June 2015
Volume 56, Issue 7
Free
ARVO Annual Meeting Abstract  |   June 2015
Magnetic Resonance Imaging (MRI) Suggests New Non-Muscular Constraints on Ocular Rotation in High Myopia
Author Affiliations & Notes
  • Joseph L Demer
    Ophthalmology, University of California, Los Angeles, Los Angeles, CA
    Neurology, University of California, Los Angeles, Los Angeles, CA
  • Footnotes
    Commercial Relationships Joseph Demer, None
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Investigative Ophthalmology & Visual Science June 2015, Vol.56, 1329. doi:
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    • Get Citation

      Joseph L Demer; Magnetic Resonance Imaging (MRI) Suggests New Non-Muscular Constraints on Ocular Rotation in High Myopia. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):1329.

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      © ARVO (1962-2015); The Authors (2016-present)

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Abstract
 
Purpose
 

Traditional concepts of motility assume a spherical globe freely rotated by extraocular muscles (EOMs). This study employed MRI to investigate the hypothesis that globe irregularity and the optic nerve (ON) length may constrain ocular rotation in axial high myopia..

 
Methods
 

High resolution, surface coil axial MRI using T1 or T2 fast spin echo sequences was obtained in 8 normal adults, 12 adult esotropes (ET) with normal axial length (AL) <25.3 mm, 11 myopic esotropes with mean AL 28.7±3.6 (SD) mm, and 7 exotropes (XT). ON length was measured in eccentric horizontal gaze positions where it became straight.

 
Results
 

ON straightening occurred only in adduction, at 23.6±9.0° in normal subjects, not significantly different from XT at 22.2±11.8°, but significantly greater in ET at 36.3±9.3°, and in myopic ET at 34.1±11.7° (P<0.003). ON length at straightening was 30.4±3.4 mm in normals, not significantly different from XT at 30.7±2.6 mm, but less in ET at 27.8±2.7 mm and 25.5±2.0 mm in myopic ET (P<0.03). ET was associated with globe retraction and elongation in adduction, suggesting ON tethering with forces concentrated at the scleral canal. Eight globes of the myopes exhibited prominent irregular nasal posterior, temporal posterior, nasal and temporal posterior, or equatorial staphylomata. These scleral ectasias were positioned to contact and elongate horizontal rectus EOM paths in some gaze positions.

 
Conclusions
 

Concepts of strabismus in axial high myopes should be expanded to include irregular posterior staphylomata, and tethering in adduction by the ON. Staphylomata act like "cams" affixed to the normally spherical globe, exerting no mechanical effect until ecentrically rotating against EOMs. After rotational contact, staphylomata would stretch and thus add tension to the EOM that increases non-linearly with further duction. Tethering by the ON can add a further strong nonlinear force opposing adduction, even sufficient to deform the posterior pole.  

 
Axial MRI in a highly myopic esotropic subject demonstrates optic nerve straightening in adduction with globe retraction seen as posterior shift of the corneal surface (green line). Posterior staphylomata in this subject are evident from angulated posterior contour of each globe. Ghost images anterior to the corneal surface are motion artifacts due to retraction as the extraocular muscles pull against the tethering optic nerve.
 
Axial MRI in a highly myopic esotropic subject demonstrates optic nerve straightening in adduction with globe retraction seen as posterior shift of the corneal surface (green line). Posterior staphylomata in this subject are evident from angulated posterior contour of each globe. Ghost images anterior to the corneal surface are motion artifacts due to retraction as the extraocular muscles pull against the tethering optic nerve.

 
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