Symposium on low frequency harmonic acceleration, the rotatory chair. Responses to low-frequency harmonic acceleration in patients with acoustic neuromas.

There has been increased interest in harmonic acceleration (HA) for the evaluation of vestibular pathology in recent years. The purpose of this study was to determine the value of harmonic acceleration testing in 24 patients with surgically proven acoustic neuromas. All patients were tested preoperatively and postoperatively, and none were lost to follow-up. Thirteen were tested one year or more after surgery. For rotational evaluation, each patient was seated in an enclosed chair driven by a D.C. torque motor turntable system and rotated utilizing five low frequency sinusoids (.01 - .16 Hz). Slow eye movements in response to acceleration were analyzed by analog and digital computer techniques. Output measures were the phase relationships between the input stimulus velocity and the slow phase eye movement velocity and labyrinthine preponderance (LP) or asymmetry. Preoperative results showed that caloric examination identified 79% of patients with tumors and HA 67% (2 standard deviation criteria). When both tests were evaluated together they identified 91% of all patients with tumors. Of the 4 small tumors (less than 2 cm), HA identified 4 and caloric examination 2. All patients were tested approximately 7 days postoperatively and had statistically significant shifts in their phase and LP measures. The shifts in phase were stable up to 3 years after surgery; however, the LP returned toward normal values. These findings indicate that the vestibular system can compensate but not repair itself after the loss of one labyrinth. In those patients who continued to be symptomatic 1 year or more after surgery, the LP did not return to normal values. Harmonic acceleration testing complemented caloric testing in the identification of patients with tumors and objectively demonstrated patients with continued symptomatology after surgery.