TRANSTEMPORAL BONE MICROSURGICAL REMOVAL OF ACOUSTIC NEUROMAS. FACIAL NERVE INVOLVEMENT AND TESTING IN ACOUSTIC NEUROMAS

Effect of Vestibular Schwannoma Size and Nerve of Origin on Posterior External Auditory Canal Sensation: A Prospective Observational Study

OBJECTIVE

Posterior external auditory canal (EAC) hypesthesia (Hitselberger's sign) has been previously described to occur in all vestibular schwannomas (1966) but has not been studied since. We hypothesized that sensory loss may be related to tumor size and sought to determine if this clinical sign could predict preoperative characteristics of vestibular schwannomas, intraoperative findings, and/or surgical outcomes.

STUDY DESIGN

Prospective observational study.

SETTING

Tertiary referral center.

PATIENTS

Twenty-five consecutive patients who underwent surgery for vestibular schwannoma.

INTERVENTION

Patients were tested for the presence of EAC hypesthesia or anesthesia.

MAIN OUTCOME MEASURES

Preoperative, intraoperative, and postoperative findings were recorded, including facial nerve function, hearing function, tumor size, tumor nerve of origin, and extent of resection.

RESULTS

Twelve patients (48%) demonstrated either posterior EAC hypesthesia (11 patients) or anesthesia (1 patient). Sensory loss was a significant predictor of size (tumor maximal diameter) (p = 0.004). Median tumor diameter was 1.7 cm in the cohort with intact sensation versus 2.9 cm in the cohort with sensory loss. Patients with sensory loss were also significantly more likely to be associated with a superior vestibular nerve origin tumor (p = 0.01). Preoperative sensory loss did not significantly predict postoperative facial outcome (p = 0.10).

CONCLUSION

Neurological exam findings may be overlooked in the workup of brain tumors. Posterior EAC hypesthesia is a predictor of tumor size and superior vestibular nerve origin. These findings may have implications for patient selection, particularly with the middle cranial fossa approach. Furthermore, given this relationship with tumor size, this clinical biomarker should be studied as a potential predictor of tumor growth.

Clinical neurophysiology of cranial nerve disorders

Electrophysiologic techniques are available to measure many of the cranial nerves. The procedures can be done using equipment available in standard clinical neurophysiology laboratories. These studies can aid in localization of cranial nerve lesions as well help identify the underlying pathology and possibly aid in prognosis. The trigeminal pathways can be measured using the blink and masseter responses. The facial nerve is measured by the blink response and by direct facial stimulation; techniques such as lateral spread can identify specific abnormalities. The spinal accessory nerve is measured using nerve conduction techniques. Needle examination can be routinely performed on muscles innervated by cranial nerves V, VII, X, XI and XII. These studies reliably measure the functional integrity of cranial nerves and their central pathways. Intraoperative monitoring of the cranial nerves is useful in certain surgeries. This chapter reviews current techniques used to evaluate cranial nerves, emphasizing the methods available in most clinical neurophysiology laboratories.

A Cross-sectional Survey of the North American Skull Base Society: Current Practice Patterns of Vestibular Schwannoma Evaluation and Management in North America

Background  Very few studies have examined vestibular schwannoma (VS) management trends across centers and between providers. The objective of this study is to examine current practice trends, variance in treatment philosophies, and nuanced or controversial aspects of VS care across North America. Methods  This is a cross-sectional survey of North American Skull Base Society (NASBS) members who report regular involvement in VS care. Results  A total of 57 completed surveys were returned. Most respondents claimed to have over 20 years of experience and the majority reported working in an academic practice with an affiliated otolaryngology and/or neurosurgery residency program. Sixty-three percent of respondents claimed to evaluate VS patients in clinic with both an otolaryngologist and neurosurgeon involved. Eighty-six percent of respondents claimed to operate on VS with both an otolaryngologist and neurosurgeon involved, while only 18% of neurosurgeons and 9% of otolaryngologists performed surgery alone. There was a wide range in the number of cases evaluated at each center annually. Similarly, there was wide variation in the number of patients treated with microsurgery and radiation at each center. Additional details regarding management preferences for microsurgery, stereotactic radiosurgery, stereotactic radiotherapy, and conservative observation are presented. Conclusion  VS management practices vary between providers and centers. Overall, most centers employ a multidisciplinary approach to management with collaboration between otolaryngology and neurosurgery. Overall, survey responses concur with previous studies suggesting a shift toward conservatism in management.

