Three numb tongues, two MRIs and a gold weight in an eyelid: a case series of acoustic neuromas and their relevance to dental practice

Acoustic neuromas (AN), also known as vestibular schwannomas or cerebellopontine angle (CPA) tumours are benign tumours originating from the Schwann cells of the vestibulocochlear nerve (CN VIII) (Figure 1).^1,2 Although usually benign, it must be noted that there are reports documenting de novo malignancy and malignant transformation.³ A 2010 study of 1621 patients showed an AN incidence rate of 1.1:100,000 with the average age of presentation 53 years, and no reported differences in laterality or affected sex.¹ AN account for 8% of all intracranial tumours,⁴ with the majority of cases being unilateral, less than 5% present bilaterally.¹ Bilateral AN is characteristic of neurofibromatosis type 2 (NF2), a genetic condition predisposing the patient to neural, cutaneous and ocular lesions,⁵ with 7% of AN cases presenting on a background of NF2.⁴ Other risk factors for AN development are poorly understood, but potentially include high-dose ionizing radiation, atopic disease, mobile phone use, and dental radiographs, although interestingly, tobacco has demonstrated an inverse relationship.^6,7,8,9

AN can impinge upon intracranial structures, potentially resulting in hydrocephalus, otological symptoms and neurological features.² The most common presenting feature is unilateral hearing loss (80% of cases), which is progressive in 90% of cases. Other common symptoms include tinnitus, ataxia, vertigo, facial numbness, facial pain and headaches.¹⁰ Of particular dental interest, there are case reports of acoustic neuromas presenting as toothache, temporomandibular disorders and orofacial paraesthesia.^11,12,13 Management of these lesions is coordinated by the neurosurgery team, although the surgery is carried out by a specialist ENT surgeon because the approach is through the ear.

This case series presents three patients, referred to Glasgow Dental Hospital and School's oral medicine department, that highlight the importance of considering an acoustic neuroma in the differential diagnosis of unexplained orofacial symptoms.

Patient 1

Patient 1 initially presented to the oral medicine department in February 2017, following referral from his general dental practitioner with a 9-month history of numbness and tingling of his tongue. His primary complaint was of sudden onset sensory loss affecting the right lateral aspect of his tongue, oral mucosa and skin on the right side of his face. His taste was unaffected, but he reported occasional pains radiating from his ear and angle of his mandible, lasting for seconds at a time. Medically, he had mild asthma and acid reflux, but was otherwise a fit and healthy 63 year old. Interestingly, not long before his initial presentation, he experienced loss of hearing in his right ear and was recommended to use a hearing aid by a hearing aid practitioner. He also reported occasional loss of balance.

A cranial nerve examination confirmed his reported symptoms, with reduced sensation over the distribution of the mandibular branch of the trigeminal nerve, coupled with decreased hearing acuity on the right. All other cranial nerve function appeared intact.

Given the unilateral and widespread nature of his symptoms, there was a high suspicion of AN. An urgent MRI was arranged, which was later substituted by a CT owing to the patient's claustrophobia. He was referred onwards to the specialist ENT neurosurgical team with a 30-mm mass noted on the right side, consistent with the appearance of an acoustic neuroma (Figure 2). During this time, his symptoms had evolved to include headaches upon waking, together with marked ataxia.

His tumour was entirely resected (Figure 2), with subsequent surgery to manage the resulting facial nerve palsy, including the insertion of a gold weight to aid passive closure of his right eyelid. There remains residual asymmetry and reduced facial nerve function; however, at over 2 years post-operatively, there has been no evidence of AN recurrence (Figure 3).

Patient 2

Patient 2, a fit and healthy 31-year-old female, was referred by her dentist in April 2019 with a history of right-sided lip paraesthesia. She reported progressive hypoaesthesia over the right side of her face over the previous 18 months, but it had extended to include reduced sensation over the right side of her tongue.

Upon questioning, she reported no changes to her hearing or vision. There were no specific problems with balance, although she did report an increasing awareness of ‘clumsiness’. There was no objective hypoaesthesia noted upon examination to either light or sharp touch, and the cranial nerve examination was otherwise unremarkable. This led the differential diagnoses to include an oral dysaesthesia. However, the specific correlation of sensory change to the right divisions of the trigeminal nerve raised suspicion of a mass, or of degenerative changes, and an MRI was arranged. This revealed a 25-mm mass lesion on the right side, with associated compression of pons and partial effacement of the fourth ventricle (Figure 4a, b).

