the craniocervical junction ( ccj ) comprises the region located between the skull - base and the upper cervical spine ( atlas and axis ) , along with its neural components ( brainstem and spinal cord ) . its stability is mainly based on strong complex ligamentous and bony structures that are also responsible for the majority of axial rotation ( at the atlantoaxial complex ) and flexion - extension movements ( at the occiput - c1 joint ) . one of the most common ccj anomalies is basilar invagination ( bi ) , a radiographic finding represented by a prolapsed spine into the skull - base , diagnosed when the tip of the odontoid process is located at least 2 mm above of chamberlain 's line . many osseous anomalies are associated with bi , such as clivus and condyle hypoplasia , atlas assimilation and subaxial vertebral body fusion . tonsillar herniation is described as an associated neural finding that can lead to neurological impairment and can drive surgical treatment . in this paper , we review a case series of patients treated for symptomatic bi , discussing the different strategies required to manage this complex ccj disease , as well as complications and patients neurological outcomes . a retrospective evaluation of a consecutive series of patients surgically treated for treatment of symptomatic bi between january 2009 and september 2013 by one of the authors ( afj ) was performed . institutional ethical committee approval was obtained prior to the study ( caae : 17337313.7.0000.5404 ) . clinical data included : age , sex , follow - up , surgical indication , surgical complications , and neurological status before and after surgery . the pre- and post - operative functional status was classified according to the nrick scale [ table 1 ] . nrick scale for evaluation the neurological status radiological evaluations consisted in a craniocervical magnetic resonance ( mr ) and a computed tomography ( ct ) with sagittal and coronal reconstructions . a flexion and extension ct scan was also required when instability could not be clearly inferred based on static examinations . patients were diagnosed as having bi when the tip of the odontoid was located 2 mm above the chamberlain 's line on sagittal ct scan reconstructions . our protocol of treatment included foramen magnum decompression when tonsillar herniation was compressing the medulla or the spinal cord . in selected cases , craniocervical realignment and stabilization was performed in patients with static or dynamic atlanto - axial instability . realignment consists of preoperative traction or direct c1-c2 manipulation or occipital c2 distraction [ figure 1 ] . surgical decision - making flow chart for patients with basilar invagination radiological data were used to assess the surgical procedure , technique of fixation used ( when applied ) , and late radiological complications . age ranged from 15 to 67 years old ( mean 38 and median of 38.5 years old ) . ten patients were male ( 38% ) , and 16 ( 62% ) were female . follow - up ranged from 1 to 25 months ( average of 9 and median of 6 ) . most of the patients had clinical improvement , as shown in table 2 , according to the nrick grade system . of note , some patients had also axial cervical pain with significant improvement after surgery , although this change did not alter the nrick grade ( 11 patients with nrick grade i still grade i , although they were satisfied with surgery , with less clinical symptoms ) . eight patients improved one grade in the nrick system and three improved within two grades . clinical evolution of the patients before surgery and after final follow - up assessment according to the nrick grade system all the patients underwent foramen magnum decompression , except patient 5 , who underwent an occipitocervical fixation and fusion in mild extension . nine patients underwent craniocervical instrumentation and fusion with concomitant foramen magnum decompression : three patients for atlanto - axial instability secondary to evident lysthesis of the facet joints in static ct and/or mr imaging ; four patients for abnormal subluxation detected on dynamic ct scan between c1 and c2 ( increased atlanto - dens interval in flexion and extension ) and two for severe craniocervical kyphosis ( clivus - canal angle of < 90 ) . six patients of our series were had surgery prior to presentation to our institution and underwent a revision surgical procedure for progression of their neurological symptoms . one had a prior craniocervical fixation without foramen magnum decompression in flexion with pseudarthrosis and failed instrumentation . finally , one patient had a prior occipitocervical fusion without reduction of the anterior brainstem compression secondary to c1-c2 listhesis . minor complications included wound dehiscence in five patients : three were treated with antibiotics only , one required surgical debridement besides antibiotics and one required a cranial flap due to poor skin coverage associated with prior surgical treatment ( patient 13 ) . one patient who underwent a transoral dens resection developed meningitis , even though a cerebrospinal fluid ( csf ) leak was not noted during the procedure ( patient 6 ) , and was treated with antibiotics only without neurological compromise . there was mortality in this series , 1 month after surgery associated with removal of a tracheal cannula . the head and neck department weaned her tracheotomy cannula , but she had a tracheal