Kitakanto Med. J. (S1) : 23 `28, 1998 23 DIRECT SURGERY FOR INTRA-AXIAL BRAINSTEM LESIONS Kazuhiko Kyoshima, Susumu Oikawa, Shigeaki Kobayashi Department of Neurosurgery, Shinshu University School of Medicine, Matsumoto, Japan Abstract : Direct surgery for intra-axial lesions of the brainstem was done for 9 cases : seven cases in the pons (2 cavernous angiomas, 3 gliomas, 2 metastatic tumors) and two in the medulla oblongata (one cavernous angioma, one glioma), by suboccipital craniectomy. For the lesions in the pons, the supra or infrafacial approach was made. Cavernous angiomas were totally removed. Metastatic tumors were gross-totally removed. Gliomas were partially removed except one case in the medulla oblongata which was removed gross-totally. Suprafacial approach was taken in 5 cases and infrafacial approach was in 2. As surgical results, 7 cases showed improvement of symptoms, and 2 cases, which were operated by infrafacial approach, showed worsening. Cavernous angiomas are good candidate for total removal. Infrafacial approach will required more meticulous procedures than the suprafacial triangle. Key words : Brainstem, Cavernous angioma, Glioma, Metastatic tumor, Surgical anatomy (Kitakanto Med. J. (Si) : 23 `28, 1998) INTRODUCTION The recent development of magnetic resonance (MR) imaging has renewed interest in surgical approaches to the brainstem. However, direct surgery for intra-axial lesions of the brainstem is still considered hazardous. We report our surgical experiences of brainstem lesions. Anatomical Considerations The dorsal aspect of the fourth ventricular floor, after removal of the cerebellum, is referred to as the rhomboid fossa. It is 3 ~ 2 cm in size, is bordered rostrally by the cerebellar peduncles and caudally by the tubercles of the gracilis nucleus and cuneate nucleus, and the tuberculum cinereum (Fig. 1 and 2). Other external structures include the median sulcus in the midline, sulcus limitans, facial colliculus, and striae medullares. The striae medullares, running across the floor of the fourth ventricle, roughly indicate the border between the medulla oblongata and pons (Fig. 1). As internal structures, the medial longitudinal fascicle runs just lateral to the median sulcus, the nucleus of the abducens nerve courses under the facial colliculus, and the facial nerve originates from Fig. 1 Schematic drawing of the anatomy and the safe entry zones to the brainstem via the floor of the fourth ventricle. Important neural structures are sparse in these safe entry zones. A = suprafacial triangle ; B = infrafacial triangle ; MLF = medial longitudinal fascicle ; trochlear = trochlear nerve ; facial = facial nerve ; sup.inf. = superior and inferior ; ambiguus = nucleus ambiguus ; vagus= dorsal nucleus of the vagus nerve ; hypoglossal = nucleus of the hypoglossal nerve. its nucleus, which is located laterally deep under the Received : March 2, 1998 Address : KAZUHIKO KYOSHIMA Department of Neurosurgery, Shinshu University School of Medicine Asahi 3-1-1, Matsumoto 390, Japan
24 Kyoshima, Oikawa, Kobayashi striae medullares and runs up to and around the nucleus of the abducens nerve (Fig. 1 and 2). The nuclei of the hypoglossal and vagus nerves are located just caudal to the striae medullares. Fig. 2 Schematic drawing with a three-dimensional view of of the brainstem showing the relation of the structures. Abducens= nucleus of abducens nerve ; cuniatus = nucleus cuniatus ; gracilis= nucleus gracilis ; MLF = medial longitudinal fascicle ; facial = facial nerve ; ambiguus = nucleus ambiguus ; vagus dorsal nucleus of the vagus nerve ; hypoglossal = nucleus of the hypoglossal nerve. A Safe Entry Zones Into the Brainstemn Based on symptomatology and surgical anatomy, there is two safe entry zones into the brainstem through the fourth ventricular floor via suboccipital approach. These safe entry zones are located where important neural structures are sparse (Figs. 1 and 2). One is the suprafacial triangle, which is bordered medially by the medial longitudinal fascicle, caudally by the facial nerve (which runs in the brainstem parenchyma), and laterally by the cerebellar peduncles. The second is the infrafacial triangle, which is bordered medially by the medial longitudinal fascicle, caudally by the striae medullares, and laterally by the facial nerve. At the bottom of these triangles are the medial lemniscus and corticospinal tract. The brainstem parenchyma is vascularized by the perforating arteries coursing from the ventral or lateral aspect of the brainstem, but not from the surface of the fourth ventricular floor2,3). This means that on medullary incision from the fourth ventricular floor there is little possibility of causing brainstem dysfunction