development of the craniovertebral junction is complex and developmental anomalies due to abnormal re - segmentation of the fourth occipital sclerotome are named proatlas segmentation anomalies . these proatlas segmentation anomalies are being increasingly recognized after the routine use of three - dimensional computed tomography ( ct ) for the evaluation of craniovertebral junction abnormalities . in this study , we report five patients with proatlas segmentation anomalies who presented to us with different radiological and clinical findings and discuss their surgical management . the embryological origin of these anomalies is also discussed . a 19-year - old male presented with 6-month history of progressive difficulty in walking , clumsiness of both upper extremities , and occasional difficulty in swallowing . plain radiographs revealed completely assimilated anterior arch of atlas , partially assimilated posterior arch , and atlanto - axial subluxation [ figure 1a and b ] . magnetic resonance imaging ( mri ) revealed features of basilar invagination , horizontally oriented clivus with features of ventral brainstem compression , and chiari i malformation [ figure 1c ] . ct of the craniovertebral junction revealed horizontally oriented clivus , basilar invagination , fused anterior arch of atlas , and an accessory ossicle interposed between the clivus and the dens [ figure 1d ] . there was also an evidence of partial assimilation of the posterior arch of atlas which was present only on the right side [ figure 2 ] . on the basis of these radiological features , a diagnosis of basilar invagination with proatlas segmentation anomaly causing ventral brainstem compression was made . it was believed that the chiari malformation encountered in this patient was the result of reduced posterior fossa volume induced by the above - mentioned abnormalities . hence , craniovertebral re - alignment by intraoperative traction and occipito - cervical fusion was planned . the posterior midline from the occiput up to the mid - cervical region was exposed , and occipito - cervical fusion was done using an occipital plate , c2 crossing translaminar screws , and lateral mass screws of c4 and c5 . c3 lateral masses were skipped during screw placement as the lateral mass fractured during drilling . after intraoperative fluoroscopic verification of proper craniovertebral realignment , bone grafts were placed on the exposed bony areas after denuding them . postoperative plain radiographs and ct showed proper craniovertebral re - alignment [ figures 3 and 4 ] . ( a and b ) plain radiographs flexion and extension views showing partial assimilation of atlas , basilar invagination , and atlanto - axial subluxation ; ( c ) sagittal t2-weighted magnetic resonance imaging sequence showing basilar invagination with ventral compression at the cervicomedullary junction ; ( d ) sagittal computed tomography of the craniovertebral junction showing assimilated anterior arch of atlas ( white arrowhead ) , accessory ossicle ( white arrow ) interposed between the clivus , and the invaginated odontoid ; the partially assimilated posterior arch of atlas ( black arrowhead ) three - dimensional computed tomography of the craniovertebral junction showing the absence of the posterior arch on the left side with partial assimilation of the right half of the posterior arch on the right side ( white arrow ) postoperative lateral radiograph obtained after occipito - cervical fusion ( a ) preoperative sagittal computed tomography showing the accessory ossicle ( black arrow ) above and anterior to the odontoid process ; ( b ) postoperative sagittal computed tomography showing the accessory ossicle ( black arrow ) with the odontoid process ( white arrow head ) below the ossicle and the double arrows point to the increased distance between the occiput and c2 a 19-year - old female presented with head tilt since childhood , neck pain , and stiffness of both lower extremities of 1-year duration . examination revealed a moderately built female with head tilt to the right side and features of cervical myelopathy . plain radiographs revealed features of assimilation of atlas , basilar invagination , and klippel  feil anomaly [ figure 5a ] . mri revealed basilar invagination with ventral brainstem compression and klippel  feil anomaly with cervical canal stenosis at c3c4 and c4c5 [ figure 5b ] . ct of the craniovertebral junction showed the characteristic comma - shaped clivus characteristic of proatlas segmentation anomaly , basilar