What is the initial workup for a patient suspected of having an acoustic neuroma?

Initial Workup for Suspected Acoustic Neuroma

MRI with dedicated internal auditory canal (IAC) protocol is the gold standard for initial workup of a patient suspected of having an acoustic neuroma. 1

Clinical Presentation

Patients with acoustic neuroma (vestibular schwannoma) typically present with:

• Unilateral sensorineural hearing loss (80% of cases, usually progressive) 2
• Unilateral tinnitus (6.3% of cases) 2
• Vestibular symptoms such as vertigo or unsteadiness (3.4% of cases) 2
• Headache (2% of cases) 2
• Facial weakness or altered facial sensation (less common, associated with larger tumors) 2

Diagnostic Algorithm

Step 1: Imaging

• First-line imaging: MRI with dedicated IAC protocol
  • Include standard T1 and T2-weighted sequences
  • T1-weighted sequences before and after gadolinium administration
  • Thin slice spin echo or turbo spin echo T1-weighted sequences
  • Axial submillimetric heavily T2-weighted sequences (FIESTA, CISS, or DRIVE) 1
  • Diffusion-weighted imaging (DWI) to differentiate VS from arachnoid or epidermoid cysts 1

Step 2: Additional Imaging (if needed)

• CT temporal bone with contrast may provide complementary information:
  • Useful for preoperative surgical planning
  • Provides information about skull base anatomy, especially petrous bone 1
  • Consider when MRI is contraindicated (pacemakers, metallic implants, claustrophobia) 1

Step 3: Audiological Assessment

• Audiometry to document degree and type of hearing loss
• Auditory Brainstem Response (ABR) may be considered but has limitations:
  • May miss 20% (range 8-42%) of intracanalicular vestibular schwannomas 1
  • More sensitive for tumors >1 cm in size 1
  • Not a replacement for MRI but may be useful in specific clinical scenarios

MRI Findings in Acoustic Neuroma

Typical MRI appearance:

• Solid nodular mass with intracanalicular component in the internal acoustic canal
• Often causes widening of the internal acoustic canal
• Isointense on T1-weighted imaging
• Strong enhancement after gadolinium administration
• Heterogeneously hyperintense on T2-weighted imaging
• Larger lesions may show cystic degenerative changes and hemorrhagic areas 1

Important Considerations

1. Differential diagnosis includes meningioma, epidermoid, metastases, other peripheral nerve sheath tumors, neuritis, and lipomas 3

2. False positives can occur with both CT and MRI:

   • Vascular loops around vestibular nerves can mimic small acoustic neuromas 4

3. Neurofibromatosis Type 2 (NF2) screening:

   • Consider NF2 when a patient presents with unilateral vestibular schwannoma at <30 years 1
   • Bilateral vestibular schwannomas are a hallmark of NF2 1

4. Incidental findings on MRI are common:

   • 57% of MRI studies in patients with sudden sensorineural hearing loss show some abnormality
   • Only 11% of these findings are directly related to the hearing loss 1

Pitfalls to Avoid

1. Relying solely on ABR: Can miss up to 42% of small tumors, particularly those confined to the internal auditory canal 5

2. Using standard CT head: Provides insufficient detail of inner ear structures and is insensitive for detecting small acoustic neuromas 1

3. Delayed diagnosis: Despite improved imaging techniques, many tumors are still diagnosed at advanced stages due to ignoring the significance of unilateral progressive sensorineural hearing loss 6

4. Unnecessary contrast: While gadolinium-enhanced T1-weighted MRI is considered the gold standard, there is debate about whether contrast is always necessary 1

By following this diagnostic algorithm, clinicians can ensure timely and accurate diagnosis of acoustic neuromas, which is critical for appropriate management and preservation of neurological function.