Progressive cerebellar tonsillar herniation with recurrent divergence insufficiency esotropia.

The Chiari malformations are characterized by herniation of posterior fossa contents through the foramen magnum. Chiari I malformation is currently defined as ectopia of the cerebellar tonsils more than 5 mm below the foramen magnum. Extension of the cerebellar tonsils up to 3 mm may be found in the normal population. Although Chiari malformations are congenital, symptoms often do not manifest until the third and fourth decades of life, or even later. Patients usually present with headache, lower cranial nerve palsies, downbeat nystagmus, ataxia, or dissociated anesthesia of the trunk and extremities. Definitive diagnosis is made by magnetic resonance imaging (MRI), which shows the compressed tonsils extending through the foramen magnum into the cervical subarachnoid space. One of the rare presenting signs of Chiari I malformations is acquired esotropia with a divergence insufficiency pattern. We report such a case in which the initial neuroimaging showed tonsillar herniation, but of insufficient magnitude to meet diagnostic criteria for Chiari I malformation. When the strabismus recurred after initially successful eye muscle surgery, follow-up scan showed progressive tonsillar herniation.