Objectives: Atlanto-occipital assimilation (AOA) is one of the most common skeletal anomalies of the craniovertebral junction (CVJ). Because its clinical symptomatology is non-specific and it has several variations, many cases go unnoticed which may lead to additional and unnecessary radiological examinations. In this study, we aimed to present CVJ abnormalities with MRI to improve diagnostic accuracy of AOA. Methods: Cervical MRIs of the patients registered in PACS between January 2008 and October 2011 were scanned and AOA was detected in 40 cases. Sagittal FSE T1 and T2-weighted cervical MRIs and axial T2*-GRE sequence images were re-evaluated for AOA typing, anterior atlantodental interval (AADI), posterior atlantodental interval (PADI) measurements, spine fusion anomalies, basilar invagination, tonsillar herniation, myelomalacia, suboccipital muscles and vertebral arteries (VAs). Results: CVJ abnormalities were present in all cases and the most frequent association was observed in suboccipital muscles (100%) and VAs (95%). 60% of the cases had decreased PADI, 32% C2–3 vertebrae fusion, 25% increased AADI, 22.5% basilar invagination, 15% myelomalacia and 5% tonsillar herniation. Conclusion: Suboccipital muscle abnormality was found in all AOA cases whatever the severity and type of the bony fusion. VA anomaly was observed as the second most common abnormality and accompanied preferably the cases with lateral body involvement. Being aware of additional CVJ abnormalities in MRI examinations may reduce unnecessary radiological examinations by increasing the AOA diagnosis rate.
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