Diagnosis of basilar impression prior to operation depends primarily on radiography. The abnormality may be missed clinically because of presence of deceptive symptoms and signs or absence of symptoms. MRI is the criterion standard, and flexion extension MRIs have a higher yield. Frontal and lateral skull plain radiography, as well as coronal and mid-sagittal reconstructed CT scanning, can also document this, but MRI provides better information. Somatosensory evoked potentials (SSEPs) may have false-positive results and are not routinely recommended. Radiologically, basilar impression is defined by reference to numerous parameters. These include :
The Chamberlain line. This is defined as the line joining the posterior edge of hard palate to the posterior lip of the foramen, caudal to which all parts of the atlas and axis should lie. If the dens is more than 3 mm above this line, the patient has basilar invagination. The dorsal lip, however, can be difficult to define radiologically and can itself become invaginated.
The McGregor line. This is a modification of the Chamberlain line proposed by McGregor in 1948. It is drawn from the back of the hard palate to the lowest point of the occipital squama and normally passes just above the tip of the dens peg. In diagnosing basilar invagination, the dens should be more than 4.5 mm above this line.
The McCrae line. This is drawn from the middle of the anterior lip of the foramen magnum to the posterior lip of the foramen magnum. This landmark was proposed by McRae in 1953. The dens tip should lie below this point.
The Bull 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 first cervical vertebrae in a lateral radiograph of the skull and cervical spine. In 1955, Bull and colleagues performed a comparative study of 120 patients, measuring the distance of the dens tip above the Chamberlain line and McGregor line, and postulated that exceeding three standard deviations in any one of these criteria is sufficient to diagnose basilar impression.[1,3,4,6,7,8]
Treatment of basilar impression depends on the symptomatology of the patient. In the absence of symptoms, a conservative approach may be undertaken, consisting of a collar, nonsteroidal anti-inflammatory drugs (NSAIDs), and simple neck traction.In the presence of neurologic symptoms and signs and confirmation of cord compression on neuroimaging, surgery is recommended.
Neurosurgical treatment is difficult and involves anterior decompression, followed by posterior stabilization for irreducible invagination. For reducible invagination, neurosurgical treatment involves posterior decompression and stabilization. In persons with Chiari malformation, occipital decompression surgery may also be necessary. In patients who are considered poor surgical risks, neurologic progression is likely and the 1-year prognosis is poor.
The patient in this case was diagnosed with basilar impression. He had a history and examination findings suspicious for osteogenesis imperfecta, including blue sclera and a history of repeated fractures. Skin biopsy for biochemical testing or DNA sequencing for associated gene mutations could have been pursued for diagnostic confirmation and counseling. He was subsequently referred to the neurosurgery department for surgical decompression.
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