Parietal Craniotomy Free
General Considerations and Operative Anatomy
Parietal craniotomy is designed to provide an operative exposure of the mid to posterior hemisphere while sparing the highly functional anteriorly located sensorimotor cortices and the posteriorly located visual cortex. The approach can be devised to lateral and mesial parietal lobe lesions as well as to interhemispheric median or paramedian lesions.
The variations of this corridor allow access to lesions through the transcortical route (through the more functionally “silent” superior parietal lobule) or the interhemispheric fissure. The parasagittal veins are often less numerous in the posterior parietal region, therefore providing an opportunity to reach deep lesions through the interhemispheric trajectory.
The right or nondominant parietal lobe (see Wikipedia) is implicated in spatial awareness and navigation. Operative interventions that place the entire right lobe at risk are associated with hemibody neglect. This neglect does significantly improve over time, but some residual disability persists.
The left or dominant parietal lobe (see Wikipedia) is involved in symbolic functions in language and mathematics. Damage to the left lobe results in problems with mathematics, long reading, writing, and understanding symbols. Gerstmann's syndrome is associated with lesions in the dominant inferior parietal lobe, whereas Balint's syndrome (simultanagnosia, oculomotor apraxia, optic ataxia) is associated with bilateral lesions.
The posterior parietal cortex can be subdivided into the superior parietal lobule (Brodmann areas 5 + 7) and the inferior parietal lobule (39 + 40), separated by the intraparietal sulcus.
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Parietal lobe veins are classified according to surface of drainage (medial or lateral group) and to the direction of drainage (ascending group: drain to the superior sagittal sinus or descending group: drain to the inferior sagittal sinus or the sylvian fissure). On the lateral surface of the lobe, the ascending veins are the central, postcentral, anterior and posterior parietal veins, while parietosylvian veins form the descending group. On the medial surface of the lobe, the ascending veins are the paracentral, anteromedial and posteriomedial parietal veins. Finally, the descending group is formed by the posterior pericallosal veins.
The lateral group also includes the vein of Trolard, also known as superior anastomotic vein, which crosses the frontal and parietal lobes on its way from the sylvian fissure to the superior sagittal sinus. The most common location of the vein of Trolard is the postcentral region, but it can also be found at the central or precentral region. The cortical veins drain directly to the superior sagittal sinus or may join a parasagittal meningeal sinus or lacunae in the dura, which is the drainage channel of meningeal veins commonly located at the parietal and posterior frontal areas.
Indications for the Approach
The parietal craniotomy is used for both intra- and extra-axial lesions of the region, including neoplasms such as metastases, gliomas, and meningiomas, and vascular lesions such as arteriovenous malformations and cavernous malformations. The parietal interhemispheric corridor is used to approach parafalcine, medial parietal, and splenial lesions.
The parietal craniotomy is most often performed for convexity, falcine, and parafalcine meningiomas. In these cases, the patent superior sagittal sinus and the associated draining veins are at risk and should be spared to avoid disabling venous infarcts. Any dissection around the tumor capsule should protect en passage veins and arteries. As discussed above, vascular injuries in the parietal lobe can cause deficits in spatial awareness, sensorimotor function, and visual processing, and also risk injury to the nearby motor cortex and deep white matter tracts.
Parietal craniotomy can also be used to approach paramedian (periatrial) lesions of the atrium of the lateral ventricle. The traditional approach to the atrium involves a transcortical route through the superior parietal lobule with a risk of deficits in spatial awareness such as astereognosia and speech or visual processing. Recent studies have suggested that, depending on the patient’s occupation and activities, quality of life may indeed be significantly impacted by such deficits.
To avoid these risks, lesions of the atrium can be approached through a paramedian posterior parietal craniotomy and contralateral interhemispheric transfalcine approach through the precuneus. This approach provides a longer and more technically challenging path to the atrium, but involves less white matter tract disruption and brain retraction.
Cortical stimulation mapping under “awake,” “sleep” conditions or phase reversal mapping may be considered for localizing the sensorimotor cortex for intraparenchymal lesions situated along the anterior parietal area. Since early access to the basal cisterns is not available during parietal craniotomies, I have a low threshold for placing a lumbar drain, even for large lesions with significant mass effect. To avoid transtentorial herniation in the case of massive lesions with midline shift, I open the drain to remove cerebrospinal fluid (CSF) during dural opening. This CSF drainage significantly assists with brain relaxation and manipulation of edematous brain.
If the interhemispheric corridor is considered and large parasagittal veins are suspected on preoperative contrast-enhanced magnetic resonance (MR) imaging, an MR or CT venogram guides the location of craniotomy. The venogram will also confirm the patency of the superior sagittal sinus in the presence of an infiltrating meningioma. If numerous parasagittal veins prohibit the ipsilateral interhemispheric corridor, the contralateral interhemispheric transfalcine route may be considered for parafalcine lesions.
If the tumor partially infiltrates the lumen of the venous sinus and the risk of air embolism is significant, a preoperative cardiac diagnostic workup is necessary to exclude the risk of a paradoxical air embolism. A transesophageal echocardiogram and transthoracic Doppler may be used and there should be a low threshold of suspicion for air embolism during the procedure.
The degrees of the patient’s head turn and tilt are dependent on the exact location of the lesion with respect to the midline, coronal, and lambdoid sutures. For parafalcine parietal lesions, the side of the lesion can be placed in the dependent position to use gravity retraction while tilting the head away from the floor to permit a more ergonomic sitting position for the operator during microsurgery. Similarly, when approaching the atrium or periatrial region through the contralateral transfalcine route, I prefer to place the patient in a three-quarters prone position and the normal hemisphere on the dependent side.
An axillary roll supports the contralateral axilla. The ipsilateral shoulder is gently pulled anteriorly and inferiorly and secured with tape to keep it out of the operator’s working zone. For convexity lesions, it is advantageous to tilt the patient’s head enough to place the lesion at the highest point in the operative field.
Upon elevation of the bone flap, mild to moderate bleeding from the sinus wall may be controlled with thrombin-soaked gelfoam or SURGICEL Fibrillar (Somerville, NJ). The latter is left in place untouched during closure. Please refer to the chapter on the Repair of Dural Venous Sinus Injury in the Principles of Cranial Surgery Volume for further details regarding managing injuries to the sinus.
Once the pathology is handled, hemostasis is achieved and the surgeon’s attention turns to closure. If the ventricle is entered, a ventricular drainage catheter may be placed to clear debris within the ventricles during the immediate postoperative period.
I do not routinely close the dura in a watertight fashion for supratentorial craniotomies. I avoid allograft dural substitutes for their risk of aseptic inflammation or infection. Dural closure should not “kink” or compromise flow within the parasagittal veins.
Pearls and Pitfalls
- The three-quarters prone position is a reasonable option for parietal lesions as it facilitates access to the ipsilateral lesion and allows gravity retraction to expand the interhemispheric operative corridor.
- Injury to the superior sagittal sinus during a paramedian craniotomy should be prevented at all costs. Keep a low threshold of suspicion for air embolism.
- Parasagittal bridging veins often do not receive the respect they deserve. A venous infarction in this region can be catastrophic.
Contributor: Marcus A. Acioly, MD, PhD
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