular importance in operations on a dominant hemisphere side. A combination of the above and orbito-zigomatic approaches permits to minimize traction of a temporal lobe and facilitates further stages of surgical intervention.
As for giant neoplasms with a wide zone of initial growth (Fig.6), wee use a subtemporal transtentorial transmastoid access, directed from above through a mastoid part of a temporal bone, which forms posterior segments of the fundus of the middle cranial fossa. It allows to approach nerves of acoustic-facial and caudal groups, trigeminal, oculomotor and trochlear nerves. Visualization of structures of the interpeduncular, ambient and cerebellopontine cisterns, upper segment of the clivus and oculomotor triangle is possible. Upper and middle petroclival regions are exposed in a lateral direction; it permits to visualize the inferior petrous sinus, Darello's canal and the 6th cranial nerve, entering it. A peculiar feature of this approach lies in preservation of a facial nerve and function of a middle ear in spite of considerable volume of resection of the petrous pyramid. The superior petrous sinus is ligated extradurally; it permits to avoid a conflict with Labbe's vein and, as a result, traction and dyshemopoietic injuries of a temporal bone.
Processes, typical of a cerebral wound in a postoperative period, differ in their significance and are heterogeneous in their pathogenetic essence. It is quite evident, that the less serious injuries of intact tissues during an operation, the easier and quicker reparative processes. Thus, a course of a postoperative period is greatly dependent on severity of surgical trauma, sustained during both an approach and tumor resection. According to our opinion, it is groundless to perform extensive, durable and rather traumatic approaches, whose only result is neoplasm biopsy.
A choice of an adequate approach determines possibility of tumor removal under conditions of direct visualization of important neural structures and, in the long run, their anatomic and functional integrity. However, even skillful surgeons can experience certain difficulties, while identifying cranial nerves during removal of basal extracerebral tumors. When function of a cranial nerve (especially a locomotor one) is lost, it can lead to considerable reduction of life quality and a poor level of social adaptation of a patient. Intraoperative monitoring of locomotor cranial nerves can become an important means of nerve preservation; it especially concerns cases, when a tumor changes its localization, strains and thins a nerve to a considerable extent. As for our study, we used electroneuromyography as the most sensitive and specific method of diagnosis of nerve lesions. Both spontaneous and evoked bioelectric activity were estimated. A reduced amplitude or disappearance of EMG activity during intervention turned out to be an unfavorable prognostic sign.
Today there is multimodality neurophysiologic equipment, which makes it possible to carry out intraoperative evaluation of many parameters, characterizing a state of vital cerebral structures. Here one can mention auditory stem evoked potentials, somatosensory evoked potentials, EEG and a set of devices for intraoperative monitoring and electrostimulation of locomotor cranial nerves.
As far as our study is concerned, data of electrophysiologic monitoring were analyzed during an operation by a neurosurgeon, anesthesiologist and electrophysiologist. Neurophysiologic monitoring helped to detect appearance of signs of the stem lesion, conditioned by a surgeon's activity. In such situations an operation was stopped for a while and a new manipulation zone was chosen. Sometimes their appearance led to cessation of surgical intervention.
Technique of surgical intervention is standard to some extent and does not depend on an approach to neoplasm. First of all, it is important to preserve venous outflow in brain areas, subject to traction; it refers both to basal segments of a temporal lobe in a supratentorial access or lateral segments of the cerebellar hemisphere in a retrosigmoid approach. As for the latter, it should be noted, that after introduction of intraoperative methods of microcirculation estimation, we make resection of lateral segments of the cerebellar hemisphere less and less frequently. Early devascularization of neoplasm parenchyma provides a "dry" operative field and promotes performing an operation under direct visual control. It is wise to use an ultrasonic aspirator at this stage of surgical intervention. Stage-by-stage and fragment-by-fragment decrease of a tumor size allows to get a sufficient reserve space for further manipulations in a narrow operative field. When the notch of the cerebellum tentorium is packed by a tumor, one should strive for dissection of the tentorium at early stages of intervention, irrespective of a chosen approach; it ensures better visualization and brain stem decompression. In our opinion, such tactics allows to avoid stem dysfunction in a postoperative period. Removal of a tumor capsule should be performed under continuous electrophysiologic monitoring. It is useful to stop intervention even in stable indices of this monitoring and to irrigate a wound with saline. We consider, that it makes capsule dissection less risky and prevents reperfusion complications. In general, balanced and purposeful maneuvers of a surgeon within an operative wound, good knowledge of anatomy of a lateral cistern of the pons and a notch of the cerebellum tentorium , as well as experience, gained in performing operations in this zone, are a pledge of success.
We consider results of surgical treatment of meningiomas of the petrous pyramid apex in our series of patients to be favorable: 2 out of 95 operated cases died and the mortality rate was equal to 2.1%. Fatal outcomes were caused by development of ischemic lesions of the brain stem due to neoplasm invasion into pia mater and inclusion of perforating branches of the basilar artery into neoplasm stroma. Such biological behavior of a tumor was the main factor, limiting its radical removal.
Thus, use of different (including combined) approaches together with modern technologies of surgery of the skull base, microsurgical technique, adequate anesthesiologic support and continuous neurophysiologic monitoring conditioned achieving minimum mortality (2.1%). Total and subtotal tumor removal (with adequate repeated expansion of the stem) was performed in 76% of cases.