Chiari malformation type II included protrusion of the medulla, fourth ventricle, cerebellar vermis, and cerebellar tonsils into the spinal canal.The insertion of the tentorium was low, the tentorial hiatus widened, and posterior fossa small in these patients.To reverse this process, Gardner performed a surgical procedure that removed the bone from the posterior aspect of the foramen magnum, opened the fourth ventricle to the subarachnoid space, and plugged the obex.In the 1970s Logue introduced a less-invasive alternative to Gardner’s procedure.Chiari type IV malformation was extremely rare and was characterized by severe cerebellar hypoplasia, but with the cerebellum and brain stem remaining within the posterior fossa.Mortality in infancy is the rule with this malformation.

His group performed a clinical study comparing Gardner’s procedure with their procedure of simple decompression and duraplasty and demonstrated that there was no difference in syrinx resolution between the procedures, although Gardner’s operation resulted in a higher complication rate.

Another mechanism of syrinx formation and progression has been proposed that does not require a patent central canal.

The mechanism is initiated by the Chiari malformation partially obstructing CSF pathways at the foramen magnum, which prevents the normally rapid efflux and influx of CSF between the head and the spine that compensates for brain expansion and contraction during the cardiac cycle.

The CSF cisterns are poorly developed in these patients, theoretically because the CSF cisterns did not expand during development because the outlets of the fourth ventricle are obstructed Drainage of CSF through the myelomeningocele in utero encourages herniation of the posterior fossa structures through the foramen magnum.

Following repair of the myelomeningocele, obstruction of the CSF pathways in the basilar cisterns, and often at the cerebral aqueduct, prevent normal CSF flow, which results in hydrocephalus.