foramen magnum ( fm ) meningiomas are challenging lesions because of the vicinity of the medulla oblongata , the lower cranial nerves , and the vertebral artery7 ) . when diagnosed , these tumors are often large . however , due to their slow - growing rate and indolent course , partly attributable to the wide subarachnoid space at this level , clinical diagnosis is difficult , leading to a long interval since the onset of the first symptom5 ) . dorsally situated fm meningiomas are clearly approached through a posterior approach , while ventrally located tumors have been considered to require other complex approaches to ensure surgical corridor such as the postero - lateral approach , called the far - lateral approach7 ) , or the antero - lateral approach , also named the extreme - lateral approach6 ) . since goel et al.8 ) introduced the posterior approach for fm meningiomas , there have been only a few reports of the posterior approach in ventral fm meningiomas9,28,29 ) . lateral bony exposure also may pose risk to the vertebral artery and low cranial nerve injury . the occipital condyle drilling may sometimes be necessary in the far lateral approach . therefore , conventional posterior approach without lateral bony extension may be good to address these problems . in this report , we aimed to present our experience of conventional posterior approach without far lateral approach for ventral fm meningioma and tried to evaluate the approach is applicable to ventral fm meningioma . from november 1999 to march 2011 , we operated on 14 patients with fm meningioma in our clinic . we defined a ventral tumor as its dural origin was anterior to the dentate ligament that displaced the brain stem and spinal cord dorsally or dorsolaterally . and 11 of these patients had ventral fm meningioma . magnetic resonance imaging ( mri ) or computed tomography ( ct ) were taken pre- and postoperatively . the greatest axial tumor size was measured on axial cross - sections using gadolinium - enhanced t1-weighted mri using picture archiving communication system ( marosis , version 5438 ; infinitt , seoul , korea ) . each patient 's pre- and postoperative clinical status was evaluated with nurick myelopathy grading system , which ranges from 0 to 5 , with 0 denoting normal walking and 5 denoting complete paraplegia19 ) . the extent of tumor removal was evaluated by operative findings ( simpson grade ) and that was confirmed by postoperative mri . gross total removal was defined as no visible enhancing / non - enhancing tumor on postoperative imaging , with the surgeon recording no remnant tumor on microscopic operative views . recurrence was defined as a new onset or worsening of symptoms and/or radiological confirmation of tumor growth4 ) . this retrospective study was approved by the institutional review board of seoul national university hospital ( h-0910 - 014 - 296 ) . all operations were performed using a posterior approach consisting of suboccipital craniotomy without far lateral approach and c1 laminotomy . after the dura was opened , care was taken not to violate the arachnoid membrane . we usually were able to find the tumor lateral to the medulla and cervical spinal cord . thereafter , the dentate ligament was severed and c1 posterior root was cut , if needed . as most tumors were a little eccentric , we could expose the tumor from one side ( fig . the tumor was removed little by little , using an ultrasonic aspirator . with this gradual removal of the tumor the tumor was progressively delivered into the space made by decompression , with little displacing or handling the medulla , spinal cord or lower cranial nerves ( fig . all operations were performed using a posterior approach consisting of suboccipital craniotomy without far lateral approach and c1 laminotomy . after the dura was opened , care was taken not to violate the arachnoid membrane . we usually were able to find the tumor lateral to the medulla and cervical spinal cord . thereafter , the dentate ligament was severed and c1 posterior root was cut , if needed . as most tumors were a little eccentric , we could expose the tumor from one side ( fig . the tumor was removed little by little , using an ultrasonic aspirator . with this gradual removal of the tumor the tumor was progressively delivered into the space made by decompression , with little displacing or handling the medulla , spinal cord or lower cranial nerves ( fig . paresthesia was the most common symptom and all patients were able to ambulate at the time of surgery . median follow - up period was 55 months ( range , 20 - 163 months ) . preoperatively , six patients were of nurick grade 1 , three were of grade 2 , and two were of grade 3 . the mean size of maximal tumor diameter was 24.56.3 mm ( range , 15 - 36.4 mm ) . preoperative t2 mri did not show the absence of clear separating plane in all patients . the extent of resection was simpson grade i in one case and simpson grade ii in remaining 10 cases ( table 1 ) . this encased vertebral artery was trapped through preoperative endovascular intervention in order to decrease the tumor vascularity , because the tumor was hypervascular . three patients with nurick grade 2 improved to grade 1 . in six patients with nurick grade 1 , three patients were in the same grade 1 and the other three patients improved to grade 0 ( table 1 ) . postoperative dysphagia with dysarthria resolved progressively for 3 weeks following surgery ( table 1 ) . paresthesia was the most common symptom and all patients were able to ambulate at the time of surgery . median follow - up period was 55 months ( range , 20 - 163 months ) . preoperatively , six patients were of nurick grade 1 , three were of grade 2 , and two were of grade 3 . the mean size of maximal tumor diameter was 24.56.3 mm ( range , 15 - 36.4 mm ) . preoperative t2 mri did not show the absence of clear separating plane in all patients . the extent of resection was simpson grade i in one case and simpson grade ii in remaining 10 cases ( table 1 ) . this encased vertebral artery was trapped through preoperative endovascular intervention in order to decrease the tumor vascularity , because the tumor was hypervascular . three patients with nurick grade 2 improved to grade 1 . in six patients with nurick grade 1 , three patients were in the same grade 1 and the other three patients improved to grade 0 ( table 1 ) . two complications occurred . in patient 2 , postoperative pseudomeningocele was found . however , additional treatment was unnecessary . he had no further neurological deficits . in patient 6 , there was transient dysphagia with dysarthria . postoperative dysphagia with dysarthria resolved progressively for 3 weeks following surgery ( table 1 ) . in this series , posterior surgery was a safe and effective method for fm meningiomas as neither operative morbidity nor permanent neurological deficit occurred in all patients . gross total removal was achieved in all patients with minimal morbidity and in terms of simpson grade , we could achieve grade 2 in all but one patient who got simpson grade 1 . first , posterior approach is more familiar to surgeons , as well as being easier than the far lateral or extreme lateral approach . although the working space is narrow at first , the posterior approach can provide enough working space after a tumor has started to be decompressed . even though occipitocervical fusion is rarely necessary , occipital condyle resection was needed in between 0% and 66% in the far - lateral approach14,18,20,21,23,26 ) . fourth , in our experience , postoperative morbidity may be minimal with the conventional posterior approach . in our series , there was only one occurrence of transient neurological deficit ( 9% ) and one incidence of pseudomeningocele . various neurological complications have been reported in the studies using the far lateral or extreme lateral approaches as transient and permanent neurological morbidity occurred in 6 - 61% and 2 - 25% , respectively2,10,11,15,22,29 ) . although direct comparison of figures from each approach is inappropriate , we could assume that the complication after conventional posterior approach is not inferior to the far lateral approach . in addition , csf fistula has been mentioned in the various series1 - 3,10,15,29 ) . both cranial and spinal meningiomas originate from dural attachments and stretch the arachnoid membrane , which can then be exploited as an anatomical dissection plane24 ) . even though fm meningiomas were located closely with low cranial nerves and vertebral artery , the tumor mostly did not adhere to adjacent neurovascular structures because of arachnoid membrane . after internal debulking of the mass , tumor could be delivered into the working space made by decompression . vertebral artery was located in the superior side of ventral fm meningiomas , except in one case . this tendency was similar to that presented in previous literature8,10,28 ) . if the vertebral artery is encased by the tumor , preoperative endovascular intervention is recommended . preoperative embolization or trapping if possible can be helpful when preoperative angiography revealed hypervascular tumor such as in patient 7 . it has been reported that small fm meningiomas are more difficult to approach than large ones7,9 ) . based on our study results , however , further studies may be necessary to indicate an applicable cut - off minimum size for posterior approach . most tumors were removed with simpson grade ii in our series even though the working space to remove the original dura can be large ( fig . our previous studies with those of other investigators have demonstrated that for spinal meningiomas , gross total removal of a tumor and coagulation of its dural origin ( simpson grade ii ) appears to be sufficient2,30 ) . on the other hand , nakamura et al.17 ) recently reported that 6 out of 13 simpson grade ii resected spinal meningiomas underwent reoperation . the average follow - up period for reoperation was 12.2 years . in our study , the anterior landmarks for fm have been the lower third of the clivus and upper edge of the body of c26 ) . the anterior tubercle of the clivus is derived from the hypocentrum of the proatlas , while the apex of the odontoid process of the axis is derived from the centrum of the proatlas16 ) . hence , fm meningiomas are more similar to spinal cord meningiomas than to skull base meningiomas . through our experience , ventral intradural extramedullary spinal cord tumors