Children born to mothers prescribed the heroin substitute methadone during pregnancy may be at risk of wide-ranging sight problems, indicates a small study published ahead of print in the British Journal of Ophthalmology.
There may be as many as 350,000 children in the UK whose parents are problem drug users, say the authors.
Methadone is a synthetic opioid, which provides a longer lasting "high" than most opioids, and is much less likely to be misused.
It is usually prescribed as a substitute for heroin, and is associated with a more stable maternal lifestyle and less likelihood of stunted fetal growth or preterm birth.
But most babies born to mothers, who are prescribed it during their pregnancy, have significant withdrawal symptoms, known as neonatal abstinence syndrome or NAS. These symptoms are severe enough to warrant treatment in up to 80% of cases.
They assessed the eyesight of 20 children with vision problems, whose mothers had taken methadone during the pregnancy.
Most of the children had also been exposed to either benzodiazepines (55%) or heroin (40%) while in the womb.
Virtually all the children (95%) had poor eyesight in addition to which seven out of 10 had involuntary eye movement (nystagmus), while in half vision had not yet developed fully (delayed visual maturation).
Eleven out of 12 children who had been treated for NAS had nystagmus, compared with only three out of eight whose NAS had not been severe enough to warrant treatment.
One in three (35%) also had a squint (strabismus), while a similar proportion (30%) had blurred vision or long or short sightedness problems (refractive errors). And one in four had impaired brain function relating to sight.
One in four children also had significant developmental problems, including developmental delay and cerebral palsy.
The underlying causes of the children's visual problems are unclear, say the authors, but the developing visual system is particularly sensitive to unexpected stressors before birth, they point out.
The incidence of substance misuse during pregnancy is on the rise, say the authors, citing an anonymous screening study from one UK clinic which indicated that between 11% and 16% of expectant mothers were taking at least one illicit drug during their pregnancy.