Electrically Evoked Taste Threshold

How accurately can the electrically evoked taste threshold be determined? Why have standards of normal values not yet been generally accepted? In order to answer these questions, 80 normal subjects were examined by means of the method of adjustment. The results showed a greater variability of threshold values and side differences than found by other authors. In explanation, it is suggested that the method of adjustment minimizes the biasing effect of the examiner, which normally leads to an underestimation of the threshold variability as well as of the side differences. The method of adjustment is therefore suitable for certain experimental purposes. For clinical use, a simple procedure like Krarups should be preferred. Due to its limited validity, however, only side differences of 100% or more should be considered pathological. A log-scale with rather large units (25% increment steps) is recommended.

Auditory evoked blink reflex in peripheral facial paresis

PURPOSE

The auditory blink reflex (ABR) is a teleceptive reflex consisting of an early brief muscle contraction of the orbicularis oculi in response to sound stimuli. Constriction of the orbicularis oculi in response to auditory stimulation is accepted as a part of the startle reaction. The blink reflex and ABR might share a final common pathway, consisting of facial nerve nuclei and the facial nerve and may have common premotor neurons.

METHODS

In this study, the authors evaluated the value of the ABR in patients with peripheral facial palsy (PFP), cross-checking the results with commonly used blink reflex changes.

RESULTS

In total, 83 subjects with PFP and 34 age-matched healthy volunteers were included. Auditory blink reflex was elicited in all control subjects and in 36 PFP cases on the paralytic sides (43.3%), whereas it was asymmetric in 30.1% of the patients. Auditory blink reflex positivity was significantly lower in PFP cases with increasing severity. Blink reflex results were largely correlated with ABR positivity.

CONCLUSIONS

Auditory blink reflex is a useful readily elicited and sensitive test in PFP cases, providing parallel results to blink reflex and being affected by disease severity.

Electrophysiological Evaluation of Cranial Neuropathies

BACKGROUND

Cranial nerve abnormalities occur frequently in both focal and diffuse neurologic disorders and can be evaluated by electrophysiological techniques available in most clinical neurophysiology laboratories.

REVIEW SUMMARY

The optic nerve is evaluated by visual evoked potentials. Measurements of latency, amplitude, and waveform morphology are especially useful in detecting demyelinating lesions. Brain stem auditory evoked potentials evaluate the auditory portion of the eighth cranial nerve. Using an auditory stimulus, a number of waveforms are generated, and changes in the normal patterns of response can detect abnormalities. Assessment of the trigeminal and facial nerves is done using a series of electrical stimulation techniques including the blink, masseter, and masseter inhibitory reflexes and facial motor nerve conduction studies. The blink reflex detects lesions of the first division of the trigeminal nerve and the facial nerve. The masseter reflex evaluates the third division of the trigeminal nerve. Changes in responses are measured and, using a combination of these techniques, localization of lesions at specific sites can be made. Accessory motor nerve conduction is useful not only in focal nerve injury, but repetitive stimulation on the accessory and facial nerves is used in diagnosing neuromuscular junction disorders. In addition, many of the voluntary muscles innervated by the cranial nerves are accessible to needle electrode examination, and evaluation can aid in identification of focal nerve lesions, as well as diagnosis in diffuse nerve and muscle disorders.

CONCLUSION

Electrophysiological techniques offer reliable means of measuring the integrity of the cranial nerves and their central pathways.

Cranial nerve conduction and needle electromyography in patients with acoustic neuromas: a model of compression neuropathy

Of 45 patients with acoustic neuromas (0.3-5.0 cm), 73% had facial nerve impairment on electrophysiologic testing, but only 16% had facial weakness. Cranial nerve conduction was the most sensitive measurement, especially prolongation of the ipsilateral R1 latency of the blink reflex compared with that of the contralateral reflex. The severity of nerve conduction abnormality was highly correlated with tumor size. Our results confirm and quantitate the sensitivity of nerve action potential latency in response to chronic nerve compression.

