Magnetic resonance imaging of tissues compatible with supernumerary extraocular muscles

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

Prospective, observational case series.

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.

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.

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.