N Tuong-Vi, Jutras B, Monnier P, Muckle G, Velez MP, Arbuckle TE, Saint-Amour D. Psychoneuroendocrinology 2019 Feb 14;104:33-41. doi: 10.1016/j.psyneuen.2019.02.015.
In infants, sex differences in hearing have been known for several years; however, the mechanisms remain unclear. Increasing evidence has indicated that sex hormones, including the “male” hormone androgen, may be primarily responsible for the sex differences in hearing. Androgens are produced early in the development of male human foetuses and cause a masculinization of the prenatal brain and body. There are a range of postnatal physical characteristics that can be measured as indicators of fetal androgen exposure. These include anogenital distance (AGD) (the distance from the anus to the genitalia), the ratio of the length of the second and fourth fingers (2D:4D ratio), length and width of penis, and degree of skin pigmentation of the areolar (nipple) and genitals. The prenatal-androgen exposure theory suggests that early exposure to high levels of androgens may weaken auditory function of the inner ear.
Otoacoustic emissions (OAEs) tests are widely used as a method to test auditory function in newborns. OAEs are sounds coming from the inner ear (cochlea) in response to a stimulus sound, and can be recorded by a microphone fitted into the ear canal. Click-evoked OAEs (CEOAEs) and distortion-product OAEs (DPOAEs) are two of the various measurable forms of OAEs. CEOAEs are brief, echo-like sounds that can be recorded after an auditory stimulus. These sounds are typically stronger in females than males, and stronger in the right vs. left ears. DPOAEs also show small sex differences, though these are less pronounced than those seen in CEOAEs.
This study was designed to examine the relationship between physical markers of prenatal androgen exposure and hearing in infants. These markers were measured at birth (AGD, penile length/width, areolar/genitalia pigmentation) or at 6 months of age (2D:4D, two types of OAEs) in 25 boys and 24 girls from the MIREC-ID study.
This study found that in boys, an inverse relationship was observed between inner-ear function and two markers of androgen exposure – length of penis and scrotal pigmentation. These relationships tended to be stronger in the right ear. There was also a positive association in boys, between AGD and DPOAEs in the left ear. There were no associations between the physical androgen markers and auditory function in girls.