invagination , complete assimilation of atlas , and occipital condyle hypoplasia on the right side [ figure 5c and d ] . a diagnosis of proatlas segmentation anomaly with associated basilar invagination , unilateral occipital condyle hypoplasia , atlas assimilation , and klippel  feil anomaly was made . during surgery , the patient was placed prone with 5 kg of traction , and the posterior midline was exposed from occiput to the mid - cervical region and an occipito - cervical fusion was done with occipital plates and c2 pars screws and lateral mass screw fixation of c4 and c5 . this was followed by laminectomy of c3 and c4 to relieve the canal stenosis at that level . postoperatively , the patient noticed a significant decrease in the neck pain , and the head tilt had disappeared along with the relief of lower extremity spasticity . postoperative radiographs and ct of the spine showed a reduction of basilar invagination with proper craniovertebral alignment [ figure 6 ] . ( a ) plain radiograph showing klippel  feil anomaly , basilar invagination , and assimilated atlas ( b ) sagittal t2-weighted magnetic resonance imaging showing ventral compression at the cervicomedullary junction with cervical canal stenosis at c3c4 and c4c5 ( c ) sagittal computed tomography of the craniovertebral junction showing the proatlas segmentation anomaly ( white arrow ) and the invaginated odontoid process ( black arrow ) ( d ) coronal computed tomography of the craniovetebral junction showing hypoplasia of the occipital condyle on the right side ( white arrow ) ( a ) preoperative sagittal computed tomography showing the invaginated odontoid ( single arrow ) and the narrow space between occiput and c2 ( double arrows ) ( b ) postoperative sagittal computed tomography showing the odontoid ( single arrow ) outside the foramen magnum and the increased space ( double arrows ) between the foramen magnum and c2 ( c ) posoperative radiograph showing occipito - cervical fusion a 14-year - old male was admitted with a history of fall while playing followed by weakness of all four extremities . plain radiographs revealed hypoplastic odontoid with evidence of atlanto - axial subluxation ( not shown ) . mri revealed compression at the cervicomedullary junction with intramedullary signal changes [ figure 7a ] . ct spine showed features of proatlas segmentation anomaly and the anterior arch of atlas was located directly above the hypoplastic dens which was devoid of its cap [ figure 7b ] . the patient underwent c1 lateral mass  c2 pars screw fixation [ figure 7c ] . ( a ) sagittal t2-weighted magnetic resonance imaging showing compression at the cervicomedullary junction , there is also evidence of os avis ( white arrow ) ( b ) sagittal computed tomography of the craniovertebral junction showing the os avis attached to the basion ( arrow ) and the anterior arch of atlas ( arrow head ) that sits directly above a hypoplastic odontoid process ( c ) postoperative lateral radiograph showing c1 lateral mass and c2 pars screw fixation a 16-year - old male was admitted to another institution after a road traffic accident with an admission glasgow coma score of 13/15 . his ct brain showed mild cerebral edema for which he was treated with anti - edema agents and recovered consciousness in 24 h. two days after the injury , he complained of neck pain , and plain radiographs obtained at that time showed atlanto - axial subluxation ( not shown ) . ct of the craniovertebral junction showed a hypoplastic odontoid along with an abnormally elongated clivus with a typical  comma  shape that is characteristic of proatlas segmentation anomaly [ figure 8a ] . the patient underwent c1 lateral mass fusion with the lateral mass screws being placed through the posterior arch of atlas and c2 pars screw fixation uneventfully . postoperatively , the patient 's neurological status continued to remain normal , and the ct spine showed appropriate screw placements at c1 and c2 [ figure 8b and c ] . ( a ) preoperative sagittal computed tomography of the craniovertebral junction showing the prebasioccipital arch ( double white arrows ) and the hypoplastic odontoid ( black arrow ) and evidence of atlanto - axial subluxation ( b ) postoperative sagittal computed tomography showing the c1 lateral mass screw through the posterior arch of atlas ( c ) postoperative axial computed tomography showing the c2 pars screws a 23-year - old female presented to us with progressive difficulty in walking of 1-year duration . mri revealed features of platybasia with a