MRI demonstration of midbrain deformity in association with Chiari malformation.

The differentiation of Chiari malformation from intrinsic brainstem neoplasm in adults can be difficult. We report three patients presenting with brainstem signs, in whom midbrain abnormality was detected on computed tomography and interpreted as evidence of intrinsic tumour. Subsequent investigation by magnetic resonance imaging revealed evidence of Chiari I malformation in all three cases and a syrinx in two. The association of Chiari I with deformity of the midbrain or pons has not been described previously.