Evolution of tonsillar ectopia associated with frontal encephalocoele.

INTRODUCTION: Chiari I malformation has been traditionally considered a congenital malformation. However, there is growing clinical evidence suggesting that it is an acquired phenomenon as also exemplified by this case. CASE: Fetal magnetic resonance imaging (MRI) at 28th week gestation revealed a frontonasal encephalocoele with no hindbrain abnormalities. Post-natal MRI of brain and cervical spine of the 7-week-old infant showed the presence of tonsillar ectopia in the absence of hydrocephalus. The normally developed cerebellar tonsil has herniated through the foramen magnum during the third trimester and neonatal period. DISCUSSION: We hypothesise that the presence of the encephalocoele resulted in dampening expansile forces, produced by the growing brain as well as the cerebrospinal fluid pulsation required to stimulate of the skull growth. As a result, cranial growth is diminished producing a small posterior fossa. The subsequent growth spurt of the cerebellum at the end of the third trimester and during the neonatal period has resulted in cerebellar tonsillar ectopia consequent upon the state of cephalocranial disproportion.