Cushing (1917) on the other hand described a bilateral suboccipital access and stated the unilateral suboccipital approach as disadvantageous.
Altough the lateral suboccipital approach associated to resection of posterior arch of C1 is considered feasible, definitely, it is not the best way to achieve an adequate exposure of vertebral artery, anterior and lateral medulla regions.
In order to perform a perfect exposure with minimal retraction of neural structures there have been purposed, adding to the lateral suboccipital approach, a partial mastoidectomy with lateral exposure of sigmoid sinus as well as the resection of the posterior arch of atlas and exposition of its transverse foramen beyond the neural and vascular structures inside the sub occipital triangle 1).
Indications for this approach are variable such as resection of meningiomas, acoustic neuromas and epidermoid tumors, treatment of vascular lesions of vertebrobasilar system, vascular decompression of cranial nerves (V, VII, IX, X), cranial nerve neurectomies, and intrinsic lesions of the cerebellum and brainstem.
Microvascular decompression (MVD) via lateral suboccipital craniotomy is the standard surgical intervention for trigeminal neuralgia (TN). For recurrent TN, difficulties are sometimes encountered when performing reoperation via the same approach because of adhesions and prosthetic materials used in the previous surgery.
Normally in a semisitting position for the suboccipital lateral route.
The operation can also be carried out with the patient in a lateral position at the surgeon's discretion. After local shaving of the hair behind the pinna and disinfection of the surgical field.
The nuchal muscles are split with the monopolar knife.
In this approach, it is important to expose the margin of the transverse and sigmoid sinus (T/s) safely and quickly. adequate exposure of these margins by placing a keyhole on the junction of the T/s junction minimizes bone loss. it is considered ideal if half of the T/s junction can be seen in the keyhole and many methods have been described for preoperative identification of the positions of the transverse sinus, T/s junction, and the sigmoid sinus based on external skull bony landmarks 3) 4) 5) 6) 7) 8).
Historically, the asterion has been viewed as a landmark for the T/S junction 9) 10). However, the asterion is now seen as unreliable for this purpose because of difficulties with observation, palpation, and anatomical variety.
Tubbs et al. suggested that the mastoid process and the zygomatic arch were more reliable landmarks for prospective identification of the T/s junction 11).
Methods for distinguishing the T/s junction from other superficial structures, including the semispinalis capitis muscle, superior nuchal line, inion, and mastoid process, have also been described.
Osteoplastic craniotomy in retrosigmoid approaches to the cerebellopontine angle region has been suggested as an alternative to traditional osteoclastic craniectomy. It is important both for prevention of postoperative complications and for cosmetic purposes. It provides acceptable results both clinically and radiologically. More comparative studies are required to evaluate possible advantages of this technique over osteoclastic craniotomy 12).
The suprajugular extension of the retrosigmoid approach will permit removal of tumors located predominantly in the cerebellopontine angle but also extending into the upper part of the jugular foramen without any additional skull base approaches 13).
The dura is opened under the microscope in semilunar fashion parallel to the course of the sigmoid sinus.
The internal auditory canal IAC can be open with a high-speed diamond drill from lateral to medial, opening the canal for 180° of its circumference 14).
Look for the safe zone of posterior semicircular canal resection in suboccipital retrosigmoid sinus approach for acoustic neuroma surgery 15).
The intrameatal part of the vestibular schwannoma is partially removed and the facial nerve identified. Thereafter, opening of the capsule and debulking of the tumor with an ultrasonic surgical aspirator in the cerebellopontine angle CPA. Once the tumor's mass is significantly reduced, a bimanual dissection of the cleavage plane between capsule and the surrounding arachnoid is performed. Starting from below, the capsule is elevated with a tumor grasping forceps and the arachnoid membrane is peeled off. Following the cleavage plane, the facial nerve is separated in a medial to lateral direction from the VS's capsule. Throughout the whole procedure the field is irrigated with warm Ringer's solution. Its important to seal the drilled posterior lip of the IAC as well as eventually opened mastoid air cells with a free muscle or fat patch 16).
