Smoking during pregnancy weakens the optic nerve of the child

​Children whose mothers have smoked during pregnancy have a weaker retina-brain connection. This is the result of new research by Rigshospitalet, Zealand University Hospital and the University of Copenhagen.

​Danish researchers have conducted the first major study of children's optic nerves. The study shows that the optic nerve is thinner in children aged 11-12 years whose mothers had smoked during pregnancy compared with children whose mothers had not smoked. The study has just been published in the prestigious journal JAMA Ophthalmology. A total of 1,323 Danish children were examined during the study.

"We found a 5% deficiency in the tissue mass in the optic nerve when we compared children of mothers who had smoked during pregnancy with children of mothers who had not smoked. This difference was independent of other differences between the children, for example, their birth weight," said Håkan Ashina, medical student, who conducted the study as his bachelor-degree project, and is first author of the article.

The researchers do not know the biological mechanism behind the effects of smoking on the optic nerve. However, they know that the number of nerve fibres in the optic nerve is highest in early pregnancy, and that around half of the nerve fibres are discarded later in pregnancy as part of natural maturation of the optic nerve.
"This may be the process that is being affected by the substances ingested with the tobacco smoke," said Professor Michael Larsen from Rigshospitalet's Department of Ophthalmology, and advisor on the project. 

Weak connection may affect vision

A thinner optic nerve has no bearing on whether a child will need glasses, but it may have some long-term effects," explained Inger Christine Munch, a research associate professor from the Department of Ophthalmology at Zealand University Hospital, and senior author on the study: 
"The optic nerve loses nerve fibres throughout life, and if the number of nerve fibres reaches below a certain level, the patient will develop glaucoma, with blind spots in the field of vision. Therefore, it must be assumed that a more developed optic nerve will better protect against the development of glaucoma and blindness."

The eye is part of the central nervous system

The optic nerve, which connects the eye to the brain, is part of the central nervous system.  
"We still do not know whether smoking during pregnancy selectively affects the optic nerve, or whether it also affects the brain. In other contexts, the optic nerve is very sensitive to congenital weaknesses in the energy metabolism and to drugs that weaken the energy metabolism, e.g. tuberculosis medicine. We have data to look at these contexts as well, and we are doing this in other ongoing research projects," said Michael Larsen.

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