By the nature of their origin, acoustic neuromas always result in some degree of vestibular dysfunction. The implications of this are typically more notable postoperatively, rather than preoperatively or intraoperatively. However, preoperative vestibular assessment can have implications on operative approach and postoperative rehabilitation. This paper details the preoperative vestibular findings that correlate with differing stages of acoustic neuroma growth. It also describes the findings that help localize the tumor origin to either the inferior or superior divisions of the vestibular nerve. Finally, and probably most importantly, we discuss the causes of persistent postoperative vestibular symptoms.
| AN | Acoustic neuroma |
| BPPV | Benign paroxysmal positional vertigo |
| CDP | Computerized dynamic posturography |
| CPA | Cerebellopontine angle |
| ENG | Electronystagmography |
| IAC | Internal auditory canal |
| IVN | Inferior vestibular nerve |
| SVN | Superior divisions of the vestibular nerve |
| SOT | Sensory organization test |
| VEMP | Vestibular evoked myogenic potentials |
| oVEMP | Ocular vestibular evoked myogenic potentials |
Acoustic neuroma (AN) is a misnomer for the benign tumor discussed in this article, because it does not originate from the acoustic nerve and is not a neuroma. AN is a benign tumor arising from the Schwann cells of the vestibular nerve, more appropriately called a vestibular schwannoma. Consequently, the vestibular nerve is always involved with AN and there is virtually always some vestibular system pathology secondary to AN development. Although by far the most common presenting complaints in AN patients are hearing loss and tinnitus, this disorder is primarily one of the vestibular system, specifically the vestibular nerve. It is important to keep this in mind even though most patients do not complain of vestibular symptoms before intervention. However, vestibular complaints are common among patients after intervention.
Because intervention for AN by nature results in loss of vestibular function beyond the damage already inflicted by the tumor, it behooves the surgeon to have some knowledge of the preoperative status of the vestibular system because this can, on occasion, affect the treatment mode. Intraoperatively, the origin of the AN from either the inferior (IVN) or superior divisions of the vestibular nerve (SVN) can have some prognostic importance regarding hearing preservation and possible residual vestibular function. Postoperatively, in terms of morbidity from AN treatment, balance dysfunction is one of the most common consequences.
As already mentioned, the AN arises from the Schwann cells surrounding the vestibular division of the eighth cranial nerve. Specifically, it has been proposed that ANs arise at the transition zone of the central myelin and peripheral myelin sheaths, known as the Obersteiner-Redlich zone. This location is also in the same vicinity as the vestibular (Scarpa) ganglion. It should be noted that although this region appears to be the most common site of AN origin, the site of this zone is variable and there are many examples of AN arising distal to this region as well. In any event, the vast majority of ANs appear to arise from within the medial portion of the internal auditory canal (IAC), with only a small minority originating in the lateral end of the IAC. Even fewer appear to originate at the porus acusticus or in the cerebellopontine angle (CPA) cistern itself. There is a small proportion that even arises within the vestibular end organ. AN can arise from either the IVN or SVN, giving somewhat differing functional vestibular pathology.
AN growth results in compression of the surrounding structures. Presumably the resulting compression of the associated vestibular nerve, and/or its blood supply, results in primary neural dysfunction; as already mentioned, it is rare for direct end-organ involvement. However, the degree of compression, or whether compression of the associated nerve occurs at all, is dependent on the size of the tumor and its location. Medially based AN, especially if there is minimal or no IAC component, will tend to have less impact on function, whereas laterally based tumors will tend to have more profound effects on vestibular function. Such effects are likely attributable to the limited degree of expansion for IAC tumors because of their bony confines, compared with the relatively larger area for expansion within the CPA for a medially based AN. This difference explains the incongruity of a large medial tumor with no significant vestibular loss in contrast to a small IAC tumor that may demonstrate near total loss of vestibular function.
Of course, at its outset a small tumor may cause no neural compression and cause no dysfunction, and the patient will be asymptomatic. As the tumor grows and causes surrounding compression, dysfunction occurs as the associated structures are compressed. The 4 anatomic stages of AN growth are:
- 1.
Intracanalicular
- 2.
Cisternal
- 3.
