A 47‐YEAR OLD MALE WITH A CEREBELLOPONTINE ANGLE TUMOR

CLINICAL HISTORY AND IMAGING STUDIES

A 47‐year‐old male patient presented with a fluctuating hearing impairment in his left ear over the past 5 years. Tinnitus or vertigo was not observed. Audiometric analysis showed an inner ear deficit of 50 dB between 1500 and 6000 kHz on the left side. Hearing in the right ear was normal. The facial nerve was clinically and by means of electrophysiological testing without pathological findings. BEAP (brainstem evoked auditory potentials) revealed a latency increase between J1 and J3 up to 2.5 ms for the left side, whereas only 2.3 ms on the right side. MRI (magnetic resonance imaging) scanning showed a tumor of the cerebellopontine angle in the left inner auditory canal (IAC) of 1.2 x 0.7 x 0.9 cm in size (Figure 1). After application of contrast media, the tumor showed clear signal enhancement.

Figure 1.

The tumor was entirely removed by a transtemporal approach to the IAC. Surgical exploration found the cochlear nerve embedded in a tumorous mass, whereas the vestibular and the facial nerve were normal (Figure 2a, vestibular nerve arrow A and cochlear nerve with tumor arrow B).

Figure 2.

Nerve and tumor (Figure 2b) were removed and sent to histopathological examination. The patient lost his hearing after the operation due to the removal of the cochlear nerve, whereas a regular postoperative vestibular function was observed. The postoperative course as well as the 5‐year follow‐up examination was unremarkable and control MRI scanning showed no recurrence.

PATHOLOGIC FINDINGS

Histopathological examination revealed a tumor composed of loosely packed irregular nerve fibres embedded within loose connective tissue (Figure 3a, H&E‐stain, x 100). Mature adipose tissue elements and numerous capillary‐ to medium‐sized thin‐walled vascular channels were diffusely dispersed between nerve fibres. In addition, isolated adipocytes were seen within the endoneurium of some nerve fibres, but no skeletal muscle cells, ganglionic cells, glial elements or other tissue derivatives were seen. Also, hemosiderin pigment, scarring or inflammatory cells as would be anticipated in amputation (traumatic) neuroma were not observed. Immunohistochemistry revealed a strong expression of protein S100 and neurofilament protein in the nerve axons and Schwann cells (Figure. 3b, x 100). An intact perineurial sheath was highlighted by reactivity for epithelial membrane antigen (Figure 3c, EMA).

Figure 3.

DIAGNOSIS

Angiolipomatous hamartoma of the cochlear nerve

DISCUSSION

Most tumors of the cerebellopontine angle are schwannomas and over 90% originate from the superior branch of the vestibular nerve. Tumors originating from the cochlear nerve are rare 1, 2, 3. This case raises two major issues. The first one relates to the precise allocation of the tumor to the affected nerve and the probable histological diagnosis based on clinical and imaging findings. The second issue concerns the pathogenesis of this peculiar lesion at this unusual location.

The allocation of tumors to the cochlear nerve within the IAC is most likely when they expand towards or even into the cochlea 4, 5, 6. In this case there was no signal enhancement detectable within the cochlea, so we did not presume a pathology arising from the cochlear nerve. The only denominator was a progressive hearing impairment, a non‐specific symptom for cochlear nerve involvement. After surgical exploration it became apparent that the tumor was clearly originating from the cochlear nerve and association to the surrounding nerves could be excluded. This contrasts with schwannomas of the vestibular nerve as they are known to merge with surrounding nerves and therefore often hamper an easy determination of their origin.

Since the majority of tumors originating in the IAC are schwannomas, alternative diagnoses at this anatomic site seem quite unusual. The histopathological differential diagnosis of our case included traumatic neuroma, true circumscribed “palisaded” neuroma, neuromuscular choristoma and lipomatous hamartoma of the cochlear nerve. Schwannoma and neurofibroma could be primarily excluded based on well defined criteria. Against a diagnosis of circumscribed neuroma argue the irregular borders of the lesion and the prominent angiolipomatous component. Likewise, absence of a history of previous surgery or trauma of any kind, and lacking of typical microfascicular pattern of traumatic neuroma as well as signs of scarring, old bleeding and inflammatory cells preclude a diagnosis of traumatic neuroma. A more plausible explanation would be a non‐traumatic “choristomatous” or “hamartomatous” origin. In fact, the finely distributed adipocytic elements and prominent vascular component represent a strong clue to the choristomatous nature of the lesion. However, no skeletal muscle fibers were seen, thus excluding a neuromuscular hamartoma (choristoma) of cranial nerves.

Hamartomas of the IAC are exceedingly rare lesions and most were described as case report or small series 7, 8, 9. Lesions described by Wu et al (9) under the rubric of “lipochoristoma” showed a heterogeneous histology, some of them contained skeletal muscles and thus represent neuromuscular choristomas. Other rare lesions described as lipomas of the IAC are probably lipomatous hamartomas, as they were less circumscribed and lacked the encapsulation typical of true benign lipomatous tumors of the soft tissue. However, given the finely dispersed fatty and vascular components and absence of other tissue elements, we suggest the term “angiolipomatous hamartoma” for this peculiar lesion to alert to its high vascularity that might suggest a more serious pathology on imaging procedures.

In summary, we described an unusual case that we believe to represent the first description of an angiolipomatous hamartoma of the cochlear nerve. The pathogenesis of this rare lesion at this location remains unknown. Hamartomas should be included in the pre‐operative differential diagnosis of acoustic or vestibular schwannoma.

ABSTRACT

A 47‐year old man presented with a five‐year history of fluctuating hearing impairment in the left ear. There was no tinnitus or vertigo. Imaging studies demonstrated a contrast‐enhancing cerebellopontine angle mass in the left internal auditory canal. Surgically the lesion was attached to the cochlear nerve. Pathological evaluation revealed what is best described as an angiolipomatous hamartoma of the cochlear nerve. Similar lesions have only rarely been described.