TITLE: Acoustic Neuroma and =
Hearing
Loss
SOURCE: Grand Rounds Presentation, UTMB, Dept. of Otolaryngology
DATE: December 6, 2006
RESIDENT PHYSICIAN: K. Kevin HO, M.D.
FACULTY PHYSICIAN: Vicente A. Resto, M.D., Ph.D=
.
SERIES EDITORS: Francis B. Quinn, Jr., MD and Matthew W. Ryan, MD
"This
material was prepared by resident physicians in partial fulfillment of
educational requirements established for the Postgraduate Training Program =
of
the UTMB Department of Otolaryngology/Head and Neck Surgery and was not int=
ended
for clinical use in its present form. It was prepared for the purpose of
stimulating group discussion in a conference setting. No warranties, either
express or implied, are made with respect to its accuracy, completeness, or
timeliness. The material does not necessarily reflect the current or past
opinions of members of the UTMB faculty and should not be used for purposes=
of
diagnosis or treatment without consulting appropriate literature sources and
informed professional opinion."
Introduction:
This manuscript serves to review the evidence-ba= sed literature on hearing loss in acoustic neuroma = (AN) patients. The focus will be on the natural history of acoustic neuroma and how hearing progression may be influenced= by factors such as tumor growth.
Historical Perspectives:
The first documented case of AN was recorded in = 1777 by Dr. Sandifort. The mortality rate associated= with surgery was alarmingly high at that time up to 80% due to delayed diagnosis= and primitive instrumentation. Major progress in the surgical management of AN = was made during the early twentieth century owing to the contribution of three surgeons. Dr. Harvey Cushing employed meticulous dissection and hemostasis, successfully lowering surgical mortality = to 4%. His student, Dr. Walter Dandy, further refined technique with use of vessel clips and ligatures. Dr. Dandy was also the first surgeon to perform a comp= lete resection of AN. Dr. William House introduced operating microscope and surg= ical drills to revitalize the once-condemned translabyrinth= ine approach in the 1960s. In addition, Dr. House introduced the concept of combining the expertise of neurosurgeons and otologist= s in the management of AN patients. The current state-of-the-art micro-instrumentation and intraoperative neural= monitoring have enabled surgeons to achieve complete tumor removal with a very low mortality rate (<2%).
Anatomy of the Cerebellopontine= Angle (CPA):
The cerebellopontine=
angle
is a rather small area located in the posterior fossa<=
/span>
near the origins of several vital cranial nerves. The medial and lateral borders of =
this
space are the brainstem and the petrous portion=
of
the temporal bone, respectively.
Superiorly, it is bounded by the middle cerebel=
lar
peduncle, and inferiorly by the arachnoid tissu=
e of
the lower cranial nerves.
Acoustic Neuroma:
Epidemiology:
AN represents about 6% of all intracranial tumor= s and 80-90% of all CPA masses. Its incidence in the States is about 10 cases per million every year. It predominantly presents in adulthood. There are two f= orms of AN: 1) the sporadic form represents about 95% of all ANs and it presents unilaterally; 2) Neurofibromatosis type 2 (NF-2) has a stro= ng genetic predisposition on chromosome 22, presenting as bilateral ANs. There is currently no known gender and race predilection in the development of AN. AN is a benign tumor arising from t= he vestibular nerve (hence it is also known as vestibular schwannoma). Majority of ANs arise from the internal auditory canal (IAC).
Hearing in AN patients:
Hearing loss is the most common symptoms of AN a= nd can be found in about 95% of AN patients. Classically, AN patients present with asymmetric sensorineural hearing loss (SNHL) at= high frequencies (down-sloping). Speech discrimination scores (SDS) typically decrease out of proportion of pure tone thresholds (PTT).
According to the American Academy of Otolaryngology-Head and Neck Surgery classification published in 1995, pati= ents falling into categories of either class A (>70 % SDS and < 30 dB PTT loss) or B (>50% SDS and < 50 dB PTT) have serviceable hearing. Heari= ng of Class C (> 50% PTT and >50% SDS) and class D (< 50% SDS) patien= ts are considered non-serviceable.
The etiology of hearing loss in AN is not known.= Some proposed mechanisms include compression on the cochlear nerve, vascular disruption of the internal auditory artery, and biochemical changes in inner ear fluids.
The importance of following hearing progression = in AN patients cannot be overstated. Preoperative hearing status has been shown t= o be the most important predictor for the success of hearing preservation surgery (middle fossa or retrosigm= oid approach). A number of studies addressing the relationship between hearing progression, tumor size, and tumor growth are described in the following sections.
Tumor size:
AN can be classified by size according to the
2) small < 10 mm; 3) medium 11-25 mm; 4) large 25-40 mm; 5) giant > 40
mm.
