Purpose
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 to the sound fellow eye and normal control eyes.
Design
Prospective case control study
Methods
Thirty-four amblyopic patients and 60 normal control patients were evaluated using surface coil MRI. Retrobulbar ON cross section, maximum globe cross section, globe non-circularity (globe major axis/minor axis), axial length (AL), and the ratio of AL to ON (AL/ON) were measured.
Results
Corrected logmar acuity of all 34 amblyopic eyes averaged 0.43±0.30. Mean retrobulbar optic nerve cross section was 9.7±2.4 mm2, 9.5±2.3 mm2, and 10.7±2.6 mm2 for amblyopic, fellow, and normal ONs, respectively. While amblyopic and fellow ONs had similar cross sections, both were significantly (p=0.02) subnormal. AL/ON was 2.7±1.2 mm-1, 2.7±1.0 mm-1, and 2.3±0.5 mm-1, respectively. While AL/ON significantly exceeded normal in amblyopic eyes (p=0.01), there was no significant difference between amblyopic and fellow eyes. Globe non-circularity of amblyopic (1.17±0.07, p=0.002) and fellow eyes (1.15±0.04, p<0.001) was significantly greater than control (1.11±0.04), but amblyopic and fellow eyes did not differ significantly.
Conclusions
Unilateral amblyopia is associated with bilaterally but subclinically hypoplastic ONs, greater than normal AL/ON, and abnormally non-circular globe cross section. These factors evidently do not determine which of the two 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.