Eur J Anat, 12 (3): 159-167 (2008) Occipitalization of atlas with other associated anomalies of skull M. Sharma1, B. Singh1, A. Abhaya1 and H. Kumar2 1- Department of Anatomy, Govt. Medical College & Hospital, Chandigarh, India 2- Department of Anatomy, Vardhman Mahavir Medical College, New Delhi, India SUMMARY cephalic). Basal angles were within the normal range (120°-140°). Congenital fusion of the atlas with the occipi- These variations are discussed in light of tal bone is the common denominator of a developmental aspects and their clinical signifi- galaxy of skull defects. Some skull anomalies cance. may result in sudden unexpected death. Seventy dry skulls were obtained from the Key words: Atlas – Foramina transversaria – Department of Anatomy. Of these, 2 skulls Occipitalization – Basal angle showed occipitalization. These were pho- tographed and radiographed. Several measure- INTRODUCTION ments, including cranial index, dimensions of the foramen magnum, basal angle and basal Occipitalization of the atlas is defined as impression, were taken. Both skulls showed congenital bony fusion of the atlas vertebra to near by complete fusion of the atlas with the the base of the occipital bone of the skull. The occipital bone. A continuation of the conflu- incidence of atlanto-occipital fusion ranges ence of the sinuses only to the left transverse from 0.14% to 0.75% of the population, both sinus was observed in both skulls. sexes being equally affected (Lanier, 1939a, b; Al-Motabagani and Surendra, 2006). One skull was asymmetrical in shape. The Occipitalization of the atlas can produce a foramina transversaria of atlas in this specimen wide range of neurological signs and symp- were bilaterally reduced to admit only the toms, which vary from transitory headache to diameter of a safety pin. The thickness of the a full blown neurological syndrome. wall of the other skull was asymmetrical on Cervical cord compression is due to the soft the two sides. It also showed a shelf-like bony tissue and bony abnormalities associated with triangular projection (dural band/ spur), occipitalization of the atlas. This leads to which was attached to the upper margin of the weakness and ataxia of the lower extremities. groove for the transverse sinus on the right Numbness and pain in the upper extremities side. The cranial indices of both the skulls is a prominent complaint. Occipital headache were greater than 80 (brachycranial/brachy- may be present. Correspondence to: Dr Mahesh Sharma. Department of Anatomy, Govt. Medical College & Hospital, Sector-32, Sarai Building, Chandigarh-160030, India. Phone: 0172-2665253-60 ext. Submitted: August 21, 2008 1034, 1032 (office); 0172- 2620180 (residence); Fax: 0172-2609360, 0172-2608488. Accepted: November 14, 2008 E-mail: [email protected] 159 M. Sharma, B. Singh, A. Abhaya and H. Kumar Objective findings chiefly comprise hyper- brief embryological explanation. Knowledge reflexia, positive Babinski and Hoffmann of this rare anatomical variation in the cran- reflexes, weakness and other long-tract signs iovertebral juncture is important for radiolo- in both the upper and lower extremities. gists, neurologists and neurosurgeons. Ataxia may be present to a marked degree, and nystagmus is frequent. Sensory findings are less frequent. MATERIAL AND METHODS Several examples of bony distortion of the foramen magnum were reported by early Seventy human adult skulls from the anatomists. There are two principal types of northwest region of India were studied for evi- congenital anomalies: the first is atlanto-occipital dence of occipitalization of the atlas vertebra. fusion or occipitalization / assimilation of atlas, Each specimen was examined carefully for any first described by Rokitansky in 1844 and associated anatomical variations/anomalies. In demonstrated roentgenographically by Schüller the event of atlas assimilation being detected, in 1911, and second is the manifestation of the degree of fusion between the first cervical occipital vertebra, described by Meckel in 1815 vertebra and occipital bone was examined. and again by Kallman in 1905. Several other Skulls were photographed and X-ray films authors have also reported this bony abnormal- were also obtained in a lateral view. A vernier ity and the neurovascular complications associ- caliper was used for linear measurements. The ated with occipitalization of the atlas following details were noted: (Gladstone and Erichsen, 1915; Green, 1930; – Maximum cranial breadth: the maximum Lanier, 1939a; Mac Rae and Barnum, 1953; breadth taken at a right angle to the mid Misra, 1954; Shehata, 1964; Kalka et al., 1989; sagittal plane. Al-Motabagani and Surendra, 2006). – Maximum cranial length: the linear distance Occipitalization of the atlas is an important between the glabella and opisthocranion. congenital malformation of the cranioverte- – Cranial index = Maximum cranial breadth x 100 bral region because of its proximity to the Maximum cranial length spinomedullary region, with the possibility