Intracanalicular Stage

This stage involves hearing loss, tinnitus, and vertigo.

Cisternal Stage

Tumor involves the cistern of the cerebellopontine angle. Early growth into the cerebellopontine angle cistern affords the tumor room to grow without significant impingement by local structures. At this stage, auditory symptoms worsen, vertigo transitions into dysequilibrium, and headache may begin. This stage is unique in that there can be considerable tumor growth without, necessarily, a concordant increase in tumor symptoms.

Brainstem Compressive Stage

In this stage, hearing loss and dysequilibrium worsen and the patient may develop trigeminal symptoms.

Hydrocephalic Stage

Tumor growth causes obstruction of the fourth ventricle with associated hydrocephalus, and is accompanied by rapid clinical deterioration associated with generalized headache, facial twitch and weakness, visual loss or diplopia, lower cranial nerve dysfunction, and finally long tract signs and death due to tonsillar herniation.

In a study at The University of California, San Francisco (UCSF), it was determined that the average duration of hearing loss and tinnitus before diagnosis of acoustic neuroma was almost 4 years. Vertigo was generally noted 3.6 years before diagnosis. Headache developed on average 2.2 years before diagnosis, followed by dysequilibrium 1.7 years before diagnosis. Late symptoms, such as trigeminal dysfunction, occurred at 0.9 year before diagnosis and facial nerve dysfunction at 0.6 year before diagnosis. With this understanding of the natural history in mind, the goal of current otolaryngologists is to diagnose acoustic neuromas at the earliest stage, hopefully when a patient presents with only hearing loss, tinnitus, or vertigo, and before progression to more ominous symptoms.

Hearing Loss

Hearing loss is traditionally the index symptom in patients presenting with acoustic neuromas in the modern era. Hearing loss is typically gradual in onset and is a unilateral, asymmetric sensorineural loss of hearing. A certain number of patients, however, will present with atypical hearing loss or even normal hearing; therefore, clinicians should maintain a high level of suspicion if there are other symptoms or signs suggesting the possibility of an acoustic neuroma.

Tinnitus

Tinnitus has been described in 53% to 70% of patients with acoustic neuromas. The classical description is an ipsilateral high-pitched, continuous tinnitus, although it may also present as low-pitched, “whistling, roaring, or pulsatile” tinnitus. Tinnitus is generally accompanied by hearing loss. Few patients with acoustic neuroma will present with isolated tinnitus in the absence of hearing loss. Asymmetric tinnitus without hearing loss should raise suspicion of the possibility of retrocochlear pathology. A clinician must take into account the fact that approximately 15% of the population complains of tinnitus.

Vertigo

Vertigo is a sensation of motion that may be characterized by movement of the patient in the surrounding environment or movement of the environment itself. Vertigo tends to occur early in the sequence of symptom development and may spontaneously resolve. The reported incidence varies from 18% to 58% in assorted studies. These figures may seem surprisingly low given the origin of acoustic neuromas from the superior and inferior vestibular nerves. This is explained by the body’s ability to compensate for vestibular dysfunction by way of the central and contralateral vestibular systems. Because acoustic neuromas show slow growth, there is time to allow for gradual compensation, and many patients do not experience vertigo. The appearance of vertigo may correspond to a period of rapid tumor growth, or a vascular event that leads to an abrupt change in the capabilities of the vestibular system.

Dysequilibrium

Dysequilibrium was present in 48% of patients in the study by Selesnick and colleagues. Dysequilibrium is commonly described as an unsteady or off-balance sensation. Whereas vertigo is an indicator of involvement of the peripheral vestibular symptom, dysequilibrium suggests cerebellar involvement either alone or in combination with peripheral dysfunction. As such, dysequilibrium tends to occur later in the course of tumor growth. Once initiated, cerebellar symptoms, such as dysequilibrium, tend to be continuous and enduring.

