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  • Updated 05.06.2023
  • Released 12.28.2004
  • Expires For CME 05.06.2026

Basilar impression



Basilar impression is a skeletal malformation characterized by translocation of the upper cervical spine and clivus into the foramen magnum, giving rise to various symptoms related to brainstem, upper spinal cord, and cerebellar compression. It is generally classified as primary (familial), in which the defect is unaccompanied by intrinsic abnormalities of bone, or as secondary, reflecting an intrinsic bone defect, as is the case in osteogenesis imperfecta. In this article, the author provides a review on basilar impression, with a discussion of additional experiences on diagnostic imaging criteria and surgical management.

Key points

• Basilar impression is a skeletal malformation in which the upper cervical spine and clivus demonstrate translocation into the foramen magnum.

• Depending on the degree of osseous invagination, clinical manifestations of basilar impression vary from asymptomatic to severe dysfunction.

• Primary basilar impression, a diagnosis of exclusion, is generally considered a familial developmental abnormality showing mostly an autosomal dominant inheritance pattern with incomplete penetrance. Secondary basilar impression is attributed to metabolic or developmental abnormalities of bone that give rise to osseous softening, such as Paget disease and osteogenesis imperfecta.

• Management of basilar impression rests primarily with surgery for cases where there is progressive spinal deformity, ataxia, nystagmus, apnea, or cranial nerve dysfunction, aiming to relieve compression of the neuraxis at the affected site and to prevent future additional compression. Treatment plans may be modified based on imaging characteristics of joint indices. Posterior decompression techniques have generally reported success.

• Proposed criteria for the radiographic diagnosis of basilar impression continue to evolve as imaging techniques become more readily available. The clivopalate angle (CPA), clivodens angle (CDA), clivoaxial angle (CXA), and foramen magnum angle (FMA) as measured on midsagittal MRI images are of particular diagnostic value.

Historical note and terminology

Basilar impression, a skeletal malformation involving the relation between the cervical spine and the base of the skull, was first described as a postmortem anatomic entity by anatomists in 1790. The first ante-mortem cases in patients were described in Europe in the early 20th century (46; 24). Basilar impression emerged in the American literature in 1939, with the establishment of diagnostic criteria by Chamberlain, who defined a line, to be known as Chamberlain’s line, drawn from the dorsal margin of the hard palate to the dorsal tip of the foramen magnum, caudal to which all parts of the atlas and axis should lie (09). In 1948, McGregor proposed a modification of Chamberlain’s line by defining a line drawn from the upper surface of the posterior edge of the hard palate to the most caudal point of the occipital curve, to be known as McGregor’s line (40). Subsequently, McRae in 1953 proposed another landmark below which the odontoid tip should lie. Known as McRae’s line, this is the line drawn from the anterior border to the posterior border of the foramen magnum (41). In 1955, Bull and colleagues performed a comparative study of 120 patients, measuring the distance of the odontoid tip above Chamberlain’s line and McGregor’s line. In addition, the investigators proposed a new parameter to diagnose basilar impression. Referred to as Bull’s angle, this is the angle between the plane of the hard palate and the line formed by joining the midpoints of the anterior and posterior arches of the atlas in a lateral radiograph of the skull and cervical spine (06). This study established general means for these measurements, postulating that exceeding three standard deviations in any one of these criteria is sufficient to diagnose basilar impression. Studies have also described pathophysiologic correlations based on radiographic morphometric characteristics (17). Radiographic re-examination of this entity has proposed new diagnostic criteria (54; 55), and supplemental imaging parameters have been proposed (39; 18). Additionally, it has also been observed that craniocervical kyphosis is typically more pronounced in basilar impression than in normal subjects (05). Craniometric parameters continue to be explored radiographically, with diagnostic criteria showing good intraobserver reproducibility and interobserver agreement (13).

Basilar impression is divided into 2 basic types: type A is accompanied by atlantoaxial instability and is usually associated with ligament failure, whereas type B has atlantoaxial stability and is commonly associated with developmental malformation of the skull base (33).

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