After several refinements and modifications through different dedicated neurosurgeons (Fish, House and Seiffert , Dandy's suboccipital approach (1917) with an ipsilateral suboccipital flap evolved to the retrosigmoid transmeatal approach.
Although a number of scalp incisions and flaps for the retromastoid approach have been previously described and most commonly employed, these incisions and flaps each have their own advantages and limitations 1)
The linear incision and its variations have been the most commonly employed methods due to their simplicity and versatility. The linear incision may be simply extended superiorly or inferiorly, if necessary, to tailor suboccipital bony removal and expose the junction of the transverse sinus and sigmoid dural sinuses.
This incision is usually placed just medial to the mastoid groove.
Although easy to execute, a linear retromastoid approach inherently limits the surgical area. The resultant “bunching” of the skin and deep muscles, upon placement of the retractor parallel to the line of the incision, increases the working distance between the surgeon and the cerebellopontine angle
The incision is usually 8 to 10 cm long and is made one fingerbreadth medial to the mastoid process and digastric groove.
The linear incision requires dissection of the posterior neck muscles, including the sternoclidomastoid muscle, and this muscle dissection, often leads to an increased postoperative discomfort and long-term suboccipital headaches due to scarring and muscle fibrosis. The occipital artery and occipital nerves (neurovascular bundle) may be easily sacrificed in this incision technique, further increasing the risk of postoperative painful neuroma formation.
The incision extends from a line just above the top of the pinna of the ear to a point just below the mastoid tip. Care in splitting the muscle at the inferior end is important in avoiding an ectactic vertebral artery. For very large lesions, the superior end of the incision can be extended medially to a point 2 cm above the inion and curved toward the midline (inverted hockey stick)
Once the linear retromastoid incision is cleared down to the bone, it can usually be held open by one or two curved cerebellar retractors.
This incision has become popular among skull-base surgeons, partly due to its associated multilayer closure that may lead to a potential decreased risk of incisional postoperative cerebrospinal fluid (CSF) leak. The “C-shaped” skin incision is based over the pinna, and the skin flap and muscle flap are dissected in separate layers and reflected laterally and inferiorly, respectively. The advantages of this incision include the fact that the myocutaneous flap will not interfere with the working space of the surgeon as it does in the case of the linear incision. The disadvantages include a more elaborate dissection required through the subcutaneous tissues and muscle layers. The lower part of the incision places the neurovascular bundle at a significant risk. The two-layered closure is more time consuming as well.
A curvilinear retromastoid incision provides an adequate exposure of the targeted surgical area, and its base is along the suboccipital muscles as well as the neurovascular bundle and allows reflection of a single myocutaneous flap inferiorly.
The myocutaneous flap does not interfere with the working zone of the surgeon. By reflecting the flap inferiorly and not bundling it laterally (as in the case of the linear incision), the surgeon allows a closer working distance to the target in the cerebellopontine angle.
This incision was initially described by Dandy 2) and has been slightly modified based on the description below. Dr. John Tew (Mayfield Clinic).
It is necessary to identify the mastoid eminence, mastoid groove, inion and the root of zygoma. A line connecting the inion to the root of the zygoma should be drawn to approximate the transverse sinus. A vertical line is also drawn over the mastoid groove to intersect the first line. We have previously demonstrated that this point of intersection approximates the junction of transverse and sigmoid dural sinuses 3).
The incision techniques described above can be reliably compared only in a randomized controlled trial requiring a large number of patients to provide enough statistical power to allow for a meaningful comparison across the three groups. Such a trial is unlikely to be performed; nonetheless, the retrospective nature of this analysis is a major limitation.
A combination of image-guidance and micro-Doppler enhances the accuracy in localizing the margins of the transverse and sigmoid sinuses in the retrosigmoid approach, thus preventing inadvertent injury. The method could potentially be applied during other craniotomies involving the exposure of a venous sinus 4).
