Functional anatomy and physiology of the upper esophageal sphincter.

Upper esophageal sphincter (UES) refers to the high-pressure zone located in between the pharynx and the cervical esophagus. The physiological role of this sphincter is to protect against reflux of food into the airways as well as prevent entry of air into the digestive tract. UES is a musculocartilaginous structure with its anterior wall being formed by the full extent of the posterior surface of the cricoid cartilage and arytenoid and interarytenoid muscles in the upper part.

The effect of therapeutic drugs and other pharmacologic agents on activity of porphobilinogen deaminase, the enzyme that is deficient in intermittent acute porphyria.

Drugs and toxins precipitate life-threatening acute attacks in patients with intermittent acute porphyria. These materials may act by directly inhibiting enzyme activity, thus further reducing porphobilinogen (PBG) deaminase activity below the ca. 50% level that results from the gene defect.