Trigeminal Nerve Dysfunction

Patients with trigeminal nerve dysfunction may complain of hypesthesia, paresthesia, or pain, most commonly in the malar region. More frequently, patients may be without complaints referable to the trigeminal nerve but will be found on clinical examination to have decreased or absent corneal reflexes. A UCSF study found trigeminal dysfunction in 20% of a series of patients with acoustic neuroma. Earlier studies have cited higher prevalence, from 33% to 71%, which may be attributable to the higher prevalence of larger tumors in earlier studies, because the UCSF data found that the presence of trigeminal dysfunction markedly depended on tumor size. No patient in that study with tumors less than 1 cm in size had trigeminal symptoms, whereas 20% of those with tumors 1 to 3 cm in size and 48% of patients with tumors larger than 3 cm showed trigeminal dysfunction.

Headache

Headache caused by acoustic neuroma is commonly described as dull or of moderate intensity, and gradual in onset. Location can be general, and although there is a predilection for symptoms ipsilateral to the tumor, many patients cannot establish laterality of their headache. Reported incidence of this symptom varies from 19% to 85% of patients, and has been found to be associated with tumor size. This figure must be taken in the context of the incidence of headache in the general population. Point prevalence of headache has been estimated at 11% in men and 22% in women, with a lifetime prevalence of 69% in men and 88% in women.

Symptoms of Increased Intracranial Pressure

Signs and symptoms of increased intracranial pressure include headache, nausea, vomiting, decreased vision, diplopia, papilledema, anosmia, and obtundation. Although these symptoms were common in early reports, the incidence has decreased in the modern era. For example, diplopia was a presenting symptom in 66% of Cushing’s case series from 1917, but was present in only 3% of patients in a study published in 1993 from UCSF.

Facial Nerve Dysfunction

Facial nerve dysfunction may present as either hyperfunction (facial twitching or spasm) or hypofunction (facial paresis or paralysis.) Estimates of the prevalence of facial nerve dysfunction range from 10% to 18% in the literature. These numbers are relatively low given the location of the facial nerve within the internal auditory canal, and likely reflect the resilience of facial nerve fibers to tumor compression. Hyperfunction of the facial nerve may indicate a primary facial nerve tumor.

Lower Cranial Nerve Dysfunction and Long Tract Signs

Symptoms of lower cranial nerve palsies include hoarseness, dysphagia, and dysarthria. These symptoms were more common in older reports (prevalence estimates in the 20%–30% range), but are rarely reported in the modern era. Long tract signs such as hyperreflexia, hemiparesis, and hemiplegia represent advanced disease, and are mainly described in the historical literature. It is rare for patients currently to present with such advanced symptoms.

Asymptomatic Tumors

Acoustic neuromas may be diagnosed incidentally after a patient undergoes MRI evaluation for an unrelated problem. Two such patients were described in the 1993 series from UCSF. Selesnick and colleagues later described 4 cases of incidentally discovered acoustic neuromas, but did not find any cases in a prospective study of patients undergoing gadolinium-enhanced MRI for diagnoses other than acoustic neuroma or sensorineural hearing loss. As MRI technology continues to evolve and become more accessible, it is likely that this class of patients will increase in number. Of note, some patients who have had an incidental discovery of acoustic neuromas may, in fact, not be asymptomatic. On questioning, these patients may have recognized, for example, a long-standing hearing loss that they have attributed to some other cause, and so have not sought the care of a physician.

Pure Tone Audiogram

After obtaining a clinical history and performing a physical examination, the first diagnostic test generally performed is a pure tone audiogram with speech reception thresholds and speech discrimination analysis. Johnson performed a retrospective review of audiologic testing in 500 patients with acoustic neuromas. Pure tone thresholds in this group ranged from 5 to 130 dB (no response), with a mean pure tone threshold of 66.5 dB. Sixteen percent of patients had a complete loss of hearing in the involved ear that precluded classification of the audiometric pattern. In patients who had adequate hearing to obtain pure tone audiograms, pure tone loss was classified into 4 groups: high tone loss, flat, trough-shaped, and low tone loss. Sixty-five percent of patients were found to have high-frequency hearing loss, followed by flat type loss in 22% of patients.

Poor speech discrimination, particularly when out of proportion to pure tone threshold responses, has traditionally been an indicator of retrocochlear pathology. This finding was corroborated by Johnson’s study, in which 24% of patients with testable hearing had 0% speech discrimination. Overall, 44% of patients with testable hearing were able to achieve a speech discrimination score of 62% or more, whereas the remaining 56% had speech discrimination scores of 60% or less. The correlation between acoustic neuroma and poor speech discrimination was less impressive in a study by Hirsch and Anderson, which found that only 45% of patients with acoustic neuromas had abnormal speech discrimination.

