Childhood hearing impairment is a serious health problem. 1-2 children per 1 000 have so-called sensorineural hearing loss (lesion of the inner ear or the central nervous system), and may require a hearing aid. It has been observed that genes and effects on the foetus such as infections, alcohol or toxic drugs can affect normal development of the ear. Knowledge about the causes of this type of hearing loss is lacking.
– Our study shows that sensorineural hearing loss is related to low birthweight says Bo Engdahl, researcher with the Division of Mental Health at the NIPH. The results are recently published in the journal Paediatric and Perinatal Epidemiology.
Link with low birthweight
The risk was largest when the child had a birthweight lower than 1 500 gram.
– Perhaps the ear is not completely developed if the foetus grows little in the womb, says Engdahl.
Babies with low birthweight were compared with babies who weighed 3 500-3 999 grams.
The study shows that the length of pregnancy had no independent effect on the risk for hearing loss.
– The results indicate therefore that the fetal growth is important for normal hearing development, says Engdahl.
– Our findings put the focus on prevention of known factors that affect fetal growth, such as mother’s illness, smoking, drug abuse and specific pregnancy complications such as pre-eclampsia.
Also risk with birthweight of 2500-3500 grams
Babies that weigh 2 500-3 500 grams are considered to be of normal weight. WHO defines low birthweight as weight less than 2 500 grams. However there was also a risk for sensorineural hearing loss among babies who weighed between 2 500 – 3 500 grams.
– This indicates that it is difficult to set a meaningful threshold in birthweight concerning who should be followed up for potential hearing damage, says Engdahl.
The risk for hearing loss became lower as birthweight increased.
Data from Medical Birth Registry and county register
In this study, 327 children with sensorineural hearing loss were identified through the Norwegian county register in the period 1978-1998. These children were diagnosed with sensorineural hearing loss before they were five years old. The children with hearing loss were compared with all children without hearing loss in the same counties, in total 391 992. Information was collected from the Medical Birth Registry.
Mild to moderate hearing loss is defined as an average hearing loss of 35-70 decibels whilst severe/profound hearing loss is defined as a hearing loss of more than 70 decibels.
Bo Engdahl & Anne Eskild (2007). Birthweight and the risk of childhood sensorineural hearing loss. Paediatric and Perinatal Epidemiology 21, 495–500.