Hyperventilation, which occurs in some patients with severe mental handicap, is a prominent feature in the histories of most girls with Rett syndrome but its mechanism and effects have not been established. Respiratory function was therefore studied in 18 patients with Rett syndrome and 23 healthy controls. Ten of the patients (56%), but none of the controls, hyperventilated only when awake, and began doing so after a period of normal breathing without hypoxaemia. After hyperventilation was established it was interspersed with prolonged periods of apnoea (over 19 seconds) accompanied by Valsalva manoeuvres. Hypoxaemia (less than 90%) occurred in 47% of these periods of apnoea and five (50%) of the patients had oxygen saturation values of under 50%. During hyperventilation severe hypocapnia developed in every patient, and recorded arterial pH measurements ranged from 7.47 to 7.60. A further four patients (22%) did not hyperventilate, but had clear histories of hyperventilation when younger. All had frequent apnoeic pauses accompanied by Valsalva manoeuvres. The remaining four girls (22%) neither hyperventilated nor gave a clear history of doing so. Three had occasional apnoeic pauses associated with the Valsalva manoeuvres. All but one of the 18 patients had increased quantities of periodic apnoea compared with the control subjects. The hypocapnic alkalaemia and hypoxaemia resulting from hyperventilation may contribute to the cerebral impairment in Rett syndrome. Since the hyperventilation is 'primary', and not secondary to preceding apnoea, it is potentially treatable. Further studies will determine if treatment is practical and of benefit.
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