Brainstem compressive
- 4.
Hydrocephalic
In direct opposition to what one may expect, the worst vestibular symptoms are typically seen at the intracanalicular stage. During this stage, as the vestibular nerve is being progressively destroyed the patients may be asymptomatic or may have episodic vertigo suggestive of a peripheral vestibular disorder. As further growth occurs, the vertigo diminishes and the vestibular symptoms usually subside. Later, as the cisternal phase progresses and the brainstem compressive phase begins, onset of dysequilibrium occurs and signs of central vestibular dysfunction arise. The dysequilibrium worsens as the hydrocephalic stage of growth takes place.
Although the primary means of vestibular dysfunction caused by AN appears to be neural compression, distinct end-organ effects have been demonstrated. Specifically, histopathology of the inner ear in some AN patients has demonstrated endolymphatic hydrops, and eosinophilic proteinaceous precipitate in the perilymphatic and endolymphatic spaces. In addition, there is degeneration of other inner ear structures, including hair cells, the stria vascularis, and the spiral ligament. This process may possibly explain how a minority of patients with small AN present with episodic vertigo.
Preoperative vestibular testing
Vestibular testing has been used in the past as a screening procedure for AN. Although vestibular testing will frequently be abnormal in AN patients, there is a lack of sensitivity or specificity for the purpose of diagnostic screening tests. For vestibular testing to demonstrate an abnormality in an AN case, the tumor must be of adequate size to compromise either the blood supply (to the vestibular nerve or end organ) or the nerve itself to a sufficient degree so as to reduce the vestibular function on the side affected. In addition, the degree of vestibular loss must be sufficient to appear significant on vestibular testing; it must be borne in mind that the state of vestibular testing is such that minor differences in vestibular test responses are often considered insignificant. Consequently, small tumors may not have attained adequate size to result in vestibular loss, thus resulting in false-negative outcomes for vestibular testing as a screening device. By contrast, although vestibular test abnormalities are often the rule among AN patients, abnormalities on vestibular studies are a common finding among a host of other vestibular disorders, resulting in a relatively low degree of specificity in a large screening population. However, this does not mean that vestibular testing in AN patients is without merit.
The values of preoperative vestibular testing are mainly to help distinguish the nerve of origin for the AN, to establish a baseline function in not just the tumor ear but also the contralateral ear, and as a means to help prognosticate about functional postoperative vestibular status. The vestibular-nerve origin has more than just academic significance. SVN tumors have a better prognosis for hearing preservation than IVN tumors. Differentiation between IVN-based and SVN-based tumors can help determine whether a surgical approach to hearing preservation may or may not be warranted. Establishing normal vestibular function in the non-AN ear is important for postoperative rehabilitation. Severely diminished or absent vestibular function in the non-AN ear may warrant a more conservative approach to maintaining vestibular function in the AN ear.
Electronystagmography
Electronystagmography (ENG) has been the most studied vestibular test battery among AN patients, and its most useful subtest is the bithermal caloric test. On caloric testing, the vast majority of AN patients will demonstrate reduced or absent response on the affected side. Because the caloric test is a test of the horizontal semicircular canal function and the SVN, tumors arising from the SVN have a higher rate of reduced caloric response than tumors arising from the IVN. Of course, once tumors of the IVN attain sufficient size they could compromise the adjacent SVN. In addition, the larger the tumor is the more likely that the caloric response will be diminished. As alluded to earlier, this is not a good screening test for AN, but from a practical standpoint a large AN without a corresponding hypoactive caloric response would lead one to suspect that the pathology may be something else, such as a meningioma.
As one would expect, spontaneous nystagmus frequently accompanies AN. Most typically, it is a nystagmus of the paretic type from the loss of unilateral vestibular function. However, other types of nystagmus can also be seen. As the AN grows and causes central compression, signs of brainstem and cerebellar dysfunction may manifest with Bruns nystagmus. Bruns nystagmus is a combination of a gaze-evoked nystagmus from cerebellar compression and a paretic nystagmus from unilateral deafferentation. The resulting nystagmus is an asymmetric one with slow, large-amplitude beats of nystagmus looking to the side of the AN and fast, small-amplitude nystagmus when gaze is directed away from the side of the AN. As the tumor continues to grow, causing bilateral cerebellar compression, a more pure gaze-evoked nystagmus may predominate. Alternatively, with brainstem compression and increased intracranial pressure, vertical down-beat or up-beat nystagmus may be present.