horizontally oriented clivus , ventral brainstem compression at the cervicomedullary junction with tonsillar ecotpia extending up to c2 , and a cervical syrinx [ figure 9a and b ] . ct of the cervical spine showed platybasia with shortened basiocciput , anterior arch of atlas located immediately subjacent to the clivus , and an  exuberant  looking apical dental segment which was causing kinking of the cervicomedullary junction [ figure 9c ] . the patient was placed under traction intraoperatively , and the c2 ganglion was sectioned bilaterally exposing the atlanto - axial joints which were then curetted and 4 mms titanium spacers were inserted into both the joints followed by placement of c1 lateral mass and c2 pars screws . this was followed by bony foramen magnum decompression without duraplasty to address the chiari malformation . postoperative ct and mri obtained 1 month later showed craniovertebral re - alignment with decompression of the cervicomedullary junction , resolution of syrinx , and tonsillar ascent [ figures 1012 ] . ( a ) sagittal t1-weighted ( b ) sagittal t2-weighted magnetic resonance imaging showing severe platybasia , basioccipital dysgenesis , ventral compression at the cervicomedullary junction , tonsillar ectopia up to c2 and cervical syrinx ( c ) preoperative sagittal computed tomography of the craniovertebral junction showing severe platybasia with basioccipital dysgenesis ( white arrow head ) , the anterior arch of atlas ( single white arrow ) that is immediately subjacent to the horizontally oriented clivus and the  exuberant  apical segment of the dens ( double white arrows ) ( a ) preoperative sagittal computed tomography to be compared with the ( b ) postoperative sagittal computed tomography showing the descent of the odontoid with increase in the basion - dens interval ( black arrow ) and the foramen magnum decompression ( white arrow ) ( a ) postoperative sagittal computed tomography showing the c1 lateral mass and c2 pars screws in situ ( b ) postoperative sagittal computed tomography showing the titanium spacer in the atlanto - axial joint ( a ) postoperative sagittal t1-weighted magnetic resonance imaging showing tonsillar ascent ( white arrow ) ( b ) postoperative sagittal t2-weighted magnetic resonance imaging showing ventral decompression of the cervicomedullary junction as evidenced by opening up of the anterior subarachnoid spaces ( double white arrows ) and complete resolution of syrinx ( single white arrow ) a 19-year - old male presented with 6-month history of progressive difficulty in walking , clumsiness of both upper extremities , and occasional difficulty in swallowing . plain radiographs revealed completely assimilated anterior arch of atlas , partially assimilated posterior arch , and atlanto - axial subluxation [ figure 1a and b ] . magnetic resonance imaging ( mri ) revealed features of basilar invagination , horizontally oriented clivus with features of ventral brainstem compression , and chiari i malformation [ figure 1c ] . ct of the craniovertebral junction revealed horizontally oriented clivus , basilar invagination , fused anterior arch of atlas , and an accessory ossicle interposed between the clivus and the dens [ figure 1d ] . there was also an evidence of partial assimilation of the posterior arch of atlas which was present only on the right side [ figure 2 ] . on the basis of these radiological features , a diagnosis of basilar invagination with proatlas segmentation anomaly causing ventral brainstem compression was made . it was believed that the chiari malformation encountered in this patient was the result of reduced posterior fossa volume induced by the above - mentioned abnormalities . hence , craniovertebral re - alignment by intraoperative traction and occipito - cervical fusion was planned . the posterior midline from the occiput up to the mid - cervical region was exposed , and occipito - cervical fusion was done using an occipital plate , c2 crossing translaminar screws , and lateral mass screws of c4 and c5 . c3 lateral masses were skipped during screw placement as the lateral mass fractured during drilling . after intraoperative fluoroscopic verification of proper craniovertebral realignment , bone grafts were placed on the exposed bony areas after denuding them . postoperative plain radiographs and ct showed proper craniovertebral re - alignment [ figures 3 and 4 ] . ( a and b ) plain radiographs flexion and extension views showing partial assimilation of atlas , basilar invagination , and atlanto - axial