Basilar impression, often grouped in with a similar condition (basilar invagination), is a very common craniocervical junction anomaly, in which the top of the C2 vertebra is displaced upward and posteriorly, prolapsing into the foramen magnum and causing its narrowing. The two terms, basilar impression and basilar invagination, are used interchangeably because both are caused by upward displacement of the cervical spine into foramen magnum. The only difference between the two is that upward migration of cervical vertebrae in basilar impression is the result of bony softening at base of the skull, whereas in basilar invagination, it occurs in the presence of normal bone.[1,2,3,4,5,6] The term basilar impression was first described by anatomists as a postmortem entity in the 18th century, but it was not until the early 20thcentury that the first antemortem cases in patients were noted and described in Europe. In American literature, the term emerged in 1939, with the establishment of diagnostic criteria by Chamberlain.
Basilar impression is frequently associated with other craniocervical junction malformations, such as atlanto-occipital assimilation, incomplete ring of C1, and hypoplasia of the basiocciput, occipital condyles, and atlas. Chiari malformation, syringomyelia, syringobulbia, and hydrocephalus are among the common neural axis associates of basilar impression.[1,2,3,4,5] It is seen in around 40% of all adults treated for symptomatic Arnold-Chiari malformation,[7,8] and herniation of cerebellar tonsil may occur in both primary and secondary basilar impression.
Primary basilar impression is a congenital anomaly inherited as an autosomal dominant trait with incomplete penetrance and is a diagnosis of exclusion. Metabolic or developmental skeletal abnormalities causing generalized bone disease, such as Paget disease, osteogenesis imperfecta, osteomalacia, rickets, osteoporosis, rheumatoid arthritis, and hyperparathyroidism result in secondary basilar impression.[2,10,11] Trauma secondary to falls or vehicular accidents can also result in basilar impression.
The onset of symptoms of primary basilar impression is usually in the third to fifth decade.[10,12] Basilar impression causes a wide variety of clinical manifestations, depending on the structures involved. The structures that may be affected include the upper spinal cord, mid brain, pons, medulla, medial cerebellum, and the vertebrobasilar system. Patients become symptomatic when the displaced cervical segment causes sufficient pressure on the upper cervical cord, brainstem, nerves, or their blood supply. These symptoms may be progressive, transient, or even fatal.
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