Rationale and Objectives
To evaluate neurochemical alterations in Heschl’s gyri and determine the most affected side in case of unilateral acoustic neuroma using magnetic resonance spectroscopy (MRS).
Materials and Methods
Fifteen patients with unilateral acoustic neuroma were studied. Following routine cranial MRI sequences, MRS of Heschl’s gyri on tumor and nontumor sides was obtained. MRS metabolite values of both Heschl’s gyri were statistically compared.
Results
The values of N-acetylaspartate (NAA) and Cr on nontumor side Heschl’s gyrus (HG) were significantly lower than that on tumor side.
Conclusions
We found nontumor side HG more affected with lower NAA and Cr values, suggesting neuronal damage and decreased energy metabolism compared to the tumoral side.
Acoustic neuromas arise from the vestibular nerve and are caused by an overproliferation of Schwann cells. The incidence of acoustic neuroma is estimated to be about 0.7–1 per 100,000 per year. The mean age of those affected is usually about 45–50 years . The most common symptom observed in patients with acoustic neuroma is a progressive asymmetric or unilateral sensorineural hearing loss. About 90% of patients present with gradual progressive hearing loss in one ear. However, about 5% will present with a sudden hearing loss and many of them experience tinnitus in one ear . About 3% of patients with acoustic neuroma will have normal hearing at presentation . Tumor growth within the internal auditory canal (IAC) and resultant compression of cranial nerves VII and VIII cause associated symptoms . There is a significant correlation between the site of origin of the tumor and the incidence of subjective hearing loss .
The mechanisms of hearing loss and affected regions by the disease are still unclear. Hearing impairment in patients with acoustic neuroma has been associated with a retrolabyrinthine disturbance because the tumor originates in the vestibular nerve and impairs cochlear nerve function. Tumor-related hearing loss may be due to direct cochlear nerve damage, sensory organ impairment resulting from disruption of the circulation by vascular compression in the IAC, or a combination of these two factors . Because of the bilateral projections from the ears, it is believed that unilateral lesions have no effect on hearing, although minor and transient hearing loss in the contralateral ear resulted from acoustic neuroma. The effect of acoustic neuroma on the primary auditory cortex is critical. Because lesions in the Heschl’s gyrus (HG) may cause symptoms similar to those caused by lesions in other parts of the auditory pathway, radiologists may need to localize the lesions in relation to the HG .
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Material and methods
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Results
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Table 1
Mean Values of Metabolites of Heschl’s Gyrus From Tumor and Nontumor Sides in Patients with Acoustic Neuroma ∗
Groups N-acetylaspartate Choline Creatine Mean ± Standard Deviation (SD) Mean ± SD Mean ± SD Tumor side 137.87 ± 29 69.91 ± 10 79.99 ± 11 Nontumor side 120.67 ± 32 62.29 ± 14 69.68 ± 13P <.05 † >.05 <.05 †
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Discussion
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