Retrosigmoid Versus Extended Retrosigmoid Approach

Preoperative Considerations

• The standard retrosigmoid approach allows for expeditious access to the posterior fossa, specifically to the cerebellopontine angle (CPA).

• The extended retrosigmoid approach includes the skeletonization of the transverse–sigmoid sinus and an optional partial mastoidectomy to the standard retrosigmoid craniotomy. This provides a wider corridor in between the cerebellum and petrous bone in those patients with tight cerebellopontine and cerebellomedullary cisterns and allows more anterior access to the CPA, pre-pontine cistern and tentorium ( Figure 21.1 ).

  

• Several preoperative tests are important for the optimal planning of the approach:

  • In addition to standard MR imaging, all patients undergo MR venography in order to rule out contralateral sinus occlusion prior to surgical manipulation of the sinus ipsilateral to the approach.

  • A high-resolution CT scan of the petrous bone can provide information regarding the bone pneumatization, and position of the vestibule and cochlea. This can help determine the boundaries of the bone resection, especially in the extended retrosigmoid approach.

  • A preoperative transthoracic echocardiogram is required for the semi-sitting position, to exclude a patent foramen ovale (increased risk of cerebral stroke in case of intraoperative air embolism).

• Intraoperatively, central venous access is obtained and precordial doppler is placed for monitoring due to the potential risk of air embolism during the venous sinus skeletonization in this approach.

• In younger patients with good quality dura, a craniotomy is performed; in older patients a craniectomy is preferred.

• Preoperative audiogram and speech discrimination test, together with intraoperative somatosensory evoked potentials (SSEPs) and brainstem auditory evoked potentials (BAEPs), are considered in patients with pathology in the vicinity of the facial and vestibulocochlear nerves.

Indications

• The traditional retrosigmoid craniotomy is typically used for lesions located in the cerebellopontine angle (CPA) and can provide access in a cranial–caudal direction from the tentorium and trigeminal nerve to the jugular bulb and its associated cranial nerves (IX, X and XI).

• The “extended” version of the retrosigmoid craniotomy includes a standard retrosigmoid approach, but also includes a limited posterior mastoidectomy in order to skeletonize the transverse–sigmoid sinus junction and additional mastoidectomy to expose the jugular bulb (if needed).

• The “extended” retrosigmoid craniotomy provides additional access not granted by a traditional approach, but is not as extensive as the posterior petrosal approaches. The extended approach is used for lesions that extend medially to the petroclival junction or in patients with tight cerebellopontine and cerebellomedullary cisterns (between the lateral surface of the cerebellum and the petrous bone) where, otherwise, extensive brain retraction would be necessary to visualize the lateral structures.

Contraindications

• Patients with a non-patent contralateral transverse–sigmoid sinus (relative).

• The semi-sitting position is contraindicated in patients with a patent foramen ovale.

Surgical Procedure