Facial Nerve Schwannomas are rare tumors arising from the Facial Nerve. They arise from the Schwann cells, which form the protective sheath around nerves. These slow growing benign tumors occur anywhere along the course of the Facial Nerve from Cerebellopontine angle (CPA) to its extratemporal course in the Parotid gland
ANATOMY
Facial nerve is a mixed Cranial nerve consisting of 2 roots- Motor root and Nervus Intermedius. Motor root arises from Facial Nucleus at the level of Pons. Part of Facial nerve which arises from the Sensory root is called Nervus Intermedius or Nerve of Wrisberg. Special Sensory taste fibres arise from Nucleus Tractus Solitarius whereas Parasympathetic secretomotor fibres arise from Superior Salivatory Nucleus. The two roots form the Facial Nerve after they enter the anterosuperior quadrant of Internal Acoustic Meatus (IAM). The next Labyrinthine segment is the narrowest and shortest and ends in the Geniculate Ganglion which gives rise to the first branch, Greater Superficial Petrosal Nerve. The nerve runs anteromedially on the surface of the temporal bone, enters the Vidian canal and carries secretomotor fibres to the lacrimal glands. The Geniculate Ganglion contains cell bodies of the sensory neurons in the Facial Nerve. The Facial Nerve takes a 75° anteroposterior turn and becomes the Tympanic segment which then runs inside the Fallopian Canal. The Nerve makes a second turn downwards at the Pyramidal Eminence to form the mastoid segment. Nerve to Stapedius and Chorda Tympani are the other intratemporal branches. The Nerve then exits at the Stylomastoid Foramen and gives rise to two muscular branches to Posterior belly of Digastric and Posterior auricular muscles. The Nerve enters the Parotid gland where it divides into 5 distal branches –Temporal, Zygomatic, Buccal, Marginal Mandibular and Cervical.
Understanding the anatomy of the Facial Nerve is important to diagnose tumors arising from the Facial Nerve. Facial Nerve Schwannomas usually involve multiple segments of the Facial Nerve. Skip lesions can also occur. The Perigeniculate area is the commonest site of involvement followed by tympanic and mastoid segments1.
SYMPTOMATOLOGY
Symptoms are dependent on the location of the tumor. Patients usually present with Facial Paralysis2. Progressive Facial Paralysis with Facial twitching is a classical symptom of this condition. Some patients may get misdiagnosed as Bell’s Palsy. Patients can remain completely asymptomatic until the tumor becomes large enough to cause clinically observable Facial dysfunction. Perigeniculate and Intracanalicular tumors can lead to Sensorineural hearing loss. These patients can also present with tinnitus and vertigo. Lesions arising from tympanic and mastoid segment can present as middle ear masses with Conductive hearing loss. Extra-temporal involvement results in a mass in the Parotid region. A tumor involving the Greater Superficial Petrosal Nerve can present as an extra-axial mass in the Middle Cranial Fossa along with dry eye. Dysgeusia can occur if the nerve is involved near the origin of Chorda Tympani.
RADIOLOGY
High Resolution Computed Tomography of the Temporal bone would show smooth osseous widening of the Fallopian Canal. Schwannomas usually appear iso- to hypointense relative to gray matter on T1-weighted images and hyperintense on T2 weighted images. T1 Contrast Magnetic Resonance Imaging would usually show FNS as well-circumscribed fusiform enhancing masses along the course of the Intratemporal Facial Nerve. Enhancement in MRI with widening of the Fallopian Canal is diagnostic of FNS3. Larger tumors may undergo cystic degeneration seen as focal intramural low signal intensity on contrast-enhanced T1 images4. FNS involving the CPA-IAM facial nerve segments cannot be distinguished from Vestibular Schwannomas if extension into the labyrinthine segment of the facial nerve is not present. The presence of a Labyrinthine tail is indicative of a FNS. Extensive FNS involving CPA-IAM and Perigeniculate area do not conform to the usual radiological presentation and cause dumb bell shaped masses on imaging4. Tympanic segment FNSs are often multi-lobular and they extend into the middle ear to present as an avascular retrotympanic mass with conductive hearing loss. Finally, mastoid segment FNSs break into the surrounding thin wall septations of the mastoid air cells and can seem as locally aggressive masses on MR imaging.
MANAGEMENT
Surgery remains the mainstay of treatment. Surgical decision making depends on the site, size of the tumour , preoperative hearing and Facial nerve status. Some form of Facial paresis is expected postoperatively. Hence an informed decision should be taken in such cases. In cases where there is a slow growing tumour ( <1.4mm/year)5 with preserved Facial nerve function, watchful waiting would be the best option until Facial nerve function worsens to House Brackmann ( HB ) grade III. In cases with HB > III Facial nerve dysfunction, Surgery would be preferred.
Surgical approach would depend on Hearing status. A Middle Cranial Fossa approach would be suitable for a tumor proximal to the geniculate ganglion with serviceable hearing, provided the tumor does not extend far into the cerebellopontine angle. A Retrosigmoid approach gives the best chance of hearing conservation in CPA lesions >1 cm. With non-serviceable hearing, a translabyrinthine approach is the procedure of choice. This approach also provides the best access for Facial Nerve grafting.
Management of Facial nerve can range from preservation of nerve integrity to grafting to anastomosis3. Good Facial nerve function cannot be guaranteed even in cases where the surgeon tries to preserve the integrity of the nerve. Primary anastomosis even with nerve re-routing may compromise neural blood supply. Interposition grafting is usually performed with Greater Auricular or Sural nerves, but this doesn’t give favourable results if the Facial nerve paresis has been present > 1 year. A XII to VII or a V to VII anastomosis is preferred in such cases. If this fails, dynamic facial reanimation techniques such as Labbe’s surgery is used. It has to be noted that full facial recovery isn’t possible in all these procedures and the maximum function achieved ranges from HB III-1V.
CONCLUSION
Management should be tailored according to tumor location and size, existing facial nerve function, patient priorities and age, with priority placed on function preservation over complete tumor removal.
BIBLIOGRAPHY
1. Wilkinson EP, Hoa M, Slattery III WH, Fayad JN, Friedman RA, Schwartz MS, Brackmann DE. Evolution in the management of facial nerve schwannoma. The Laryngoscope. 2011 Oct;121(10):2065-74.
2. Chung JW, Ahn JH, Kim JH, Nam SY, Kim CJ, Lee KS. Facial nerve schwannomas: different manifestations and outcomes. Surgical neurology. 2004 Sep 1;62(3):245-52.
3. Prasad SC, Laus M, Dandinarasaiah M, Piccirillo E, Russo A, Taibah A, Sanna M. Surgical management of intrinsic tumors of the facial nerve. Neurosurgery. 2018 Oct 1;83(4):740-52.
4. Wiggins RH, Harnsberger HR, Salzman KL, Shelton C, Kertesz TR, Glastonbury CM. The many faces of facial nerve schwannoma. American journal of neuroradiology. 2006 Mar 1;27(3):694-9.
5. Perez R, Chen JM, Nedzelski JM. Intratemporal facial nerve schwannoma: a management dilemma. Otol Neurotol 2005; 26(1):121-126.