gms | German Medical Science

Objective: The edges of the transverse and the sigmoid sinus mark the border for the most common approach to the posterior cranial fossa and the cerebello-pontine angle, the retrosigmoid approach. Injury to these anatomic structures bears a potentially lethal risk. The asterion, i.e. the junction of the lambdoid, temporooccipital, and occipitomastoid bone sutures is not a safe outer landmark for the transition of the transverse to the sigmoid sinus inside the skull. The image guidance based on pre-operative magnetic resonance (MR) and computed tomography (CT) scans is superior to other methods in locating these landmarks before the craniotomy. The dural coverings of the sinuses may obscure these venous structures. The margins of error of current image guidance systems in daily routine demand for real-time guiding methods to enhance precision. We have investigated the applicability of a micro-Doppler ultrasound probe for this purpose.

Methods: In a series of 25 patients undergoing a surgery for lesions in the cerebello-pontine angle we have dispatched both image guidance and a micro-Doppler ultrasound probe (16Mhz, Multi-Dop pro®, Compumedics, Germany) to the task of locating the transverse and sigmoid sinus. The mean maximum error of the image guidance system (Kolibri®, Brainlab, Germany) was calculated from the measured distance between the location of the sinuses as indicated given by the navigation system and the real location as indicated by the Doppler probe. The depth of the ultrasound probe was set at 2mm. The shape and the cross-section area of the sinus were determined by ultrasound measurements. Visual detectability of the sinuses under the operating microscope was also noted. Finally, the time required to prepare and apply image guidance and ultrasound Doppler was measured for this specific procedure.

Results: Accidental incision of the transverse or sigmoid sinuses did not occur in any case when the two localizing methods have been used in combination. Image guidance was off-target by a mean of 2.64 mm (range 0-6mm, SD 1.55 mm, D in Fig. 1). The exact location of the transverse sinus was invisible in 7 cases, while the sigmoid sinus was visually undetectable in one case. The shape of the cross section of the sinuses resembled a flat triangle in 23 of the cases. The ultrasound Doppler indicated blood flow outside the visible borders of the sinuses in 5 cases.

Conclusion: A combination of MR- or CT-based image-guidance and micro-Doppler allows for precise location of the transverse and sigmoid sinus in retrosigmoid approaches. The method prevents injury to these important venous structures by restricting the craniotomy strictly to the very edges of the sinuses and by indicating blood flow outside the borders of the sinuses as detected by preoperative imaging methods before the dural incision is performed.