Short-term axial eye length changes after imposed defocus in emmetropes, myopes and hyperopes

Short-term axial eye length changes in response to imposed positive and negative defocus were compared in hyperopic, emmetropic and myopic young adults to learn about possible differences in emmetropization. Thirty-seven subjects (average age: 28 ± 4 years) participated: emmetropes (n = 15), myopes (n = 15), and hyperopes (n = 7). They viewed a 30-minute movie on a large TV screen (65″) at 2-meter distance (equivalent to -0.5D) with optical corrections, while a + 3.5 D (myopic defocus) or -3.5 D (hyperopic defocus) lens was added in the right eye. A subset of myopes (n = 14) was also tested with -2 D lenses. Axial length was measured in both eyes before and after viewing using the Haag-Streit Lenstar LS 900. With + 3.5 D myopic defocus, emmetropes (-9.7 ± 13.1 µm) and hyperopes (-8.9 ± 5.6 µm) exhibited significant axial shortening, while myopes (-1.1 ± 10.2 µm) did not. Hyperopes and myopes differed significantly, but emmetropes and hyperopes did not. With -3.5 D hyperopic defocus, both emmetropic (+5.0 ± 7.5 µm) and hyperopic eyes (+6.6 ± 6.9 µm) elongated, indicating that their retina distinguished positive from negative defocus. Myopic eyes displayed paradoxical axial eye shortening (-7.1 ± 8.6 µm). Hyperopes and myopes differed significantly, but emmetropes and hyperopes did not. Reducing the lens power to -2 D abolished the shortening in myopes. Group data indicate bidirectional, sign-of-defocus-dependent axial length changes occur in both emmetropic and hyperopic eyes, indicating similar retinal function. Myopes showed reduced or even reversed responses, showing a functional deficiency in the myopic retina.