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Intracochlear Schwannoma

Summary

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  • Rare benign tumour arising from Schwann cells of cochlear nerve branches within the cochlea
  • Presents with progressive unilateral sensorineural hearing loss, tinnitus, and vertigo
  • MRI demonstrates filling defect within cochlear turns with enhancement on post-contrast sequences

Pathophysiology

  • Arises from Schwann cells of peripheral branches of cochlear nerve
    • Most commonly from osseous spiral lamina or modiolus
    • Can originate from scala tympani or scala vestibuli nerve fibres
  • Slow-growing benign tumour (WHO Grade 1)
  • May remain confined to cochlea or extend to internal auditory canal (IAC)
  • Distinct from vestibular schwannoma which arises in IAC/cerebellopontine angle

Demographics

  • Extremely rare (<1% of temporal bone schwannomas)
  • Age range: 20-70 years (mean age 40-50 years)
  • No significant gender predilection
  • Usually unilateral and sporadic
  • No association with neurofibromatosis type 2

Diagnosis

  • Clinical presentation:
    • Progressive unilateral sensorineural hearing loss (most common)
    • Tinnitus (60-80% of cases)
    • Vertigo or disequilibrium (30-40%)
    • Symptoms typically develop over months to years
  • Audiometry:
    • Asymmetric sensorineural hearing loss
    • Poor speech discrimination scores
  • Differential diagnosis:
    • Labyrinthitis
    • Meniere's disease
    • Intralabyrinthine haemorrhage
    • Labyrinthine ossificans
    • Cochlear otosclerosis

Imaging

  • High-resolution T2 (CISS/FIESTA):
    • Hypointense filling defect within normally hyperintense cochlear fluid
    • Loss of normal fluid signal in affected cochlear turns
    • May see extension into IAC or vestibule
  • T1:
    • Isointense to hypointense relative to brain parenchyma
    • Obliteration of normal cochlear anatomy
  • T1+C:
    • Intense homogeneous enhancement within cochlea
    • May show linear enhancement along cochlear nerve if extending to IAC
    • Enhancement helps differentiate from haemorrhage or fibrosis
  • DWI:
    • No restricted diffusion (unlike cholesteatoma)
    • ADC values similar to other schwannomas
  • CT temporal bones:
    • Normal bony cochlear architecture initially
    • May show cochlear expansion in larger tumours
    • Possible erosion of modiolus or osseous spiral lamina in advanced cases

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  • A 40-year-old patient presented with hearing loss, vertigo and tinnitus.
  • High resolution T2-weighted imaging showed loss of the normal fluid signal within the middle turn (red arrow) and scala tympani of the basal turn of the cochlea (red arrow).
  • With enhancement on post-gadolinium imaging, the appearances were consistent with an intracochlear schwannoma.

Treatment

  • Conservative management:
    • Observation with serial MRI for small, minimally symptomatic tumours
    • Hearing aids for serviceable hearing
  • Surgical options:
    • Cochlear implantation (hearing preservation not possible)
    • Translabyrinthine or transotic approach for complete excision
    • Middle fossa or retrosigmoid approach if IAC extension present
  • Radiation therapy:
    • Stereotactic radiosurgery for poor surgical candidates
    • May halt growth but unlikely to improve hearing
  • Prognosis:
    • Hearing loss typically irreversible
    • Complete surgical excision usually curative
    • Recurrence rare after complete resection

Differential diagnosis

Differential diagnosis Differentiating feature
Labyrinthitis Temporal course - labyrinthitis typically resolves within weeks, while intracochlear schwannoma shows progressive symptoms
Meniere's disease Episodic vertigo attacks with symptom-free intervals, unlike the progressive unilateral hearing loss in intracochlear schwannoma
Intralabyrinthine haemorrhage T1 hyperintense signal on MRI without enhancement, whereas schwannoma shows enhancement
Cochlear otosclerosis CT shows hypodense focus at fissula ante fenestram; schwannoma shows filling defect in cochlea on MRI
Vestibular schwannoma with cochlear extension Primary mass in internal auditory canal/cerebellopontine angle with secondary cochlear involvement
Labyrinthine ossificans CT shows ossification; MRI shows loss of normal T2 fluid signal without enhancing mass
Autoimmune inner ear disease Bilateral involvement and response to steroids; schwannoma is unilateral without steroid response
Sudden sensorineural hearing loss Acute onset without mass on MRI; schwannoma shows enhancing intracochlear lesion
Cochlear nerve aplasia Absent cochlear nerve on high-resolution T2 MRI; schwannoma shows enhancing mass
Temporal bone metastasis Bone destruction on CT; multiple lesions; irregular margins; no intracochlear confinement