Parasagittal Craniotomy Free
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The parasagittal craniotomy exposes lesions along the paramedian hemispheres and superior sagittal sinus. This basic cranial exposure provides access to the superior sagittal sinus itself and can be extended across the midline for complete control over the sinus. Because of numerous bridging veins and the inconsistent location and size of parasagittal venous lakes, operating in this region can be difficult. The risks of profuse blood loss and life-threatening venous air embolism are significant and therefore appropriate precautions are necessary.
Contrary to common knowledge, the sagittal suture does not define the exact location of the superior sagittal sinus. If fact, the sinus is typically deviated to the right of the sagittal suture; the maximum deviation is usually no more than 11 mm.
The interhemispheric modification of the parasagittal approach is very flexible and practical in reaching midline deep-seated lesions through exploitation of the interhemispheric fissure. This route will be discussed in its own dedicated chapter: Interhemispheric Craniotomy.
Indications for the Approach
The parasagittal approach is used most often for parasagittal meningiomas, but it can be adapted for metastases, gliomas, arteriovenous malformations, and cavernous malformations. It is typically reserved for lesions that are intimately associated with the superior sagittal sinus and the superior falx.
Neuronavigation guides the extent of the craniotomy. If large bridging veins are suspected on preoperative contrast-enhanced magnetic resonance (MR) imaging, an MR or CT venogram is indicated to plan the location and extent of the craniotomy based on the exact locations of large veins. These imaging modalities will also confirm the patency of the superior sagittal sinus in the presence of infiltrating meningiomas. If numerous parasagittal veins prohibit exploration of the ipsilateral interhemispheric corridor, the contralateral interhemispheric transfalcine route may be considered for parafalcine lesions.
If the tumor has infiltrated the lumen of the sagittal sinus, the risk of air embolism is significant. In this situation, preoperative cardiac diagnostic tests should be performed to exclude the risk of a paradoxical air embolism. Transesophageal and transthoracic Doppler echocardiography should be used during surgery. The operator should have a low threshold for suspicion of an air embolism, especially for the patients in the semi-sitting position. In high-risk cases, a central venous catheter and a precordial Doppler are necessary.
Since the basal cisterns are not readily accessible, large tumors with associated cerebral edema can lead to cerebral herniation during the dural opening. Such tension can lead to cortical injury, and the surgeon may be tempted to use aggressive retraction to expose the tumor. In addition, during the craniotomy, the Cerebrospinal fluid (CSF) drainage via a lumbar drain decompresses the dura away from the bone and protects the venous sinuses during drilling.
I use lumbar CSF drainage to facilitate brain relaxation regardless of the large size of the tumor. For large tumors with midline shift, the CSF is drained while the dura is being opened; this timing of CSF drainage decreases the risk of brain shift. If there is a significant amount of vasogenic edema, mannitol and steroids are useful adjuncts.
An understanding of parasagittal venous anatomy is important for safe execution of the craniotomy.
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A Penfield #3 dissector is used to generously dissect between the inner table of calvarium and the wall of the superior sagittal sinus. If the wall of the sinus is adherent, a third burr hole should be placed; all burr holes should be readily in continuity within the epidural space. Cerebrospinal fluid drainage through the lumbar drain mobilizes the wall of the venous sinus and the dura away from the bone, therefore preventing their injury by the footplate of the drill.
Upon elevation of the bone flap, mild to moderate bleeding from the sinus wall may be controlled with thrombin-soaked gelfoam or SURGICEL Fibrillar (Medline Industries, Somerville, NJ). The latter is left in place untouched during closure. Please refer to the chapter on Repair of Dural Venous Sinus Injury for details related to managing severe injuries to the sinus. Aggressive coagulation of the dura is avoided as this maneuver may increase the tear along or the hole within the sinus wall.
This step has the highest risk of venous air embolism. Unexplained hypotension, decreased end-tidal Pco2, and a dramatically “dry” operative field should increase the operator’s suspicion of this complication.
Paramedian extensions of the superior sagittal sinus or venous lakes are frequently encountered in this region. They may limit dural opening close to the midline and restrict access to the interhemispheric fissure. In this situation and near the midline, the dural incision may be extended parallel to the venous sinus or lake rather than perpendicular to it.
Once the pathology is handled, hemostasis is achieved and attention turns to closure. A watertight dural closure is preferable, but I usually approximate the dura and do not insist on a watertight closure for supratentorial lesions. Tack-up stiches are not used close to the superior sagittal sinus. Hemostasis along the sinus is cardinal for avoidance of compressive postoperative epidural hematoma formation.
Pearls and Pitfalls
- Injury to the superior sagittal sinus during a parasagittal craniotomy should be prevented at all costs. The operator should have a low threshold of suspicion for an air embolism, especially if the sitting or semi-sitting position is used.
- Parasagittal bridging veins often do not receive the respect they deserve. The risk of venous infarct is higher for any injury to the parasagittal veins along the middle and posterior third of the superior sagittal sinus.
Pradilla G, Solero C, DiMeco F. Parasagittal meningiomas, in DeMonte F, McDermott M, Al-Mefty O (eds): Al-Mefty’s Meningiomas, 2nd ed. New York: Thieme Medical Publishers, 2011.
Rhoton AL Jr. The cerebral veins. Neurosurgery. 2002;51(Suppl 1)159-205.
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