Chiari I malformation A Chiari I malformation is a condition characterised by an abnormal inferior projection of the cerebellar tonsils through the foramen magnum, into the upper cervical spinal canal. If the degree of projection is small then the condition is generally referred to as tonsilar ectopia. Chiari I malformations may be associated with other anomalies, most commonly hydrosyringomyelia (which in turn can lead to neurologic deficits and scoliosis) and hydrocephalus. Symptoms of a Chiari I malformation most often begin during infancy, although they may be delayed until adolescence or adulthood. Symptoms are often non-specific and usually include headache, vomiting, muscle weakness in the head and face, difficulty swallowing, and varying degrees of mental impairment. Paresis or paralysis of the arms and legs may also occur. As they grow older, adolescents and adults with Chiari I malformation who previously were asymptomatic may show signs of progressive brain impairment, such as involuntary, rapid, downward eye movements. The pathophysiology of syrinx development (hydrosyringomyelia) is understood to be a state of chronic interstitial edema of the spinal cord due to an accumulation of extracellular fluid.