Physiological Aspects of Vestibular Schwannoma Treated with Radiosurgery

Vestibular schwannomas have been considered to be a one of the best indication for radiosurgery. In reality, tumor control after gamma knife treatment are reportedly very excellent. In spite of excellent tumor control, functional outcomes like hearing ability are not so great. Hearing preservation have been achieved mostly for the cases within normal range at the first radiosurgery. Persistent disequilibrium is the another problem for the patients before and during follow-up period.

Among 50 cases treated in our institute since July 2016, the most popular symptoms at the radiosurgery are hearing disturbances including hearing loss, tinnitus or sudden deafness (Figure 1). In addition, dizziness, vertigo, ataxia and floating sensation are known to be next familiar symptoms of this disease. In addition to the radiological studies with CT or MRI, the functional studies after radiosurgery, like pure-tone hearing study (G-R), auditory brainstem response (ABR), vestibular evoked myogenic potential (VEMP), stabilometry are routinely performed. After the initial study at the radiosurgery, the subsequent examinations are carried out periodically with every 3 to 6 month intervals.

In first 50 cases treated in our hospital, hearing disturbance with G-R study is most remarkable. In the studies with ABR, low amplitude of the affected side is prominent in many cases and delayed propagation time from I to V waves are often noted. The specific changes of VEMP are available in many cases, chiefly in terms of delayed latency of P13, and the decreased amplitude, which is estimated by the hight from the bottom of P13 to the top of N23 as shown in (Figure 2). Subsequent changes of latency of P13 as well as the amplitude are marked in many cases. These changes are often moving parallel to the recovering clinical symptoms like dizziness, floating sensation and ataxia, as shown one example in (Figure 3) (Table 1).

Table 1: Summary of the finding of physiological studies. Pure tone hearing and ABR were not significantly improved. Conversely, VEMP study demonstrated a remarkable recovery of p13 latency.

The most popular symptoms of vestibular schwannoma are the hearing impairment and related signs. However, many patients are often suffering from disequilibrium before and after the treatment [1-4]. Functional studies with hearing ability, ABR and VEMP, Stabilometry may indicate the serial changes of neurophysiological function [5]. Special importance is the findings with VEMP study, indicating the effects of radiosurgery better than those after microsurgery [6, 7]. Since the P13 is readily and constantly captured, these objective changes can be detected quantitatively after radiosurgery, which may be more accurate than the subjective evaluation with Dozziness Handicap Inventry (DHI) [8]. In conclusion, neurophysiological studies show us various functional informations after radiosurgery. Among them, VEMP study demonstrates the changing and improving function of disequilibrium clearly.

None.

No conflict of interest.

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