A nighttime fear

It’s not uncommon for a young child to be afraid of the dark, but on rare occasions it could be a tip-off to an unusual vision problem -- congenital night blindness.

“Everyone has difficulties seeing when room lights are switched off, but most people’s eyes adjust and a degree of vision returns after 10 minutes or so. For those with night blindness, this adjustment either does not happen or happens to only a limited degree,” says Ruth Hamilton, coauthor of a report on the problem in the Jan. 25 issue of the British Medical Journal. A profound fear of the dark may be the result, she says.

In the report, Hamilton, a senior medical physicist at the Tennent Institute of Ophthalmology at Gartnavel General Hospital in Glasgow, Scotland, describes two children with congenital night blindness, or nyctalopia.

Congenital night blindness is an inherited disorder in which the rods of the retina, which normally allow us to see in near darkness, don’t work properly. A child may have otherwise normal vision, which is one reason the disorder might not be detected until he or she is older, says Michael Redmond, president of the American Academy of Ophthalmology. He says a child with night blindness may have nystagmus, in which the eyes appear to jiggle, as one of the children in the report did. That condition is usually obvious. There is a test for night blindness, called an electroretinogram, which measures rod function, but it is not always accurate in very young children, says Redmond. There is no treatment, but keeping a low light on in the child’s room will certainly ease the fear, he says.