skeletal system disorder primary basilar invagination https://github.com/monarch-initiative/mondo/issues/6752 https://www.malacards.org/card/basilar_impression_primary https://www.malacards.org/card/primary_basilar_invagination Primary basilar impression (PBI) is a very rare skeletal developmental defect characterized by congenital upward translocation of the upper cervical spine and clivus into the foramen magnum. PBI can be asymptomatic or associated with severe neurological dysfunction. MEDGEN:400018 OMIM:109500 Orphanet:2285 Bull-Nixon syndrome MESH:C566226 primary basilar impression UMLS:C1862299 MONDO:0007188 GARD:0001037 disorder of development or morphogenesis