Chapter 51 Transotic Approach

The transotic approach to the cerebellopontine angle (CPA) was first introduced in 1979 by one of us (U.F.) in response to the limitations of the translabyrinthine technique. The objective of this approach is to obtain a direct lateral exposure and the widest possible access to the CPA through the medial wall of the temporal bone, from the superior petrosal sinus to the jugular bulb, and from the internal carotid artery to the sigmoid sinus. The tympanic and mastoid portions of the fallopian canal are left in situ. This transtemporal access is achieved at the expense of bony exenteration, rather than cerebellar retraction.

Despite well-documented technical details,¹ there is a general misconception equating the transotic approach with the transcochlear approach of House and Hitselberger.² Significant differences exist between the two approaches in the extent of exposure, the management of the facial nerve, and the obliteration of the surgical cavity. As a natural extension of subtotal petrosectomy, which forms the basis of lateral and posterior skull base surgery at the University of Zurich,¹ the transotic approach was initially designed for acoustic neuromas and has since expanded to include other pathology. Several modifications were also made over the years to optimize its use.³^–⁵

INDICATIONS

Acoustic Neuroma

Although the transotic approach, similar to the translabyrinthine approach, can be used for tumors of all sizes, it is ideal for tumors 2.5 cm or less in their mediolateral extent, in patients with no serviceable hearing. In this clinical setting, the transotic approach offers the best possible exposure for tumor extirpation and preservation of facial nerve with minimal morbidity.

At the University of Zurich, tumors larger than 2.5 cm that cause significant brainstem compression are managed by the neurosurgery department as a matter of departmental policy. Small intracanalicular tumors in patients with good hearing (using the 50/50 rule of at least 50 dB hearing loss and 50% discrimination score) are managed through a middle cranial fossa (transtemporal-supralabyrinthine) approach (see Chapter 35).

PREOPERATIVE EVALUATION

The evaluation for retrocochlear lesions, such as acoustic neuromas, is standard at the University of Zurich and includes routine audiometry, auditory brainstem response, electronystagmography, and magnetic resonance imaging (MRI) with gadolinium enhancement. High-resolution computed tomography (CT) is still performed for bony assessment of lesions within or invading the temporal bone. Facial nerve status is recorded clinically using the Fisch grading system ⁶ and quantified by electroneuronography before surgery.

In addition to a candid discussion of surgical and postoperative complications, patients are informed of the advantages of the transotic approach specifically with regard to the preservation of the facial nerve and the complete obliteration of the surgical cavity, with blind sac closure of the external auditory canal to minimize cerebrospinal fluid (CSF) leak. The option of conservative management by close monitoring with serial MRI to follow tumor growth is presented to all patients and is recommended for patients with nonprogressive long-standing symptoms (especially elderly patients), patients with small tumors and normal hearing, patients with significant medical illnesses, or patients who refuse to undergo surgery. These patients are told that rapid tumor growth ultimately requires surgical attention, and that facial nerve function and hearing preservation may be compromised as a result of the delay in surgery.

SURGICAL TECHNIQUE

Preoperative Preparation

The patient is premedicated with clonidine, metoclopramide, and midazolam before surgery. A perioperative antibiotic, ceftriaxone (Rocephin), 2 g intravenously for 24 hours, is given at the time of surgery until the removal of intravenous infusion, usually by the 3rd day after surgery.

Surgical Site Preparation, Positioning, and Draping

The surgical site is prepared in the operating room after induction of anesthesia. Hair over the temporal area is shaved 9 cm above and 5 cm behind the pinna. The skin is washed with povidone-iodine. The abdomen and the contralateral leg are also shaved and prepared for fat harvesting and the possible need of a sural nerve graft.

The positioning and draping for this procedure are similar to positioning and draping described in Chapter 1, with some minor differences. The patient is secured in supine position on the Fisch operating table (see Chapter 35), with the head turned away from the surgeon. A large plastic bag is incorporated into the draping to catch excess irrigation and blood.

Intraoperative Monitoring and Concerns

Intraoperative facial nerve monitoring using the Xomed nerve integrity monitor (NIM-II) and percutaneous electromyography needles is standard with this approach. Intracranial pressure is controlled by deep anesthesia induced intravenously before introduction of inhalational anesthetics. Partial pressure of carbon dioxide is maintained at 30 to 40 mm Hg. Pharmacologic manipulation with dexamethasone (Decadron), 4 mg every 8 hours perioperatively and 4 days postoperatively, and mannitol, 0.5 mg/kg intravenously intraoperatively, is also standard. Furosemide is added when necessary. Lumbar CSF drainage is not routinely performed. Hypotensive anesthesia with nitroglycerin or clonidine (Catapres), or both, is used in most cases to maintain a systolic blood pressure of 80 to 100 mm Hg.

Skin Incision

A postauricular incision is placed along the hairline to keep it behind the operative cavity (Fig. 51-1). The incision is made from the mastoid tip to the temporal region for the surgical approach; its superior extension is made at the time of wound closure for the exposure of the temporalis muscle flap.