INTRODUCTION

The Acoustic Neuroma (AN) is a benign, slow growth tumor that usually originates from the upper portion of the vestibular nerve. It is considered the most common cause of retrocochlear pathologies, representing 2 to 7% of all intracranial tumors (1).

The use of Brain Stem Auditory Evoked Potential (BSAEP) in the AN diagnosis has been established for years, however the existence of some false negatives and the development of magnetic resonance imaging (MRI) for the same purpose lead us to review the values of this method (2). Within the specialized literature there are different sensitivity indexes for BSAEP in AN screening, despite some divergence concerning the preferable method used for this goal. Our present study aims at describing the current aspects related to the BSAEP importance in detecting AN, approach its basic definition, symptoms, ways it appears and treatment modalities for such pathology.

LITERATURE REVIEW

Acoustic neuroma definition and symptoms: The acoustic neuroma (AN) originates from Schwann cells, and it may rise in the peripheral portion of the facial, cochlear, superior vestibular or inferior vestibular nerves. Its most frequent origin is the vestibular nerve, having been occasionally spotted on the facial nerve and even more rarely on the cochlear (3). These are benign, fibrous growth tumors that do not give off metastasis. They start in the internal acoustic meatus and may expand all the way to the brain. They may be deeply inserted within the skull and close to brain vital centers (4).

Its incidence varies from 6 to 18/1.000.000 per year (5). It is estimated that in our country
there could be 1,700 cases diagnosed per year (1). There is certain higher prevalence in
women and the most frequent age at diagnosis is between 35 and 60 years (6).
Thanks to an increase in the use of magnetic resonance imaging (MRI), asymptomatic ANs may be identified early on, thus increasing this tumor incidence estimates (7).

Although population studies and autopsies broadly diverge in these estimates, ANs make up approximately 6% of all intracranial tumors.

Usually most patients have mild and almost unperceivable symptoms, many do not show any change in their clinical picture for years, and this is seen through MRI (4). Clinical symptoms may include unilateral or asymmetrical sensorineural hearing loss, unilateral tinnitus, poor unilateral or bilateral vocal recognition if compared to the degree of hearing loss, distorted sound perception in cases of essentially normal peripheral hearing (8).

Sudden hearing loss, even when fully recovered, may be an alert signal as to the presence of an AN. The reason for this recovery after hearing loss is still unknown (9). Although there are other affections that may involve the auditory nerve, such as infections, inflammations, demyelination and vascular dysfunction, the AN is the most common, being considered a classic cause for retrocochlear hearing loss (10).

Acoustic neuroma forms: AN occurs sporadically and unilaterally (95% of the cases) or associated to type II neurofibromatosis (NF II), bilaterally (5% of the cases). Sporadic AN patients tend to have symptoms onset in their middle ages, in average they are diagnosed around 50 years. NF II patients develop their symptoms in earlier ages, around 30 years. However, there is great variability and patients may start showing symptoms and be diagnosed during childhood, when young adults or older (10).

As far as size is concerned, AN are classified as small, medium or large tumors. The small AN (diameter < 1 cm) is usually located within the internal acoustic meatus (IAM). As the AN grows and reaches middle size (between 1 and 3 cm in diameter), it extends outside the IAM, towards the brain stem (BS). The large tumors are the ones measuring at least 3 cm in diameter. They apply pressure on the BS and the cerebellum involving other sensitive nerves coming from the BS (11).

Diagnostic Methods: Early AN diagnosis is paramount in order to install proper treatment,
specially because there is a close relationship between surgical morbidity and tumor size (12). BSAEP is the most specific and sensitive audiometric exam in the acoustic neuroma detection, it enjoys a fundamental role in screening programs for retrocochlear pathologies (13).

When the BSAEP was developed, many professionals counted on it as a first step in AN screening. BSAEP was considered the most accurate non-invasive method until image diagnostic techniques were developed. CT scan with contrast injection was able to detect approximately 30% of AN, and only tumors larger than 5mm in the cerebellopontine angle were detected (14). Now, magnetic resonance is the only exam that bears 100% sensitivity even for small tumors, challenging the BSAEP importance in AN screening (15). The only drawback MRI has is its high cost. Notwithstanding, new technologies have brought prices down substantially (16). This has contributed to reducing even more the use of BSAEP, leading some otolaryngologists to indicate this exam only in cases where there is AN clinical suspicion, in cases of MRI contra-indication or as a pre-operative exam to predict hearing preservation in AN patients (14). In general, BSAEP has 90% sensitivity and 70 to 90% specificity (4).

Some authors (17,18) mention the BSAEP as a non-reliable and insuficient method for the detection of small tumors, basing themselves on the fact that its sensitivity varies according to tumor size, in other words, the smaller the tumor, the less sensitive the exam becomes (19). This sensitivity/tumor size ratio is shown in the following studies.

In 1994, Dornhoffer et al. (20) reported 93% sensitivity for tumors smaller than 1 cm.

In 1995, Chandrasekhar et al. (21) found 83.1% sensitivity for less than 1cm tumors and 100% for 1 to 1.5 cm tumors, 86% for tumors between 1.6 and 2 cm, and 100% for tumors larger than 2cm.