subluxation ; ( c ) sagittal t2-weighted magnetic resonance imaging sequence showing basilar invagination with ventral compression at the cervicomedullary junction ; ( d ) sagittal computed tomography of the craniovertebral junction showing assimilated anterior arch of atlas ( white arrowhead ) , accessory ossicle ( white arrow ) interposed between the clivus , and the invaginated odontoid ; the partially assimilated posterior arch of atlas ( black arrowhead ) three - dimensional computed tomography of the craniovertebral junction showing the absence of the posterior arch on the left side with partial assimilation of the right half of the posterior arch on the right side ( white arrow ) postoperative lateral radiograph obtained after occipito - cervical fusion ( a ) preoperative sagittal computed tomography showing the accessory ossicle ( black arrow ) above and anterior to the odontoid process ; ( b ) postoperative sagittal computed tomography showing the accessory ossicle ( black arrow ) with the odontoid process ( white arrow head ) below the ossicle and the double arrows point to the increased distance between the occiput and c2 a 19-year - old female presented with head tilt since childhood , neck pain , and stiffness of both lower extremities of 1-year duration . examination revealed a moderately built female with head tilt to the right side and features of cervical myelopathy . plain radiographs revealed features of assimilation of atlas , basilar invagination , and klippel  feil anomaly [ figure 5a ] . mri revealed basilar invagination with ventral brainstem compression and klippel  feil anomaly with cervical canal stenosis at c3c4 and c4c5 [ figure 5b ] . ct of the craniovertebral junction showed the characteristic comma - shaped clivus characteristic of proatlas segmentation anomaly , basilar invagination , complete assimilation of atlas , and occipital condyle hypoplasia on the right side [ figure 5c and d ] . a diagnosis of proatlas segmentation anomaly with associated basilar invagination , unilateral occipital condyle hypoplasia , atlas assimilation , and klippel  feil anomaly was made . during surgery , the patient was placed prone with 5 kg of traction , and the posterior midline was exposed from occiput to the mid - cervical region and an occipito - cervical fusion was done with occipital plates and c2 pars screws and lateral mass screw fixation of c4 and c5 . this was followed by laminectomy of c3 and c4 to relieve the canal stenosis at that level . postoperatively , the patient noticed a significant decrease in the neck pain , and the head tilt had disappeared along with the relief of lower extremity spasticity . postoperative radiographs and ct of the spine showed a reduction of basilar invagination with proper craniovertebral alignment [ figure 6 ] . ( a ) plain radiograph showing klippel  feil anomaly , basilar invagination , and assimilated atlas ( b ) sagittal t2-weighted magnetic resonance imaging showing ventral compression at the cervicomedullary junction with cervical canal stenosis at c3c4 and c4c5 ( c ) sagittal computed tomography of the craniovertebral junction showing the proatlas segmentation anomaly ( white arrow ) and the invaginated odontoid process ( black arrow ) ( d ) coronal computed tomography of the craniovetebral junction showing hypoplasia of the occipital condyle on the right side ( white arrow ) ( a ) preoperative sagittal computed tomography showing the invaginated odontoid ( single arrow ) and the narrow space between occiput and c2 ( double arrows ) ( b ) postoperative sagittal computed tomography showing the odontoid ( single arrow ) outside the foramen magnum and the increased space ( double arrows ) between the foramen magnum and c2 ( c ) posoperative radiograph showing occipito - cervical fusion a 14-year - old male was admitted with a history of fall while playing followed by weakness of all four extremities . plain radiographs revealed hypoplastic odontoid with evidence of atlanto - axial subluxation ( not shown ) . mri revealed compression at the cervicomedullary junction with intramedullary signal changes [ figure 7a ] . ct spine showed features of proatlas segmentation anomaly and the anterior arch of atlas was located directly above the hypoplastic dens which was devoid of its cap [ figure 7b ] . the patient underwent c1 lateral mass  c2 pars screw fixation [ figure 7c ] . ( a ) sagittal t2-weighted magnetic resonance imaging showing compression at the cervicomedullary junction , there is also evidence of os avis ( white