of – Antero-posterior diameter of the foramen a neurological compression syndrome. It is magnum: the linear distance between the one of the most common skeletal abnormali- basion and opisthion. ties of the upper cervical spine. According to Rowe and Yochum (1987), occipitalization – Antero-posterior length behind the odon- represents the majority of «blocked» cephalic toid process: the linear distance between vertebra encountered in the spine. the opisthion and midpoint taken on an To obtain a diagnosis of morphological imaginary line drawn between the tubercles assimilation of the atlas, it is necessary to to which the transverse ligament of the demonstrate some degree of bony union atlas is attached. between the skull and the atlas (Mac Rae and – Breadth of foramen magnum: the greatest Barnum, 1954). Such assimilation usually transverse breadth of the foramen magnum. involves the anterior arch of the atlas, the lat- – Antero-posterior length of the vertebral eral masses, or the entire atlas. Partial fusion is foramen of the atlas vertebra: the maximum more common. Probable criteria based on the antero-posterior distance in the midline of position of the hypoglossal and suboccipital the vertebral foramen. nerves and the vertebral artery are the most – Maximum breadth of the vertebral foramen reliable for differentiating occipitalization of of the atlas vertebra: the linear distance the atlas from manifestations of an occipital between two points of the vertebral fora- vertebra (third occipital condyles, a para- men of the atlas vertebra on the most later- condyler process, a transverse basioccipital fis- ally placed margins. sure, basilar process, bipartite atlantal facets). – Length at the upper border of the dural Barge demonstrated that the atlanto-occip- spur: the linear distance between the apex ital joint is intersegmental and not segmental and a point at the upper end of the base of (Mac Rae and Barnum, 1953). the spur. The aim of the present study was to deter- – Length at the lower border of the dural mine the prevalence of atlanto-occipital spur: the linear distance between the apex fusions and to describe the skull changes asso- and a point at the lower end of the base of ciated with such an anomaly, along with a the spur. 160 Occipitalization of atlas with other associated anomalies of skull – Length of the base of the dural spur: the lin- Features ear distance between the upper and lower The bilateral presence of a carotido-clinoid ends of the base of the dural spur. foramen was noted (Fig. 1). The thickness of – Basal angle (on X-ray film): measured parietal wall was asymmetrical on both sides. between lines drawn from the tuberculum Thickness was greater on the right side as sellae to the nasofrontal suture and to the compared to the left side. The hypophyseal anterior lip of the foramen magnum on the fossa was shallow. X-ray film. The groove for the left sigmoid sinus was – Basilar impression: a transverse line drawn larger and spacious as compared to the right between the groove for the posterior belly side. The confluence of sinus was continuous of the digastric muscle of one side to the with the left transverse sinus. The groove for same point on the other side. The position the occipital sinus split at the posterior aspect of this line in respect of the atlanto-occipi- of the foramen magnum and joined the groove tal joint or sqamous part of the occipital for the sigmoid sinuses of both sides. bone was noted. If the line lies at or below A bony triangular shelf-like projection, these structures, the basilar impression is whose base was attached between the right present. parietal and sqamous part of the right tempo- – Cranial capacity: cranial capacity was meas- ral bone was seen. This is the dural spur pres- ured using sand: all holes were plugged ent at the upper margin of the attachment of with plasticine and the skull was filled with the tentorium cerebelli (Fig. 1). sand. The sand was then measured volu- metrically. Occipitalization of the Atlas – Atlas: The atlas vertebra was almost completely RESULTS fused with the occipital bone at the base of the Of seventy skulls, only two exhibited the skull, except at the transverse processes on anomaly of occipitalization of the atlas verte- both sides. bra. Both skulls were sexed subjectively using Anterior arch: This part of the atlas was morphological features given in standard text fused with the anterior margin of the foramen books of anatomy, anthropometry and foren- magnum, leaving a single anterior midline sic science (Al-Motabagani and Surendra, (0.4 cm) opening between the anterior tuber- 2006; Singh and Bhasin, 1989; Standring et cle and the basiocciput (Fig. 2). A small circu- al., 2005), showing the skulls probably lar facet of approximately 6-7 mm in belonged to approximately 40-45-year old diameter, located on the posterior surface of males. the anterior arch in the midline for articula- tion with the odontoid process of the axis ver- Skull-I measurements: tebra was seen.