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Epidermoid Cyst, CPA-IAC


KEY FACTS

Terminology

  • Definition: Congenital inclusion of ectodermal epithelial elements during neural tube closure

Imaging

  • CPA cisternal insinuating mass with high signal on DWI MR

    • 90% intradural, 10% extradural; margins usually scalloped or irregular; cauliflower-like margins with “fronds” possible

  • TI and T2: Isointense or slightly hyperintense to cerebrospinal fluid

  • DWI: Restricted diffusion makes diagnosis

Top Differential Diagnoses

  • Arachnoid cyst in CPA

  • Cystic neoplasm in CPA

    • Cystic vestibular schwannoma

    • Cystic meningioma

    • Infratentorial ependymoma

    • Pilocytic astrocytoma

  • Neurenteric cyst

  • Neurocysticercosis, CPA

Pathology

  • Surgical appearance: Pearly white CPA cistern mass

  • Cyst wall: Internal layer of stratified squamous epithelium covered by fibrous capsule

Clinical Issues

  • Clinical presentation

    • Principal presenting symptom: Dizziness and headache

      • –

        Sensorineural hearing loss also common

    • If extends to lateral pons: Trigeminal neuralgia

    • Rarer symptoms: Facial palsy, seizure, hemifacial spasm

  • Treatment: Complete surgical removal is goal

    • If adherent to neural structures, complete removal may not be possible; if recurs, takes many years to grow; DWI MR key to diagnosing recurrence

Axial graphic shows a large cerebellopontine angle (CPA) epidermoid cyst within a typical “bed of pearls.” Note that the 5th
, 7th
, and 8th
cranial nerves along with the anterior inferior cerebellar artery loop
are characteristically engulfed by this insinuating mass.

Axial CECT shows a large CPA epidermoid cyst
. Note that this nonenhancing low-density lesion appears to invade the left cerebellar hemisphere
. Minimal rim enhancement is visible along the posterior margin of the cyst
.

Axial FLAIR MR of the same patient shows “incomplete” or partial nulling of the signal of this large epidermoid cyst. Associated high signal
along the deep margins of the lesion is most likely due to gliosis of the cerebellar hemisphere.

Axial DWI MR in the same patient reveals the expected high signal from epidermoid cyst diffusion restriction. DWI sequence allows straightforward differentiation of this epidermoid cyst from an arachnoid cyst.

TERMINOLOGY

Synonyms

  • Epidermoid tumor, primary cholesteatoma, or epithelial inclusion cyst

Definitions

  • Congenital inclusion of ectodermal epithelial elements during neural tube closure

IMAGING

General Features

  • Best diagnostic clue

    • CPA cistern insinuating mass with high signal on DWI MR

      • –

        Engulfs cranial nerves (7th and 8th) and anterior inferior cerebellar artery (AICA) loop

  • Location

    • 90% intradural,10% extradural

      • –

        Posterior fossa location most common

        • □

          CPA ~ 40%; 4th ventricle ~ 20%

  • Size

    • Wide range: 1-8 cm or more in diameter

  • Morphology

    • Insinuating mass in cisterns

    • Margins usually scalloped or irregular

      • –

        Cauliflower-like margins with “fronds” possible

    • When large, compresses or invades brainstem ± cerebellum

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