The type of contact lens a patient uses may influence his risk of microbial keratitis, according to a new study.

Dr. Kam Cheng and colleagues in Holland conducted telephone surveys between 1994 and 1997 to see how many Dutch people wore rigid gas permeable (RGP) contact lenses and how many wore soft lenses, and how long they wore their lenses without removing them. They then surveyed all ophthalmologists in Holland to discover the number of microbial keratitis cases diagnosed within 3 months in 1996.

The microbial keratitis incidence in RGP wearers was 1.1 per 10,000 per year. However, for soft-lens patients, it was much higher. Soft lens patients who wore their lenses for 24 hours or less had a risk of microbial keratitis 3.3 times higher than RGP wearers. Those who wore their soft lenses for more than 24 hours at a time had a risk almost 20 times higher.

The researchers recommend that patients not wear contact lenses overnight, and especially not soft contact lenses.