I started a 48-year-old bifocal spectacle wearer on multifocal soft contact lenses. He was getting on very well with his bifocal spectacles but wanted to have the option of wearing contact lenses when socialising and reading menus in restaurants. The multifocal contact lenses fit is good and he achieved 6/5 in each eye and N5 print at near. He has been wearing them for two weeks and has returned complaining of headaches and eye aches when he is reading and doing close work when wearing the contact lenses. What is going on?
It is sounds like the change from bifocal spectacles to multifocal soft contact lenses has caused these symptoms.
When people with myopia look through a negative spectacle lens at near, base in prism is induced as the eyes are not looking through the optical centre of the lens but at points nasal to the optical centres. The induced base-in prism means that the person needs to exert less convergence in spectacles than is required for the near working distance. The induced base-in prism allows this shortfall in natural convergence.
Moderate levels of myopia are required for the base-in prism effect to be significant. Using Prentice’s rule (prism=decentration [mm] x lens power) with an inward eye movement of 2mm in each eye for near vision, a person with-5 DS myopia can obtain 1Δ induced base-in prism in each eye.
When wearing well-fitting soft multifocal contact lenses as the eyes there is no induced prism because the eyes are always looking through the optical centre of the lens. The assistance that the induced base-in prism gave to the natural convergence is lost when wearing the contact lenses and this means that the under convergence is very likely to cause symptoms such as headache and eye ache with close work.
This is a very difficult clinical challenge to solve. One solution is to use single vision close work spectacles with the appropriate level of base-in prism in the lenses over the contact lenses. This may not be acceptable to the patient as the reason for having the soft multifocal contact lenses was to avoid having to use spectacles. Some patients however may accept this solution and use the over spectacles only for those tasks that involve close detailed reading.
Another solution is to continue with the contact lenses and put up with the symptoms when and if they occur.
One other solution is to cease using the contact lenses and to return to using bifocal spectacles for all activities. Perhaps considering varifocals for their better appearance.
The only other thing I can think of is to advise the patient about eye exercises to increase the natural convergence ability. In my experience this is not something that patients are keen on and even when they are willing to try because they are presbyopic it is difficult to improve convergence because of the natural degradation of accommodation.
Watch out for this possibility the next time you fit a person with myopia with soft contact lenses and perhaps give them an indication that this problem occurs with some patients.