collapse some hours later , with acute respiratory failure . two illustrative cases are presented in figures 2 and 3 and table 3 content the general results of these series illustrative case patient 9-presented with a severe neurological deterioration 3 years after a posterior fossa decompression for treatment a tonsillar herniation . ( a ) sagittal t2 sequence magnetic resonance imaging showing odontoid compression over the brainstem and upper spinal cord . ( b and c ) flexion and extension ct scan sagittal reconstructions showing atlanto - axial instability . ( d ) ct scan 3d reconstruction of a unilateral c1-c2 fusion and unilateral occipito - c2 fusion . ( e ) note the c1 lateral mass screws and the fusion of the occipital condyle with c1 . ( f ) laminar screws at c2 . ( g ) coronal ct scan showing the left c1 screw in the lateral mass . patient 25-presented with cervical neck pain after moderate walking and signs of cord compression ( hyperreflexia , a positive babinski sign ) without gait complaints . ( a ) sagittal t2 sequence magnetic resonance imaging showing tonsillar herniation and syrinx in the cervical spinal cord . ( b and c ) flexion and extension ct scan sagittal reconstructions showing atlanto - axial stability . ( d ) sagittal ct scan showing a normal facet joints congruence despite an important clivus hipoplasia . ( e and f ) postoperatory sagittal ct scan after posterior fossa decompression and dura mater expansion with fascia graft . the patient reported significant relief of cervical axial pain after 4 months of follow - up clinical data of the 26 patients surgically treated for basilar invagination basilar invagination refers to a radiological finding secondary to a developmental anomaly of the occipital bone and upper cervical spine where the tip of the odontoid process is located at least 2 mm above chamberlain 's line . this neurodysplasia results in a prolapsed vertebral column into the skull - base that can lead to neural compression or craniocervical instability . cmi is the most common ccj malformation , represented by tonsillar herniation into the spinal canal and can be associated with bi in more than 90% of the cases . in an analysis of 100 cases with cmi , menezes and traynelis have reported that 92 ( 92% ) were associated with bi , and 34 ( 34% ) had reducible ventral compression due to atlantoaxial instability which received craniocervical fixation concomitant to posterior fossa decompression . syrinx is presented in about 50% of the cases , probably due to the csf blockade at the foramen magnum region . management of bi , most of the times should be different than those cases of tonsillar herniation present in isolation . goel proposed a classification for bi based on the presence ( group a ) or absence ( group b ) of clinical and radiological instability between c1 and c2 . the importance of this classification is that it helps surgeons to choose the best strategy to manage these patients . patients with bi and c1-c2 instability can be treated with three posterior approaches described in the literature : 
 preoperative traction with reduction followed by craniocervical fixation and fusion;c1-c2 direct joint manipulation with distraction of the joints and spacers for maintaining reduction anddirect intraoperative distraction between the occipito and c2 using pedicle screws instrumentation . preoperative traction with reduction followed by craniocervical fixation and fusion ; c1-c2 direct joint manipulation with distraction of the joints and spacers for maintaining reduction and direct intraoperative distraction between the occipito and c2 using pedicle screws instrumentation . anterior approaches are only used in patients with irreducible ventral compression as we performed in two of our patients ( patients 6 and 10 ) . instability can be misdiagnosed in patients if flexion and extension radiological examination are not performed 3 ( 11% ) patients of our series had instability detected on dynamic ct scan reconstruction . this assures our policy of performing dynamic ct scan reconstruction in flexion and extension in all patients with cmi or bi before surgery , especially in patients who underwent a posterior foramen magnum decompression . we have treated three patients with bi and c1-c2 lysthesis ( goel group a ) . patients 15 and 16 received preoperative traction with reduction of c1-c2 instability followed by craniocervical fixation . in patient 9 , an anterior approach was used once the patient had a previous craniocervical fusion without c1-c2 reduction and a persistent irreducible anterior brainstem compression . patients with bi secondary to clivus hypoplasia and without c1-c2 instability can be treated with posterior foramen magnum decompression  the most common treatment for patients in this group  based on the principle of increasing the posterior fossa volume . in such cases , where instability is not present , treatment is similar to pure cmi , with controversies regarding the size of the craniectomy  a small versus a large suboccipital decompression  and the extension of the laminectomy to the cervical spine ( c1 , c2 or even c3 ) . controversies regarding bone decompression with or without concomitant duraplasty or even tonsils resection to preservate the arachnoid are also found . in our series , all the patients except one ( patient 5 ) underwent a suboccipital bone decompression from below the inferior nucal line to the posterior