Which condition is associated with Chiari malformation presenting with progressive hydrocephalus?

Chiari malformation, particularly Chiari type I, is characterized by the downward displacement of the cerebellar tonsils through the foramen magnum. This anatomical change can obstruct the normal flow of cerebrospinal fluid (CSF), leading to a condition known as secondary hydrocephalus. In this case, the blockage of CSF flow can result in an accumulation of fluid within the brain's ventricles, leading to increased intracranial pressure and progressive enlargement of the ventricles.

Progressive hydrocephalus associated with Chiari malformation occurs due to the herniation of the cerebellar tonsils, which directly impacts CSF dynamics. This is a key feature of Chiari malformation as it can alter the normal flow patterns of CSF, causing an obstruction and subsequent hydrocephalus.

The other options presented do not have a direct relationship with Chiari malformation or hydrocephalus. Atrial fibrillation and chronic arrhythmia are cardiac conditions that primarily affect the heart's rhythm and have no known association with the anatomical changes seen in a Chiari malformation. Osteoporosis pertains to bone density and strength and is not relevant to the neurological aspects of Chiari malformations or hydrocephalus. Thus