including meningiomas could be removed gross totally through conventional posterior approach12 ) . therefore , it is quite natural to use a conventional posterior approach for removing fm meningiomas like ventral intradural extramedullary spinal cord tumors . therefore , the dural origin site can be manipulated only after the tumor was removed . if tumor vascularity is very high , a far lateral approach may be more suitable in terms of controlling the dural origin site first . when this type of tumor adheres to the spinal cord or brain stem , it is impossible to remove it completely using posterior approach . this type of tumor might be anticipated because preoperative mri and/or ct show a very intimate connection to the spinal cord and the absence of the typical clear separating plane24 ) . preoperative t2 mri did not show the absence of clear separating plane even in patient 1 and 9 whose preoperative nurick grade were 3 . although we have not experienced these cases , we agree that the posterior approach would be risky in these cases because the normal dissection plane along the arachnoid membrane is absent2,13,25 ) . other approaches such as far lateral approach or anterior approach should be considered , even though gross total removal is probably impossible even with these approaches . in the present study although all these operations were consecutive series , such a study may lead to inevitable selection bias . nonetheless , this study confirmed the feasibility of the conventional posterior approach for ventral fm meningiomas . the anterior landmarks for fm have been the lower third of the clivus and upper edge of the body of c26 ) . the anterior tubercle of the clivus is derived from the hypocentrum of the proatlas , while the apex of the odontoid process of the axis is derived from the centrum of the proatlas16 ) . hence , fm meningiomas are more similar to spinal cord meningiomas than to skull base meningiomas . through our experience , ventral intradural extramedullary spinal cord tumors including meningiomas could be removed gross totally through conventional posterior approach12 ) . therefore , it is quite natural to use a conventional posterior approach for removing fm meningiomas like ventral intradural extramedullary spinal cord tumors . therefore , the dural origin site can be manipulated only after the tumor was removed . if tumor vascularity is very high , a far lateral approach may be more suitable in terms of controlling the dural origin site first . when this type of tumor adheres to the spinal cord or brain stem , it is impossible to remove it completely using posterior approach . this type of tumor might be anticipated because preoperative mri and/or ct show a very intimate connection to the spinal cord and the absence of the typical clear separating plane24 ) . preoperative t2 mri did not show the absence of clear separating plane even in patient 1 and 9 whose preoperative nurick grade were 3 . although we have not experienced these cases , we agree that the posterior approach would be risky in these cases because the normal dissection plane along the arachnoid membrane is absent2,13,25 ) . other approaches such as far lateral approach or anterior approach should be considered , even though gross total removal is probably impossible even with these approaches . in the present study , we have described the feasibility of a posterior approach for ventral fm meningiomas . although all these operations were consecutive series , such a study may lead to inevitable selection bias . nonetheless , this study confirmed the feasibility of the conventional posterior approach for ventral fm meningiomas . ventral fm meningiomas can be removed gross totally using a conventional posterior surgery without far lateral approach .
objectivewe present our experience of conventional posterior approach without fat lateral approach for ventral foramen magnum ( fm ) meningioma ( fm meningioma ) and tried to evaluate the approach is applicable to ventral fm meningioma.methodsfrom january 1999 to march 2011 , 11 patients with a ventral fm meningioma underwent a conventional posterior approach without further extension of lateral bony window . 
 the tumor was removed through a working space between the dura and arachnoid membrane at the cervicomedullary junction with minimal retraction of medulla , spinal cord or cerebellum . 
 care should be taken not to violate arachnoid membrane.resultspreoperatively , six patients were of nurick grade 1 , three were of grade 2 , and two were of grade 3 . 
 median follow - up period was 55 months ( range , 20 - 163 months ) . 
 the extent of resection was simpson grade i in one case and simpson grade ii in remaining 10 cases . 
 clinical symptoms improved in eight patients and stable in three patients . 
 there were no recurrences during the follow - up period . 
 postoperative morbidities included one pseudomeningocele and one transient dysphagia with dysarthria.conclusionventral fm meningiomas can be removed gross totally using a posterior approach without fat lateral approach . 
 the arachnoid membrane can then be exploited as an anatomical barrier . however 
 , this approach should be taken with a thorough understanding of its anatomical limitation .