Our years of experience in treating skull base lesions have led us to realize that in a significant number of cases, enlarging the skull base approach does not improve the surgical outcome or even increases the risk.
Preface
Since the description of the unilateral approach to the CPA in 1903, et al. have developed several techniques to expose the CPA structures. the CPA procedure consists of three approaches: the transtemporal epidural approach, the transmammillary vagus approach, and the posterior sigmoid inferior lateral occipital sinus approach.
The posterior sigmoid sinus approach provides adequate visualization of the CPA structures and can be expanded downward to a distal lateral transcondylar approach and a distal lateral postcondylar approach, and upward to a posterior subdural superior auditory approach (RISA) to the sigmoid sinus. Expanding the skull base approach has the potential to increase the risk of surgical disability and postoperative neurological deficits. Therefore, a simple posterior sigmoid sinus approach is increasingly preferred.
”Major, partial, or total lithotomy should be applied only if it does improve prognosis by facilitating tumor resection, increasing the rate of total resection, improving survival, and decreasing complications.” This is Sammi’s philosophy that has been implemented in daily practice.
Here, we will discuss the technique of posterior sigmoid sinus approach to reach the middle and posterior cranial fossa and its application.
Preoperative evaluation
In patients proposed for CPA, both cranial CT and MRI are performed. for microvascular decompression, radiological examination is required to exclude secondary causes of neurovascular injury such as tumors or arachnoid cysts. mri also evaluates multiple sclerosis as well as the involved vessels.
For hemi-sitting procedures, preoperative evaluation should include a cervical spine x-ray to rule out cervical instability and a thoracic echocardiogram to clarify foramen ovale non-occlusion. In patients with neurofibromatosis type 2, cervical MRI is required for the presence of other central nervous system tumors in combination.
Pure-tone audiograms, speech discrimination tests, somatosensory evoked potentials, and brainstem auditory evoked potentials are used for lesions near the facial and vestibulocochlear nerves. These patients were classified according to Hannover hearing and brainstem auditory evoked potential classification criteria.
High-resolution thin-section CT of the rock bones (1 mm) provides the location of the vagus and endolymphatic vessels as well as the height of the jugular venous bulb. This is essential information for the extent of osteotomy, especially for auditory neuroma surgery or for those who choose the RISA approach. The size of the guiding vessels should also be considered during surgical planning. For auditory neuromas, T2-phase MRI is useful to see if the tumor reaches the end of the internal auditory canal, because of the potential impact on hearing from extensive grinding of the internal auditory canal.
Cerebral angiography is sometimes necessary for meningiomas. Sometimes, embolization of the blood vessels supplying the tumor helps to reduce intraoperative bleeding and postoperative complications.
Posterior sigmoid sinus approach to reveal the posterior cranial fossa
Surgical technique
The procedure is performed under general anesthesia, and the patient may be positioned in the supine, park bench, or semisitting position. The authors prefer the semisitting position. However, in patients with microvascular decompression and unclosed foramen ovale, the supine position is preferable. Intraoperatively, electrophysiological monitoring of bilateral somatosensory evoked potentials, unilateral facets, and auditory nerve should be given for upper CPA lesions; posterior group cranial nerve electrophysiological monitoring should also be provided for large tumors or low CPA lesions.
The head is fixed with a Mayfield head frame with a single nail located near the temporal line on the side of the lesion and anterior to the external auditory canal; the double nail is located near the contralateral temporal line. The patient was placed in a semi-sitting position with the head hyperextended and turned 30° toward the affected side in flexion. The legs are elevated at or above the level of the heart and the knees are slightly flexed so that the venous pressure can be elevated and the risk of air embolism reduced. Somatosensory evoked potentials are recorded continuously on both sides during position placement. If there is a change in latency or amplitude, the operator should be alert and adjust the patient’s position. The precordial Doppler allows monitoring of air emboli. More recently, air emboli can be monitored by transesophageal cardiac ultrasound.
The hair is shaved to facilitate identification of anatomic landmarks. A slightly curved incision line is drawn with a marker pen, with the upper end 2 cm posterior to the auricle, passing through the star point and terminating 1-2 cm medial to the mastoid tip. the lower border of the incision depends on the need for caudal exposure. No local anesthesia is required. The flap is lifted with the periosteum and retracted with a skin retractor. The neck muscles are incised along the incision line and retracted with the Leyla automatic retractor.
The bone hole is located below the star point. The suboccipital opening is completed with biting forceps towards the transverse and sigmoid sinuses and the occipital scales. The star point and the posterior part of the parietal mammary suture reveal the projection of the transverse-sigmoid sinus turning point and therefore constitute the outer superior border of the craniotomy range.
Imaging virtual techniques can be used to clarify the relationship between venous sinus location and bony landmarks. A suboccipital craniotomy range of 3-4 cm in diameter is sufficient to reveal the CPA.
High-speed diamond drilling is used to reveal the transverse sinus, the transverse sinus-sigmoid sinus turning point, and the edge of the sigmoid sinus. Continuous flushing to cool down during the application of the grinding drill. Bone wax is used to fill the mastoid airspace and guiding veins. Particular attention should be paid to the possibility of air embolism facilitated by a tear of the guiding vein in patients in a semisitting position. If preoperative thin-section CT of the temporal bone reveals a thick guiding vein, we recommend dissecting out the guiding vein with a fine diamond drill and electrocoagulating it under direct vision. For microvascular decompression, the craniotomy can be reduced to reveal the upper or lower CPA, as appropriate.
An arm rest is fixed to the operating table to ensure stability and accuracy of the operator’s movements. The dura is cut microscopically in a C-shape with the base along the transverse and sigmoid sinuses. The incision should be several millimeters from the venous sinus to facilitate closure. An auxiliary oblique incision at the corner of the incision can increase exposure but is not routine. Draping 3-4 stitches further expands the exposure to reduce the strain on the cerebellum. Separate superior and inferior incisions are chosen depending on whether the lesion is located superior or inferior to the CPA.
The cerebral pool was opened with a narrow cerebral pressure plate and pointed forceps, and the cerebellum was protected with a tampon. As a result, cerebrospinal fluid is released and the cerebellum is relaxed.
Further relaxation of the cerebral tissue can be done by taking a sedative drop of mannitol while cutting the skin. Afterwards, the cerebral pressure plate can be used for protection without applying pressure to the cerebellum. Oblique and smooth application of the cerebral compression plate is preferable to longitudinal pulling of the cerebellum through the facial auditory nerve bundle, which may lead to vestibulocochlear nerve injury. In addition, in auditory neuromas and tumors in the upper CPA, smooth application of the cerebral pressure plate may prevent the occurrence of trigeminal heart reflexes. Gentle lifting of the cerebellar vermis can reveal the lateral foramen of the fourth ventricle and the lateral crypt. The next step in the procedure will depend on the lesion.
The advantage of the semisitting position is the automatic outflow of blood and cerebrospinal fluid, which is aspirated by saline irrigation, providing a clear operative field and reducing dissection time. Finally, the anesthesiologist needs to be allowed to compress the jugular vein and check for venous bleeding, especially in the semisitting position. Tight hemostasis.
The dural flap needs to be sutured continuously with a single, non-absorbable thread to achieve a water seal. Patching is rarely required for C-shaped incisions. Cranioplasty is performed with methyl methacrylate. The mastoid airspace is closed with muscle and fibrin glue. The neck muscles are sutured at anatomical levels. No drainage is left in place. Compression bandages are applied.