by damaging perforating arteries. Furthermore, on parenchymal procedures approached from the fourth ventricular floor there is a low possibility of causing ischemic brainstem damage to the remote area from the operative field. Thus, these two triangles can be designated safe entry zones in the brainstem for surgical treatment of a lesion located dorsal to the medial lemniscus. B Fig. 3 Schematic drawing of the safe entry zones via the fourth ventricular floor. A : Suprafacial triangle approach (suprafacial approach). The triangle is bordered by the medial longitudinal fascicle (MLF) medially, the facial nerve caudally, and the superior and inferior cerebellar peduncles laterally. A 1-cm longitudinal medullary incision is made caudally from the edge of the superior cerebellar peduncle and 4 to 5 mm laterally from the median sulcus. The brainstem can be retracted either laterally or rostrally with relative safety (arrows). B : Infrafacial triangle approach (infrafacial approach). The triangle is bordered by the MLF medially, the striae medullares caudally and the facial nerve laterally. A 1-cm longitudinal medullary incision is made rostrally from the caudal margin of the striae medullares and 4 to 5 mm laterally from the median sulcus. The brainstem can be retracted only laterally (arrow). PPRF = paramedian pontine reticular formation ; abducens = nucleus of the abducens nerve ; facial = nucleus of the facial nerve.
25 Direct surgery for intra-axial brainstem lesions Suprafacial approach. On the suprafacial triangle (Fig. 3A), the most feasible brainstem incision is made longitudinally, up to 1 cm in length, caudal from the edge of the cerebellar peduncle and about 5 mm lateral to the median sulcus. The length of the linear incision may be about 7 mm, because it is extended by brainstem retracion. In order to minimize the retraction-related damage to important brainstem structures, the brainstem should be retracted either laterally or rostrally. Retraction to the rostral side is possible because the oculomotor nuclei are located far from the triangle. Infrafacial approach. The most feasible brainstem incision in the infrafacial triangle (Fig. 3 B) is made longitudinally, rostral from the caudal margin of the striae medullares and about 5 mm lateral to the median sulcus. The incision should be less than 1 cm in length. Brainstem retraction is possible only laterally, because the facial nucleus is located deeper and the facial nerve courses upward to the abducens nucleus. This triangle is narrow and surrounded by more important structures compared to the suprafacial triangle. An initial medullary incision approximately 5 mm length is suitable. It is important that surgical procedures not extend over the caudal edge of the striae medullares. A MATERIALS AND METHODS Direct surgery for intra-axial lesions of the brainstem was carried out for 9 cases seven cases in the pons (2 cavernous angiomas, 3 gliomas, 2 metastatic tumors), and two in the medulla oblongata (one cavernous angioma, one glioma) (Table 1). Surgery was performed by suboccipital craniectomy. For the lesions in the pons, intra-axial approach was made via the suprafacial approach (5 cases) or infrafacial approach (2 cases) (Table 4). Table 1 Operated cases Illustrative Cases Case 1 Cavernous angioma in the pons, 31-year-old man (Fig. 4). Preoperative neurological examination revealed left abducens paresis, nystagmus, numbness and hypalgesia Fig. 4 Case 1, cavernous angioma in the pons treated via the suprafacial approach. Pre (A) and postoperative (B) magnetic resonance images. of the right side of the face, right sensory disturbance for touch and pain, and cerebellar signs predominantly on the right side, with gait disturbance. Computerized tomography revealed bleeding in the brainstem, 28mm in size. A heterogeneous high-intensity mass with thin low-intensity rim was seen on a T 1-weighted MR image and a high-intensity core with thick lowintensity rim on a T2-weighted MR image. The mass extended from the lower pons to the midbrain on the left side (Fig. 4A right). The lesion was removed via left suprafacial approach. The patient was alert immediately postoperatively and showed no further neurological deficits except increased gait ataxia, which improved gradually. Over the following several months, he became ambulatory without disability in daily life. The left abducens paresis and nystagmus improved gradually and resolved completely over sev- B
