Congenital Deformities in the Region of the Foramen Magnum: Basilar Impression

Basilar impression, platybasia, and basilar invagination are terms used to designate a deformity of the bony structures about the foramen magnum. The basilar, condylar, and that part of the squamous portion of the occipital bone which surrounds the posterior part of the foramen magnum are displaced cranialward (1), the foramen magnum is irregular in outline instead of the usual oval shape, and most often it is smaller (2) than normal, with its margins directed upward into the cranium (3, 4, 5). The downward tilt or angle of the clivus with the horizontal plane is decreased (6, 7, 8); it is broad and flat, and the petrous portion of the temporal bone is pushed upward (7). The floor of the posterior fossa is thin (1, 7, 9,10). There is a variable degree of fusion of the atlas with the occipital bone. This condition, according to Grawitz (9), was first mentioned by Rokitansky (1844), but the earliest detailed description was by Virchow (1876), who collected all the known cases. Many specimens of this and lesser anomalies of the base of the skull and cervical spine have been described, and a number of clinical case reports of basilar impression have appeared in the foreign literature. In the American literature Chamberlain (11) has contributed to the subject, a short review has been published by List (12), and Schüller (13) has reiterated some of the material in his previous writings2. It is therefore quite obvious that, at least in this country, little attention has been paid to basilar impression. Cases are not being recognized because clinicians are not familiar with the condition and are diagnosing them incorrectly as syringomyelia, multiple sclerosis, cerebellar tumor, Klippel-Feil's syndrome, etc., fitting them, according to the variable symptoms and findings, into what seems the most probable category, often as atypical examples. There has been much discussion in the literature concerning the etiology of basilar impression. Rokitansky believed that hydrocephalus was the cause, but although an associated hydrocephalus is found at times (6, 14), the relationship is not clear. It is more probable that basilar impression is the cause and not the effect of the hydrocephalus. Trauma, rickets, and Paget's osteitis have been given as the cause in some of the case reports. Shüller says that basilar impression is most common in countries where it is the custom to carry burdens on the head, but Russell (15) finds it most common in Eskimo skulls. The evidence in favor of a congenital maldevelopment is so convincing that it is here assumed that basilar impression is a congenital anomaly, although it is’ possible that deformities simulating basilar impression may occur from other processes. There are many varieties of anomalous development about the base of the occipital bone and the upper cervical spine, of which basilar impression is one of the more extensive. The other lesser anomalies may occur alone or in combination with basilar impression.