crowding phenomenon

The crowding phenomenon is the loss of visual acuity that occurs when a letter is surrounded by other contours, as in a line of letters, compared with the same letter seen alone. Contour interaction between neighboring targets causes the loss, and the effect is also called separation difficulty.^1,2

Many amblyopic eyes recognize much smaller symbols presented singly than symbols presented in a row, so these eyes effectively have two acuities, line acuity and single-optotype acuity. The difference can be large, and a line acuity of 6/30 may correspond to 6/6 with single symbols. About two thirds of amblyopic patients show crowding at the first examination. Crowding is not specific to amblyopia, because adjacent contours also reduce form recognition in normal eyes.¹ The effect is exaggerated in strabismic amblyopia, although strabismic and anisometropic amblyopes probably form a continuum.^2,3

Line acuity is the clinical standard, and an amblyopic eye cannot be considered cured until it reaches standard line acuity. Single-optotype tests lack crowding and overestimate acuity, which makes them unreliable for amblyopia screening.¹ Real text is more crowded than a letter chart, and that may explain why a successfully treated amblyope can still have trouble reading passages of text.³