Table 3 Primary and secondary outcomes

  Extent of tumour resection

Gross total resection: total resection under the microscope, imaging without residual.

Note: The evaluation of total resection under the microscope and determination of microscopic residual tumours depend largely on subjective observations from experienced neurosurgeons according to the operation video.

Near-total resection: microscopic residual (a few tumour capsules, < 5%), imaging no residual.

Subtotal resection: microscopic residual (small tumour remnant, 5–10%), imaging small tumour remnant residual.

Partial resection: microscopic nodular residual tumour (≥ 10%), imaging nodular residual tumour.

 Facial nerve function

As one of the inclusion criteria, facial nerve function was documented photographically at rest and while performing standardized facial expressions at defined time points (preoperatively, discharge, and 6 and 12 months after surgery).

This feature was evaluated by two experienced neurologists and classified according to the HB grading system [29].

 Hearing function

Hearing level will be analysed and evaluated using pure-tone audiometry (PTA) and speech discrimination score (SDS) according to the guidelines of the American Association of Otolaryngology–Head and Neck Surgery (AAO-HNS) classification [30].

Serviceable hearing was defined as either class A (PTA ≤ 30 dB, SDS ≥ 70%) or class B (30 < PTA ≤ 50 dB, SDS ≥ 50%) and non-serviceable hearing as class C (PTA > 50 dB, SDS ≥ 50%) or class D (any PTA, SDS < 50%).

  Time of operation

  General complications

  Special complications