The term acoustic neuroma defines a benign tumor of the Schwann cell neurilemma, which grows mostly in the lower vestibular nerve of the 8th cranial nerve. From 1991 on, vestibular schwannoma (VS) became the most appropriate term, representing the real situation for the majority of cases.
Acoustic neuroma is a misnomer as the tumor usually arises from the vestibular division of the vestibulocochlear nerve, rather than the cochlear division and it is derived from the Schwann cells of the associated nerve, rather than the actual neurons (neuromas).
Etiology
Site of origin:
Obersteiner-Redlich zone (60-80%): Superior and inferior division of vestibular nerve in the medial internal auditory canal at the site of transition from central to peripheral myelin
Yagihashi, A., Sekiya, T., & Suzuki, S. (2005). Macrophage colony stimulating factor (M-CSF) protects spiral ganglion neurons following auditory nerve injury: morphological and functional evidence. Experimental Neurology, 192(1), 167–177. https://doi.org/https://doi.org/10.1016/j.expneurol.2004.10.020
Risk factors:
Ionizing radiation (only known risk factor)
Congenital predisposition: Neurofibromatosis type II (bilateral acoustic neuromas are neurofibromatosis type II until proven otherwise)
Classification
Small neuromas: Only in pons (nerves responsible for auditory performance, equilibrium and movements of motor muscles as well as some vessels of the inner ear)
Medium-sized neuromas: Stretch from pons to cranial cavity without compressing any brain structure
Large neuromas: Stretch outside the internal canal towards the cranial cavity producing some pressure on the brain and thus altering important vital centers
Tumor stages according to Samii | T1: intracanalicular; T2: intra-/extrameatal; T3: tumor reaches the brainstem; T4: tumor compresses the brainstem. | Rosahl SK, Samii M. Tumore des Kleinhirnbrückenwinkels. In: Moskopp D, Wassmann M, editors. Klinische Neurochirurgie. 2nd ed. Stuttgart, New York: Schattauer; 2004. pp. 461–472. | Samii M, Matthies C, Tatagiba M. Management of vestibular schwannomas (acoustic neuromas): auditory and facial nerve function after resection of 120 vestibular schwannomas in patients with neurofibromatosis 2. Neurosurgery. 1997 Apr;40(4):696–705
Presentation
Vestibular features:
Unilateral sensorineural dysacusis (90% cases)
High-frequency pattern of loss (70% cases)
Unilateral tinnitus (ringing or hissing in the ears) (#2 M/C symptom)
Balance/vertigo (50% cases)
Sensation of fullness of ear
Cranial nerve involement
Other cranial nerve imnvolvement leads to other deficits
Cochleovestibular symptoms (CN VIII): Earliest CN symptoms when the tumour is still intracanalicular and presents due to pressure on cochlear/vestibular nerve fibres on the internal auditory artery
Trigeminal symptoms (CN V):
Trigeminal neuralgia: Unilateral facial pain, resembling an electric discharge, limited to one or more branches of the trigeminal nerve path
Facial nerve involvement:
Hitzelberger sign: Hypoaesthesia of posterior meatal wall
Facial weakness or paralysis (rare): Motor fibres are more resistant than sensory fibres
Cerebellar and intracranial pressure symptoms:
Increased intracranial pressure: Headache, vomiting, clumsy gait and mental confusion
Illustration of acoustic neuroma. | Blausen.com staff (2014). “Medical gallery of Blausen Medical 2014”. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. – CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=31339194
Illustration of acoustic neuroma. | Blausen.com staff (2014). “Medical gallery of Blausen Medical 2014”. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. – CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=31339194
Diagnosis
Audiology:
Stapedial reflex: Absent in retrocochlear lesion (acoustic neuroma) but present in cochlear lesions (Meniere’s)
Recruitment phenomenon: Sounds heard rapidly louder with increasing sound level, leading to the somewhat paradoxical but common request of people with cochlear disorders “to speak louder” followed by the complaint to “stop shouting”
Absent in retrocochlear lesions
Short increment sensitivity score (SISI): Low in retrocochlear lesions
Rollover phenomenon: Speech discrimination steadily increases with intensity but suddenly drops in intensity beyond a particular threshold
Facial nerve function:
Grading of the facial nerve function, simplified according to House & Brackmann mentioning the percentage of the deficit. | House JW, Brackmann DE. Facial nerve grading system. Otolaryngol Head Neck Surg. 1985 Apr;93(2):146–147. doi: 10.1177/019459988509300202. Available from: http://dx.doi.org/10.1177/019459988509300202.
