Diagnosis and treatment of lipomas of the internal auditory canal.
A Chinese woman came to the otolaryngology department with a complaint of a longstanding progressive left hearing loss and intractable tinnitus. A finding of asymmetry on sensorineural hearing loss and auditory brainstem response testing provided evidence of a retrocochlear lesion on the left side. Computed tomography and magnetic resonance imaging detected a tumor in the left internal auditory canal (IAC). In addition to these radio-logic features, our suspicion of the possibility of an IAC lipoma was raised by the observation of multiple lipomas over the patient's trunk and limbs. The patient underwent a complete tumor resection via the translabyrinthine approach. Pathology confirmed the diagnosis of an JAG lipoma. Although we were not able to preserve the hearing in her left ear, the patient was satisfied that we had relieved her constant tinnitus. In this article, we review the particulars of this case and discuss the clinical, radiologic, and pathologic features of IAC lipomas.
Lipomas of the internal auditory canal (IAC) are rare, benign neoplasms that occur primarily in white men during the third to fifth decades of life. To our knowledge, the first reported case of an IAC lipoma was seen on autopsy by Klob in 1859.  Until now, only 84 cases of either IAC or cerebellopontine angle (CPA) lipoma have been reported in the English-language literature.  We describe a new case of IAC lipoma in a patient who also had multiple lipomas over the trunk and extremities. We also discuss the clinical symptoms, radiologic characteristics, and pathologic features of IAC lipomas, along with treatment options and surgical complications.
A 41-year-old Chinese woman reported that she had been experiencing a progressive left hearing loss with intermittent tinnitus for many years. The tinnitus had become constant and intractable despite medical treatment during the previous 6 months.
An otoscopic examination revealed no abnormalities. A general physical examination, however, revealed that the patient had many elastic and nodular subcutaneous lesions over her left forearm, abdomen, and buttocks. She also had many pigmented nevi over her forearm.
An audiogram detected a left sensorineural hearing loss. The patient's pure-tone average was approximately 50 dB, her speech reception threshold was about 40 dB, and her speech discrimination score was 94%. Electronystagmography and caloric testing found no unilateral weakness or directional preponderance. Her auditory brainstem response showed a 0.26-msec interaural wave V delay on the left. A finding of asymmetry on sensorineural hearing loss and auditory brainstem response testing provided evidence of a retrocochlear lesion on the left side.
High-resolution computed tomography (CT) detected a lesion in the left IAC with a signal of -29 to -109 Hounsfield units (figure 1A).T1-weighted magnetic resonance imaging (MRI) demonstrated a 10 x 5-mm lesion in the same location (figure 1B). On TI-weighted axial and coronal MRI, the lesion emitted a hyperintense signal. Conversely, on T2-weighted coronal MRI, it emitted a hypointense signal (figure 1C). After gadolinium administration, there was no significant contrast enhancement (figure 1D). These findings are compatible with the presence of a lipoma.
The patient underwent a resection of the IAC tumor via a translabyrinthine approach. During surgery, we noted that the soft, yellowish tumor appeared to arise from the inferior vestibular nerve and adhere tightly to the facial nerve. We removed the 8 x 8 x 4-mm tumor with a sharp dissection, but we had to sacrifice the auditory nerve to do so. Later, we also removed a 3.0 x 2.7 x 1.4-cm soft tissue mass from the left forearm.
Following surgery, the patient experienced vertigo, cerebrospinal fluid leakage, and facial weakness (House Brackmann grade IV). Re-exploration with fat obliteration was performed. Once the leak was corrected, the vertigo subsided. On histology, the IAC tumor showed nerve tissue with fatty infiltration (figure 2), while the forearm tumor showed an angiolipoma. At the 6-month followup, the patient's tinnitus had disappeared, her facial weakness had improved to House-Brackmann grade II status, and a repeat MRI detected no evidence of tumor recurrence.
Intracranial lipomas are very rare, accounting for only 0.1% of all cerebral tumors.  Lipomas also make up less than 0.14% of all tumors in the CPA.  Prior to the publication of this article, only 36 cases of IAC lipoma have been reported, most of them in whites.  No case has ever been reported of a patient with an IAC lipoma in addition to multiple lipomas over the trunk and limbs.
On histologic study, typical JAG and CPA lipomas contain mature adipose cells and varying degrees of fibrovascular tissue.  The adipose tissue thoroughly infiltrates and separates the involved nerve fibers. These lesions are probably malformative rather than neoplastic in nature.  Lipomas usually grow extremely slowly, and no malignant transformation has been reported. If a patient with a definite diagnosis exhibits no clinical symptoms, conservative followup is sometimes the best option.
Unlike other intracranial lipomas, IAC and CPA lipomas can intimately involve the cranial nerves and produce progressive focal symptoms. According to a study by Bigelow et al, the three most common symptoms of IAC and CPA lipomas are hearing loss (in 64% of patients), dizziness (44%), and tinnitus (43%).  The most frequently involved cranial nerves are the auditory nerve (98%), facial nerve (85%), and trigeminal nerve (24%).  Because these lesions are so infiltrative in nature, it is sometimes difficult to preserve hearing during surgery. When a total resection of the tumor is performed, it is usually undertaken via the translabyrinthine approach, in which hearing cannot be preserved. Because there is a high risk of postoperative neurologic sequelae, surgical intervention is indicated only when the patient has significant progressive or disabling symptoms.
Before proceeding to surgery or deciding on no treatment, it is necessary to confirm the diagnosis with imaging studies. The characteristic findings of lipomas on CT and MRI are well described.  Fat is hypointense on CT, which is reflected by the negative Hounsfield units (usually -25 to -100). On MRI, fat is hyperintense on T1-weighted images and hypointense-to-isointense on T2-weighted images. Gadolinium contrast offers no significant enhancement. Fat-suppression techniques can be applied to MRI to suppress the strong signals from fatty tissue, which interfere with the signals from adjacent areas. Nowadays, fat-suppression MRI is the best choice for diagnosing lipomas of the temporal bone. 
The authors thank Yee-Jee Jan, MD, of the Department of Pathology at Taichung Veterans General Hospital and Shien-Chi Kong, MD, of the Department of Radiology for their assistance in the preparation of the figures that accompany this article.
From the Department of Otolaryngology, Taichung Veterans General Hospital, Taichung, Taiwan, Republic of China.
Reprint requests: Chen-Chi Wang, MD, Department of Otolaryngology, Taichung Veterans General Hospital, 160 Taichungkang Rd., Sec. 3, Taichung 40705, Taiwan, R.O.C. Phone: +886-4-2359-2525, ext. 5401; fax: +886-4-2359-6868; e-mail: firstname.lastname@example.org
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|Comment:||Diagnosis and treatment of lipomas of the internal auditory canal.|
|Publication:||Ear, Nose and Throat Journal|
|Date:||May 1, 2001|
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