Elsevier

World Neurosurgery

Volume 113, May 2018, Pages e88-e92
World Neurosurgery

Original Article
A Comparison of Cerebellar Retraction Pressures in Posterior Fossa Surgery: Extended Retrosigmoid Versus Traditional Retrosigmoid Approach

https://doi.org/10.1016/j.wneu.2018.01.173Get rights and content

Highlights

  • The retrosigmoid approach is a widely used and highly versatile approach.

  • The retrosigmoid approach can be extended by skeletonizing the sigmoid sinus.

  • Retraction pressures were reduced significantly with the extended approach.

  • Patients should be selected carefully to avoid complications.

Background

The retrosigmoid approach is broadly applicable to many posterior fossa procedures. However, cerebellar retraction is often necessary for lesions in the cerebellopontine angle, which can lead to complications. An extended retrosigmoid approach skeletonizes the sigmoid sinus and allows a wider corridor with less retraction. This study investigated the differences in retraction pressure between the retrosigmoid and extended retrosigmoid approach in a cadaveric model.

Methods

Anatomic dissection of 2 cadaveric heads was performed for comparison of surgical approaches. Bilateral measurements were obtained on each head, providing 4 sets of data. Retrosigmoid craniotomy was first performed with recording of retraction pressure necessary for 1.5-cm exposure. The exposure was then expanded to an extended retrosigmoid approach, and retraction pressures were recorded.

Results

Mean retraction pressure in cadaver 1 for retrosigmoid and extended retrosigmoid approaches was 20.25 ± 5.9 mm Hg and 10.25 ± 3.8 mm Hg, respectively; in cadaver 2, values were 11.75 ± 3.1 mm Hg and 4.75 ± 1.8 mm Hg, respectively. This corresponded to a mean relative reduction in retraction pressure of 49.4% in cadaver 1 and 59.6% in cadaver 2 by using the extended retrosigmoid approach. Retraction pressures were also significantly less (P < 0.05) for the extended retrosigmoid group when comparing all surgical approaches (N = 4).

Conclusions

The extended retrosigmoid approach gains better visualization with reduced brain retraction. In our study, cerebellar retraction pressures were greatly reduced when using the extended retrosigmoid approach in a cadaveric model.

Introduction

The field of skull base surgery was developed to minimize brain retraction and increase exposure of the required surgical corridor by selectively removing bone using high-speed drills as surgeons' tools. The standard retrosigmoid approach is a very common and familiar approach used to access a wide variety of pathology in the posterior fossa, including tumors of the cerebellopontine angle, petroclival region, and proximally within the internal auditory meatus. It is a highly versatile approach, with potential exposure to cranial nerves IV–XI depending on the extent of the craniotomy.1 Retrosigmoid craniotomy classically exposes the inferior margin of the transverse sinus, the posterior margin of the sigmoid sinus, and a variable amount of exposure inferiorly toward the foramen magnum depending on the operation being performed. Despite the wide applicability of this approach, occasionally it does not provide sufficient exposure without significant brain retraction. In these instances, options include using skull base approaches through presigmoid or middle fossa corridors or suprameatal extension of the retrosigmoid approach.2, 3, 4, 5, 6

In the traditional retrosigmoid approach, the operative corridor can be widened by further retracting the petrosal surface of the cerebellum with fixed retractors, dynamic retraction,7 or cerebrospinal fluid (CSF) relaxation through arachnoidal dissection or lumbar drainage. CSF relaxation methods provide sufficient relaxation of the cerebellum to avoid retraction in most cases; however, working angles and viewing trajectories may still be suboptimal in select cases. Retraction, particularly with fixed retractors, also has potential deleterious effects if not done with caution.8, 9 The use of brain retractors causes a localized reduction or cessation of perfusion and is possibly a component of direct injury.10 The degree of injury depends on retraction pressure, size of the retractor, number of retractors used, and duration of retraction.8

The use of brain retractors and resultant regional reduction in blood flow with risk of focal cerebral ischemia has been investigated in animal models.10, 11 The pressure threshold in canine models to induce histologic changes after 1 hour of retraction was 20 mm Hg during normotension, but only 10 mm Hg under hypotensive conditions.11 Hongo et al.12 evaluated this from a clinical perspective with pressure-recording retractors in patients with varying pathology and a wide variety of approaches. They showed initial retractor pressures of 26.6 mm Hg, which gradually subsided. Drainage of CSF and use of multiple retractors were successful in reducing retraction pressure.12 Intermittent retraction was also compared with continuous retraction in a canine model showing that intermittent retraction induces less damage to brain tissue.13 This logically has transitioned into dynamic retraction with the use of suction and microinstruments to minimize the risk of injury even further.7

To prevent the complications associated with retraction when a wider exposure is required, the sigmoid sinus can be skeletonized and the dural flap reflected anteriorly to create a direct line of visualization down the petrous bone in an extended retrosigmoid approach.14, 15, 16 Skeletonizing the sigmoid sinus is not without an increased risk of potential complications17, 18, 19, 20, 21, 22; therefore, careful consideration must be given to whether this increased risk is justified to decrease retraction on the cerebellum. Although injuries from excessive retraction and methods to help ameliorate effects of retraction on the brain are documented in the literature,8, 9 there are as yet no investigations comparing retraction pressure for surgical approaches to a specific anatomic location. We compared cerebellar retraction pressures in posterior fossa surgery for the extended retrosigmoid approach versus traditional retrosigmoid approach.

Section snippets

Materials and Methods

Anatomic dissection of 2 fresh, latex-injected cadaveric heads was performed for comparison of the surgical approaches. Bilateral measurements were obtained on each head, providing 4 sets of data. Retrosigmoid craniotomy was first performed on each head,1 and measurements were recorded for retraction pressure, as will be described in more detail subsequently. Following data collection, the craniotomy was extended to skeletonize the sigmoid sinus for the extended retrosigmoid approach.

Results

Cerebellar retraction pressures were greatly reduced to achieve the same 1.5-cm exposure with the extended retrosigmoid approach compared with the traditional retrosigmoid approach. Localized cerebellar parenchymal pressure adjacent to the retractor for the retrosigmoid approach was 14.7–25.3 mm Hg in cadaver 1 and 10–13.3 mm Hg in cadaver 2. For the extended retrosigmoid approach, pressures decreased to 7.3–14 mm Hg in cadaver 1 and 3.7–5.7 mm Hg in cadaver 2. The mean retraction pressure in

Discussion

The principles of skull base surgery entail removing bone to enhance operative exposure and thereby reduce retraction on the brain. This can be accomplished in various ways depending on the location of pathology, including extensive drilling of the sphenoid wing to expose the proximal carotid and optic nerve; removal of the orbital roof to enhance the upward view needed for anterior communicating aneurysms and suprasellar lesions; transpetrosal approaches for access to the brainstem; and, as in

Conclusions

The retrosigmoid approach can be widely used to approach various pathologies in the posterior fossa. The extended retrosigmoid modification is a technique to gain better visualization of the cerebellopontine angle and deep vascular structures with reduced brain retraction,14 providing superior views along the petrous temporal bone. In our study, cerebellar retraction pressures were greatly reduced (49.4%–59.6%) when using the extended retrosigmoid approach. This approach can be tailored to a

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    Conflict of interest statement: The authors declare that the article content was composed in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

    Portions of this work were presented in poster form at the North American Skull Base Society Annual Meeting, February 2016, in Scottsdale, Arizona.

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