arrow ) ( b ) sagittal computed tomography of the craniovertebral junction showing the os avis attached to the basion ( arrow ) and the anterior arch of atlas ( arrow head ) that sits directly above a hypoplastic odontoid process ( c ) postoperative lateral radiograph showing c1 lateral mass and c2 pars screw fixation a 16-year - old male was admitted to another institution after a road traffic accident with an admission glasgow coma score of 13/15 . his ct brain showed mild cerebral edema for which he was treated with anti - edema agents and recovered consciousness in 24 h. two days after the injury , he complained of neck pain , and plain radiographs obtained at that time showed atlanto - axial subluxation ( not shown ) . the patient was referred to our institution for further management . on admission , he had no neurological deficits , but complained of neck pain . ct of the craniovertebral junction showed a hypoplastic odontoid along with an abnormally elongated clivus with a typical  comma  shape that is characteristic of proatlas segmentation anomaly [ figure 8a ] . the patient underwent c1 lateral mass fusion with the lateral mass screws being placed through the posterior arch of atlas and c2 pars screw fixation uneventfully . postoperatively , the patient 's neurological status continued to remain normal , and the ct spine showed appropriate screw placements at c1 and c2 [ figure 8b and c ] . ( a ) preoperative sagittal computed tomography of the craniovertebral junction showing the prebasioccipital arch ( double white arrows ) and the hypoplastic odontoid ( black arrow ) and evidence of atlanto - axial subluxation ( b ) postoperative sagittal computed tomography showing the c1 lateral mass screw through the posterior arch of atlas ( c ) postoperative axial computed tomography showing the c2 pars screws a 23-year - old female presented to us with progressive difficulty in walking of 1-year duration . mri revealed features of platybasia with a horizontally oriented clivus , ventral brainstem compression at the cervicomedullary junction with tonsillar ecotpia extending up to c2 , and a cervical syrinx [ figure 9a and b ] . ct of the cervical spine showed platybasia with shortened basiocciput , anterior arch of atlas located immediately subjacent to the clivus , and an  exuberant  looking apical dental segment which was causing kinking of the cervicomedullary junction [ figure 9c ] . the patient was placed under traction intraoperatively , and the c2 ganglion was sectioned bilaterally exposing the atlanto - axial joints which were then curetted and 4 mms titanium spacers were inserted into both the joints followed by placement of c1 lateral mass and c2 pars screws . this was followed by bony foramen magnum decompression without duraplasty to address the chiari malformation . postoperative ct and mri obtained 1 month later showed craniovertebral re - alignment with decompression of the cervicomedullary junction , resolution of syrinx , and tonsillar ascent [ figures 1012 ] . ( a ) sagittal t1-weighted ( b ) sagittal t2-weighted magnetic resonance imaging showing severe platybasia , basioccipital dysgenesis , ventral compression at the cervicomedullary junction , tonsillar ectopia up to c2 and cervical syrinx ( c ) preoperative sagittal computed tomography of the craniovertebral junction showing severe platybasia with basioccipital dysgenesis ( white arrow head ) , the anterior arch of atlas ( single white arrow ) that is immediately subjacent to the horizontally oriented clivus and the  exuberant  apical segment of the dens ( double white arrows ) ( a ) preoperative sagittal computed tomography to be compared with the ( b ) postoperative sagittal computed tomography showing the descent of the odontoid with increase in the basion - dens interval ( black arrow ) and the foramen magnum decompression ( white arrow ) ( a ) postoperative sagittal computed tomography showing the c1 lateral mass and c2 pars screws in situ ( b ) postoperative sagittal computed tomography showing the titanium spacer in the atlanto - axial joint ( a ) postoperative sagittal t1-weighted magnetic resonance imaging showing tonsillar ascent ( white arrow ) ( b ) postoperative sagittal t2-weighted magnetic resonance imaging showing ventral decompression of the cervicomedullary junction as evidenced by opening up of the anterior subarachnoid spaces ( double white arrows ) and complete resolution of syrinx ( single white arrow ) proatlas segmentation anomaly was described in antemortem for the 1 time by menezes