At a follow up MRI 6 months later, there was no further growth of the tumour; however, the patient elected to have the tumour removed due to anxiety surrounding its potential impact in later life. It was successfully removed through a suboccipital approach (Figure 4c) by the ENT team. Full preservation of facial nerve function was achieved and facial sensation returned. The patient continues to have intermittent headaches and swelling at the surgery site, and she remains under the care of the surgical team.

Patient 3

A 71-year-old female presented following referral from her general dental practitioner, with an 8-week history of persistent numbness and an intermittent tingling sensation affecting the right lateral border of her tongue. No significant pain was reported, and lingual motor function remained intact. Concurrently the patient experienced an intermittent dull throbbing pain of the right posterior maxilla. These symptoms presented on a complex medical background of hypertension, severe osteoporosis, primary hyperparathyroidism from a parathyroid adenoma and a history of surgically managed breast cancer. The patient was under investigation for disturbed balance.

On soft tissue examination, there was evidence of soft palate candida infection, and on dental examination, UR6 showed periodontal clinical attachment loss with furcation involvement, as well as grade 1 mobility. An OPT was taken revealing alveolar bone loss associated with the UR6, and an incidental finding of hypercementosis of LR4 (Figure 5a). No deficit was detected on a cranial nerve exam and the findings could not exclude the right maxilla pain being due to a dental cause.

Routine blood investigations were within normal limits. Owing to the unilateral nature of the reported paraesthesia, an MRI of the patient's brain and trigeminal nerve was performed; revealing a 23 x 18 x 15-mm cerebellopontine mass immediately adjacent to the trigeminal nerve root (Figure 5b). The radiographic features and lesion locality were consistent with an acoustic neuroma. Prior to assessment in oral medicine, a head CT scan had been taken, and no mass was reported. However, when the CT scan was reviewed following the MRI, the mass was noted in the same location (Figure 5b, c). This provided some benefit as it demonstrated a potential increase in the size of the lesion.

The patient was referred to the surgical team and, following discussion at a multidisciplinary team meeting, she was recommended for stereotactic radiotherapy, on account of the AN increasing in size and progression of her symptoms causing numbness around her right ear.

Discussion

Imaging

Acoustic neuromas are diagnosed radiologically. Tumours, if excised, are likely to be correlated histopathologically; however, no biopsy is required for the diagnosis, which is based on widely accepted radiological findings.¹⁴ These tumours are not seen on dental radiographs because they are related with the internal auditory meatus, deep within the petrous temporal bone at the cerebellopontine angle. Cone beam CT (CBCT) will also not be of any benefit for assessment of these tumours because this type of scan provides only hard tissue detail and soft tissue is required.

The main imaging modality for assessing for the presence of an acoustic neuroma is MRI, owing to its provision of good soft tissue and peri-tumour detail. The differential diagnosis of a cerebellopontine mass would include an acoustic neuroma, meningioma, arachnoid cysts and other neural tumours.¹⁴ Different MRI sequences can help to differentiate between most of these lesions without the need for biopsy.¹⁵ The advantages of MRI over CT for AN diagnosis are clearly seen in Patients 1 and 3, where both were taken prior to surgery. MRI provided much more detail and, in the case of Patient 3, the tumour was not detected initially on the CT scan alone.

Treatment

AN are best managed by a multidisciplinary team (MDT) comprising otolaryngology (ENT), neurosurgery, neuroradiology and audiology. This means that care is often provided at tertiary care centres, and initial referrals are usually to local otolaryngologists because of the hearing loss with which the majority of cases present. The three primary treatment options for AN are radiological surveillance, stereotactic radiosurgery or surgical resection.¹⁴ For patients with NF2, or worsening neurological function, bevacizumab, a monoclonal antibody, has been employed with some success.¹⁶ Factors that are considered when determining treatment options include the size and growth rate of the tumour, as well as level of disability resulting from the tumour.² As would be expected, there are complications with both intervention and monitoring.¹⁴ Left unchecked, AN can cause worsening symptoms and hydrocephalus owing to the tumour mass effect, which in turn could lead to death.² However, there are also risks associated with conventional and radiosurgery (Table 1). Furthermore, conventional skull base surgery approaches are associated with multiple non-tumour specific risks, including CSF leakage, infection, meningitis and epilepsy. Cases are therefore regularly discussed at MDT meetings to decide upon appropriate management pathways (Table 1).^2,17

