AUDITORY BRAINSTEM RESPONSES IN PATIENTS WITH CEREBELLOPONTINE ANGLE TUMORS

Haralampiev K, Kitanoski B, Jakovijevic B, Milenkovic J.

ENT Department, Military Medical Academy

11000 Belgrade, Yugoslavia

The authors present their experience in recording ABR in 126 confirmed cerebellopontine angle tumors during the period from 1981 to 1995. The most of them were acoustic neuromas (76%). The ABR were recorded on an Amplaid MK-6 unit, mostly using a click stimulus at rate 21 per second and analysis time of 10 ms. The needle electrodes were applied to the vertex and each earlobe.

The normal ABR has three major composite peaks (I,III, V). Wave V is the most robust of the ABR component in normal hearing subjects and inpatients with cochlear hearing loss. The absence of waves III and V with a well-defined wave I is highly suggestive of auditory brainstem impairment. This type of ABR is significantly more often associated with large CPA tumors. Out of 33 tumors with such ABR, in our patients 75% were larger than 2 cm. Such bilateral ABR abnormality is commonly found in patients with multiple sclerosis, but it can also indicate bilateral acoustic neuromas. The interaural difference of the wave I to V interval (beyond 0.5 ms) is an important parameter in detecting the cerebellopontine angle tumors. The most reliable ABR manifestation of an acoustic neuroma is the increase of the I – III interpeak latency in the affected side. This type of the ABR were recorded in 34 tumors. Half of them were smaller than 2 cm.

Acoustic neuromas significantly more often than meningiomas and other CPA tumors affected contralateral response in the second segment of the ABR.

Auditory brainstem responses are very reliable for detecting CPA tumors, particularly Small acoustic neuromas. Associated with caloric vestibular testing they are the best way of selecting patients for further neuroradiological investigations.