The Keyhole Retrosigmoid Approach to the Cerebello-Pontine Angle: Indications, Technical Modifications, and Results

ABSTRACT

Objectives: Establish the versatility and usefulness of the keyhole retrosigmoid approach to the cerebello-pontine angle (CPA) in various pathologies. Design: Prospective clinical study. Setting: Tertiary referral university hospital. Methods: One hundred twenty-one patients with various pathologies of the CPA underwent the relevant investigations and were operated upon by the retrosigmoid microendoscopic approach. The technical modifications and progression of our technique are described. Results: This group consisted of 121 patients (103 men and 28 women). The pathologies included were 60 acoustic neuromas, 28 vestibular neurectomies, nine meningiomas, and four arachnoid cysts. For nonmass lesions, no additional facial nerve injury or deterioration of hearing occurred. Total excision of mass lesions was achieved in 94.5% of cases. Facial nerve integrity was preserved in 92.3% of cases with mass lesions and permanent facial paralysis occurred in 8.3%. There were no mortalities, and the most frequent complication was a delayed cerebrospinal fluid leak from the site of the wound (15%), which was managed conservatively in all cases. Conclusions: The keyhole retrosigmoid approach is a versatile one. It can be used to deal with different pathologies through a unified access, and with the increasing exclusive use of endoscopes, a truly minimally invasive surgery can be achieved.

The cerebello-pontine angle (CPA) is the seat of different pathologies. Multiple approaches have been developed to permit the surgeon to tailor the procedure to a particular patient's pathology and physiological status. Each has its own limitations, morbidities, and indications.¹ To minimize the morbidity inherent to more extensive approaches and to try to find a versatile approach suitable for most indications, the minimal retrosigmoid approach was conceived.^2,3 The surgical exposure extends from the trigeminal nerve and tentorium superiorly to the foramen magnum and jugular foramen inferiorly. Many variations of the technique have been described, and many authors advocate it as the procedure of choice during vestibular nerve section, microvascular decompression, surgery for vestibular schwannomas, nonacoustic tumors, or insertion of auditory brain stem implants.^{4,5,6,7,8,9,10} It gives a better view of the CPA and allows early identification of various neurovascular structures, more complete removal of lesions, less morbidity, and better functional results in terms of hearing and facial nerve preservation.^11,12,13,14 The exclusive use of an endoscopic approach for a variety of lesions in the CPA is evolving.^5,8,14,15,16

We present our technical modifications and our results in various CPA lesions illustrating the versatility and usefulness of this approach.

PATIENTS AND METHODS

This series of 121 patients underwent the keyhole retrosigmoid approach to the CPA for various lesions (Table 1).

Table 1.

Types of Lesions and Sex Distribution

Lesion	Number	Males	Females
Acoustic neuromas	60	40	20
Vestibular neurectomy	28	28	0
Vascular loops	20	28	2
Meningiomas	9	4	5
Arachnoid cysts	4	3	1
Total	121	103	28

All patients underwent full audiovestibular assessment, contrast enhanced computed tomography scan of the temporal bone and contrast-enhanced magnetic resonance imaging of the posterior fossa with different protocols depending on the suspected type of lesion.

TECHNIQUE

Under hypotensive general anaesthesia, brain dehydration is started with mannitol with induction. The patient is supine with the head turned to the opposite side and the neck flexed to bring the chin close to the contralateral shoulder. Two lines are marked on the skin: the orbitomeatal line and a tangent along the posterior edge of the mastoid. A 3-cm straight incision is made perpendicular to the orbitomeatal line 0.5 to 1 cm posterior to the intersection of the initial landmarks (Fig. 1). An inferiorly based U-shaped musculoperiostial flap is designed, and a 2-cm² square craniotomy is performed posterior to the sigmoid. Care is taken to bring the craniotomy to the bottom of the posterior fossa and to the edge of the sigmoid. The dura is opened as an anteriorly based U-shaped flap, and the cerebrospinal fluid (CSF) is drained from the cisterna magna by exposing the lower cranial nerves at the anteroinferior angle of the craniotomy. By that time, the cerebellum is well away from the operative field, and the whole CPA is exposed from the tentorium to the lower cranial nerves (Fig. 2). No cerebellar retractors are needed.

Figure 1.

Skin landmarks.

Figure 2.

Overview of the cerebello-pontine angle.

For minimal lesions (vascular loops, vestibular neurectomy, and small neuromas), an exclusive endoscopic technique is used (Figs. 3, and 4). For larger lesions, a combined microscopic and endoscopic approach was initially used. In the last few cases, even large lesions were managed by a purely endoscopic approach. A “three-hand” technique is used rather than an endoscope holder to allow easier manipulations with more flexibility and mobility. For these large lesions, after maximal cerebellar retraction, a custom-made silastic cerebellar protector is inserted, and a 0-degree endoscope is used to get an overview of the lesion. Intracapsular debulking then follows until the capsule collapses. Then the 30-degree endoscope is substituted, and dissection of the lesion off the neurovascular structures proceeds. The internal auditory canal (IAC) may be drilled to expose the fundus (Figs. 5, and 6). At the end of surgery, CSF is replenished with Ringer's solution, the dural flap is replaced with an underlay fascia lata graft and sealed with bone pate and abdominal fat, and the wound is closed in two layers without drainage.

