Background: Due to the synergic role of the facial nerve and the nervus intermedius in the mechanical protection of the eye and taste, vestibular schwannomas and/or their treatment may prove to be dangerous for the visual function and taste. Our goal was to evaluate and compare the impact of the tumor itself and the impact of microsurgery (MS) or Gamma Knife radiosurgery (GKS).
Materials and methods: A functional questionnaire evaluating, among other items, patient complaints related to the eye and taste has been given out to a series of 200 patients 3 years after the GKS of a unilateral vestibular schwannoma not previously resected. Their answers were compared with those of a group of 200 patients operated on microsurgically. A Schirmer test was additionally performed before radiosurgery (RS) and more than 2 years after RS in 66 patients.
Results: The risk of dry eye and burning eye is much higher in patients operated by MS compared to patients operated by GKS due to the high incidence of facial palsy (FP) in the former (57/99) and its absence in the later (0/80). In the population operated on microsurgically, the presence of a permanent FP (57 patients among 99 responding to the questionnaire) was, of course, associated with a high rate of complaint, with burning eye in 27 and crying eye in 39. In patients from the two arms with no FP, a dry eye was reported in 8/64 after GKS and 7/42 after MS (not significant) and a burning eye in 9/64 after GKS and 9/42 after MS (not significant). Thus, 14% of patients with no clinical signs of impairment of the VIIth motor nerve presented signs indicating the injury of the intermedius nerve, with the same probability whatever the kind of surgery. When no permanent FP was observed, a crocodile tear syndrome was more frequently observed after MS (4/42 versus 1/64; p = 0.07). This suggests an early lesion of the VIIth motor nerve and nervus intermedius and a subsequent abnormal regrowth. The only patient reporting a crocodile tear syndrome after GKS turned out to have a transiently presented mild deficit of the orbicular muscle signing a transient partial facial nerve injury. In the absence of FP, a 'crying eye' was reported more frequently after MS (16/42 vs. 9/64; p = 0.01) leading us to suspect a frequent subclinical injury of the VIIth nerve in those patients operated on using MS with no obvious FP. Patients tested with the Schirmer test before and more than 2 years later were improved in 27.3%, stable in 56.1% and worse in 16.7% of cases. The answers about taste showed that 8.1% of patients after GKS and 45.5% of patients after MS complained of taste.
Conclusions: This study is the first demonstrating that RS can induce nervus intermedius injury in a small percentage of cases (14%). These patients have been treated 11 years ago with what we can consider as 'archeo-GKS technology' compared to today's radiosurgical instruments. Influence of modern GKS on the nervus intermedius is currently under evaluation in our group. However, symptoms related to the eye and taste either due to the injury of the nervus intermedius or the VIIth motor nerve or both are much more frequent after MS than after RS.