The cause of postoperative headaches after retromastoid surgery, especially for patients who undergo resection of their vestibular schwannoma through this approach, is unknown, but scarring between the muscle and dura is implicated 5)
Hypoglossal schwannoma 6)
Ten patients with primary trigeminal neuralgia and 20 patients with hemifacial spasm (test group) were treated using retrosigmoid approach (RA) for microvascular decompression. Preoperative head magnetic resonance angiography and temporal bone computed tomography were performed and the images registered using SPM12 and fused with MRIcron to determine the relationship between mastoid air cells (MACs) and sigmoid sinuses. An O-arm was used for navigation, and the transverse sinus-sigmoid sinus was projected under a microscope to guide RA. A control group comprised 139 patients who had the same surgical procedure as the test group but without image processing or intraoperative navigation.
The relationship between mastoid air cells (MACs) and the ipsilateral sigmoid sinus was classified as follows: I, MACs did not exceed the lateral edge of the ipsilateral sigmoid sinus (10/60); II, MACs exceeded the ipsilateral lateral edge of the sigmoid sinus but did not exceed the medial edge (42/60); and III, MACs exceeded the medial edge of the ipsilateral sigmoid sinus (8/60). Test and control groups showed significant differences in the incidences of opening MACs (P = 0.003). There was no cerebrospinal fluid leakage or scalp and intracranial infection at follow-up.
Image processing and intraoperative microscopic navigation can avoid unnecessarily opening MACs and might reduce postoperative cerebrospinal leakage and scalp infection after RA craniotomy 7).
Consecutive 89 patients with untreated unilateral sporadic vestibular schwannoma undergoing tumor resection via a retrosigmoid approach during 2008-2010 were prospectively analysed. Endoscopy-assisted microsurgical (EA-MS) removal was performed in 39 cases (Grade 1: 2, Grade 2: 5, Grade 3: 9, Grade 4: 22, Grade 5: 1) and microsurgical (MS) removal was performed in 50 cases (Grade 1: 1, Grade 2: 3, Grade 3: 9, Grade 4: 34, Grade 5: 3). Minimally invasive approach with craniotomy ≤ 2.5 cm was employed for small tumors (Grade 1 and 2) in the EA-MS group. Endoscopic technique was used for monitoring of neuro-vascular anatomy in CPA, during dissection of the meatal portion of tumors, assessment of radicality and for identification of potential pathways for CSF leak formation. All cases in MS group were deemed as radically removed. In the EA-MS group, residual tumor tissue in the fundus of internal auditory canal not observable with microscope was identified with endoscope in four cases. Such cases were radicalized. Tumor recurrence was not observed during the follow-up in EA-MS group. There is a suspicious intrameatal tumor recurrence on the repeated MRI scan in one patient in the MS group. Neither mortality nor infection was observed. The most common complication was pseudomeningocele (EA-MS 20 cases; MS 23). It was managed with aspiration with or without tissue-gluing in all cases without the need for any surgical revision. Adjunctive use of endoscope in the EA-MS group identified potential pathways for CSF leak formation, which was not observable with the microscope in five patients. Improved cochlear nerve (EA-MS: 22, MS: 14; p = 0.012), brainstem auditory evoked potentials (EA-MS: 3 of 8, MS: 0 of 4) and hearing (EA-MS: 14 of 36, MS: 4 of 45; p = 0.001) preservation were observed in EA-MS group. Despite the trend for better useful hearing (Gardner-Robertson class 1 and 2) preservation (EA-MS: 8 of 26, MS: 1 of 16) there were no significant differences in the postoperative hearing handicap inventory in both groups. There were no differences in the postoperative tinnitus in both groups. Better facial nerve preservation (EA-MS: 39, MS: 44; p = 0.027) and excellent-very good (House-Brackmann 1 or 2) facial nerve function (EA-MS: 31, MS: 29; p = 0.035) were observed in EA-MS group. Postoperative compensation of vestibular lesion, symptoms typical for VS, patients assessed by dizziness handicap inventory, facial disability index were comparable in both studied groups. Adjunctive use of endoscope during the VS surgery due to its magnification and illumination enable superior view in the operative field. It is valuable for assessment of radicality of resection in the region of internal auditory meatus. Improved information about critical structures and tumor itself helps the surgeon to preserve facial nerve and in selected cases also hearing. These techniques can help to decrease incidence of postoperative complications 8).
Functional Surgery of Cerebellopontine Angle by Minimally Invasive Retrosigmoid Approach