Electroneurography and intraoperative facial monitoring in contemporary neurotology

Electrodiagnostic testing of the facial nerve has evolved beyond prognostic and topognostic testing to include preoperative assessment and intraoperative monitoring. The state of the art in facial nerve testing is herein reviewed. Electroneurography is described as a means of preoperative assessment to detect subclinical neural degeneration for temporal bone tumors, malignant external otitis, and recurring facial paralysis. Techniques of intraoperative facial nerve monitoring are presented with clinical correlation of the facial evoked responses. Finally, recent advances in electrodiagnostic testing, including antidromic recording and transtemporal magnetic stimulation of the facial nerve, are discussed.

Surgical management of acoustic neuromas during the last five years. Part I

A consecutive series of 94 patients with acoustic neuromas, who were treated in the years 1980-1984, was reviewed. Hearing loss was the most common presenting symptom. During 1980-1984, all acoustic neuromas were treated surgically; 93 cases had a total excision and only one case had a subtotal excision. Mortality and morbidity were low and patients who had a fair to poor outcome rating had large tumors. Overall, cranial nerve VII was preserved in 64.1% of cases, while cranial nerves VII and VIII were anatomically preserved in 27% of cases.

Surgical management of acoustic neuromas during the last five years. Part II: Results for facial and cochlear nerve function

Postoperative facial and cochlear nerve function in 83 consecutive patients with acoustic neuromas, who were undergoing their initial surgical procedure during 1980-1984, have been examined. The facial nerve was preserved in anatomic continuity in 71% of cases. Various nerve grafting procedures were used when the facial nerve was divided; the most common of these was a faciohypoglossal anastomosis, which was performed in 20 cases. The facial and cochlear nerves were anatomically preserved in 30.1% of all patients having their initial surgical procedure. Good speech discrimination was preserved in four patients, whereas more crude hearing was preserved in six other patients.

Electroneurography: preoperative facial nerve assessment for tumors of the temporal bone

Tumors of the temporal bone and cerebellopontine angle may be associated with subclinical facial nerve degeneration, despite clinically normal facial function. To determine if preoperative neurophysiologic testing might predict postoperative facial function after tumor surgery, preoperative facial electroneurography (ENoG) was performed in 82 patients with confirmed tumors of the temporal bone and cerebellopontine angle. In patients with acoustic neuroma, preoperative ENoG amplitude reduction varied directly with tumor size. In addition, a statistically significant association between impaired postoperative facial function and acoustic neuroma size greater than 2.5 cm was found. However, preoperative ENoG amplitude reduction did not accurately predict postoperative facial function. These findings suggest that factors, such as the type and size of tumor, the microanatomic relationship between the facial nerve and the neoplasm, and/or desynchronization of the evoked motor-unit volley, may effect results obtained with ENoG in this setting.

Acoustic neuromas (diagnostic value of testing the function of the trigeminal nerve, the cerebellum and optokinetic nystagmus)

The involvement of the trigeminal nerve, cerebellum, and optokinetic nystagmus in patients with acoustic neuromas, as well as the methods of investigation, are described. The corneal and/or facial sensibility was found to be reduced in 29 per cent of the whole series and in 53 per cent of tumors larger than 40 mm. There was a significant correlation between reduced corneal and/or facial sensibility and the findings of pressure at the trigeminal root at operation. Only three patients had a persistent reduction of trigeminal function post-operatively. Cerebellar dysfunction was found in 32 per cent, but significantly more frequently (58 per cent) in patients with tumors larger than 40 mm. Post-operatively, six patients had cerebellar symptoms in the form of gait disturbances; five of these patients had a supplementary suboccipital removal performed, after the initial translabyrinthine approach. A defective optokinetic nystagmus was found pre-operatively in 10 patients, nine of whom had tumors larger than 40 mm in diameter. All patients with a defective optokinetic nystagmus had a large anatomic impression in the pons at operation. In patients suspected of having an acoustic neuroma, symptoms from the trigeminal nerve, the cerebellum and the optokinetic nystagmus predict the presence of a large tumor and subsequent difficulties at operation. The symptoms were completely reversible in the vast majority of cases and post-operative symptoms persisted only in patients in whom tumor removal was difficult and the tumor very large. Testing of the trigeminal nerve, the cerebellum and the optokinetic nystagmus still deserves its place in the diagnostic work-up of patients with unilateral acoustic or vestibular symptoms, especially in cases with severe hearing impairment, which necessitate the use of tests that are independent of acoustic function.

