Lecithin Sphingomyelin Ratio

Excerpt

The Lecithin-to-Sphingomyelin Ratio (L/S ratio) is one of several methods for clinicians to assess fetal lung maturation. This biochemical test was first introduced in the 1970s, where a sample of amniotic fluid was collected via amniocentesis to determine the risk of the neonate developing respiratory distress syndrome (RDS). The sample was then evaluated, utilizing thin-layer chromatography to assess the size of lecithin relative to sphingomyelin. Historically, this test was useful for clinicians to attempt to time the delivery of infants prior to 39 weeks gestation in hopes of minimization of RDS. In recent years, the test has had a decline in use due to guidelines and recommendations from major medical societies.

Fetal lung development occurs as a gradual process in which the maturation of the pulmonary system progresses with increasing gestational age. There are five stages of lung development, listed in sequential order: embryonic (3 to 7 weeks), pseudo glandular (5 to 17 weeks), canalicular (16 to 26 weeks), saccular (26 to 36 weeks), and alveolar (32 weeks through childhood). The major maturation of the fetal lungs occurs during the alveolar stage when the maturation of type II pneumocytes occurs. Type II pneumocytes are responsible for the production of surfactant, which is critical for the support of alveoli in the lungs to combat surface tension.

The effect of the surfactant prevents the collapse of the alveoli during expiration. The composition of the surfactant includes phospholipids, proteins, and lipids. Of notable importance is the composition of the phospholipids. Phosphatidylcholine, also referred to as lecithin, is one of the phospholipids present in mature surfactant. It is stored and secreted by organelles called lamellar bodies.

These organelles appear after 22 to 24 weeks gestation. Before the 28th week of gestation, the fetal lung primarily synthesizes sphingomyelin, a nonpulmonary lipid. At approximately 32 weeks gestation, the number of lamellar bodies increases, which translates into increased surfactant present in the fetal lungs and amniotic fluid. At this point in lung maturity, the ratio of lecithin and sphingomyelin are relatively equal in concentration. By 35 weeks of gestation, the mature surfactant has been produced and is marked by a sharp increase in the concentration of lecithin in the fetal lungs and amniotic fluid. The lecithin to sphingomyelin ratio of 2:1 or greater is characteristic of mature fetal lungs. Fetuses delivered prior to this gestational age are at increased risk of neonatal respiratory distress syndrome.