Background: A trapped fourth ventricle (TFV) is diagnosed when the inlet and outlets of the fourth ventricle are obstructed with a resultant dilated fourth ventricle.
Case report: A 3-month-old male child had a ventriculoperitoneal shunt for post-infectious hydrocephalus. At the age of 9 years, he became symptomatic with altered sensorium. He had a CT head which showed enlarged ventricles with a large fourth ventricle. A subsequent ventriculogram revealed dilated lateral ventricles with an aqueductal obstruction in association with an enlarged fourth ventricle. Although diagnosed as a TFV radiologically, his lateral ventricle shunt was revised with a diagnosis of shunt malfunction as there was a simultaneous dilatation of the lateral ventricle. He was never treated for the TFV. At his latest follow-up at 25 years of age, his MRI brain showed well-decompressed lateral ventricles with a large fourth ventricle. On clinical examination, he had truncal ataxia noticed only during turning which, according to the mother, "was always there". His MR flow study showed obstruction at the level of the aqueduct with some evidence of bidirectional flow at the foramen of Magendie.
Discussion: The concept of compensated hydrocephalus represents a stage where a previously active hydrocephalus attains a nonprogressive state clinically and radiologically. We postulate the concept of a "compensated" TFV with a long-term clinical and radiological follow-up.
Conclusion: We demonstrate and describe a compensated TFV and reiterate that a TFV is a functional concept with imaging being at most only corroboratory.