Imaging evaluation and selection of surgical approach
Imaging evaluation
Preoperative MRI and enhancement imaging for FMM are necessary, and it is important to distinguish the tumor location, texture, and the relationship between the tumor and the dura. We named the classification ABS according to the relationship between a tumor and the brainstem on MRI imaging, and we think it is the most difficult to operate if the tumor is located ahead of the brainstem (A), followed by the side of the brainstem (S), and the back of the brainstem (B) in an actual surgical operation. It is helpful to predict the texture of a tumor and the space between a tumor and the arachnoid membrane, and it is instructive to design the surgical approach and evaluate the difficulty of tumor resection. For example, the long T2 image indicates a soft tumor; it is a relatively soft tumor that is easier to remove, and bone removal during the operation should not be too much if a tumor shows a long transversal relaxation time-weighted image (T2W), while the short T2 image is the opposite. Bone invasion, subluxation, and total dislocation of the atlantoaxial joint can be observed using 3DCT of the foramen magnum. In our experience, if the distance between the axial odontoid and the atlantoaxial odontoid is greater than 3 mm on the midsagittal plane, it is considered that the dislocation of the atlantoaxial joint exists, and the fixation of the dislocation should be performed after surgery. We named the classification SIM according to the relationship between FMM and VA based on combined head and neck 3D-CTA imaging, and we believe it is the most difficult to operate if the tumor is located in the mixed type (M), followed by superior to the VA (S) and inferior to the VA (I) in an actual surgical operation.
In addition, for the location at the back of the brainstem of the FMM (B), regular 3D-CTA imaging showed strong contrast enhanced, and there was a close relationship between FMM and blood vessels; a DSA examination needs to be performed to further distinguish the solid hemangioblastoma of the dorsal medulla oblongata [5, 11, 14, 20,21,22]. This is of great significance because there are essential differences in surgical preparation, operation scheme, operation difficulty, and prognosis of patients between these two diseases.
The selection of surgical approach
There are a variety of extensions of FLA, including B-FLA, T-FLA, S-FLA, P-FLA, and extended FLA (E-FLA) [16, 23,24,25,26,27,28], and we think that the approach should be combined to analyze ABS and SIM classifications (Table 2).
The main points during FLA
Flap formed
The skin flap of the FLA used in the study was adopted in an inverted L-shaped incision, and then, the skin flap was flipped with the cervical occipital muscles. Because the VA in the suboccipital triangle is protected by its surrounding venous plexus and muscles, the probability of injury is rare. If not necessary, the VA may not be exposed and turned over to reduce vascular injury and spasms caused by the extension of the VA [29, 30].
Bone removed
It is essential to remove the occipital condyle in FLA. The removal of the internal portion of the occipital condyle only increased the exposure by 7%, but the operating space increased significantly by 22% [19, 25, 31, 32]. The more ventral the lesion, the larger the range of the occipital condyle removal, but the range of removal of the occipital condyle was not beyond the hypoglossal canal; that is, it could not exceed the posterior 1/3 of the long axis of the occipital condyle. Some scholars believe that the ventral field of the BS has been improved by gradually pushing it backward in some long-term extramedullary subdural lesions, providing additional operating space. It is feasible to remove the ventral subdural tumor of the foramen magnum without removing the occipital condyle, and the disability rate could be reduced [15, 33]. Other studies believe that a small amount of the occipital condyle can be removed if the tumor is located in the ventral lateral of the brainstem and the superior cervical spinal cord because extra space would be exposed during the tumor resection process, and the tumor has created access pathways through the relationship with peripheral nerve and vessels [5, 34, 35]. During the operation, the ventral side of the brainstem and the high cervical spinal cord can be observed and exposed in a coronal plane of 45° or more, to reveal the relationship between the tumor and the medulla oblongata and cervical spinal cord, without important structures retraction, such as the medulla oblongata and cervical spinal cord, to effectively expose and remove the tumor [12, 36].
Among the 76 patients in this group, 54 patients were treated with FLA, including 31 cases of B-FLA, 12 cases of P-FLA, 9 cases of T-FLA (including five cases with the posterior 1/3 of the occipital condyle removed, four cases of less than 1/3), two cases of S-FLA, and no atlanto-occipital fusion.
