Basilar Impression, Primary

Description

Primary basilar impression of the skull is a developmental defect of the cranium in which there is invagination of the foramen magnum upward into the posterior cranial fossa. Basilar impression is often associated with other malformations of the notochord and craniovertebral junction, such as occipitalization of the atlas, Klippel-Feil anomaly (see 118100), Chiari type I malformation (118420), and syringomyelia (186700) (Paradis and Sax, 1972; Bhangoo and Crockard, 1999). Secondary basilar impression occurs as a result of generalized skeletal diseases, including hyperparathyroidism (see 145000), Paget disease (see 167250), and osteogenesis imperfecta (see, e.g., 166200).

Platybasia refers to a skull base with an abnormally obtuse angle between the planes of the clivus and the anterior fossa. Platybasia may occur in basilar impression, but it is not of medical significance on its own (Bhangoo and Crockard, 1999).

Historically, basilar impression was defined radiologically by numerous parameters, including the lines defined by Chamberlain (1939), McGregor (1948), and Fischgold and Metzger (1952), and the angle defined by Bull et al. (1955).

Nomenclature

Bhangoo and Crockard (1999) noted that there is confusion as to the correct usage of the terms basilar impression and invagination with some authors insisting that the terms are not interchangeable. They suggested that basilar impression and invagination are terms that can be used interchangeably and describe the invagination of the foramen magnum into the posterior cranial fossa and the concomitant translocation of the upper cervical vertebrae into this cranial depression.

Clinical Features

Although basilar impression has been recognized for centuries, basilar impression with associated neurologic symptoms was first described in modern literature by Homen (1901) (Paradis and Sax, 1972).

Bull et al. (1955) reported the radiographic finding of primary basilar impression in 20 individuals. Of 39 available relatives, 11 also showed basilar impression. Despite consanguinity in 1 case, the authors suggested autosomal dominant inheritance. Of the 20 probands, 10 were asymptomatic, 7 had a previous diagnosis of syringomyelia, and 3 had symptoms and signs explicable by a local lesion at the level of the foramen magnum. Brocher (1955) described affected mother and daughter.

Morariu and Taranu (1968) reported 2 families with primary basilar impression. Sajid and Copple (1968) reported 2 brothers with basilar impression associated with X-linked aqueductal stenosis (HSAS; 307000).

Paradis and Sax (1972) reported a family in which at least 6 individuals spanning 3 generations had primary basilar impression. The proband was a 32-year-old male who had associated syringomyelia with neurologic signs. He presented with weakness mainly in the left arm and leg. Other features included short neck, craniofacial asymmetry, left Horner syndrome, depressed reflexes in the arms, scoliosis, and lower limb hyperreflexia with extensor plantar responses. Other affected family members had short neck and atrophy of the cervical paravertebral muscles, but no other neurologic signs. Paradis and Sax (1972) suggested autosomal dominant inheritance with incomplete penetrance and variable expressivity.

In a review article, Bhangoo and Crockard (1999) stated that symptomatic basilar impression can result in upper and lower motor neuron signs, cranial nerve abnormalities, hydrocephalus, cerebellar dysfunction, syringomyelia, and syringobulbia. Symptoms likely occur when there is brainstem compression from the clivus or translocated odontoid peg impinging on the anterior craniospinal neuraxis. Other contributory factors include altered CSF dynamics, traction on the brainstem, and possibly obstruction of the vertebrobasilar blood supply.