Measurement of recti eye muscle paths by magnetic resonance imaging in highly myopic and normal subjects

Abstract

Purpose: To analyze the path of extraocular muscles (EOMs) quantitatively in highly myopic subjects with and without restricted eye motility versus control. To elucidate the cause of the acquired motility disorder in patients with high myopia.

Methods: Thirty-three orbits were imaged using a Magnetom or Siemens Vision (Siemens, Erlangen, Germany; both 1.5 Tesla) MRI (magnetic resonance imaging) scanner. Coronal T1-weighted, spin-echo images were obtained with repetition time of 550 msec and echo time of 15 msec. Subjects had to fixate in different positions of gaze. Orbits of three patient groups were analyzed: group 1 (n = 14), patients with high axial myopia and restricted eye motility (average axial length, 31.4 mm; refractive error more than -15 D); group 2 (n = 8), subjects with high axial myopia and normal eye motility (average axial length, 29.2 mm); control group (n = 11), emmetropic subjects with normal eye motility (average axial length, 23.6 mm).

Results: Highly myopic patients showed significant displacements of recti EOMs in comparison with control subjects. Mean displacements, measured in the plane 3 mm anterior to the globeoptic nerve junction in primary gaze, were in group 1, lateral rectus (LR) 2.9 mm (2.5 downward, 1.4 medial), medial rectus (MR) 1.3 mm downward and in group 2, LR 1.4 mm (1.3 downward, 0.6 medial) and MR 1.2 mm downward. In groups 1 and 2 the inferior rectus (IR) was displaced 1.3 mm medially and upward. In both groups of myopic patients the superior rectus (SR) was displaced 1.5 mm medially and downward.

Conclusions: In patients with high axial myopia, displacements of all recti EOMs can be detected by MRI. Displacements of SR, MR, and IR were very similar in groups 1 and 2 versus control. LR displacement into the lateral and inferior quadrant of the orbit was greatest in patients with restricted eye motility. Thus, LR displacement is probably the major pathophysiological factor for the restrictive motility disorder in high myopia. EOM dislocations can be explained by myopia-associated alterations in the orbital connective tissues confining EOM positions in relation to the orbital wall.