Posterior petrous meningiomas: surgical classification and postoperative outcomes in a case series of 130 patients operated via the retrosigmoid approach

Objective: There is a lack of standardized definition and classification of primary posterior petrous meningiomas (PPM), with consequent challenges in comparing different case series. The aim of our study is to provide an anatomical description and classification of PPM analysing a homogenous series of patients operated via retrosigmoid approach (RSA). Methods: PPM originate laterally to the petro-occipital fissure within the 'venous ring' composed of superior petrosal, sigmoid, inferior petrosal and cavernous sinuses. We proposed a classification based on tumour site of origin, direction of growth relative to the internal acoustic meatus and cranial nerves' displacement. Four types of PPM were defined: retro-meatal (type A), meatal (type B), pre-meatal (type C) and broad-based (type D). One hundred and thirty consecutive patients with PPM who underwent RSA as first-line treatment were retrospectively analysed. Results: The PPM classification predicted clinical presentation, postoperative morbidity and resection rates. Headache, hydrocephalous and cerebellar deficits were more common in type A (59.0%-37.7%-49.2%) and D PPM (66.7%-66.7%-33.3%), hypo/anacusia (87.5%) in type B and trigeminal hypo/anaesthesia (85.0%) in type C PPM. After surgery, patients with type A and D PPM had higher risks to develop cerebellar deficits (11.5%-22.2%), whereas patients with type B and C PPM presented hypo/anacusia (12.5%) and trigeminal deficits (10.0%), respectively. The near-total resection rate was higher in type A (91.8%), followed by types B (82.5%), C (80.0%) and D (77.8%) PPM. Conclusions: The PPM surgical classification has an operative and prognostic relevance. In expert hands, the RSA represents a safe and effective approach to remove PPM.