Figure 3.

Vestibular neurectomy. (A) Intact bundle. (B) Cut nerve with facial nerve appearing.

Figure 4.

Vascular loop.

Figure 5.

Acoustic neuroma: (A) Internal auditory meatus (IAM) drilled out; (B) preoperative magnetic resonance imaging.

Figure 6.

Acoustic neuroma (A) T, tumor. (B) Internal auditory meatus (IAM) drilled-out tumor; excised VII, facial VIII cochlear.

RESULTS

The patients were 63 men and 58 women with a mean age of 45.5 years (range 28–65). The sex distribution amongst different lesions is shown in Table 1. The mean operative time in the common approach from skin to posterior fossa was 30 minutes (15 to 42 minutes). The total operative time depended on the type of surgery. For vestibular neurectomy or vascular decompression, the mean total operative time was 60 minutes. For tumor surgery, the total time depended on the tumor size but averaged 3 hours. All patients fully recovered after surgery and none required intensive care unit admission. The average postoperative hospital stay was 5 days (range, 3 to 7 days).

Of 73 patients with CPA mass lesions, total excision was achieved in 69 (94.5%). Residual lesions were at the fundus in an acoustic neuroma, on the posterior surface of the petrous bone, on the facial nerve, and on the brain stem in three cases with meningioma. In cases of vestibular neurectomy and microvascular decompression, there was no additional hearing loss or facial paralysis. In patients with mass lesions, and no preoperative facial paralysis (65 patients), the anatomic integrity of the nerve could be preserved in 60 (92.3%); temporary facial paralysis occurred in 35 patients with complete recovery of function (House-Brackman 1 to 2) in 28, and partial recovery (House-Brackman 3 to 4) in seven. Permanent paralysis occurred in five patients (8.3%), all of whom had large meningiomas.

There were no major neurological complications such as quadriparesis, hemiparesis, lower cranial nerve deficits, or deaths. The most frequent complication was a delayed (3 to 9 weeks) CSF leak from the operative site (15%). All cases were managed conservatively with dehydration and compression, and none required shunting or reintervention. This is mostly due to retraction of the sealing soft tissue.

Postoperative meningitis occurred in two patients (1%); all were treated conservatively with parenteral antibiotics and none suffered additional morbidity.

DISCUSSION

Surgery of the posterior fossa has evolved over the years with a shift toward a more minimally invasive approach.⁷ A variety of lesions can present, and if a single standardized approach could address most of them, this would allow better patient management and shorten the learning curve for surgeons in training.^14,15,16,17 The retrosigmoid approach provides simple and direct access to the cerebello-pontine zone. Endoscopes provide excellent illumination of a wide visual field within an anatomic site particularly rich in neurovascular structures, and their angled lenses can look around and behind important structures. Endoscopic and microsurgical techniques may be combined for better exposure and completeness of excision for mass lesions.^{2,3,6,8,16,18,19,20} We present our experience in a variety of lesions involving the CPA and posterior fossa. In the beginning, we started with a classical combined microscopic-endoscopic approach, but with more experience, we shifted to an exclusive endoscopic approach especially for small lesions (vascular decompression, neurectomies, and mass lesions < 3 cm). Although endoscopic management of nonmass lesions is well established, a fully endoscopic approach to CPA masses is still evolving.^{7,11,15,16,19,21,22} Our technique differs from other series in some points.^2,3,15,23,24 We use a shorter linear skin incision, decreasing the need for muscle retraction and thus minimizing postoperative neck pain and spasm and shortening operative times. However, we use a square craniotomy, which gives better exposure and allows better handling of the instruments. With more familiarity with the technique, the purely endoscopic approach can be applied to all pathologies provided the cerebellum is properly dehydrated and the cisterna magna completely drained. This provides a wide open space and allows introduction of the scope without the disadvantages of shadow effects on the microscope beam. It also helps keep the surgeon in a more comfortable position without overtilt of the patient.

We are presenting a variety of lesions to illustrate the versatility of the approach and the expanded possibilities offered by the use of the endoscope. Our results as regards mass lesions are comparable to those of other series, with 94.5% total excision and 91.7% facial preservation.^{6,8,9,14,21,24} However, the rate of delayed postoperative CSF leak was higher (15%) compared with other reports.^5,11 This was mostly in the beginning of our work as we do not use biological glue and we used to seal the operative cavity with fat. With substitution with fascia lata, the incidence of CSF leak has dropped significantly.

The approach is simple and straightforward, and no major structures are encountered or at risk during the initial routine phases. All important structures are easily visualized and can be protected early on. The learning curve for the approach is relatively rapid, and the initial steps easily mastered. The intracranial manipulations are also straightforward as all structures are exposed and identifiable.

This series illustrates the versatility of this approach and its adaptability to a variety of CPA lesions with minimal morbidity and results that are similar or even surpass more invasive techniques. It may eventually supplant other techniques regardless of the hearing or facial nerve status of the patient.