lag of accommodation

Lag of accommodation is the amount, in diopters, that the accommodative response falls short of the accommodative stimulus.^1,2 During near viewing the retina is usually conjugate with a point slightly behind the object of regard.¹ The average lag is between 0.25 and 0.50 D in children and young adults, and school-aged children show a reported mean of 0.34 D.^1,3

For most patients the lag measures between +0.25 D and +0.75 D, and a lag greater than +1.00 D is often found in accommodative insufficiency or infacility.^2,4 A high lag suggests a tentative near addition, accommodative therapy, or both.^2,3 A lead of accommodation, a reflex that needs a minus lens to neutralize it, suggests accommodative excess or spasm.^2,4

Clinics routinely measure the lag with monocular estimate method (MEM) retinoscopy.³ The test card and retinoscope sit at the same distance, usually 40 cm, and the plus power that neutralizes the resulting with motion estimates the lag.^1,4 Any trial lens is held for half a second or less, too briefly to change the accommodative response.¹ Nott retinoscopy keeps the target fixed and moves the retinoscope to find reversal, and it typically shows a slightly lower lag.⁴ The fused cross-cylinder test measures lag subjectively, but it is less accurate than MEM and often difficult for children under 8 years.²