Original article
Magnetic Resonance Imaging of Tissues Compatible with Supernumerary Extraocular Muscles

https://doi.org/10.1016/j.ajo.2010.06.007Get rights and content

Purpose

To determine by magnetic resonance imaging (MRI) the prevalence and anatomy of anomalous extraocular muscle (EOM) bands.

Design

Prospective, observational case series.

Methods

High-resolution, multipositional, surface coil orbital MRI was performed using T1 or T2 fast spin echo weighting with target fixation control under a prospective protocol in normal adult subjects and a diverse group of strabismic patients between 1996 and 2009. Images demonstrating anomalous EOM bands were analyzed digitally to evaluate their sizes and paths, correlating findings with complete ophthalmic and motility examinations.

Results

Among 118 orthotropic and 453 strabismic subjects, 1 (0.8%) orthotropic and 11 (2.4%) strabismic subjects exhibited unilateral or bilateral orbital bands having MRI signal characteristics identical to EOM. Most bands occurred without other EOM dysplasia and coursed in the retrobulbar space between rectus EOMs such as the medial rectus to lateral rectus, from superior to inferior rectus, or from 1 EOM to the globe. In 2 cases, horizontal bands from the medial rectus to lateral rectus muscles immediately posterior to the globe apparently limited supraduction by collision with the optic nerve. All bands were too deep to be approached via conventional strabismus surgical approaches.

Conclusions

Approximately 2% of humans exhibit on MRI deep orbital bands consistent with supernumerary EOMs. Although band anatomy is nonoculorotary, some bands may cause restrictive strabismus.

Section snippets

Methods

Between December 1, 1996, and December 31, 2009, a total of 118 orthotropic volunteers and 453 strabismic patients underwent high-resolution orbital imaging under a prospective protocol designed to optimize image resolution using the best available methods at the time. We selected for detailed analysis those cases with evidence of anomalous EOMs. Written informed consent was obtained prospectively according to a protocol approved by the Institutional Review Board of the University of

Results

From December 1996 through December 2009, a total of 118 normal subjects without strabismus and 453 strabismic subjects underwent high-resolution orbital MRI. Twelve of these subjects were identified who had a band consistent with 1 or more supernumerary EOMs. Bands had intensities equivalent to that of typical EOMs. Eleven of these subjects had underlying strabismus, giving a prevalence of supernumerary EOM bands in strabismic subjects of 2.4% and in normal subjects of 0.8%, a difference that

Discussion

We believe the present report to be the largest case series of supernumerary human EOMs, which in this series have been identified on high-resolution MRI as having signal characteristics matching those of known EOMs. These supernumerary muscular bands occurred in numerous variations, most commonly as connections between 2 EOMs or between an EOM and the globe. The prevalence of these anomalous EOM bands is higher in the strabismic (2.4%) than the nonstrabismic (0.8%) population, but nonetheless

Monica R. Khitri, MD, has recently completed her residency in ophthalmology at the Jules Stein Eye Institute, University of California Los Angeles. She is starting her fellowship in pediatric ophthalmology and strabismus at Children's Hospital of Philadelphia.

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    Case 4 is the first case reported with postoperative synergistic convergence after contralateral medial rectus recession and lateral rectus resection. Accessory bands are relatively rare orbital structures associated with restrictive strabismus, and a few DRS cases with accessory bands have been previously reported.19-23 However, only 3 DRS cases with unilateral accessory bands next to the medial rectus muscle were identified in previous reports.19,22

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Monica R. Khitri, MD, has recently completed her residency in ophthalmology at the Jules Stein Eye Institute, University of California Los Angeles. She is starting her fellowship in pediatric ophthalmology and strabismus at Children's Hospital of Philadelphia.

Joseph L. Demer, MD, PhD, is Chief of Comprehensive Ophthalmology and Professor of Neurology at the University of California Los Angeles. He holds the Leonard Apt Professorship, Directs the Ocular Motility Clinical Laboratory, and chairs the EyeSTAR Training Program.

With a PhD in Biomedical Engineering, Dr. Demer has worked for more than 30 years on neural and mechanical factors regulating ocular motility, particularly MRI of the functional anatomy of extraocular muscles and orbital connective tissues.

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