The management of acoustic tumors has become more complex through the years with the availability of multiple therapeutic options. Because of the relative limitations of early imaging techniques, most tumors were discovered only after they had grown to considerable size. During the 1970s, the development of computed tomograph (CT) air cisternography scanning permitted more accurate diagnosis of acoustic tumors. Advances in neuroradiology during the 1980s, specifically the advent of gadolinium-enhanced magnetic resonance imaging (MRI), led to earlier diagnosis of tumors. The use of MRI permits the detection of small intracanalicular tumors in patients with minimal early symptoms and represents the gold standard method for diagnosis. In addition, heightened clinical suspicion by otolaryngologists and primary care physicians attributable to an increased awareness of early symptoms has contributed to the earlier screening and eventual diagnosis of patients with tumors.

The increase in diagnosis of tumors at an early growth stage has led to controversy over management strategies. In 1994, the National Institutes of Health Consensus Development Conference determined that the treatment of patients with acoustic neuroma should be provided by an experienced multidiscipli-nary team and individualized with regard to tumor and patient characteristics.1 The individualized approach is applicable, in particular, to the management of a patient with an intracanalicular tumor. The therapeutic options available in the management of these patients include surgery, stereotactic radiotherapy, and close observation with serial MRI. The discussion in this chapter assumes a unilateral tumor with normal hearing in the opposite ear.

Hearing Status of the Patient

Before discussing the therapeutic approaches to an intracanalicular acoustic neuroma, it is important to first consider that frequently these patients will possess near normal or at least “serviceable” hearing at the diagnosis. The presence of hearing in these patients directly impacts choices in all aspects of management. Controversy exists in defining exactly what constitutes serviceable hearing for determining candidacy for hearing preservation management approaches. The most commonly employed definitions include (1) a<50-dB average of the pure tones (PTA) at 500, 1, 2, and 3 kHz with a speech discrimination score (SDS) of ≥50%; or (2) a PTA of <30 dB and an SDS of ≥70%. Beyond these definitions, there have been a multitude of complex notions of serviceable hearing, including the presence of any measurable preoperative hearing. A true consensus on the definition of serviceable hearing has not been reached within the neurotologic and neurosurgical communities, and it is not the intention of this chapter to resolve this issue. However, it is important that individual clinicians managing patients with acoustic tumors develop a working definition of serviceable hearing in order to identify candidates for hearing preservation. For purposes of candidacy for hearing preservation surgery, we at the House Ear Clinic, in general, ascribe to the 50 50 rule.

Characteristics of Intracanalicular Acoustic Neuromas

An intracanalicular acoustic neuroma is <1 cm in length and appears rounded in the early stages. Upon filling the internal auditory canal (IAC), it assumes an oblong shape. At diagnosis, three variations are encountered, depending on the extent of involvement of the IAC: (1) medial IAC location near the porus acousticus, (2) lateral IAC involvement near the fundus, and (3) involvement of the entire IAC. True intracanalicular tumors involve only the IAC, although many tumors can be considered “primarily” intracanalicular with minimal cerebellopontine angle extension, and are readily accessible by surgical approaches to the IAC. It should be noted that whereas 90% of intracanalicular lesions are indeed acoustic neuromas, patients should be counseled regarding the possibility that a facial neuroma may be radiologically indistinguishable from an acoustic neuroma.² Alternative management schemes in the event of the presence of a facial neuroma should be discussed preoperatively with the patient.

Management Options

SURGICAL OPTIONS

The modern era of acoustic neuroma surgical management began during the early 1960s with the development of micro-surgical techniques by Dr. William House. Before this, operative mortality rates approached 40%, and most patients experienced postoperative facial paralysis. With the advent of more technically advanced surgical procedures and intraoperative monitoring, the goals of acoustic neuroma surgery have evolved. Initially, the primary goal centered around decreasing operative mortality while providing total removal of the tumor mass. With technical advancements and increased knowledge of microsurgical anatomy, preservation of facial nerve function was emphasized. More recently, efforts have focused on preservation of serviceable hearing in appropriately selected patients. Three principal surgical approaches are appropriate for the removal of intracanalicular acoustic neuromas: the translabyrinthine approach, the middle fossa approach, and the retrosigmoid approach. Each of these approaches has certain advantages and disadvantages and can be considered to have different indications depending on individual patient and tumor characteristics.

In patients with a total loss of hearing or who do not meet criteria of candidacy for hearing preservation surgery, the translabyrinthine craniotomy (TLC) approach is selected. The procedure involves violation of the semicircular canals and vestibule, resulting in sacrifice of residual hearing. A major advantage of the TLC is that it provides a direct route to the internal auditory canal and cerebellopontine angle (CPA), while minimizing cerebellar retraction, thereby decreasing the incidence of postoperative ataxia.3 This approach offers full exposure of the internal auditory canal, from the fundus laterally to the porus acousticus medially. It also provides direct access to the cerebellopontine angle in the event of CPA extension. The extent of the exposure gained by this approach ensures that any intracanalicular tumor may be addressed entirely under direct vision, facilitating total tumor removal and prevention of tumor recurrence. Recurrence rates after total translabyrinthine resection of acoustic neuroma are extremely low (<1%) despite the fact that most reports include data involving tumors of all sizes and level of CPA extension.⁴ The level of exposure achieved also facilitates minimal traction on the facial nerve during tumor dissection, and facial nerve outcomes with this approach have historically been considered superior to other approaches. In patients with small tumors resected by the translabyrinthine approach, a House-Brackmann facial grade I or II was achieved in 92% of cases.⁵

PATIENTS WITH SERVICEABLE HEARING

Hearing preservation is possible by resecting intracanalicular tumors with either the middle fossa or retrosigmoid approaches. The decision to employ one of these approaches preferentially is largely dependent on the training of individual skull base surgeons. In many centers, neurotologists and neurosurgeons uniformly agree on using one approach primarily. Other skull base surgery teams tailor the decision of the approach to the precise location of the tumor within the IAC. Defining what constitutes serviceable hearing, and therefore candidacy for hearing preservation surgery, remains highly controversial. Similarly, controversy exists in reporting hearing outcomes of surgery performed to preserve hearing. These controversies contribute to the difficulty in analyzing and comparing the published rates of hearing preservation. Most investigators attempt to present postoperative results as hearing at or near preoperative levels as well as the presence of any measurable hearing. In 1995, the American Academy of Otolaryngology-Head and Neck Surgery Committee on Hearing and Equilibrium attempted to resolve this issue by establishing guidelines for reporting results of efforts to preserve hearing in the treatment of acoustic neuromas.⁶

We prefer the middle fossa approach to resect intracanalicular acoustic neuromas in patients with serviceable hearing in most cases. In our opinion, true intracanalicular tumors are best approached in this fashion, and we reserve the retrosigmoid approach for patients with serviceable hearing and tumors that involve the CPA with no extension to the fundus and minimal involvement of the porus acousticus. The middle fossa technique provides complete exposure to the contents of the internal auditory canal while eliminating any need for blind dissection.⁷ Approaching the IAC superiorly avoids violation of the cochlea and semicircular canals, permitting the opportunity to preserve hearing. With full exposure of the facial and cochlear nerves at the fundus of the IAC, complete tumor removal in this region of the IAC is possible under direct vision. Furthermore, exposure of the fundus decreases the risk to the nerves and their intimate blood supply during tumor dissection, helping to preserve postoperative neurologic function. Using modifications to the original middle fossa approach, tumors that involve the entire IAC, those isolated to the lateral IAC, and tumors in the medial IAC with up to 2 cm of cerebellopontine angle extension can be readily removed.⁸

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