Coexistence of pupillary and heart sympathergic asymmetries in cluster headache. 
Ten cluster headache patients and 10 healthy controls were subjected to electrocardiographic and pupillometric procedures in a search for cardiac and pupillary sympathergic asymmetry.  Sympathergic stimulation was provoked by hyperventilation and by instilling tyramine into both eyes.  In the control group, hyperventilation changed neither the T-wave form and polarity nor the QTc.  Tyramine provoked an equal mydriasis on the two sides.  In cluster headache sufferers, hyperventilation produced changes in the T-wave form and polarity as well as an increase of the QTc due to a disproportionate shortening of the R-R and Q-T intervals.  An unequal mydriasis was noted after tyramine instillation due to less marked response on the symptomatic side.  The observed electrocardiographic abnormalities are considered an expression of an asynchronous repolarization attributed to a sympathergic asymmetry.  It is postulated that both the cardiac and pupillary sympathetic imbalance associated with cluster headache are central in origin.