Basilar impression refers to an abnormal position of the odontoid process of C2, where its tip protrudes through the foramen magnum of the skull base. It can be a congenital or acquired abnormality – congenital causes include syndromes such as osteogenesis imperfecta and achondroplasia, while one of the most common acquired causes is rheumatoid arthritis, which this particular patient suffered from. On the sagittal CT image on the left, the odontoid process extends well above the margin of the foramen magnum (indicated by the dashed yellow line). It should be located at the same level as the anterior arch of C1 (orange arrow). MRI is routinely performed in this setting, to assess the effect of the abnormality on the brainstem and cervical cord – note the indentation of the medulla (M) in this example.