Case 1

A 21-year-old male with persistent vertigo.

Differential Diagnosis: Tonsillar Ectopia

Imaging Findings: Sagittal and coronal MR images show elongated peg-like cerebellar tonsils herniating into the upper cervical canal. Posteriorly tilted odontoid peg indenting the cervicomedullary junction is also seen. Axial images demonstrate that the cerebellar tonsils surround and compress the medulla at the foramen magnum.

Discussion: Chiari malformations are named after Hans Chiari, an Austrian pathologist, who first described these hind brain anomalies. He outlined 3-types of hindbrain herniation. Type-I and type-II are the most common forms. The Chiari malformation type-I is defined as abnormal caudal herniation of the cerebellar tonsils through the foramen magnum into the cervical spinal canal. The primary causal factor believed, in most instances, is the inadequate growth of the osseous boundaries of the posterior cranial fossa which leads to insufficient room to accommodate the neural structures. Chiari-I malformation often remains asymptomatic until adulthood. The signs and symptoms are due to compression of the posterior fossa neural structures including the medulla, cerebellum and the cranial nerves. Chief clinical complaints include headache, vertigo, blurred or double vision, vomiting and impaired movement. These patients may develop what is called as ‘Chiari-I spell’ or ‘drop attacks’, i.e. severe headache or syncope following Valsalva maneuvers, such as coughing, sneezing or straining. Chiari-I malformation is typically not associated with other congenital brain anomalies, although ventriculomegaly may be seen in as many as 30% of the cases. Craniovertebral junction anomalies ranging from platybasia, short clivus, atlanto-occipital assimilation, odontoid retroflexion to Klippel-Feil syndrome have been reported. Cervical syrinx is seen in 15–75% of cases and is believed to result from abnormal CSF flow dynamics through the central canal of the spinal cord. MRI is the imaging modality of choice. Axial images through the foramen magnum show crowding of the medulla by the tonsils. Sagittal images reveal abnormally elongated and ‘peg like’ cerebellar tonsils extending caudally for more than 5 mm below the foramen magnum. The distance is measured by drawing a line from the inner margins of foramen magnum (ophisthion to basion), and measuring the inferior most part of the tonsils. It is crucial to distinguish Chiari malformation from tonsillar ectopia, which is an asymptomatic and incidental finding in normal individuals, whereby the tonsils protrude through foramen magnum by no more than 5 mm.