In 1995, GORDON E COHEN (22) reported the following sensitivities: 69% for tumors smaller than 9 m, 89% for tumors between 1 and 1.5 cm, 86% for tumors between 1.6 and 2 cm, and 100% for tumors larger than 2 cm.

In 1997, Zappia et al. (23) showed a 89% sensitivity for tumors smaller then 1 cm, 98% for middle size tumors between 1.1 and 2 cm, and 100% for tumors larger then 2 cm. Total sensitivity was of 95%. In 1994, DORNHOFFER et al. (20) reported a 93% sensitivity.

In 1997, Chandrasekhar et al. (24) reported general sensitivity of 92.3% when they used more than 0.2 ms wave V latency difference between the ears as a criterion. In large tumors (> 3 cm) this sensitivity was of 100%, and of 83.1% in smaller or equal to 1 cm tumors.

In 2000, Robinette et al. (25) correctly identified 100% of large tumors (>2 cm), 93% of middle size tumors (1,1-2cm), and 82% of smaller tumors (<1 cm).

In 2001, Schmidt et al. (26) reported 58% sensitivity for ANs smaller than 1 cm, 94% for AN between 1.1 and 1.5 cm, and 100% for tumors larger than 1.5 cm. Total sensitivity was of 90%.

Although conventional BSAEP shows lower sensitivity for small tumors, Don et al (27) showed one excellent technique used to find small tumors undetectable through conventional BSAEP: the "stacked derived-band ABR amplitude". This method allowed a more reliable detection of small neuromas. In their study, 100% of smaller than 1cm tumors, not previously detected by conventional BSAEP, were picked up. This method requires a masking technique that may not be available to the examiner and a high noise level is necessary, making the patients complain of discomfort. Another alternative method that allowed for BSAEP manipulation was then explored by Philibert et al. (28), who used tone-bursts (TBs) to obtain a specific frequency BSAEP. The observed results were promising and many investigations were carried out with this method, which showed great sensitivity in the detection of small AN tumors.

Portier (29) increased BSAEP sensitivity from 86 to 99% by combining the results from this exam to those of the acoustic reflex and the caloric vectoelectronistagmography test. It is also indicated the use of BSAEP latency-intensity function analysis in small tumor detection, because they show a characteristic abnormality in this function. (30).

Many investigators also mentioned that the use of acoustic stimulus in high frequency may
increase the test sensitivity to identify lesions in the auditory nerve or the central auditory system (31). According to Despland and Galambos (32), in the presence of an intracranial disease, the higher the acoustic stimulus frequency, the greater is the hearing pathway difficulty in neural conduction. In his studies, retrocochlear disease bearing patients had abnormal BSAEP exam only when the stimulus was presented in high frequencies (50 to 90 clicks/s). Therefore, we recommend BSAEP recording in two frequencies (low and high) when suspecting retrocochlear alterations (33).

BSAEP findings: Besides the retrocochlear pathology, many factors may influence BSAEP results, including the degree of hearing loss, loss asymmetry, test parameters and other patient-related factors. Such influences must be considered when using and analyzing BSAEP exams. Suggestive findings of retrocochlear pathology (AN) may include one or more of the characteristics mentioned bellow (3):
- Prolonged inter-peak and absolute latencies when compared to normal data;
- No wave formation in the involved ear;
- Prolonged inter ears wave V latency difference - varies from greater than
0.2 ms to 0.4 ms, however, the one most commonly used is greater than 0.3ms (5).

Treatment: Close follow up, gama-knife surgery and microsurgery are the AN therapeutic options. Most ANs are slow growth tumors, but they may require surgery. Gama-knife radiosurgery (GKRS) and microsurgery have low morbidity rates and provide good tumor size control. Microsurgery offers the best tumor control, although it is not fully free from morbidity and mortality (34) and this is not the most indicated procedure right after diagnosis (35).

DISCUSSION

BSAEP is an important AN screening method, despite reports of its reduced tumor-size- dependant sensitivity. Different techniques may be applied to BSAEP in order to increase its AN detection sensitivity, even at early stages of development.

Although magnetic resonance imaging continues to be the most sensitive diagnostic method for the discovery of small ANs, many studies have shown the effectiveness of brain stem auditory evoked potential in the detection of this pathology.

Current literature suggests that once BSAEP alterations are detected or when there is clinical suspicion of retrocochlear disease, even with a normal BSAEP, AN presence should be confirmed by magnetic resonance imaging.

According to studies mentioned in this paper, the sensitivity for small tumors varies between 58 and 93%, and for large tumors the BSAEP is 100% sensitive.

Conventional BSAEP general sensitivity in detecting acoustic neuroma is between 90 and 95%.

CONCLUSIONS

Although the AN is a rare pathology, it is a constant concern in the differential diagnosis of
cochlear and retrocochlear hearing loss.

Many things may be done with the BSAEP, it is a promising and low cost procedure that may reduce the number of patients without Acoustic Neuroma who undergo magnetic resonance, besides being able to detect this pathology still in its early development stages.


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