Causes, Risk Factors & Associated Conditions

Arachnoid cyst (posterior fossa)

An arachnoid cyst in the posterior fossa is a fluid-filled sac that can crowd the region and contribute to a Chiari-like picture.

In short: Arachnoid cyst (posterior fossa)

It is a benign fluid-filled sac near the brain.
In the posterior fossa, it can crowd the region.
It can contribute to tonsillar descent in some cases.
Treating a problematic cyst may relieve the crowding.

What it is

An arachnoid cyst is a benign, fluid-filled sac that forms in association with the membranes around the brain. When located in the posterior fossa, it can take up space and crowd the region, potentially contributing to a Chiari-like picture.

Understanding Arachnoid cyst (posterior fossa)

Many arachnoid cysts are harmless and found incidentally. However, a cyst in the posterior fossa can crowd the already-limited space, pressing on the cerebellum and contributing to downward displacement of the tonsils — an acquired contributor to a Chiari-like picture. When a posterior fossa arachnoid cyst is causing problems by crowding the region, treating it (for example, by creating an opening to drain it, or fenestration) may relieve the crowding. Recognizing a cyst as a contributor is important so the appropriate treatment is chosen. Imaging identifies the cyst and clarifies its role. Understanding posterior fossa arachnoid cysts and Chiari clarifies one of the space-occupying conditions that can contribute to tonsillar descent.

Why it matters

Understanding posterior fossa arachnoid cysts and Chiari helps families see how a fluid-filled sac can crowd the region and contribute to tonsillar descent, and why treating a problematic cyst may relieve the crowding.

Common questions

What is an arachnoid cyst?

A benign fluid-filled sac near the brain.

How can it relate to Chiari?

In the posterior fossa, it can crowd the region and contribute to tonsillar descent.

Can it be treated?

A problematic cyst can be treated, which may relieve the crowding.

Bottom line

An arachnoid cyst in the posterior fossa can crowd the region and contribute to a Chiari-like picture, and treating a problematic cyst may relieve the crowding.

More terms in Causes, Risk Factors & Associated Conditions

Lumboperitoneal shunt / CSF overdrainageA lumboperitoneal shunt or other CSF overdrainage can lower pressure and pull the cerebellar tonsils downward, causing an acquired Chiari-like picture.
Trauma (and Chiari/instability)Trauma can contribute to craniocervical instability and, in some cases, relate to Chiari, particularly in people with connective-tissue conditions.
Basilar invagination (as associated bony anomaly)Basilar invagination is upward migration of the top of the spine into the skull base, a bony anomaly that can accompany Chiari and crowd the brainstem.
Idiopathic intracranial hypertension (association)Idiopathic intracranial hypertension is raised CSF pressure without an obvious cause, which can overlap with or mimic Chiari malformation.
Congenital (developmental) causeA congenital cause means the Chiari malformation arose during development before birth, the usual basis for types I and II.

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