It was once thought that tumor size might be correlated with hearing progression in AN. Lustig identified a higher proportion of smaller tumors in the normal hearing group compared to his overall AN group. However, he found that hearing could be normal even in larger tumors > 3 cm. A study by Sti= pkovits also failed to identify any relationship between hearing and tumor size. To date, there exists no conclusive correlation between tumor size and change = in hearing threshold in AN patients.
Tumor Growth:
Four phases of tumor growth have been described:= intracanalicular, cisternal, compressive, and hydrocephalic. Generally, patients in intracanalicular phase display unilateral loss= of vestibular or cochlear nerve function, such as hearing loss, vertigo, tinni= tus, and disequilibrium. These symptoms progress as tumor grows and expands into= the CPA cistern. Cranial neuropathies especially CN V and VII typically occur l= ate in the compressive phase. Lastly, obstruction of the fourth ventricle resul= ts in hydrocephalus and can be seen in patients with visual changes and altere= d mentation.
Numerous papers have addressed the natural histo= ry of AN by following tumor growth in patients in the observation, or wait-and-sc= an group. Despite the retrospective nature and intrinsic selection bias in mos= t of these studies, they illustrate similar results that roughly 50% of ANs grow, 40-50% remain in same size, and less than 1= 0% involute.
The relationship between tumor growth rate and h= earing was explored by Massick et al., who found a pos= itive correlation between changes in tumor volume, pure tone average as well as speech discrimination score.
An understanding of tumor growth rate is instrum= ental for the clinical management of AN. Walsh demonstrated that the risk of loss= of serviceable hearing in the observation period is much higher in the tumor growth group (80%) compared to the non-tumor growth group (14%). Undoubtedl= y, the current management of AN – observation, surgery, and radiation therapy – all hinges upon the concept of tumor growth rate. A non-gro= wing AN that is associated with stable or minimal change in hearing is likely to= be observed, while the fast growing ones with progressive deterioration of hea= ring would be resected or irradiated. Most integral = to the success of radiation as a treatment of AN is to arrest tumor growth, which occurs in no more than 10% after radiation. Various studies have attempted = to identify predictors for growth of AN without much success. There was no correlation between growth rate and patient age, gender, initial volume, and side of tumor. While potential biomarkers such as fibroblast growth factor receptor may provide an answer to tumor proliferation in the near future, t= he only reliable means to estimate tumor growth at present is serial MRI scann= ing as described below.
Diagnosis and assessment of AN:
Timely diagnosis of AN is critical because treat= ing smaller tumors either by surgery or radiation carry a smaller risk of morbi= dity and higher chance of hearing preservation compared to larger tumors. While = the frequencies and patterns of symptoms in AN have been thoroughly described in the literature, we must also be aware of the variations and often insidious disease progression. Patients often ignore the early and non-specific sympt= oms of hearing loss (~ 94%), vertigo (~39%), disequilibrium (~ 56%), or tinnitus (~64%), resulting in the delay of diagnosis. A complete otolaryngologic and neurologic exam must be performed. A high i= ndex of suspicion should be given to those with progressive unilateral SNHL, unilateral audiovestibular symptoms, facial or trigeminal nerve dysfunction, and sudden SNHL.
As described above, AN patients typically presen= t with asymmetric SNHL with decreased SDS. Therefore, a thorough audiometric assessment is a crucial first-step in diagnosing AN. Audiogram may also demonstrate features of loss of acoustic reflex and acoustic reflex decay t= hat are associated with retrocochlear pathology. The diagnostic efficiency of Audi= tory Brainstem Response (ABR) has been studied extensively. The sensitivity of A= BR has been reported to be > 95% in large tumors > 3 cm. However, the fa= lse positive rate has also been described to be as high as 33 % in intracanalicular AN and false positive rate > 80%.= More importantly, the response of = interaural latency difference for wave V (IT5) is abs= ent almost half the time in large tumors > 2 cm. Because of these limitation= s, studies on a new technique called Stacked ABR are currently underway to imp= rove its diagnostic efficiency. Otoacoustic emissions (OAE) and vestibular testing (eg. Electronystagmography, rotatory testing, posturog= raphy) have little role in the diagnosis of AN because of low sensitivities and specificities.
Gadolinium-enhanced MRI remains the gold standar= d in the diagnosing AN. It can detect tumors as small as 1 mm and differentiate = AN from many CPA lesions based on its density and enhancement on T1 and T2 ima= ges. Imaging on temporal bone lesions has been previously described by Cowan et = al. in our Grand Rounds Series and will not be discussed here. For those patien= ts unsuitable for MRI scanning (eg. Metallic impla= nt), a contrast-enhanced CT can be obtained, though it cannot reliably detect smal= ler tumors < 1.5 cm.
References:
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