Effects of contralateral white noise stimulation on distortion product otoacoustic emissions in myasthenic patients

Myasthenia gravis (MG) induces a reduction of transient evoked otoacoustic emissions (TEOAEs) and distortion product otoacoustic emissions (DPOAEs) th at reverses partially after administration of an acetylcholinesterase (AChE ) inhibitor. In normal subjects a contralateral acoustic stimulation (CAS) produces an amplitude reduction of TEOAEs and DPOAEs. This effect, called c ontralateral suppression (CS), is mediated by the efferent auditory system. Twenty subjects affected by MG underwent DPOAE recording with and without contralateral white noise in a drug-free baseline period ('basal') and 1 h ('post') after administration of a reversible AChE inhibitor. In 'basal' co ndition CAS did not induce significant DPOAE amplitude changes but a parado xical slight increase was observed. After drug administration, CAS produced a significant decrease of DPOAE amplitudes for middle frequencies (f(2) be tween 1306 and 2600 Hz). In normal controls CAS caused a significant decrea se (P < 0.001) for all frequencies. The amount of CS in controls and in the MG 'post' condition was not significantly different. The increased acetylc holine (ACh) availability following drug consumption seems to partially res tore outer hair cell function and enhances their electromotility; a further influx of ACh due to CAS yields to restoration of the CS. These findings a lso suggest that DPOAEs may be useful in the diagnosis of MG and for monito ring the effectiveness of treatment. (C) 2001 Elsevier Science B.V. All rig hts reserved.