Gadolinium-enhanced MRI
GOLD STANDARD investigation for detection, staging, and follow-up of vestibular schwannomas.
Location:
Cerebello-pontine angle (M/C, 80% cases)
Meningiomas
Transversal T1-weighted MRI after contrast: small acoustic neuroma | MBq at German Wikipedia – Selbsterstellt, Public domain. –MBq 18:36, 29. Jun 2006 (CEST), Public Domain, https://commons.wikimedia.org/w/index.php?curid=4489801Acoustic neuroma on the right with a size of 20x22x25mm | MRT-Bild (Autor ist Besitzer) – MRT des eigenen Schädels (Autor ist Besitzer), Public Domain, https://commons.wikimedia.org/w/index.php?curid=9502726
Management
Options for the management of patients with vestibular schwannomas: Generally, for individual patients a combination of the 3 management pillars and at different times a switch from one modality to another is possible. So a primarily growing and then micro- or radiosurgically treated vestibular schwannoma may first be observed and in cases of recurrent growth, the other treatment procedure may be applied. | Rosahl, S., Bohr, C., Lell, M., Hamm, K., & Iro, H. (2017). Diagnostics and therapy of vestibular schwannomas – an interdisciplinary challenge. GMS current topics in otorhinolaryngology, head and neck surgery, 16, Doc03. https://doi.org/10.3205/cto000142
Surgical management:
Treatment of choice for larger tumours
Retrosigmoid/sub-occipital approach
Middle fossa approach
Translabyrinthine approach (total sacrifice for hearing)
The 3 surgical approaches for VS resection are the retrosigmoid (A–C), translabyrinthine (D–F), and middle fossa approaches (G–I). The middle fossa approach is typically used for small tumors to preserve hearing but requires temporal lobe retraction and results in poor exposure of the posterior fossa. The translabyrinthine approach is often used for tumors with IAC extension with no serviceable hearing. The retrosigmoid approach is used primarily for cisternal tumor, but can be used for different sizes of tumors. It is also the most familiar approach to many neurosurgeons. The retrosigmoid approach is performed posterior to the sigmoid sinus and provides access to the CPA without sacrificing the labyrinth. The IAC is exposed by drilling its posterior wall. The translabyrinthine approach allows lateral access to the IAC and the CPA lesions with no cerebellar retraction. This approach, however, will sacrifice the labyrinth, and thus hearing. The Trautmann’s triangle is entered via this approach (E and F). It is demarcated by the bony labyrinth, sigmoid sinus, and superior petrosal sinus or dura. The middle fossa approach allows complete exposure of the IAC from the porus to the fundus with a limited exposure of the CPA through the superior surface of the temporal bone; thus, this approach allows for hearing preservation. The approach to Kawase’s triangle can be seen (H and I), and is demarcated by the greater petrosal nerve, trigeminal nerve (V3), the arcuate eminence, and the medial edge of the petrous ridge (or superior petrosal sinus). Lat. = lateral; n. = nerve; Post. = posterior. | Sun, M. Z., Oh, M. C., Safaee, M., Kaur, G., & Parsa, A. T. (2012). Neuroanatomical correlation of the House-Brackmann grading system in the microsurgical treatment of vestibular schwannoma, Neurosurgical Focus FOC, 33(3), E7. Retrieved Aug 28, 2022, from https://thejns.org/focus/view/journals/neurosurg-focus/33/3/2012.6.focus12198.xml
Translybyrinthine
Retrosigmoid
Middle fossa
Indications
Non-serviceable hearing; any IAC or CPA VS
VS with large CPA component; medial IAC VS
Small lateral IAC VS (<0.5 cm); small medial IAC VS with < 1 cm CPA component
Advantages
Minimal brain retraction
Panoramic CPA exposure; better facial nerve and hearing preservation for medial VS
Better exposure lateral IAC
Disadvantages
Complete hearing loss; difficult approach for CPA VS ventral to porus acusticus; risk for facial nerve injury
Limited access to lateral IAC; potential for cerebellar and brainstem injury