Impedance Testing

The acoustic reflex is analyzed in impedance testing, and represents the contraction of the stapedius muscle in response to a high-intensity sound. The absence of the acoustic reflex and reflex decay (decay of 50% of a tone administered at 10 dB over threshold) have been associated with retrocochlear lesions. Reports of the sensitivity of acoustic reflex testing vary widely, ranging from 21% to 90%. The sensitivity of reflex decay ranges from 36% to 100% in the literature. For these reasons, data from impedance testing are poor indicators of the presence of an acoustic neuroma. An advantage of this test over other audiometric tests in the basic testing battery is that the acoustic reflex is involuntary and, thus, measurements obtained are not subjective.

Auditory Brainstem Response Testing

ABR testing, also known as brainstem auditory evoked response (BAER) testing, is a recording of the synchronized response of neurons in the lower auditory pathways in response to a stimulus. ABR was first described by Sohmer and Feinmesser in 1967, followed shortly thereafter by Jewett and colleagues in 1970. The clinical applications of ABR were expanded on by Selters and Brackmann in 1977, at which point ABR became an established tool to screen patients for acoustic neuromas. ABR involves generating a stimulus and recording the evoked response. The most commonly used stimulus is a 100-microsecond click. This rapid and brief stimulus results in many auditory nerve fibers firing nearly synchronously, occurring in the first 10 milliseconds after the stimulus is presented. The response is recorded from scalp electrodes and is displayed as a predictable series of peaks. These peaks are labeled waves I through V. Waves I and V are the largest and most reliable responses obtained from the click stimulus. Wave I represents response activity from the cochlear nerve, whereas wave V represents the response from neurons of the inferior colliculus. The peak amplitudes and latencies are recorded and interaural comparisons can be drawn. The most reliable latency measurement in the diagnosis of acoustic neuroma is the interaural wave I to V latency difference, known as IT5. This latency is compared between the normal ear and the test ear. A latency difference of 0.2 milliseconds or greater generally signifies an abnormal result.

ABR testing became a key component in the diagnosis of acoustic neuromas shortly after its introduction. ABR is a noninvasive, rapidly administered, and painless test that is well tolerated by patients and is not affected by attention, sedation, or anesthesia. A notable shortcoming of the technique is the inability to obtain accurate results when a patient’s hearing threshold is greater than 80 dB at 4 kHz. Initial reports of sensitivity in diagnosing acoustic neuromas were excellent. Early publications cited sensitivity figures of 93% to 100% in diagnosing acoustic neuromas, with a specificity of 90%. Such high figures were called into question after the introduction of MRI, which allowed for detection of smaller tumors than had been identified by previous radiologic modalities. In the MRI era, new studies have shown the sensitivity of ABR in detecting acoustic neuromas to be 63% to 95%. A common thread in these studies has been the inability of ABR to diagnose small tumors with high sensitivity. Gordon and Cohen performed a prospective study of 105 patients with surgically proved acoustic neuromas who underwent preoperative ABRs. These investigators found that 100% of patients with tumors larger than 2 cm had abnormal ABRs, but only 69% of patients with tumors smaller than 1 cm had an abnormal ABR. Schmidt and colleagues performed a retrospective review of 58 patients with acoustic neuromas who had undergone both ABR and MRI. Although overall sensitivity for ABR in detecting acoustic neuromas was 90%, sensitivity decreased with tumor size and was as low as 58% in tumors measuring 1 cm or smaller. A similar study was performed by Zappia and colleagues, who examined 111 patients with acoustic neuroma who underwent preoperative ABR and MRI. These investigators found ABR to be 95% sensitive overall for recognizing acoustic neuromas; again, sensitivity was related to tumor size, with 100% sensitivity in tumors larger than 2 cm and only 89% sensitivity for tumors measuring 1 cm or less. In a retrospective series by Wilson and colleagues, ABR was found to have an overall sensitivity of 85%; sensitivity decreased to 67% in intracanalicular tumors. Hashimoto and colleagues found a 22% false-negative rate for ABR in a study of 20 small acoustic neuromas with extra-meatal size of less than 15 mm.