Internal Auditory Canal Mass in a 46-Year-Old Woman

A 46-year old woman presented with unilateral hearing loss, tinnitus, and dizziness. Magnetic resonance imaging revealed a nodule in the left distal internal auditory canal; T1-weighted images showed a hyperintense and nonenhancing nodule, and the T2-weighted images showed an isointense nodule. Pathologic studies showed adipose tissue separating nerve bundles and ganglion cells.

Intracranial lipomas are uncommon, occurring in only 0.08% of general autopsies.1 Lipomas of the IAC are rare, making up 0.15% of tumors in this area2 and only 2.9% of intracranial lipomas.3 Lipomas of the IAC, together with those of the cerebellopontine angle, constitute 9% of all intracranial lipomas.1,4 The age range of patients at presentation is 7 months to 82 years, with the tumor being most common in the third to fifth decades (mean, 40 years),5 and having a male predominance of 2–5:1.1,3,5,6 Left-sided lipomas are more common.5,6 The IAC lipomas are often symptomatic, in contrast to other intracranial lipomas, which are usually incidental findings.7 

Lipomas of the IAC may involve cranial nerves VII and VIII, whereas cerebellopontine angle lipomas may involve cranial nerves V through XII.1 Symptoms—unilateral hearing loss, tinnitus, and vertigo—are clinically and audiologically indistinguishable from those of eighth cranial nerve schwannomas.1,2,3,6 Vestibular symptoms such as gait unsteadiness have been reported.1 Symptoms are often present for years prior to presentation (average, 4.4 years; range, 5 months to 10 years).6 

The clinical differential diagnosis includes eighth cranial nerve schwannoma, the most common tumor in this area (∼90%),2,3 and meningioma, the second most common (3%) tumor, although rarely confined to the IAC.2 Other possibilities include cholesteatoma (∼2%), arachnoid cyst (0.5%), hemangioma (0.3%), metastases (0.3%), and epidermoid and dermoid cysts (0.15%).2,8 The clinical presentation of most of these lesions is similar because they all are space-occupying tumors. Hemangiomas should be suspected when there is a small lesion with severe symptoms, such as facial nerve palsy.2 

Magnetic resonance imaging findings are helpful in separating lipomas from other tumors in this region: MRI reveals a nonenhancing, often heterogeneous, hyperintense signal on T1-weighted images and hypointense or isointense signal on T2-weighted images (hyperintensity, as in our case).1,3,5,6 Fat-suppression T1-weighted images, which suppress the strong signals from fat that can interfere with signals from nearby tissue, are especially useful.3,5,6 These findings are in contrast to schwannomas, which show strongly enhancing, homogeneous, isointense signal on T1- and T2-weighted images.3 They show no diminution of signal intensity with fat-suppression techniques.9 Computed tomographic findings of a mass of very low density is highly suggestive, but not specific, of a lipoma as this may also be seen in some eighth cranial nerve schwannomas and cholesteatomas.5,6 

Grossly, these lesions are composed of soft, lobulated, yellow, fatty-appearing tissue that is difficult to dissect from the adjacent nerves because the growth pattern is infiltrative rather than expansile.8 Histologically, lipomas are composed of nerve fascicles, ganglion cells, and mature adipose tissue, which infiltrate and separate the nerve fibers.1,5 Cartilaginous elements have been described on rare occasion,5 although they are more common in other intracranial lipomas.7 When adipose tissue is present along with thick-walled blood vessels, smooth muscle, and skeletal muscle, the finding is best termed lipochoristoma.10 The primary histologic differential diagnosis is lipoblastic meningioma.1 Meningiomas, however, are typically much larger than lipomas, and only rarely are confined to the IAC.2 

The infiltrative nature of the adipose tissue into the nerve is unique to lipomas of the cerebellopontine angle and IAC (as opposed to other intracranial lipomas)1,7 and often precludes adequate dissection of the tumor from the cochlear nerve.3 The translabyrinthine approach appears to be the best approach for complete resection, although this is accomplished in only 33% of cases overall.5 Partial resolution of symptoms is achieved in 43% of patients. Many patients (68%) have postoperative deficits, most often hearing loss.5 No cases with rapid growth or malignant transformation have been reported.7 Because of the slow growth of lipomas and the inherent difficulties in their removal, excision is not always possible or necessary, and a more conservative approach, such as repeated monitoring of the lesion by MRI, is suggested.1,5 When surgical excision is performed, however, recurrence is rare.1 

The origin of lipomas in this location is uncertain, but they are believed to be a malformation rather than a neoplasm.4 Theories have been proposed that suggest that intracranial lipomas are mesenchymally derived and are associated with an abnormal persistence of primitive meninges undergoing differentiation into mature adipose tissue.1,2 The origin of the seventh and eighth cranial nerves from the brainstem at the posteromedullary junction is thought to provide an opportunity for lipomas to develop within the cerebellopontine angle and IAC.1 Although these lesions are rare, lipomas of the IAC are important to recognize because clinically they resemble the much more common schwannoma. Mature adipose tissue in the IAC is never a normal finding and its presence is diagnostic of a lipoma.7