Recent insights into hyperventilation from the study of Rett syndrome.

The medical literature presents two major groups of disorder in which hyperventilation is a presenting feature. Disorders of mood, notably panic disorder, and disorders of brain stem function—developmental, vascular, traumatic, toxic, metabolic, degenerative or neoplastic. References to the underlying brain morphology leading to these conditions are few and imprecise, perhaps reflecting the inaccessibility of the brain stem to clinical medicine and the complexity of the neural basis of respiratory control. Even in Joubert’s syndrome, where hyperventilation is a major feature, hypoplasia of the cerebellar vermis and cyst of the fourth ventricle have been described but the exact pathophysiology is unclear. The clinical medical literature gives more space to discussing the eVects of hyperventilation, including the well known respiratory alkalosis with reduction in plasma ionised calcium, tingling sensations, tetany, induction of epileptic seizures, and induction of interictal epileptogenic activities seen on the electroencephalogram (EEG). In healthy children, especially girls, voluntary hyperventilation produces slow EEG waves, usually generalised over the cerebral cortex. Hyperventilation reduces oxygen delivery to the brain, and cardiac repolarisation abnormalities including ST depression and T wave inversion have been described. From its delineation in 1937 to the 1980s, the “hyperventilation syndrome” was held responsible for much of the symptomatology of panic disorder; however, with increased accuracy of measurements, this notion was replaced by the view that autonomic instability underlies both hyperventilation and panic disorder. Although there is a clear association between panic and hyperventilation, the neurological basis for this is still unresolved.