in 1987 . kotil and kalayaci reported a 40-year - old female with ventral brainstem compression due to proatlas segmentation anomaly . menezes and fenoy analyzed their vast craniovertebral junction database of 5200 patients and identified that 7.2% had proatlas segmentation anomalies . interestingly , in their vast case series , proatlas segmentation anomalies produced a variety of compression syndromes . among their patients with proatlas segmentation anomalies , 66% had ventral compression , 37% had lateral compression , and 17% had dorsal compression of the cervicomedullary junction goel and shah reported a 19-year - old female with quadriparesis whose imaging showed a ventral brainstem compression by abnormal bony protuberance from the lower end of the clivus . xu et al . reported two patients with proatlas segmentation anomalies : a 58-year - old female and an 18-year - old male , both of whom had ventral brainstem compression due to proatlas segmentation anomaly . their first patient did not undergo surgery while the second patient underwent anterior decompression followed by posterior fixation . a brief review of the embryology of the craniovertebral junction is necessary to understand the pathogenesis of these rare malformations . at the end of the 4 week of gestation , there are 42 somites which include 4 occipital somites , 8 cervical somites , 12 thoracic , 5 lumbar , 5 sacral , and 810 coccygeal somites . the sclerotomes undergo re - segmentation to form the vertebral bodies . during re - segmentation , the caudal half of the fourth somite ( fourth occipital somite ) and the cranial half of the fifth somite combine to form the proatlas sclerotome . this proatlas sclerotome has an axial dense zone , an axial loose zone , a lateral dense zone , and an hypochordal bow . the axial dense and loose zones form the basioccipital clivus and the apical segment of the dens . the lateral dense zone becomes the occipital condyle , the lateral rim , and the opisthion of the foramen magnum . the caudal half of the fifth and the cranial half of the sixth somite form the c1 re - segmented sclerotome . this c1-resegmented sclerotome also has an axial zone , a lateral zone , and an hypochordal bow . the axial zone of the c1-resegmented sclerotome forms the basal segment of the dens , the lateral zone gives rise to the posterior arch of atlas , and the hypochordal bow gives rise to the anterior arch of atlas . the caudal half of the sixth and the cranial half of the seventh somites join to form the c2-resegmented sclerotome . the axial portion of the c2-resegmented sclerotome forms the c2 body and the lateral zone forms the neural arch . the boundary zone between the proatlas and c1-resegmented sclerotome forms the upper dental synchondrosis and the boundary zone between the c1- and c2-resegmented sclerotomes forms the lower dental synchondrosis . patient 1 represents an anterior homeotic transformation due to hox d-3 mutation as a result of which the c1 sclerotome behaves like the occipital sclerotome and hence , the anterior and posterior arches of atlas are fused to the occiput . the abnormality of re - segmentation of the proatlas is responsible for the accessory ossicle that is present behind the fused anterior arch of atlas . the abnormalities in this patient are due to defective re - segmentation of the hypochordal bow of the proatlas sclerotome which is responsible for the  comma  shaped bony mass in the clivus , and the occipital condyle hypoplasia on one side is due to an abnormality of the lateral dense zone of the proatlas sclerotome . feil anomaly is due to pax-1 gene mutation , which led to abnormal re - segmentation of the mid - cervical sclerotomes . pang and thompson believe this to be a case of errant ontogeny , recapitulating phylogeny as a separate bone between the dens and the basiocciput which is found in many birds and hence , they termed this malformation as os avis . these authors believe that os avis occurs because of abnormal re - segmentation of proatlas centrum which results in the apical dental segment being fused with the basiocciput instead of fusing with the stem of the dens . this is believed to be due to a mutation of hox a-7 gene . in patient 4 , the  comma  shaped extension of the clivus is the result of hyperplasia of the hypochordal bow of the proatlas sclerotome . the hypochordal bow of the proatlas normally forms a small midline osseous tubercle attached to the basiocciput at the ventral margin of the foramen magnum . occasionally , as in this index