[Unilateral deafness (author's transl)]

Unilateral conductive deafness has a readily determined aetiology in most cases. In contrast, unilateral sensorineural hearing loss requires more refined and extensive investigation. The most frequent causes of unilateral sensorineural hearing loss in our patients were sudden deafness, Menière's disease, cranio-vertebral dysplasia and cerebellopontine angle tumors. Early diagnosis of acoustic neuroma or other lesions of the internal auditory meatus or cerebellopontine angle requires special attention. The definitive diagnosis of these tumors often demands intensive clinical investigation but a high degree of suspicion may be entertained following modern routine audiometry, vestibular function testing and radiological examination. The importance of early diagnosis of these lesions is stressed with regard to the mortality rate for larger tumors and to the preservation of facial nerve function. The reliability of different diagnostic investigations is documented.

Course of the peripheral gustatory nerves

The multiple variations of the course of the gustatory nerves still considered possible are discussed. Recent investigations lead to the conclusion that there is only one path for the gustatory fibers for each gustatory area: 1) from the anterior part of the tongue via the tympanic cord and facial nerve to the medulla oblongata; 2) for the posterior part of the tongue in the IX cranial nerve; and 3) from the soft palate via the greater superficial petrosal nerve to the facial nerve. The trigeminal nerve carries no gustatory fibers to the brain.

[The corneal reflex stimulated pneumatically (author's transl)]

We describe an apparatus measuring the cornealreflex stimulated pneumatically. The flow of preassurised air in the range of 0.1-1.5 bar is controlled by an electromagnetic switch. The time the valve is open, is variable. The air flow is directed to the cornea of the eye. The blink reflex is recorded by means of the surface electromyogramm and displayed on the screen of a storage scope. The device allows a clean, reproducable and simple measurement of the cornealreflex.

[Evaluating function and disorders of taste]

In a first anatomical section the peripheral gustatory pathways, their central connections, nuclei and cortical projections are discussed. It is evident, that the gustatory fibres from the posterior part of the tongue run in the IX nerve and those from the soft palate reach the medulla oblongata via the petrosal and facial nerve. For the anterior part of the tongue there obviously exists only one gustatory pathway via the chorda tympani-facial nerve. About the further central pathways of taste fibres is much less known. In a second part the methods of taste testing with different taste solutions and the electrogustometry are described. Their practical use and the pitfalls of testing are considered. The disorders of the taste sense compose a third part. Genetic and endocrine abnormalities as well as the side effects of drugs and radiotherapy and the destruction of taste nerves may lead to gustatory deviations. The possibility of central gustatory disorders, especially the combined loss of taste and smell as a result of trauma are mentioned. A chapter dealing with the therapy of taste disorders and a short outlook on the genetic aspects of this oral sense complete this review.

Normal limit of the electrogustometry test

Electrical taste thresholds from the right and left anterior parts of the tongue were obtained from 249 normal subjects. Absolute differences between right and left became more variable as the threshold level increased, whereas the variability of relative differences remained more or less constant. The relative difference between right and left electrical taste thresholds was therefore chosen as the measurement of choice for clinical use. It was also found that smoking significantly increased electrogustometry (EGM) test variability, more so in men than in women. Therefore, separate EGM normal limits were defined as follows: nonsmokers, 18%; female smokers, 24%; male smokers, 37%. If the side of the lesion can be predicted, normal limits may be reduced to 15%, 20%, and 32%, respectively. An incidental finding of this study was a correlation between sudden deafness and other “idiopathic,” possibly retrocochlear, lesions and chorda tympani nerve deficits.