Embargoed until May 11, 2001
Contact: Kenneth S. Satterfield 760-776-8502 (5/11-5/16) 703- 519-1563[email protected]
The Sensitivity of the MRI Study to Delineate Acoustic Neuromas Is Less Accurate than Originally Presumed
Faulty imaging can misrepresent the size of a benign tumor that can cause of sensorineural hearing, requiring a change in surgical strategy.
Palm Desert, CA -- An acoustic neuroma (sometimes termed a neurolemmoma or schwannoma) is a benign (non-cancerous) tumor that arises on the eighth cranial nerve leading from the brain to the inner ear. This nerve has two distinct parts, one associated with transmitting sound and the other sending balance information to the brain from the inner ear. If not treated, sensorineural hearing loss, tinnitus, or a facial nerve disorder may occur.
While surgery remains the primary treatment for acoustic tumors, the choice of surgical approach (middle fossa, retrosigmoid, translabyrinthine) depends upon multiple factors, in particular the location of the tumor in the internal auditory canal (IAC). Magnetic resonance imaging (MRI) scans are highly sensitive tools for the diagnosis of acoustic tumors. However, MRI imaging may not be as accurate as previously thought for delineating the extension of tumor into the internal auditory canal. The presence or absence of tumor in the deepest portion of the IAC has implications on the selection of which surgical approach should be employed and ultimately on the surgeon's ability to preserve the patient's hearing in the affected ear.
A new study provides examples of the limitations of MRI in reliably predicting the tumor extention of acoustic neuromas deep in the IAC. The research presents the largest case series of patients in whom gadolinium enhanced T1 weighted MRI images failed to correlate with surgical findings of tumor extension deep in the IAC.
The authors of the study, Internal Auditory Canal Involvement of Acoustic Neuromas: Surgical Correlates to MRI Findings, are Samuel H. Selesnick MD, FACS, Janez Rebol, MD, Linda A. Heier, MD, Philip H. Gutin, MD, and Michael H. Lavyne, MD, all from the Weill College of Medicine of Cornell University, New York, NY. Their findings were presented Saturday, May 12, 2001, at the spring meeting of the American Neurotology Society in Palm Desert, CA.
Methodology: Dr. Selesnick performed 74 skull base surgeries from January 1997 through February 2000. Of these, 45 were for primary acoustic neuroma surgeries. Of this group, 28 patients had adequate imaging and intraoperative data for inclusion in this retrospective study.
Surgeries were performed at the New York Weill Cornell Center of New York Presbyterian Hospital and at Memorial Sloan Kettering Cancer Center. T1gadolinium enhanced axial and coronal MRI images that specifically included adequate images of the internal auditory canal (IAC) were reviewed. The magnet strength, Tr, Te, slice thickness, slice overlap, and date of examination were recorded. For the MRI images, IAC length and the length of the tumor were measured by a senior neuroradiologist who was blinded to surgical findings.
Results: Six patients were found to have IAC MRI findings inconsistent with the surgical findings and, so, are included in this analysis. All patients are males. Their ages averaged 50.2 years and ranged from 40 to 54 years. All patients underwent coronal and axial MRI scans with T1 weighting
The interval between the date of the MRI scan and the date of surgery averaged 33 days and ranged from 1--49 days. The cerebellopontine angle (CPA) component of the tumor averaged 20.3 mms and ranged from 15--25 mms. The measured IAC lengths on axial MRI scans ranged from 11-- 17 mms, and the percent of the IAC involved with tumor ranged from 33 percent--58 percent. The observed percentage of the IAC involved with tumor at surgery was 100 percent in all patients. In these six patients, the axial MRI findings did not accurately predict the IAC findings at surgery.
The measured IAC lengths on coronal MRI scans ranged from 12--17 mms, and the percent of the IAC involved with tumor ranged from 38 percent--71 percent. The observed percentage of the IAC involved with tumor at surgery was 100 percent in all patients. In these six patients, the coronal MRI findings did not accurately predict the IAC findings at surgery.
Both the axial and coronal data show an absence of involvement of the lateral third of the IAC in five out of six patients, while at surgery tumor filled the IAC in all of these patients. The sixth patient had 71 percent involvement of the IAC, and so minimally involved the lateral third on MRI. While there is no universal agreement on indications of surgical approaches for acoustic neuromas, indications are based on well-accepted reasoning and probably represent a reasonable consensus of many acoustic neuroma surgeons.
Conclusions: Despite the touted accuracy of the ability of MRI to identify small acoustic neuromas, several cases have been identified where tumor was present in the lateral internal auditory canal and fundus at surgery, while the preoperative MRI scan showed a lack of enhancement in that region.
It is not clear why absence of gadolinium enhancement on MRI in the distal internal auditory canal and fundus may still be associated with the presence of tumor, but several explanations could be invoked. First, that there is something unusual with the tumor in this region. For example, if the tumor is necrotic, it may not enhance well with the intravenous paramagnetic dye, although it should still appear different than a distal internal auditory canal filled with normal cerebrospinal fluid. An adequate understanding of these issues is not presently available for acoustic neuromas. Alternatively, the sensitivity of the MRI study is less accurate than originally presumed.
It is also unclear why so many of these tumors have been identified in this study in only the past three years and why all patients in our series were male. Ultimately, the acoustic neuroma surgeon should not be surprised to find tumor in the lateral internal auditory canal in the absence of gadolinium enhancement, and should make surgical plans accordingly.
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