DIAGNOSIS Angiolipomatous hamartoma of the cochlear nerve
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 - 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 - 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 (7-9). 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.
Contributed by Klaus Bumm, Abbas Agaimy, Gerald Niedobitek, Heinrich Iro, Helmut Steinhart