METHODS

Subjects

Baseline autonomic measurements were made in 15 subjects (11 males) with DS(7), with group average age ± SD, 14.3 ± 3.6 years; and 15 age and sex matched normal subjects (NS). Also recordings were made in these 15 DS subjects and in another 10 DS subjects (n = 25; group average age ± SD, 17.0 ± 5.4 years; 19 males) to compare baseline recordings with those during exposure to auditory stimuli.

Assessment procedure

During assessments subjects were seated erect, with back support, in a dimly lit, sound attenuated recording room. Autonomic and respiratory measures were recorded with a 4-channel polygraph (Medicaid, Chandigarh, India) in 5-minute periods (baseline and during auditory stimuli). The EKG was recorded using standard limb lead 1 configuration. The skin conductance level (SCL) was recorded using Ag/AgCl electrodes covered with electrode jel and placed in contact with the volar surfaces of the distal phalanges of the index and middle fingers of the right hand. A low-level DC preamplifier was used and a constant voltage of 0.5V was passed between the electrodes. The respiration was recorded using a nasal thermistor clipped at the more patent nostril. The spontaneous sympathetic skin response (SSR) was recorded using Ag/AgCl electrodes filled with electrode gel (8). Electrodes were placed (a) on the back of the hand, in the second interosseous space, about 3-cm proximal to the interdigital web (b) on the palm of the hand. Amplifier settings were kept at: TC 0.1 ms, high cut filter at 70 Hz and sensitivity at 2 mV/cm.

Design

(i) To compare baseline differences between DS and NS, subjects were assessed in separate 10 minute sessions. (2) To study responses to auditory stimuli in DS subjects, the baseline period of 5 minutes was followed by another 5 minute period with 10 auditory stimuli spaced 30 seconds apart evenly, to elicit habituation.

Data extraction

The following data were extracted from the polygraph records: The respiratory rate (in cycles per minute) was calculated by counting the breath cycles in 60 second epochs, continuously. Finger plethysmogram amplitude (in mm) and skin conductance level (in mS) was sampled at 20-second intervals. Values averaged across each of the periods (before, after) of a session, were used for analysis. An SSR was said to occur as a response to the sound stimulus, if it occurred within 1 s and the amplitude was greater than 2000 mV.

Analysis

(i) Baseline data of DS and NS subjects were compared using the Student t-test for unpaired data. (ii) Data of the DS group (a) before and (b) during exposure to the stimuli were compared using the t-test for paired data. (iii) Number of DS subjects who showed habituation of the SSR to sound stimuli were compared to those who did not, using the McNemar test.