Before 1960, the management of an intracanalicular acoustic neuroma was problematic. The tumor would have been almost impossible to diagnose, and microsurgical techniques and opening of the internal auditory canal were unknown. Beginning in the 1960s and continuing to the present, a number of events have made management of intracanalicular acoustic neuromas both interesting and exciting. The most significant events are the development of microsurgical techniques, the team approach, markedly improved imaging, and electrophysiologic monitoring. Background The acoustic neuroma arises from the vestibular nerve within the internal canal. The most likely point of origin is the juncture known as the glial–Schwann sheath junction. Theories have been proposed to explain this. One theory states that the vestibular nerve strikes the posterior portion of the internal auditory meatus and this trauma results in tumor formation. Other proposed theories such as genetic predisposition have not been substantiated. Regardless, the tumor begins within the canal and typically enlarges to fill the internal auditory canal before extending in the posterior cranial fossa. The length of the posterior wall of the internal auditory meatus is approximately 9 millimeters; therefore tumors classified as intracanalicular are <1.0 cm in length. In some surgical series, the intracanalicular portion of the acoustic neuroma is not included in the overall tumor size. Thus the measured tumor is only the portion in the posterior cranial fossa, and tumors within the internal auditory canal are grouped separately as intracanalicular. Other investigators include the intracanalicular portion; thus, a tumor that fills the internal auditory canal and that extends 1.5 cm into the posterior cranial fossa might be called a 1.5-cm tumor by the first group and a 2.5-cm tumor by the second group. I favor including the intracanalicular portion as part of the tumor. The symptoms associated with an intracanalicular tumor are otologic.1 The most common is unilateral hearing impairment. The hearing loss on audiometry is usually an asymmetric decrease in pure tones and some loss of speech discrimination. In addition, there is often a sense of sound distortion, which is disproportionate to the audiometric findings. Traditionally, the hearing loss has been thought to be primarily in the high frequencies, however the pure tones can assume any configuration, including a low tone loss, which resembles endolymphatic hydrops. A number of patients present with a sudden hearing loss.2 These patients can be difficult to differentiate from idiopathic sudden hearing loss, particularly if the hearing then improves to normal or near-normal. The second symptom of these tumors is unilateral tinnitus. Generally this is high pitched. When there is no clear-cut etiology for unilateral tinnitus, the symptom should be pursued. The third otologic symptom is dysequilibrium. Frequently the patient will remember unsteadiness in the past or describe some current persistent unsteadiness. Vertigo does occur but is rare. The other potential otologic symptom of an intracanalicular tumor is facial weakness. In a patient with an intracanalicular tumor and facial nerve paresis, a facial nerve neuroma should be the first consideration. Headache and diplopia are not associated with these tumors. Physical findings are uncommon among patients with intracanalicular tumors. Potentially there could be nystagmus or facial weakness, but facial numbness, papilledema, and decreased corneal reflexes are not possible. The important diagnostic tests are audiometry, vestibular testing, and imaging studies. Pure-tone audiometry, speech reception threshold, and speech discrimination are the most important initial tests. These tests are very sensitive and moderately specific. They confirm the patientâ ‘s symptoms, define the degree of cochlear loss, and may suggest retrocochlear disease. Auditory brainstem response audiometry (ABR) can be useful among patients with mild asymmetry of the pure tones or a disproportionate decrease in the speech discrimination. As the hearing loss increases it becomes more prudent to use an imaging study. The role of the ABR audiometry as a screening tool has been the subject of considerable discussion.3, 4 For patients with intracanalicular tumors, the sensitivity of ABR is highly suspect and the specificity poor. Many neurotologists (including the present author) are concerned about the low sensitivity and generally omit the ABR. Others counter that the ABR is widely available, less expensive, and likely to miss only small tumors that are not a threat to the patient. The role of vestibular testing as a diagnostic tool for intracanalicular tumors is limited. The sensitivity for use as a screening tool is marginal. When abnormal, this test might suggest the need to proceed with an imaging study. When normal, it does not exclude the need for further testing, thus like the ABR it is not sensitive or specific enough to play a major role in the evaluation of these patients.
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
