My favourite topic in eye care is binocular vision. My new interest is myopia management so I’m very pleased that there is considerable overlap between the two.
Studies have shown that people with myopia have more accommodative lag than people with emmetropia This lag of accommodation, causes a hyperopic defocus on the central retina and this may cause axial elongation and myopia progression.
Accommodative lag occurs when there is a significant reduction in accommodative response compared to the accommodative demand. A near object at 33 cm will require 3D accommodative demand, and an accommodative response of less than 3D represents an accommodative lag. The greater the accommodative lag the greater the central hyperopic defocus and thus potentially a greater stimulus to myopia development.
Unfortunately, it is not known if accommodative lag causes myopic progression or myopic progression causes accommodative lag. It is also unclear whether myopic progression is faster for those people with lags of accommodation or larger accommodative lags. As the evidence is conflicting with regard to accommodative lag and myopia progression let’s hope new studies clear this up.
Having said this measurement of accommodative response is a quick and easy test to perform and can be helpful to include as a further risk indicator for myopia development. Here are the steps to measuring accommodative lag:
- The test card and the retinoscope should be placed at the same distance from the subject’s spectacle plane, usually 40 cm.
- With the retinoscope in the plane mirror mode, with motion indicates a lag of accommodation and an against motion indicates a lead of accommodation while neutrality indicates that the accommodative stimulus and accommodative response are equal.
- The practitioner’s estimate of the amount of plus power that would be required to neutralise the with motion is the estimate of the lag of accommodation.
- The estimate of the lag can be confirmed by very briefly placing a plus lens equal in power to the estimated lag over one eye and quickly checking to see whether neutrality is observed.
- The lens should only be in place a half-second or less so that a change in accommodative response is not induced.
- Record the dioptric power of the lens that provides neutrality.
- Repeat procedure for the L eye.
- Plus lenses indicate positive accommodative lag (response < stimulus).
- Minus lenses indicate negative accommodative lag (response > stimulus).
- Lag > +1.00DS indicates accommodative insufficiency.
- Lag < 0 indicates latent hyperopia, pseudomyopia or accommodative spasm.
- Most non-presbyopic subjects have lags of 0 to 0.75D.
The other element to this is that under correcting people with myopia and creating myopic central retina defocus is also likely to cause myopic progression. As eye specialists we need to make sure that the central retina has clear focus.