The Abnormal Internal Auditory Canal: The Diagnosis of Acoustic Neuroma

THERE IS no doubt that the number of acoustic neuromas diagnosed during the past few years is greater than ever before. This is the result of two factors: (a) the higher index of suspicion for such a lesion by the physician and especially by the otologist and (b) the refinement of the diagnostic procedure for such a lesion. The possibility of an acoustic neuroma should be entertained in all cases of unilateral sensorineural hearing loss or vestibular loss of unknown origin. In the past this diagnosis was not suspected until the development of involvement of multiple cranial nerves and often increased intracranial pressure. Pathology According to the literature, acoustic neuromas account for approximately 8 per cent of all intracranial tumors and for 80 per cent of the tumors of the cerebellopontine angle. In our series of 85 surgically proved tumors of the cerebellopontine angle, 78 cases or 92 per cent were acoustic neuromas and 7 or 8 per cent other tumors. The report of Hardy and Crowe who found 6 small and unsuspected acoustic neuromas, bilateral in one case, within the internal auditory canal during the autopsy of 250 unse1ected cases, certainly suggests that these tumors are far more frequent than is generally thought. At least two-thirds of the tumors of the eighth cranial nerve originate from its vestibular division and less than one-third from its cochlear portion. Most but not all of the acoustic neuromas arise within the internal acoustic canal, presumably at the junction between the neurolemmal sheath deriving from peripheral ganglia and the neuroglial fibers extending peripherally from the brain stem. Acoustic neuromas are benign, encapsulated tumors which derive from the proliferation of neurolemmal or Schwann cells of the eighth cranial nerve. Histologically they are made up of streams of elongated spindle cells with fairly large nuclei of various shapes, often arranged in a palisade pattern. As the tumor becomes larger, degenerative changes may occur in its center with formation of cysts. Symptomatology Owing to the subarachnoid space within the internal auditory canal and in the cistern, acoustic neuromas do not produce any appreciable symptomatology in the early stage. As the tumor grows and starts to impinge upon the nerves and vessels, however, the clinical findings begin to appear. It seems obvious at this point that tumors arising within the internal auditory canal, where the subarachnoid space is relatively small, become symptomatic much earlier than tumors arising in the cistern. The latter often reach a voluminous size before producing clinical manifestations. The early symptoms due to impairment of the eighth cranial nerve are usually unilateral sensorineural hearing loss followed by slight dizziness or imbalance and less frequently true vertigo. In some cases, however, the vestibular involvement may precede the auditory findings.