History
Unilateral hearing loss is overwhelmingly the most common symptom present at the time of diagnosis and is generally the symptom that leads to diagnosis. Assume that any unilateral SNHL is caused by an acoustic neuroma until proven otherwise; an MRI scan with gadolinium contrast should be obtained. The tumor can produce hearing loss through at least two mechanisms, direct injury to the cochlear nerve or interruption of cochlear blood supply. Progressive injury to cochlear fibers probably accounts for slow progressive neurosensory hearing loss observed in a significant number of patients with acoustic neuromas. Sudden and fluctuating hearing losses are more easily explained on the basis of disruption of cochlear blood supply.
Consistent with direct injury to cranial nerve VIII, a significant number of individuals with acoustic neuroma have speech discrimination scores reduced out of proportion to the reduction in the pure-tone average—a feature typical for retro-cochlear lesions. This can often be demonstrated through audiological testing by a phenomenon called "rollover," in which speech discrimination scores decrease as the volume of the speech stimulus increases. Such marked reductions in speech discrimination scores are not invariable, however. A normal speech discrimination score does not rule out an acoustic tumor. Patients with acoustic tumors may have normal or near-normal hearing and speech discrimination scores.
A study by Lee et al of over 100 patients with acoustic neuroma found that tinnitus was the most frequent symptom accompanying hearing loss. It was also determined that approximately 65% of patients had asymmetrical hearing and that an association existed between greater severity of deafness and lower speech discrimination scores. [6]
Hearing loss associated with acoustic neuroma can be sudden or fluctuating in 5-15% of patients. Such hearing loss may improve spontaneously or in response to steroid therapy. However, a gadolinium enhanced MRI should be ordered in anyone with a sudden or fluctuating loss even if hearing returns to normal.
Not surprisingly, the discovery of acoustic neuromas in persons with normal hearing has been increasing as gadolinium-enhanced MRI is becoming more common. In addition, tumor size poorly correlates with hearing status as patients with large tumors may have normal hearing and patients with small tumors may be profoundly deaf in the affected ear. The presence of unilateral tinnitus alone is a sufficient reason to evaluate an individual for an acoustic neuroma. Although tinnitus is most commonly a manifestation of hearing loss, a few individuals with acoustic tumors (around 10%) seek treatment for unilateral tinnitus without associated subjective hearing loss.
Vertigo and disequilibrium are uncommon presenting symptoms among patients with an acoustic neuroma. Rotational vertigo (the illusion of movement or falling) is uncommon and is occasionally seen in patients with small tumors. Disequilibrium (a sense of unsteadiness or imbalance), on the other hand, appears to be more common in larger tumors. Overall, if carefully questioned, approximately 40-50% of patients with an acoustic neuroma report some balance disturbance. However, balance disturbance is the presenting symptom in less than 10% of patients. The destruction of vestibular fibers apparently is sufficiently slow as to permit compensation. If patients have persistent imbalance or dizziness, vestibular rehabilitation is often beneficial, even if the tumor remains untreated.
Headaches are present in 50-60% of patients at the time of diagnosis, but fewer than 10% of patients have headache as their presenting symptom. Headache appears to become more common as tumor size increases and is a prominent feature in patients who develop obstructive hydrocephalus associated with a very large tumor.
Facial numbness occurs in about 25% of patients and is more common at the time of presentation than facial weakness. Objective hypoesthesia involving the teeth, buccal mucosa, or skin of the face is associated with larger tumors, but a subjective reduction in sensation that cannot be documented on objective examination occurs commonly with medium-sized and small tumors. Decrease in the corneal reflex generally occurs earlier and more commonly than objective facial hypoesthesia. Although approximately 50-70% of individuals with large tumors have objectively demonstrable facial hypoesthesia, they are often unaware of it, and it is uncommonly the presenting symptom.
The motor fibers in the facial nerve can accommodate substantial stretching as long as it occurs slowly and are much more resistant to injury than sensory fibers of the trigeminal nerve. Facial weakness is sufficiently uncommon (< 1%) that facial weakness associated with a small- or medium-size tumor should raise suspicion that it is not an acoustic neuroma. Other diagnosis should be considered including facial neuroma, hemangioma, meningioma, granuloma, arteriovenous malformation (AVM), or lipoma.
Large tumors (>4 cm) can obstruct the flow of spinal fluid through the ventricular system by distorting and obstructing the fourth ventricle. In the early decades of this century, 75% of patients presented with hydrocephalus. Although hydrocephalus is not as common a presenting symptom as it once was, recognizing its symptoms (headaches, impaired vision, cognitive difficulties, seizures, neurologic deficits) is important to potentially diagnosing a larger, obstructing tumor.
Physical Examination
Although a thorough head and neck exam, including otoscopy, is recommended during evaluation of a patient, there are often no specific findings on exam that will detect an acoustic neuroma. Examination is important to help rule out other possible otologic diagnoses that may be more readily detectable on exam and may account for presenting symptoms that are overlapping those of acoustic neuroma. Subtle facial numbness or paresthesias or a lack of a corneal reflex suggest a tumor affecting the trigeminal nerve. Audiometry is recommended as an adjunct during the initial visits of a patient undergoing workup.
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This table shows the distribution of presenting symptoms, ie, the symptom that brought the patient to a physician and that constituted the patient's chief ailment.
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A small acoustic neuroma within the internal auditory canal is easily observed on postgadolinium MRI.
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These large bilateral acoustic neuromas are easily observed on MRI. This patient has neurofibromatosis II. Both tumors were eventually removed, leading to anacusis. Facial nerve function remained entirely normal bilaterally.
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The nerves of the internal auditory canal as observed in a cadaveric dissection are shown. The posterior wall of the internal auditory canal has been removed. F indicates the facial nerve. S is the superior vestibular nerve. VIII indicates the statoacoustic nerve as it leaves the brain stem, and P indicates the posterior ampullary nerve. The hollow arrow points to the posterior lip of the boney porous acusticus, and the solid arrow indicates the position of the vestibule. C indicates the cochlear aqueduct.
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The bone that must be removed for a middle cranial fossa approach is indicated in yellow. The tumor is in orange.
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The bone that must be removed for a translabyrinthine approach is indicated in yellow. The tumor is in orange.
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The bone that must be removed for a posterior fossa approach is indicated in yellow. The tumor is in orange.
