SUDOMOTOR SYMPATHETIC HYPOFUNCTION IN DOWN'S SYNDROME
Naveen K. V. and Shirley Telles.
Vivekananda Kendra Yoga Research Foundation, Bangalore, India.
(Received on June 28, 1999)
Abstract : General sympathetic dysfunction has been proposed as an explanation for the inability to reach normal heightened attention in Down's syndrome (DS). The present study on 15 DS subjects (group average age ± SD, 14.3 ± 3.6 years; 11 males) and in an equal number of age - and gender - matched normal subjects (NS), evaluated activity in different subdivisions of the sympathetic nervous system. DS subjects had (i) lower skin conductance levels (i.e., lower sudomotor sympathetic activity) and (ii) higher heart rates than NS. In response to auditory stimuli, DS subjects showed abnormal SSR responses (also indicating sudomotor sympathetic activity) but normal cutaneous vasoconstriction. Hence the results suggest that sympathetic dysfunction in DS is restricted to the sudomotor subdivision, activity of which has been associated with attention and recognition.
Key words : Down's syndrome; sudomotor sympathetic; autonomic dysfunction; focused attention.
Autonomic nervous system dysfunction is frequent in persons with Down's syndrome (DS) and is constituted mainly by an imbalance between sympathetic and vagal systems (1). Standard tests (i.e., postural change and the cold pressor test) revealed no differences between DS subjects and age matched controls, with respect to heart rate, BP, and plasma catacholamines (2). A decrease in heart rate variability and breath amplitude, which is normally associated with focused attention while on a task, was not seen in retardates, who showed an opposite trend, which is incompatible with sustained attention (3). Also, autonomic activity during the phases of sleep has been studied in DS to understand the greater incidence of central sleep apnoea in these persons (1). DS subjects showed lower sympathetic activity during REM sleep compared to control subjects (4). A series of studies reported by Luria in 1963 (5), suggested that mentally retarded children could be distinguished from non retarded children on the basis of the orienting response (OR). This
"what is it?" response, reflects a very basic reaction to inputs from the surroundings. Some retarded individuals were shown to have unstable or weak OR, which, if they occurred, extinguished rapidly. Children with DS showed electrodermal hyporesponsivity in their ORs to auditory stimuli(6).
In the studies cited above (1-4), autonomic activity in the mentally retarded was described in relation to a task or a change in arousal, as in sleep. The present study compared autonomic and respiratory measures at baseline in subjects with DS compared with age-matched, normal subjects. Also, studies on habituation of the ORs (5-6) described changes in electrodermal reactivity (EDR). The present study reported responses of other autonomic measures (e.g., heart rate and cutaneous blood flow) during exposure to auditory stimuli, in subjects with DS.