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Hemifacial Spasm


KEY FACTS

Terminology

  • Definition: Vascular loop compressing facial nerve at its root exit zone within cerebellopontine angle (CPA) cistern causing hemifacial spasm

Imaging

  • High-resolution T2WI MR or source MRA images show serpentine asymmetric signal void (vessel) in medial CPA

    • Anterior inferior cerebellar artery (50%) > posterior inferior cerebellar artery (30%) > vertebral artery (15%) > vein (5%)

Top Differential Diagnoses

  • Aneurysm, CPA-IAC

  • Arteriovenous malformation, CPA

  • Developmental venous anomaly, posterior fossa

Pathology

  • CN7 bundle experiences “irritation” from vessel

  • Rare, nonvascular causes of hemifacial spasm (HFS)

    • Multiple sclerosis

    • Cisternal masses

      • –

        Epidermoid, meningioma, schwannoma

    • T-bone and parotid lesions

      • –

        Perineural CN7 malignancy

Clinical Issues

  • Unilateral involuntary facial spasms (HFS)

  • HFS begins with orbicularis oculi spasms

  • Tonic-clonic bursts become constant over time

Diagnostic Checklist

  • Positive MR findings present in ~ 50% HFS patients

  • 1st look for cisternal mass lesions, multiple sclerosis

  • Then follow CN7 distally into T-bone and parotid

    • Exclude CN7 venous malformation, parotid malignancy

  • Determine whether MRA source images or high-resolution T2WI identify causal vessel

    • Negative MR does not preclude surgical therapy

(L) Clinical photograph shows a patient with severe right hemifacial spasm, which is synchronous right eyelid and face twitching. (R) In-between spasms, his face is normal.

Axial MRA in a patient with right hemifacial spasm shows a tortuous right vertebral artery
and associated posterior inferior cerebellar artery (PICA)
pushing on the root exit zone of the facial nerve, visible in the cerebellopontine angle (CPA) cistern
.

Axial CISS MR through the CPA cisterns in a patient with right hemifacial spasm demonstrates a PICA loop
pushing the cisternal CN7 and CN8 posteriorly, causing them to drape over the posterior margin of the porus acusticus
.

Axial thin-section T2WI MR reveals a dolichoectatic vertebral artery
impinging on the root exit zone
of the facial nerve in the medial CPA cistern in this patient with hemifacial spasm.

TERMINOLOGY

Abbreviations

  • Hemifacial spasm (HFS)

Synonyms

  • Facial nerve vascular loop syndrome, facial nerve hyperactive dysfunction syndrome

Definitions

  • Vascular loop compressing facial nerve at its root exit zone within CPA cistern causing hemifacial spasm

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