Cerebellopontine (CP) Angle Tumor: Imaging and Treatment

Introduction

Clinical Background (brief)

• Progressive unilateral sensorineural hearing loss with disproportionate loss of speech discrimination (retrocochlear pattern)
• Tinnitus and disequilibrium (rather than true vertigo, since compensation is gradual)
• Facial hypesthesia (5th nerve) as the tumor enlarges beyond ~3 cm
• Later: ataxia, headache, and hydrocephalus from brainstem/4th ventricle compression

IMAGING TECHNIQUES

1. MRI with gadolinium contrast - investigation of choice

• T1-weighted pre- and post-gadolinium: Acoustic neuroma is isointense/mildly hypointense to brain before contrast, and enhances markedly (often inhomogeneously) after gadolinium.
• T2-weighted / CISS (constructed interference in steady state) sequences: The tumor appears moderately hyperintense; CISS clearly delineates the cochlea, nerve, and CSF cleft around the tumor without needing contrast - useful as a screening protocol and in patients where gadolinium should be avoided.
• Classic "ice-cream cone" sign: A globular cisternal component continuous with a narrow intracanalicular extension into a widened IAC - highly characteristic of vestibular schwannoma.

2. CT scan

• Shows bony anatomy - IAC widening, erosion, hyperostosis (helps distinguish meningioma from schwannoma), and honeycomb/spiculated bone in facial nerve tumors.
• Useful when MRI is contraindicated (cardiac pacemaker, cochlear implant, severe claustrophobia).
• Contrast CT can miss tumors smaller than 1.5 cm, especially purely intracanalicular ones - a major limitation that led to MRI becoming the standard.

3. Audiometric and electrophysiological tests

• Pure tone audiometry (PTA): unilateral, asymmetric sensorineural hearing loss - the commonest audiometric abnormality.
• Speech discrimination score: disproportionately poor relative to the pure-tone loss - a classic retrocochlear sign.
• Auditory brainstem response (ABR): prolonged interpeak latency (wave I-V) or absent response on the affected side; used historically as a screening tool for small intracanalicular tumors, though MRI has superseded it in accuracy. Stacked ABR protocols improve sensitivity for small tumors.
• Tone decay and reflex decay tests, electronystagmography/caloric testing: show retrocochlear/vestibular nerve involvement (reduced or absent caloric response on the tumor side) but are non-specific and largely of historical value now.

4. Other investigations

• Oxygen cisternography - historically used for small intracanalicular lesions before MRI; now obsolete.
• Genetic testing - for suspected NF2 in bilateral or young-onset cases (chromosome 22, merlin/schwannomin gene).

TREATMENT

1. Observation ("Wait and Scan")

• Suitable for small (<1.5-2 cm), asymptomatic or minimally symptomatic tumors, especially in elderly or high-risk patients, since average growth rate is only about 0.2 cm/year and many tumors do not grow at all.
• Protocol: serial gadolinium MRI (e.g., at 6-12 months, then annually) for at least 5 years; intervention is offered if there is significant growth or new symptoms.
• Advantage: avoids surgical risk in tumors that may never progress; disadvantage: delayed treatment may reduce chances of hearing preservation later.

2. Stereotactic Radiosurgery (Gamma Knife / CyberKnife)

• Best suited to tumors less than 3 cm in maximum diameter, predominantly solid, especially in elderly patients or those unfit for surgery, and for residual/recurrent tumor after surgery.
• Delivers a single high dose of focused radiation to arrest tumor growth (not necessarily to shrink it) while sparing surrounding structures.
• Excellent tumor control rates with a lower incidence of cranial neuropathy (facial nerve palsy) than open surgery, though hearing preservation still declines over time.
• Not ideal for large tumors causing significant brainstem compression, as it does not provide immediate decompression.

3. Microsurgical Excision

• Via transmastoid labyrinthectomy, sacrificing hearing on that side.
• Indicated for medium/large tumors, or any tumor in a patient who already has no serviceable hearing, since hearing preservation is unlikely for tumors >2 cm regardless of approach.
• Advantages: direct, early identification of the facial nerve at the fundus of the IAC, giving the highest rate of facial nerve preservation; avoids cerebellar retraction.
• Also used for meningiomas, non-acoustic schwannomas, gliomas, chordomas needing CPA exposure.

• Transmastoid decompression of the sigmoid sinus with a retrosigmoid craniotomy, without disturbing the labyrinth.
• Indicated when hearing preservation is desired, in patients with serviceable hearing and limited IAC involvement (any size tumor medial to IAC can be approached).
• Drawback: cerebellum lies between surgeon and tumor, requiring cerebellar retraction after CSF drainage; risk of postoperative headache is higher than with other approaches.

• Temporal craniotomy exposing the IAC from above, via the floor of the middle fossa.
• Ideal for small, purely intracanalicular tumors in patients with good hearing; CPA extension beyond 1 cm is a relative contraindication because exposure is limited.
• Preserves hearing while permitting decompression of the internal auditory canal.

• Extension of the translabyrinthine approach with posterior transposition of the facial nerve and removal of the cochlea, for petroclival meningiomas, petrous apex epidermoids, or extensive non-acoustic tumors extending to the clivus.

Complications of surgery

Summary Note for Exam

Note on current evidence