Basilar impression is an abnormality of the skull base in which the angle between the basisphenoid and the basilar portion of the occipital bone—normally between 110° and 140°—is widened. In the congenital form, the foramen magnum is deformed and the medulla is unusually low, so that it and the upper part of the cervical spinal cord may be compressed by the odontoid process of the axis. In a lateral radiograph, the line drawn from the posterior end of the hard palate to the posterior lip of the foramen magnum normally lies above the cervical spine, but in basilar impression it crosses the odontoid process at some point (Chamberlain 1939); Bull et al. (1955) pointed out that the plane of the axis relative to that of the hard palate is a more reliable guide.
The condition is often identified, particularly in American literature, by the term ‘platybasia’, but the latter term, which simply means a flat base of skull, can equally be applied to the changes that occur, for instance, in Paget's disease. Basilar impression may be present without platybasia (Spillane et al. 1957). Basilar impression is the name best reserved for the congenital abnormality of the occipital bone, the posterior part of the atlas vertebra being partially invaginated into the cranial cavity.
Basilar impression may lead to hydrocephalus and perhaps to the Arnold–Chiari syndrome (Gustafson and Oldberg 1940), but the latter is more probably an associated congenital abnormality. It may occur secondarily to osteogenesis imperfecta or be part of a wider craniocervical anomaly, e.g. Klippel–Feil syndrome (Hurwitz and McSwiney 1960). The spinal cord may show hydromyelia, but there is also a clear association with syringomyelia (Foster et al. 1969; Barnett et al. 1974).
In adults the clinical picture may resemble multiple sclerosis, syringomyelia, or a high cervical tumour. The most common clinical features are those of a spastic tetraparesis with marked impairment of position and joint sense in both hands, and to a lesser extent in the legs, but in some cases there are also signs of involvement of lower cranial nerves, cerebellar ataxia, and/or hydrocephalus due to obstruction to the outflow of CSF from the fourth ventricle (O'Connell and Aldren Turner 1950; Michie and Clarke 1968). Symptoms suggesting dysfunction of the C8–T1 segments of the spinal cord in some such cases have been attributed to venous obstruction and stagnant hypoxia in the cervical cord (Taylor and Byrnes 1974).
The head is sometimes mushroom-shaped and the neck abnormally short, but the diagnosis can only be made by X-ray examination. If symptoms occur, the treatment of choice is surgical decompression (Gordon 1969; Harkey et al. 1990).
In children, basilar impression may be asymptomatic until the second decade. Vertigo, ataxia, and occipital pain may precede the long-term symptoms and signs outlined above. Hydromyelia is often associated with localized scoliosis at the site of the cord lesion.
The Klippel–Feil syndrome consists of fusion of the bodies of some cervical vertebrae. It may, however, occur without associated craniovertebral malformation and is often an incidental finding on X-ray; some such patients show ‘mirror movements’ in the upper extremities, so that a voluntary movement carried out with one hand is mimicked spontaneously by the other (Gunderson and Solitare 1968).
Plain X-rays may be showing failure of segmentation of lower cervical vertebrae. This is frequently associated with other vertebral anomalies, in this case of the cervico-cranial junction and thoracic hemivertebrae.
Achondroplasia may cause not only spinal-cord compression but also internal hydrocephalus without platybasia or basilar impression, possibly due to shortening of the skull base (Spillane 1952). Symptoms of high cervical-cord compression may result from atlantoaxial subluxation with separation of the odontoid process of the axis; this may be a congenital maformation or the result of trauma, but also occurs spontaneously in rheumatoid arthritis (Stevens et al. 1971). It is also an important complication of Down's syndrome (Pueschel 1983). The radiology of the craniovertebral anomalies was reviewed in detail by Wackenheim (1974). Transoral decompressive surgery has vastly improved the outlook for many of these patients (Crockard et al. 1990).