**2. Pathogenesis of hydrocephalus in patients with myelomeningocele**

Several obstructive and absorptive factors act together to cause hydrocephalus in patients with myelomeningocele.

The unified theory proposed by McLone and Knepper is the most popular postulation for the evolution of hydrocephalus in patients with myelomeningocele. It postulates that persistent CSF loss from the neural tube defect impairs brain and CSF pathways development, which results in the downward displacement of the brain stem and crowding of the posterior fossa that leads to hydrocephalus [6].

Type II Chiari malformation with an overcrowded posterior fossa causes obstruction of the fourth ventricular outlets and disturbance in the flow of cerebrospinal fluid at the craniocervical junction, and it is the major factor responsible for obstructive hydrocephalus in patients with myelomeningocele [7]. There may be associated stenosis or forking of the cerebral aqueduct, also causing obstruction to CSF flow. The crowded posterior fossa results in venous compression, which leads to increased venous pressure that impedes CSF absorption [7, 8]. Furthermore, there is a higher resistance to the flow of CSF across the tentorial hiatus and there may be associated underdevelopment of the arachnoid granulations, which results in impaired or inadequate CSF absorption [9].

These factors result in progressive ventriculomegaly and raised intracranial pressure if unchecked, which are responsible for the clinical and radiologic features of hydrocephalus seen in these patients.