Removal of the tumor
The operation was performed among the brainstem, posterior cranial nerves, C1, C2, and C3, and blood vessels under neural electrophysiological monitoring [11, 20, 22, 37,38,39,40]. Most tumors should usually be removed first within the tumor, the tumor root is cut off after the tumor volume is reduced, and the tumor tissues can then be removed piecemeal because the root of the tumor is adjacent to the running position of the VA and the tension of the tumor is high. Special attention should be paid to distinguishing and protecting the VA and its branches because their color is very similar to the color of the tumor, and they overlap with each other.
The contents and significance of the four triangles in operation
-
(1)
Triangle SOT: In this triangle, the content is the V3 segment of the VA. In this study, the FLA flap was flipped together with the occipital neck muscles, at the end of which was the position of the suboccipital triangle. When the posterior arch of the atlas is separated laterally, the VA in this area should be noted (Fig. 10a).
-
(2)
Triangle VOT: In this triangle, the contents are also the V3 segment of the VA, in which the average distance between the bottom line of the TV was 18.62 mm [41]. Therefore, it is important to understand the anatomy and relevant data of this location to guide the scope of the removal of the posterior atlas nodules during the operation and to protect the V3 segment of the VA [42] (Fig. 10b).
-
(3)
Triangle JVV: In this triangle, the contents are brainstem, tumor, V4 segment of the VA, posterior inferior cerebellar artery, glossopharyngeal nerve, vagus nerve, accessory nerve, hypoglossal nerve, etc. The triangle is the most important functional region with the most complex structure of the nerves and vessels. Any problem with the abovementioned nerves and vessels will show the corresponding clinical symptoms and manifestations. The S and part of M of the SIM classification will involve the above important structures. We must be very familiar with the anatomical position and function of each structure during the operation and strive to achieve functional reservation, preferably anatomical reservation (Fig. 10c)
-
(4)
Triangle TVV: In this triangle, the contents are the brainstem, high cervical spinal cord, tumor, accessory nerve, C1 nerve, C2 nerve, C3 nerve, etc. There are also many important nerves and vessels clustered in the triangle TVV, except for the V4 segment of the VA, which is easily confused with tumor tissue in terms of similar color and location. The nerves and vessels in this area also need to be preserved functionally, even anatomically. Although the hypoglossal nerve is not included in this region, it can also be seen on the abdominal side of the tumor when the tumor is removed completely if the tumor tissue grows upward and the VA is lifted upward, and attention must be paid to separate and protect it carefully (Fig. 6k). In addition, we do not advocate cutting off the C1, C2, and C3 nerves proactively; otherwise, complications after surgery may make patients very uncomfortable (Fig. 10d).
The main points during the suboccipital midline approach
The incision was performed strictly according to the median, and the foramen magnum and posterior arch of the atlas were opened gently. The latch of the medulla oblongata is the respiratory regulation center; therefore, pathological adhesion and double-click electrocoagulation should be carefully considered. Attention should be paid to the protection of the bilateral VA, posterior inferior cerebellar artery (PICA), and branches. The upper and lower vermis must be moderately dissected. The cerebrospinal fluid flow of the four ventricles should be ensured after tumor resection.
Postoperative complications and management
In the group of 76 cases, the function was significantly improved after administration of a nasogastric diet within 3 months of postoperative hoarseness, hoarse drinking water choking, and dysphagia; hypodermic fluidification was cured after puncture and compression bandage; pneumonia was cured after effective anti-inflammatory therapy; poor cough reflex was recovered 1 month after tracheostomy and regular sputum aspiration; neck and occipital numbness was relieved after expectant treatment; incision infection was cured after incision dressing and anti-inflammatory treatment; great occipital neuralgia was relieved after oral carbamazepine treatment; dyspnea was recovered after tracheostomy and ventilator-assisted respiration for 2 weeks, and tracheal cannula was extracted 6 weeks later; cerebrospinal fluid leakage recovered after 2 weeks of lumbar cisternal drainage; intracranial infection was recovered after 2 weeks of lumbar cisternal drainage and 4 weeks of effective anti-inflammatory therapy; hydrocephalus were recovered after ventriculoperitoneal shunt; and the muscle strength of the contralateral limb decreased. The cerebral stem and spinal cord vasospasm were considered, which was relieved after 2 weeks of antivasospasm treatment and functional training.