The main aim of surgery is subtotal tumour resection because this reduces the risk of facial nerve damage associated with total tumour resection, while reducing the impact of AN mass effect. Importantly, recurrence rates still remain low following subtotal resection. Total excision is, however, undertaken in some circumstances.^14,18 There are three surgical approaches that can be employed to resect acoustic neuromas, and depend on the size, location and proximity to particular cranial nerves.

Middle cranial fossa approach

Approaching AN through the middle cranial fossa is best suited for small AN situated within the internal auditory canal, as well as for patients with good hearing, where preservation of this is a priority. However, due to the limited access to the cerebellopontine angle, this is not suitable for larger tumours.¹⁵ The main advantage of the middle cranial fossa approach is hearing preservation, with 33–76% of patients reporting functional hearing following surgery with this approach; however, the chance of hearing preservation directly relates to tumour size and position.¹⁹

Retrosigmoid craniotomy

The retrosigmoid craniotomy, also known as the suboccipital approach, affords good access to the cerebellopontine angle, while preserving hearing. However, there is reduced access to the internal auditory canal. Thus, this is suitable for large tumours and is associated with potential hearing preservation and often a favourable position of the facial nerve. The main disadvantages are limited access to the internal auditory canal, thus suboptimal excision of tumours in this region may result from this approach.^14,15

Translabyrinthine craniotomy

This approach affords good exposure of the internal auditory canal and posterior fossa, and is preferred if the tumour has significant extension into the posterior fossa of the skull or with labyrinth or cochlear involvement. Importantly, it is also associated with the lowest rates of tumour recurrence. However, this approach results in loss of hearing and is, therefore, reserved for patients with a poor hearing prognosis.^14,15,20

Patient 1 experienced damage to his right facial nerve as a result of his surgery. Subsequent intervention was undertaken to manage this, including the insertion of a fascia lata sling, to improve the symmetry of his mouth when at rest. A tarsorrhaphy, and more recently, a right upper eyelid gold weight insertion, and lateral tarsal strip procedure were undertaken to aid the closure of the patient's right eye, preventing corneal desiccation and ulceration (Figure 3).

Importance for dentistry

Acoustic neuromas are rare; however, given their potential to present with orofacial symptoms, it is important for dentists to be aware of this potential diagnosis. Unilateral symptoms, as well as symptoms very well circumscribed over a specific nerve distribution, would increase the suspicion of a space occupying lesion, such as AN, being responsible for these symptoms. In all patients who report these symptoms, a robust systemic enquiry is essential, with specific questioning regarding auditory disturbance, tinnitus and balance disturbance. Furthermore, a cranial nerve exam is of paramount importance, as a range of AN signs and symptoms may manifest in any of the twelve cranial nerves (Table 2). In a small number of patients, as was the case for Patient 3, dental pain may be reported and can be a symptom of AN. However, a common-sense approach should be taken; if there is dental pathology consistent with the symptoms reported, it is unlikely there is a coincident AN. However, in the absence of dental pathology, further investigation is warranted and onward referral to secondary care is indicated. For Patient 3, although there was clear dental pathology occurring simultaneously with AN, it remained unclear whether the symptoms experienced were due to her UR6 or the AN, as at the time of writing, she was yet to complete her dental treatment. If a dentist is suspicious of AN following comprehensive examination and exclusion of dental pathology, then rapid onward referral is recommended to explore these symptoms with diagnostic imaging.

Conclusions

Although AN are rare, they are associated with significant morbidity, despite being benign. Common presenting features include hearing loss, balance disturbance, facial nerve weakness and altered sensation of the orofacial region. Consequently, AN may present to their general dental practitioner and a patient with such symptoms and no dental pathology should be urgently referred to a local oral medicine or oral and maxillofacial surgery unit.