Bilateral Abnormalities of Optic Nerve Size and Eye Shape in Unilateral Amblyopia

Abstract

To evaluate the optic nerve (ON) size and globe shape in amblyopic eyes using high-resolution magnetic resonance imaging (MRI) and to compare these values with those of the sound fellow eye and of normal control eyes. Prospective case-control study. Thirty-four amblyopic patients and 60 normal control patients were evaluated using surface coil MRI. Retrobulbar ON cross-section, maximum globe cross-section, globe noncircularity (globe major axis/minor axis), axial length (AL), and the ratio of AL to ON (AL/ON) were measured. Corrected logarithm of the minimum angle of resolution acuity of all 34 amblyopic eyes averaged 0.43 +/- 0.30. The mean retrobulbar ON cross-section was 9.7 +/- 2.4 mm(2), 9.5 +/- 2.3 mm(2), and 10.7 +/- 2.6 mm(2) for amblyopic, fellow, and normal ONs, respectively. Although amblyopic and fellow ONs had similar cross-sections, both were significantly (P = .02) subnormal. AL/ON was 2.7 +/- 1.2 mm(-1), 2.7 +/- 1.0 mm(-1), and 2.3 +/- 0.5 mm(-1), respectively. Although AL/ON significantly exceeded normal in amblyopic eyes (P = .01), there was no significant difference between amblyopic and fellow eyes. Globe noncircularity of amblyopic eyes (1.17 +/- 0.07; P = .002) and fellow eyes (1.15 +/- 0.04; P < .001) was significantly greater than that of control eyes (1.11 +/- 0.04), but amblyopic and fellow eyes did not differ significantly. Unilateral amblyopia is associated with bilaterally, but also with subclinically hypoplastic ONs, greater than normal AL/ON, and abnormally noncircular globe cross-section. These factors evidently do not determine which of the 2 eyes will become amblyopic. Reduced circularity of amblyopic and fellow eyes may reflect optical causes of amblyopia or bilateral dysregulation of globe shape secondary to amblyopia.