Developmental effects of ultrasound exposure in mammals

Abstract

A consideration of the principles of teratology indicates a number of important factors: • •|teratogenic susceptibility depends on a genotype • •|teratogenic susceptibility varies with developmental stage at the time of treatment • •|adverse influence of developing tissue depend on the nature of the agent • •|teratogens act in specific ways to initiate abnormal development • •|developmental toxicity is dose-related • •|there are four primary manifestations of deviant development-developmental toxicology Embryotoxicity is assessed in terms of lethality, prenatal and post-natal weight reduction, malformations, or behavioural/neuromuscular impairment. The effects of exposure depend on cellular organisation: • •|for independent cells such as germ cells and other cells in culture there is no threshold and the percentage of cells killed depends on dosage received. • •|when cells are associated in embryos before organogenesis a threshold for embryo death occurs when a critical number of cells is killed, otherwise regeneration occurs. • •|for associated cells in embryos during organogenesis a threshold for teratogenesis occurs when a critical number of cells in particular organs is affected, otherwise regeneration occurs. Hyperthermia is a known teratological agent and experiments have shown exposure of the whole body of mice for 1 h at temperatures above 42°C results in an 8% increase in microcephaly for every increase in temperature by one degree centigrade (Edwards et al.). The main effect of ultrasound irradiation on mammals results from a temperature increase due to the absorption of the ultrasonic energy. Hyperthermia resulting from continuous wave insonation of the fetus through the maternal abdomen reduces intrauterine fetal growth, and experiments in mice have shown that a linear relationship exists between reduced weight and I2t. Insonation of the fetus through the exteriorised uterus significantly reduces the amount of tissue available for absorption of the incident energy, thereby producing less heating. In these types of experiments mice were unaffected at intensities up to 1 W/cm2 SATA.