Abstract
Objectives
Magnetic resonance imaging (MRI) is the definitive investigation for detection of an acoustic neuroma. It is however an expensive resource, and pick-up rate of a tumor can be as low as 1% of all patients scanned. This study aims to examine referral patterns for MRI screening for patients presenting with asymmetrical sensorineural hearing loss (ASHL). A second aim was to suggest appropriate screening criteria.
Method
All 132 MRI scans performed for ASHL in the year 2005 were reviewed retrospectively along with their case records and audiograms. In addition, MRI scans and case records were reviewed for the last 30 patients diagnosed with acoustic neuromas. Information was analyzed using 2 published protocols and additional frequency-specific defined criteria.
Results
Two acoustic neuromas were picked up out of 132 scans performed. Of the scans performed for ASHL, a third did not fit with any of the published criteria. Of the 30 positive scans for a tumor, the patients/audiograms revealed that 10% did not fit the published criteria despite the patients having no other audiovestibular symptoms.
Conclusions
There appears to be no universally accepted guidelines on screening in ASHL with clinical acumen being used by most ENT consultants in this region. Applying protocols may reduce the amount of scans performed, but up to 10% of tumors may be missed by this approach.
1
Introduction
An acoustic neuroma (AN) is a slow-growing benign tumor of the vestibular portion of the VIIIth cranial nerve. Patients typically present with unilateral or assymetrical sensorineural hearing loss . Tinnitus, vertigo, facial paralysis and paraesthesia, and dysequelibrium can also coexist, making it a potentially challenging diagnosis . The true incidence of AN remains unknown, and figures provided by epidemiologic and postmortem studies vary markedly . Gadolinium-enhanced magnetic resonance imaging (MRI) is the gold-standard investigation of patients with suspected AN . This has led to an increase in cases diagnosed since MRI was introduced in the late 1990s with tumors as small as 4 mm being identified .
In the pre-MRI era, clinicians used a variety of approaches to investigate patients with unilateral sensorineural hearing loss. Diagnostic tests included pure tone and speech audiometry, electronystamography, auditory brainstem response, and contrast-enhanced computed tomography (CT) .
Since the efficiency of MRI in detection of AN has been established, various attempts have been made to design a widely accepted protocol for targeted screening of patients with unilateral sensorineural hearing loss . None of these has achieved universal recognition however and various regional modifications of these protocols are still used in everyday clinical practice, such as the popular Oxford and Charing Cross protocols.
This study looked at the efficacy of these 2 existing screening protocols, by applying them to 132 patients with asymmetrical sensorineural hearing loss (ASHL) sent for MRI and also to 30 positive diagnoses of AN at a tertiary teaching hospital.
2
Method
The authors examined the case records and audiograms of all patients who had underwent MRI scans of the Internal Auditory Meati at Aberdeen Royal Infirmary, a tertiary teaching hospital in the north of Scotland in 2005. Two hundred two case records were found, and of these, 132 MRI scans had been performed for either unilateral sensorineural hearing loss or ASHL.
Exclusion criteria included trigeminal or facial nerve symptoms and requests where hearing loss was not the main symptom. There were 70 cases excluded .Of these, 42 patients had unilateral tinnitus as the predominant symptom and 25 had vertigo as the predominant symptom. Two cases were scanned for V11 nerve symptoms and 1 for V nerve symptoms.
In addition, MRI scans, case records, and audiograms were reviewed for the last 30 patients diagnosed with ANs in the preceding 6 years.
Information from the audiograms was collected by a single researcher and was tabulated on Microsoft Excel. This information from the audiograms was analyzed in 3 different ways.
First, information from the patients’ audiogram was used to assess whether it would have fitted with referral for an MRI scan using 2 major published protocols: (1) the Oxford protocol, developed by Sheppard et al in 1996, which suggested a minimum of 15 dB difference between the averages of all frequencies between 250 Hz and 8 kHz (in patients up to 70 years with unilateral audiovestibular symptoms, or unilateral tinnitus with normal hearing), and (2) the Charing Cross protocol, recommended by Obholzer et al in 2004, which suggested screening for an interaural difference at 2 adjacent frequencies of more than 15 dB in unilateral hearing loss and more than 20 dB in bilateral asymmetric hearing loss.
The audiogram results were then further analyzed in a third way by an additional frequency-specific criteria devised by the authors. This was created in an attempt to develop an improved screening protocol and involved a formula that incorporated the average of the results of hearing levels between the right and left ears at all frequencies.
The hearing level (decibels) was plotted against frequency (hertz), as in a standard audiogram, and the area difference between the 2 ears (dBHz) was then calculated. The easily calculated equation, and accompanying graph, is as follows:
Area between ears = 1 2 ( G − A ) + ( H − B ) + ( J − C ) + ( K − D ) + ( L − E ) + 1 2 ( M − F ) ( dBHz )
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