and lateral edges of the foramen magnum , including the posterior arch of the atlas , followed by duraplasty with autologous graft ( muscle fascia ) without tonsil resection . it 's our policy to perform duroplasty in all cases ; although , we did not perform routine tonsils resection to avoid arachnoid scarring and late syrinx to csf blockage . in cases of severe tonsillar herniation ( tonsils displaced beyond c1 ) , we then performed tonsil coagulation ( two patients in this series ) . in selected cases ( patients with severe craniocervical kyphosis ) , a posterior craniocervical decompression followed by craniocervical fixation in extended position in proposed to increase the clivus canal angle and decrease brainstem compression . two patients with bi secondary to clivus hypoplasia underwent craniocervical fusion in slight extension for a treatment of craniocervical kyphosis ( clivus - canal angle of < 90 ) . one of these patients ( patient 5 ) did not undergo posterior fossa decompression , once his symptoms were clearly exacerbated by head flexion . despite that although we performed nine craniocervical fixations in this series , we used c1 lateral mass screws in just one case ( patient 13 ) . this patient was operated previously , underwent a foramen magnum decompression , dural expansion , and tonsils coagulation , but developed a late c1-c2 instability with neurological deterioration . noteworthy , our craniocervical fixations were extended caudally up to the level of c3 with most of them being occipito - c2 fixations . in the out - patient clinics , we did not have significant complaints of head movement restrictions , probably because small constructions seem to do better than long ones . as shown in table 2 , almost all the patients classified as nurick grade ii or more had some degree of the neurological improvement during follow - up . most patients with nurick grade i had improvement in pain symptoms , although the score remains the same . this was a limitation of our retrospective study , which could be solved using others outcome instruments , but it is our perception that patients are satisfied with surgery after hospital discharge . there was mortality in this series 1 month after surgery associated with management of the tracheal cannula . it is important to remember that patients with severe craniocervical malformations can also have craniofacial abnormalities that can lead to difficult airways . an experienced team on difficult airways is mandatory when managing these patients to avoid catastrophic complications such as this . although anterior decompression use is decreasing , some patients with persistent symptoms can benefits from it . patient 6 had a prior craniocervical fixation at another facility without correction of c1-c2 lysthesis and symptomatic anterior compression remained , so we decided to perform a transoral decompression of the dens with some significant neurological improvement . patient 10 also had an anterior decompression for persistent symptomatic anterior compression after posterior occipital - c2 fixation and swallowing problems , despite a good craniocervical arthrodesis . management of congenital craniocervical anomalies requires a deep analysis of the neural and bony anatomy of the ccj . surgical treatment has a high rate of morbidity and morbidity , especially in previously treated patients in nonreferral centers , although the benefits of surgery overweight the risks . treatment should be individualized based on the patient anatomy , presence or absence of instability and surgeon 's experience . most of the times , bi is managed differently than isolated cmi , especially regarding atlanto - axial instability . attention to the principles above has proven to be helpful in improving surgical results in the treatment of these patients .
introduction : basilar invagination ( bi ) is a congenital craniocervical junction ( ccj ) anomaly represented by a prolapsed spine into the skull - base that can result in severe neurological impairment.materials and methods : in this paper , we retrospective evaluate the surgical treatment of 26 patients surgically treated for symptomatic bi . bi was classified according to instability and neural abnormalities findings . 
 clinical outcome was evaluated using the nrick grade system.results:a total of 26 patients were included in this paper . 
 their age ranged from 15 to 67 years old ( mean 38 ) . 
 of which , 10 patients were male ( 38% ) and 16 ( 62% ) were female . 
 all patients had some degree of tonsillar herniation , with 25 patients treated with foramen magnum decompression . 
 nine patients required a craniocervical fixation . 
 six patients had undergone prior surgery and required a new surgical procedure for progression of neurological symptoms associated with new compression or instability . 
 most of patients with neurological symptoms secondary to brainstem compression had some improvement during the follow - up . 
 there was mortality in this series , 1 month after surgery , associated with a late removal of the tracheal cannula.conclusions:management of bi requires can provide improvements in neurological outcomes , but requires analysis of the neural and bony anatomy of the ccj , as well as occult instability . 
 the complexity and heterogeneous presentation requires attention to occult instability on examination and attention to airway problems secondary to concomitant facial malformations .