The reason why medications such as theophylline and terbutaline (T&T) have been given to patients with SCLS, if my understanding is correct, is because they are compounds that strengthen or prevent the breakdown of cells, including of capillary walls -- in medical jargon, they prevent a decrease, or foster an increase, in cyclic adenosine monophosphate (cAMP) levels in endothelial cells that line the interior surface of blood vessels.
They have been used for decades to prevent and treat wheezing, shortness of breath, and difficulty breathing caused by asthma, chronic bronchitis, emphysema, and other lung diseases. They relax and open air passages in the lungs, making it easier to breathe -- but they do not cure asthma. Both have serious side effects, however. Singulair (montelukast), on the other hand, is a more modern asthma medication with few side effects that is supposed to have the same effect of strengthening cell walls.
Doctors at the Mayo Clinic -- including Dr. Greipp -- were among the first to give T&T to patients with SCLS, and now also Singulair, in the hope that these medications would prevent capillary leakages. Experience shows that sometimes they seem to work, sometimes they clearly do not.