Upon dural opening, the endoscope is inserted into the operative field along the petrotentorial junction. Cerebrospinal fluid drainage provides a wider space for introduction of the endoscope and surgical instruments by opening the cerebellomedullary cistern for cerebellar relaxation.
Traditional microsurgical techniques are used during the entire procedure.
A fully endoscopic retrosigmoid approach is a safe and effective procedure for cerebellopontine angle tumor resection and cranial nerve microvascular decompression 17).
The retrosigmoid intradural suprameatal approach includes the standard retrosigmoid suboccipital route with drilling of the petrous bone above and anterior of the internal auditory meatus, allowing for exposure of the trigeminal nerve within the Meckel's cave and of the middle fossa to approach the posterior cavernous sinus for microsurgery of, e.g., petroclival meningiomas and trigeminal schwannomas.
The extended retrosigmoid intradural suprameatal approach allows exposure of the posterior cavernous sinus and may be used to remove lesions of the posterior fossa extending into the Meckel's cave and into the cavernous sinus 18).
A slightly curved surgical incision is planned two fingers behind the ear extending from the level of the tip of the ear to 1 cm below the mastoid tip.
Asterion identification and burr hole placement
Transverse sinus identification
The retrosigmoid craniotomy in two steps
Video 1:40s Sigmoid sinus unroofing exposes the sinus knee, the inferior border of the transverse sinus, the medial border of the sigmoid sinus and horizontal segment of the occipital squama.
A stepwise illustration of the retrosigmoid approach for resection of a cerebellopontine meningioma
A slightly curved surgical incision is planned two fingers behind the ear extending from the level of the tip of the ear to 1 cm below the mastoid tip. The retrosigmoid craniectomy exposes the sinus knee, the inferior border of the transverse sinus, the medial border of the sigmoid sinus and horizontal segment of the occipital squama. The dura is opened under the microscope in semilunar fashion parallel to the course of the sigmoid sinus. We open the IAC with a high-speed diamond drill from lateral to medial, opening the canal for 180° of its circumference. The intrameatal part of the vestibular schwannoma is partially removed and the facial nerve identified Thereafter, we open the capsule and debulk the tumor with an ultrasonic surgical aspirator in the CPA. Once the tumor's mass is significantly reduced, a bimanual dissection of the cleavage plane between capsule and the surrounding arachnoid is performed. Starting from below, the capsule is elevated with a tumor grasping forceps and the arachnoid membrane is peeled off. Following the cleavage plane, the facial nerve is separated in a medial to lateral direction from the VS's capsule. Throughout the whole procedure the field is irrigated with warm Ringer's solution. We seal the drilled posterior lip of the IAC as well as eventually opened mastoid air cells with a free muscle or fat patch 20).
The retrosigmoid transmeatal technique remains the approach of choice for hearing preservation during the removal of acoustic neuromas that protrude from the porus acusticus. However, encroachment into the bony labyrinth in an effort to remove the tumor in the lateral end of the internal auditory canal (IAC) continues to compromise hearing in certain cases. The limits in the safe removal of the posterior wall of the IAC are not generally agreed on. To address this problem,
Day et al,performed a morphometric analysis of 32 fixed cadaveric temporal bones by microsurgical dissection and measurement with fine-cut bone window computed tomographic (CT) scans. The morphometric relationships of identifiable surface landmarks were first determined. Fine cut bone window CT scans were next performed on each bone and the distances between the fundus, the vestibule, and the common crus (CC) with the internal auditory meatus (IAM) were determined. Additionally, the thickness of the bone overlying the posterior semicircular canal at the CC was measured. From a retrosigmoid trajectory, employing a 4-cm craniotomy, the posterior wall of the IAC was removed with a high-speed drill, limiting removal to the distance from the vestibule to the IAM, as determined by CT measurement. Preservation of the integrity of deep structures was confirmed by inspection. The length of the actual IAC unroofed was measured and was compared with the IAC length, from IAM to fundus, measured by CT. The average canal length by CT measured 10.0 mm +/- 1.8 (range, 6.6-14.0). The length of the canal uncovered averaged 5.9 mm +/- 1.4 (4.0-8.5).21).