case , it forms a well - formed osseous structure known as the prebasioccipital arch . however , as in this patient , this structure may not cause neural compression in every patient . in this patient , the clinical symptomatology was due to the hypoplastic dens , which resulted in atlanto - axial instability . patient 5 had basioccipital dysgenesis and hypertrophic apical segment of the dens , both of which are caused by defects in the axial dense zone of the proatlas sclerotome . management of proatlas segmentation anomalies is aimed at ( 1 ) relieving the neural compression , if any and ( 2 ) stabilizing the craniovertebral junction , if there is instability of this region . in patients 1 and 2 , this compression was relieved by craniovertebral re - alignment by distraction under intraoperative traction and occipito - cervical fusion . in patients 3 and 4 , the neurological symptomatology was due to instability and hence , this instability was addressed by posterior atlanto - axial fixation using the goel the neurological deficits were due to congenital anterior basilar impression and the  exuberant  apical dental segment . the small volume of the posterior fossa induced by the platybasia had led to tonsillar ectopia . the ventral compression was addressed by craniovertebral re - alignment using spacers in the atlanto - axial joints and atlanto - axial fusion . chiari malformation was addressed by foramen magnum decompression to increase the volume of the posterior fossa as the tonsillar descent was up to c2 because it was believed that the increase in the volume of posterior fossa produced by ventral decompression alone might not be sufficient to adequately deal with the chiari malformation that had extended up to c2 . a brief review of the embryology of the craniovertebral junction is necessary to understand the pathogenesis of these rare malformations . at the end of the 4 week of gestation , there are 42 somites which include 4 occipital somites , 8 cervical somites , 12 thoracic , 5 lumbar , 5 sacral , and 810 coccygeal somites . the sclerotomes undergo re - segmentation to form the vertebral bodies . during re - segmentation , the caudal half of the fourth somite ( fourth occipital somite ) and the cranial half of the fifth somite combine to form the proatlas sclerotome . this proatlas sclerotome has an axial dense zone , an axial loose zone , a lateral dense zone , and an hypochordal bow . the axial dense and loose zones form the basioccipital clivus and the apical segment of the dens . the lateral dense zone becomes the occipital condyle , the lateral rim , and the opisthion of the foramen magnum . the hypochordal bow of the proatlas forms the clival tubercle . the caudal half of the fifth and the cranial half of the sixth somite form the c1 re - segmented sclerotome . this c1-resegmented sclerotome also has an axial zone , a lateral zone , and an hypochordal bow . the axial zone of the c1-resegmented sclerotome forms the basal segment of the dens , the lateral zone gives rise to the posterior arch of atlas , and the hypochordal bow gives rise to the anterior arch of atlas . the caudal half of the sixth and the cranial half of the seventh somites join to form the c2-resegmented sclerotome . the axial portion of the c2-resegmented sclerotome forms the c2 body and the lateral zone forms the neural arch . the boundary zone between the proatlas and c1-resegmented sclerotome forms the upper dental synchondrosis and the boundary zone between the c1- and c2-resegmented sclerotomes forms the lower dental synchondrosis . patient 1 represents an anterior homeotic transformation due to hox d-3 mutation as a result of which the c1 sclerotome behaves like the occipital sclerotome and hence , the anterior and posterior arches of atlas are fused to the occiput . the abnormality of re - segmentation of the proatlas is responsible for the accessory ossicle that is present behind the fused anterior arch of atlas . the abnormalities in this patient are due to defective re - segmentation of the hypochordal bow of the proatlas sclerotome which is responsible for the  comma  shaped bony mass in the clivus , and the occipital condyle hypoplasia on one side is due to an abnormality of the lateral dense zone of the proatlas sclerotome . in addition , the klippel feil anomaly is due to pax-1 gene mutation , which led to abnormal re - segmentation of the mid - cervical sclerotomes . pang and thompson believe this to be a case of errant ontogeny , recapitulating phylogeny as a separate bone between