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Arachnoid Cyst, CPA-IAC


KEY FACTS

Terminology

  • Arachnoid cyst (AC): Developmental arachnoid duplication anomaly creating cerebrospinal fluid (CSF)-filled sac

Imaging

  • Sharply demarcated ovoid extraaxial cisternal cyst with imperceptible walls with CSF density (CT) or intensity (MR)

  • AC signal parallels (is isointense to) CSF on all MR sequences

  • Complete fluid attenuation on FLAIR MR

  • No diffusion restriction on DWI MR

Top Differential Diagnoses

  • Epidermoid cyst in cerebellopontine angle (CPA)

  • Cystic vestibular schwannoma

  • Neurenteric cyst

  • Cystic meningioma in CPA

  • Cystic infratentorial ependymoma

  • Cerebellar pilocytic astrocytoma

Clinical Issues

  • Clinical presentation

    • Small AC

      • –

        Asymptomatic, incidental finding (MR)

    • Large AC

      • –

        Mostly asymptomatic

      • –

        Symptoms may arise from direct compression ± ↑ intracranial pressure

  • Natural history

    • Vast majority of ACs do not enlarge over time

  • Treatment options

    • Most cases require no treatment

    • Treatment is highly selective process

Diagnostic Checklist

  • Differentiate AC from epidermoid cyst

    • AC: No restriction on DWI = best clue

Axial graphic of an arachnoid cyst (AC) in the cerebellopontine angle (CPA) shows a thin, translucent wall. Notice the cyst bowing the 7th and 8th cranial nerves anteriorly
and effacing the brainstem
and cerebellum
.

Axial T2WI MR reveals a right CPA AC causing bowing of the facial and vestibulocochlear nerves anteriorly
, small bridging veins posteriorly
, and flattening of the lateral margin of the brachium pontis
.

Coronal graphic of a CPA AC depicts a typical translucent cyst wall. CN7 and CN8 are pushed by the cyst
without being engulfed by it. In epidermoid cyst, cranial nerves are usually engulfed.

Coronal T1WI MR demonstrates a small CPA AC
that is almost identical with cerebrospinal fluid in the adjacent cistern
. Note subtle mass effect on the adjacent brainstem
. Complete fluid attenuation on FLAIR MR helps differentiate AC from an epidermoid cyst, which is the primary imaging differential diagnosis.

TERMINOLOGY

Abbreviations

  • Arachnoid cyst (AC)

Synonyms

  • Primary or congenital AC, subarachnoid cyst

Definitions

  • Developmental arachnoid duplication anomaly creating intraarachnoid CSF-filled sac

IMAGING

General Features

  • Best diagnostic clue

    • Sharply demarcated ovoid or lentiform extraaxial cisternal cyst with imperceptible walls with CSF density (CT) or signal intensity (MR)

    • AC signal parallels CSF (is isointense to CSF) on all MR sequences

      • –

        Complete fluid attenuation on FLAIR MR

      • –

        No diffusion restriction on DWI MR imaging

  • Location

    • 10-20% of all ACs occur in posterior fossa

      • –

        Cerebellopontine angle (CPA) = most common infratentorial site

      • –

        10% found in CPA

    • Spread patterns

      • –

        Most remain confined to CPA (60%)

      • –

        May spread dorsally along brainstem (25%)

      • –

        Rarely spread into internal auditory canal (IAC)

  • Size

    • Broad range: 1 cm to giant (> 8 cm)

    • In posterior fossa, when very large may be symptomatic

    • When large, will exert mass effect on vestibulocochlear and facial nerves, adjacent brainstem and cerebellum

  • Morphology

    • Sharply demarcated with broad-arching margins

    • Displaces, does not engulf, surrounding structures

    • Pushes cisternal structures but does not insinuate

      • –

        Epidermoid cyst insinuates adjacent structures

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