Testing visual acuity under reduced illumination can be used to distinguish amblyopia (strabismic or anisometropic) from retinal or optic nerve dysfunction.14 If the illumination to a normal eye is decreased using a neutral density filter (no. 96 2-Log filter; Kodak), the vision will be mildly reduced from 20/20 to 20/40. In an eye with a retinal or optic nerve lesion, the vision will be reduced dramatically (20/40 to 20/200) whereas an eye with amblyopia will exhibit little or no reduction (20/40 to 20/50). (Neutral-density filters can be obtained in most camera shops.) Newer electrophysiological methods of assessing
visual function include visual evoked potentials and pattern electroretinography.
Very young children (1-3 months of age) will exhibit marked lid retraction when the lights are suddenly turned off (Fig. 2-2). This primitive reflex can be used to assess function in the apparently blind infant. If little or no visual function is present, this reflex will not occur. However, if visual acuity is 20/400 or better, marked lid retraction will occur immediately after the room lights are turned off. The lids may stay retracted for 20 to 30 s but will eventually return to the normal position. This reflex will occur repeatedly every time the lights are turned off. The presence of this reflex offers reassurance that some visual function exists.
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