I am new to the practice I’m working in. I was surprised to see an optical assistant take non-contact tonometry readings from a patient wearing soft contact lenses. When I queried this the optical assistant told me that the patient was attending for a contact lens check-up and an eye examination and removing the lenses for non-contact tonometry would affect the ability of the eye specialist to judge the fit of the lens as well as slow things down. This was standard practice. I was surprised and thought that a soft contact lens on the eye would increase the readings taken with a non-contact tonometer. Is my thinking correct?
I was taught at optometry school that all contact lenses must be removed prior to non-contact tonometry as the lenses would increase the pressure readings. Initially I agreed with you view. However, even after a brief review of the literature on this topic I can see that it is not a straight forward answer to your question.
I thought that a contact lens in place would add to the thickness of the cornea and result in an artificially increased pressure value. It is well-known that thicker corneas result in higher pressure readings but it seems that not all soft contact lenses do. One study found that soft contact lenses with power -5.00 DS and -0.50 DS did not affect non-contact tonometry readings. However, a +5.00 DS lens resulted in higher pressure values. Another study, found that non-contact tonometry can be performed with sufficient accuracy over a soft lens on condition that the lens centre thickness is no more 0.30 mm and the power is not greater than +3.00 DS.
As long as these limitations are taken into account it seems that some soft contact lenses can be left on the eye during non-contact tonometry.
However, my own preference is to remove all contact lenses prior to conducting any form of tonometry. It saves optical assistants and other eye specialists from having to remember which contact lenses are okay to leave in the eye during non-contact tonometry and which to remove. Also, the patient can then have the eye examination, including slit lamp and fluorescein examination.
After retinal examination and completion of the clinical records, the contact lenses can be reinserted. It only takes five minutes for them to settle to a good enough level to allow examination of the contact lens fit. Seems like a better system to me than measuring intraocular pressure through contact lenses and no time is lost.