Other abnormalities may be seen on ENG, including positional nystagmus, which can be seen with tumors of all sizes, and central findings that are virtually exclusive to the larger tumors presenting with compressive features. Among the central findings seen with larger tumors are impaired fixation suppression, optokinetic and smooth-pursuit abnormalities, and the aforementioned gaze-evoked nystagmus and vertical nystagmus.
Rotational Studies
Similar to ENG, rotational studies lack both specificity and sensitivity as a means for making the diagnosis of AN ; however, rotational chair studies are frequently abnormal, particularly when the tumor has grown in sufficient size to compromise the SVN. Tumors arising from the IVN that are small and do not compromise the SVN should have no identifiable abnormality on rotational chair testing, because this is a test of the horizontal semicircular canal and the corresponding SVN. There is some evidence to suggest that high-frequency, vestibular autorotation testing in the vertical plane may identify vestibular abnormalities in this subgroup of AN patients. When abnormalities are identified, the findings typically reflect evidence of a unilateral vestibular deficit with varying evidence of central compensation. In general, gain reduction is seen, especially if there is a concomitant caloric hypofunction. However, in some of the large tumors an increase in gain may be seen, representing central vestibular dysfunction, which is the inability to suppress vestibular eye movements. Asymmetry tending to the side of the AN is most characteristically seen, but eye asymmetry may occasionally go to the non-AN side.
Computerized Dynamic Posturography
Within the standard vestibular test battery, computerized dynamic posturography (CDP) probably is the least sensitive to identification of AN. However, it is not unusual to see reduced scores among AN patients on CDP. There seems to be a general correlation between the size of the AN and outcomes on sensory organization test condition 5 (SOT 5), with larger tumors resulting in poorer performance. It has also been suggested that CDP may be a means to help identify AN of IVN origin as opposed to SVN origin. CDP is a measure of the vestibulospinal reflex arc, but is more sensitive to end-organ components innervated by the IVN and is relatively less sensitive to the SVN-innervated components. Consequently, one would expect to see more frequent CDP abnormalities (SOT 5) among patients with IVN-based tumors than among those with SVN-based tumors. However, in practice this is not so straightforward and has not yet been definitively shown in study. This finding may possibly be explained by the slow growth of AN and concomitant central compensation/adaptation, resulting in somewhat better performance for some patients with IVN tumors and the eventual compromise of the IVN by tumors arising from the SVN.
Vestibular Evoked Myogenic Potentials
Vestibular evoked myogenic potentials (VEMP) is a testing technique that has recently made the transition from the research laboratory to the clinical laboratory, due to its popularity in the evaluation of patients with superior semicircular canal dehiscence. VEMP is a means of measuring otolithic function. Delivery of sound stimuli to the ear results in stimulation of the otolithic organ, which initiates reflex muscular contraction that can be measured and quantified for sensitivity, latency, and amplitude. The 2 most common methods are the cVEMP (cervical) with measurement of the sternocleidomastoid muscle and the oVEMP (ocular), which measures extraocular muscle contraction.
The generally accepted view at present is that the saccule may be more amenable to air-conducted sound stimuli, whereas the utricle is more sensitive to bone-conducted sound stimuli. Because of this differential sensitivity to stimuli, the otolithic organs can be assessed separately for responsiveness. Consequently, the use of bone-conducted VEMP and air-conducted VEMP may be useful for distinguishing between IVN and SVN tumor origins. Air-conducted VEMP would be expected to more frequently demonstrate a deficit in IVN-based tumors, whereas bone-conducted VEMP would be more likely to demonstrate abnormalities in SVN-based tumors. Of course, the issue found with other distinguishing vestibular tests arises here as well. Once a tumor is of sufficient size, whether it arises from the SVN or the IVN, it has a tendency to compromise both divisions of the vestibular nerve. Therefore, the differential utility of this test is most useful with the smaller tumors.
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