Diagnosis: Facial Schwannoma
When faced with a patient who has an ear canal mass and facial paralysis, a clinician's first question is typically: What kind of tumor does this represent? A patient with this condition is very concerning for squamous cell carcinoma of the ear canal. However, taking a step back, it is important to note that a vast majority of patients with Bell's palsy recover some level of function. If a patient experiences paralysis for over six months, MRI scans of the internal auditory canals with and without contrast are warranted. It is best to look for a single diagnosis that can explain all of the patient's symptoms. While this is not always true for all patients, most patients do have a single cause for their multitude of signs and symptoms.
Bell's palsy, also called idiopathic facial nerve paralysis, has been shown to be most likely caused by a reactivation of the herpes simplex virus in the geniculate ganglion. Herpes simplex virus, which also causes cold sores, is dormant in neuron cell bodies. The virus may activate and replicate for unknown reasons, though these are most likely related to a temporary decrease in immune function. This viral replication creates an inflammatory response that causes edema and compression of the nerve fibers. The facial nerve, as it travels through the temporal bone, has a narrow section within the bony fallopian along the labyrinthine segment, which is a 3 to 4 mm portion that connects the internal auditory canal to the geniculate ganglion. In this section, the facial canal has a width of approximately 0.68 mm. The presence of a bony wall surrounding the facial nerve causes edema from the Bell's palsy, resulting in the compression of the neural fibers within the nerve trunk. The degree of inflammation and the rapidity of treatment onset dictate the degree of damage to the nerve. Mild edema, which causes some compression, can cause electrical conduction blockage of the nerve. Once the edema subsides, the nerve returns to its normal function. In more severe cases of edema, the nerve fibers may be crushed and completely disrupted. This means the nerve fibers need to regenerate to reach the facial muscles. In this situation, the nerve fibers do not necessarily find the axon across the damaged area, and end up crossing over to another side. For example, fibers that were intended to go to the eye go to the mouth, and vice versa. This causes synkinesis, wherein a person tries to smile but his or her eye closes. It is very rare for a patient with Bell's palsy to not recover some facial function. If facial function does not return within six months after the onset of paralysis, the patient would need to get an MRI of the internal auditory canal.
In the case of this patient, we obtained an MRI of the internal auditory canal, which showed that the mass in the ear canal, middle ear, and mastoid specifically involved the geniculate ganglion of the facial nerve (Figs. 2-4). The mass caused a significant bony destruction in the geniculate area, but did not widen the labyrinthine segment of the nerve (Fig. 5). The differential diagnosis of the mass, as seen on the MRI, included squamous cell carcinoma of the ear canal that may have secondarily involved the facial nerve and traveled up the facial nerve to the geniculate ganglion. The tumor could also represent a primary facial nerve tumor, the most common of which is facial nerve schwannoma. A schwannoma, sometimes incorrectly called a neuroma, is a benign tumor of the Schwann cells that cover the nerve fibers within the facial nerve trunk. A facial schwannoma most commonly occurs in the geniculate ganglion region and tends to spread along the nerve. Sometimes, the tumor can traverse the entire length of the nerve from the cerebellopontine angle into the parotid gland on the face. Tumor resection rarely helps preserve a patient's facial function. Therefore, the tumor is observed until the patient loses facial function, then treatment is undertaken. Traditionally, the treatment of facial schwannomas involved resection of the affected nerve through a nerve graft. At present, stereotactic radiosurgery has been used to treat this condition with success. Building on the experience of treating vestibular schwannomas (acoustic neuromas) with radiosurgery (focused radiation), tumor growth can be stopped.
Reconstruction of the facial nerve is somewhat complex. If the nerve has lost its function and stopped sending signals to the facial muscles for over two years, it would be extremely rare that its function can be restored through a graft. After a prolonged period of denervation (no nerve input to the muscle), the neuromuscular junction where the nerve attaches to the muscle is lost. This means that even if the nerve is replaced, it will not be able to reattach to the muscle. To determine whether a neuromuscular junction is still active, an electromyography (EMG) can be performed. An EMG can evaluate the presence of any fibrillation potentials, which indicate nerve-muscle connection. When the neuromuscular junction has been lost, no electrical activity would be seen when a needle is inserted into the muscle. This is called electrical silence. For patients who have lost their nerve-muscle connections, other forms of reconstruction have to be performed. Some examples are reconstructing static or dynamic slings using the temporalis muscle or other tissues to pull the corner of the mouth upward and create symmetry. Separate procedures may be done to enable the eyes to close. Recent animal studies have shown the great potential of developing a medical device that allows for the transfer of facial functions from the normal to the weak side (Laryngoscope. 2017 Feb;127(2):460; Otol Neurotol. 2017 Oct;38(9):e369).
The patient in this case was diagnosed with a facial nerve schwannoma. Since he had no facial function or useful hearing on the left side, a translabyrinthine surgical approach was used to remove the tumor.
BONUS VIDEOS: VISUAL DIAGNOSIS
Read this month's Clinical Consultation case, then watch the accompanying videos from Hamid R. Djalilian, MD, to review the patient's imaging for yourself.
- Video 1. Axial (horizontal) T1-weighted post-contrast MRI of the brain showing the mass and its adherence to the posterior fossa dura in the mastoid.
- Video 2. Coronal (vertical) T1-weighted post-contrast MRI of the brain showing the superior extent of the tumor and its encroachment on the temporal lobe of the brain.
- Video 3. Axial T2-weighted MRI of the brain showing the differentiation of the tumor (grey) from mastoid fluid (white).
- Video 4. Axial (horizontal) CT of the temporal bones showing the bony destruction of the posterior fossa plate and dura exposure.
- Video 5. Coronal (vertical parallel to the face) CT of the temporal bones showing the destruction of the tegmen (roof of the middle ear).
- Video 6. Sagittal (vertical parallel to the ear) CT of the temporal bones demonstrating the outside in view starting at the auricle.
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