26 Kyoshima, Oikawa, Kobayashi eral weeks, and other symptoms almost entirely resolved. Postoperative MR image confirmed 4B). total removal of the lesion (Fig. Case 2 Metastatic tumor in the pons, 42-year-old man (Fig. 5). Preoperative neurological examination showed right abducens paralysis, bilateral hearing disturbance, slight left hemiparesis with mild hyperreflexia, a left sensory disturbance, and right mild cerebellar signs. Magnetic resonance imaging demonstrated an enhanced intra-axial mass in the lower pons located on the right side (Fig. 5 upper). The tumor in the lower pons was totally removed en bloc via right infrafacial approach. Postoperatively the patient showed moderate facial paresis, lateral gaze disturbance, swallowing disturbance, and ataxic gait with loss of depth and position sense. The facial paresis and swallowing impairment resolved completely over several weeks. Pathological diagnosis was metastatic renal-cell carcinoma. Postoperative MR imaging confirmed total removal of the lesion (Fig. 5 lower). Fig. 6 Case 3, cavernous angioma in the medulla oblongata postoperative (lower) magnetic resonance images. Pre (upper) and cranial nerves palsy on the left side, left cerebellar sign, left-sided hemiparesis dominantly in the lower extremities, left-sided sensory impairment including face, and right-sided pain impairment including left face, without bowel and bladder dysfunction. Preoperative MR imaging revealed an intra-axial mass in the medulla oblongata on the left side. The lesion was totally removed by suboccipital craniectomy with Cl laminectomy. Postoperatively the patient showed complete improvement of symptoms and signs. Case 4 Glioma in the medulla oblongata, 42-yearold male (Fig. 7). Preoperative neurological findings showed paresis of left lower cranial nerves, left cerebellar sign and nystagmus. Preoperative MR imaging revealed a cystic tumor in the medulla oblongata on the left side. With the patient in the prone position, the tumor was grosstotally removed by suboccipital craniectomy with Cl laminectomy. Pathological diagnosis was astrocytoma Fig. 5 Case 2, metastatic tumor in the pons treated via infrafacial approach. Pre (upper) and postoperative (lower) magnetic resonance images. Case 3 Cavernous angioma in the medulla oblongata, 53-year-old female (Fig. 6). Preoperative neurological findings revealed lower grade 3. Postoperatively tracheotomy was required for three months because of worsening of swallowing disturbance. After irradiation and chemotherapy he was discharged without disability in daily life. The patient could spend usual life about three years without tumor recurrence (Fig. 7 B upper). But the patient died about 4 years after surgery because of tumor recurrence (Fig. 7 B lower).
27 Direct surgery for intra-axial brainstem lesions A B Fig. 7 Case 4, astrocytoma in the medulla oblongata A : Pre (upper) and postoperative (lower) magnetic resonance (MR) images. B : Postoperative MR imaging taken after 2 years and 3 months showing no tumor recurrence (upper) but MR imaging taken after 3 years and 5 months showing tumor recurrence (lower). RESULTS AND CONCLUSIONS Cavernous angiomas were totally removed. Metastatic tumors were near-gross totally removed. Gliomas were partially removed except one case in the medulla oblongata which was removed gross totally (Table 2). As surgical results 7 cases showed improvement of symptoms, and 2 cases showed worsening (one metastatic tumor and one cavernous angioma) (Table 3). Table 2 Surgica results-1 These two cases were operated by infrafacial (Table 4). Infrafacial approach meticulous surgical manipulation approach because the infrafacial approach will required more than the suprafacial triangle was smaller than the suprafacial triangle, surrounded by important structures. The supra- or infrafacial approach is indicated for localized intra-axial lesions located unilaterally and dorsal to the medial lemniscus in the lower midbrain to the pons. Magnetic resonance imaging is useful in selecting these approaches. Table 3 Surgica results-2 The suprafacial approach
28 Kyoshima, Oikawa, Kobayashi is indicated for a lesion located in the upper half of the pons and the infrafacial approach for a lesion in the lower half of the pons (Fig. 8). In intra-axial lesions of the brainstem, cavernous angiomas are good candidate for total removal. Table 4 Surgica results of lesions in the pons Fig. 8 Selection of supra- or infrafacial approach into the brainstem. Suprafacial approach indicating fora lesion located in the upper half of the pons and infrafacial approach for a lesion in the lower half of the pons. REFERENCES 1) Kyoshima, K, Kobayashi S, Gibo H, et al : A study of safe entry zones via the floor of the fourth ventricle for brain-stem lesions. Report of three cases. J Neurosurg 1993 ; 78 : 987-993. 2) Duvernoy HM : Human Brainstem Vessels. Berlin : Springer-Verlag, 1978. 3) Nakajima K : Clinicopathological study of pontine hemorrhage. Stroke 1983 ; 14 : 485-493.