the dens and the basiocciput which is found in many birds and hence , they termed this malformation as os avis . these authors believe that os avis occurs because of abnormal re - segmentation of proatlas centrum which results in the apical dental segment being fused with the basiocciput instead of fusing with the stem of the dens . this is believed to be due to a mutation of hox a-7 gene . in patient 4 , the  comma  shaped extension of the clivus is the result of hyperplasia of the hypochordal bow of the proatlas sclerotome . the hypochordal bow of the proatlas normally forms a small midline osseous tubercle attached to the basiocciput at the ventral margin of the foramen magnum . occasionally , as in this index case , it forms a well - formed osseous structure known as the prebasioccipital arch . however , as in this patient , this structure may not cause neural compression in every patient . in this patient , the clinical symptomatology was due to the hypoplastic dens , which resulted in atlanto - axial instability . patient 5 had basioccipital dysgenesis and hypertrophic apical segment of the dens , both of which are caused by defects in the axial dense zone of the proatlas sclerotome . management of proatlas segmentation anomalies is aimed at ( 1 ) relieving the neural compression , if any and ( 2 ) stabilizing the craniovertebral junction , if there is instability of this region . in patients 1 and 2 , this compression was relieved by craniovertebral re - alignment by distraction under intraoperative traction and occipito - cervical fusion . in patients 3 and 4 , the neurological symptomatology was due to instability and hence , this instability was addressed by posterior atlanto - axial fixation using the goel the neurological deficits were due to congenital anterior basilar impression and the  exuberant  apical dental segment . the small volume of the posterior fossa induced by the platybasia had led to tonsillar ectopia . the ventral compression was addressed by craniovertebral re - alignment using spacers in the atlanto - axial joints and atlanto - axial fusion . chiari malformation was addressed by foramen magnum decompression to increase the volume of the posterior fossa as the tonsillar descent was up to c2 because it was believed that the increase in the volume of posterior fossa produced by ventral decompression alone might not be sufficient to adequately deal with the chiari malformation that had extended up to c2 . abnormalities of the craniovertebral junction due to defective re - segmentation of the proatlas sclerotome are rare . their recognition requires careful thin - section ct evaluation of the craniovertebral junction and an awareness of the existence of this rare entity . management of these malformations requires neural decompression if there is ventral compression at the cervicomedullary junction and stabilization if there is instability of this region .
objective : proatlas segementation anomalies are due to defective re - segmentation of the proatlas sclerotome . 
 these anomalies of the craniovertebral junction are rare and have multiple presentations . 
 the aim of this study is to report this author 's personal experience in managing five of these patients with different radiological findings necessitating different surgical strategies and to provide a brief review of the relevant literature.materials and methods : five patients , all in the second decade of life were treated between 2010 and 2013 . 
 there were three males and two females . 
 all the patients presented with spastic quadriparesis and/or cerebellar signs . 
 patients underwent plain radiographs , mri and ct of the craniovertebral junction . 
 ct of the cranioveretebral junction was the key to the diagnosis of this anomaly . 
 postoperatively , patients were assessed with plain radiographs and ct in all patients and mri in one.results:two patients underwent craniovertebral realignment with occipitocervical fixation , two patients underwent c1-c2 fixation using goel - harms technique and one patient underwent craniovertebral realignment with c1-c2 fixation using spacers in the atlanatoaxial joint and foramen magnum decompression . 
 all patients improved during follow up.conclusions:proatlas segmentation defects are rare anomalies of the craniovertebral junction . 
 routine use of thin section ct of the craniovertebral junction and an awareness of this entity and its multivarious presentations are necessary for clinicians dealing with abnormalities of the craniovertebral junction .