Mid latency and slow vertex responses during pregnancy.

Central auditory pathways picked up electro-physiologically as mid latency responses (MLRs) and slow vertex responses (SVRs) have been studied least in women during their critical periods of life although auditory brainstem responses (ABRs) have been studied by many researchers. In the present study MLRs and SVRs were recorded in 20 pregnant women of age group 18-28 years. Their period of gestation ranged between 26-40 weeks and pregnancy had been uneventful and normal. MLRs and SVRs were recorded from Cz-A1 and Cz-A2 positions with alternating 90 dB sound pressure click stimuli delivered at 5 Hz and 0.5 Hz respectively. 256 stimuli for mid-latency and 64 stimuli for slow vertex responses were averaged and analyzed. Different waves of these auditory evoked responses were compared with 20 age matched non-pregnant females. The data obtained was analyzed for each variable by using unpaired student's T test. Present study did not reveal any difference in MLR waves during pregnancy when compared with the non-pregnant females whereas all the SVR waves were found to be significantly delayed in pregnant females. As SVR generators are found in different cortical areas, it can be said that auditory information processing at the higher centers is slow during pregnancy which in turn could be due to elevated levels of sex hormones specially estrogen and progesterone during pregnancy.

Long latency auditory evoked responses in ovulatory and anovulatory menstrual cycle.

Central nervous system pathways get influenced by the changing hormonal levels across the menstrual cycle. In an effort to see the effects on the conduction in central auditory pathways we recorded long latency auditory evoked potentials (LLAEPs) across the 4 different phases of the menstrual cycle. 20 females having normal ovulatory menstrual cycles were tested 4 times in a single cycle and LLAEPs were recorded from Cz-A1 and Cz-A2 position with alternating 90 dB sound pressure click stimuli. Twenty age matched control females having anovulatory menstrual cycles were also tested on the corresponding days. All control females were taking oral contraceptive (O.C.) pills. The LLAEPs were compared inter-group wise as well as inter-phase wise for each parameter by using hierarchal ANOVA design and Tukey test was applied to find out the significance level. Peak latencies of waves P2 and N2 were found to be delayed during mid-cycle and relatively reduced mid-luteal phase in ovulatory menstrual cycles. No such response was noticed in oral contraceptive using group. This indicates that normal cyclic variations of female sex hormones especially estrogen and progesterone modify the central processing of the auditory information. Estrogen may be responsible for delaying the conduction by influencing GABA release at the level of polysensory association areas of the brain.

Auditory evoked responses during different phases of menstrual cycle.

The electrophysiological correlates of changes in sensory function during menstrual cycle has already been studied and attributed to the hormonal influence. Effects of estrogen and progesterone on waves of auditory brainstem responses (ABR) have been reported and a hypothesis has been proposed that sex steroids have more influence on central auditory pathways. As mid-latency responses (MLRs) and slow vertex responses (SVRs) are better indicators of central auditory pathways, so MLRs and SVRs were also recorded besides ABRs in the present study. Waves of ABRs, MLRs & SVRs were recorded in 20 normal cycling females in 4 different phases of menstrual cycles from Cz-A1 and Cz-A2 position with alternating 90dB sound pressure click stimuli. Contralateral ear was masked with a white noise of -40 dBHL. With the same setting by changing the number of click stimuli, intervals of stimuli and filter bandpass the above 3 recordings were taken. The evoked responses in females having ovulatory cycles were compared within the four phases using ANOVA test. There is a trend of increase in peak latencies of ABR waves III and V and IPL I-V in estrogen-peak midcycle while decrease in latencies in progesterone-peak (interpeak latency) midluteal phase. Peak latencies of MLR waves No, Po, Na, Pa and Pb also show a same trend. SVR waves P2 and N2 are significantly delayed in mid-cycle (178.80 +/- 20.49, 276.65 +/- 18.32) while conduction is faster in midluteal phase (166.45 +/- 17.41, 261.95 +/- 21.07). Smallest latencies of all the waves are occurring during menstruation. These findings are suggesting that normal cyclical variations in the levels of estrogen and progesterone during menstrual cycle do affect the auditory pathways and effects are better seen on the central component.