Magnetic resonance imaging of tissues compatible with supernumerary extraocular muscles

Abstract

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.

Figure 1

T1 coronal MRI of deep orbit of case 1 demonstrating bilateral anomalous EOM bands coursing between the temporal edges of the superior rectus (SR) and inferior rectus (IR). LR: lateral rectus; MR: medial rectus; ON: optic nerve; SO: superior oblique; SOV: superior ophthalmic vein; SR-LPS: superior rectus-levator palpebrae superioris complex

Figure 2

T1 coronal (left) and sagittal (right) MRI of case 6, who had limited supraduction OD, demonstrating an anomalous EOM band coursing between the MR and LR in central gaze. On supraversion, the anomalous EOM band's position contacts the optic nerve (ON). Band: anomalous EOM band; IR: inferior rectus; LR: lateral rectus; MR: medial rectus; SO: superior oblique; SR-LPS: superior rectus-levator palpebrae superioris complex.

Figure 2

T1 coronal (left) and sagittal (right) MRI of case 6, who had limited supraduction OD, demonstrating an anomalous EOM band coursing between the MR and LR in central gaze. On supraversion, the anomalous EOM band's position contacts the optic nerve (ON). Band: anomalous EOM band; IR: inferior rectus; LR: lateral rectus; MR: medial rectus; SO: superior oblique; SR-LPS: superior rectus-levator palpebrae superioris complex.

Figure 3

T2 fast spin echo coronal MRI of case 8 showing an anomalous band coursing between the right inferior rectus (IR) and lateral rectus (LR). The right LR is displaced inferiorly. MR: medial rectus; ON: optic nerve; SO: superior oblique; SR-LPS: superior rectus-levator palpebrae superioris complex

Figure 4

T1 coronal MRI of case 9. Anomalous EOM band extends from left inferior rectus (IR) to the temporal globe. LG: lacrimal gland; LR: lateral rectus; MR: medial rectus; SR-LPS: superior rectus-levator palpebrae superioris complex.

Figure 5

T1 coronal MRI of case 10. Anomalous EOM band connects left SR-levator complex to the SO near the trochlea. Also detected is a disorganized left lateral rectus (LR). IR: inferior rectus; MR: medial rectus; ON: optic nerve; SO: superior oblique; SOV: superior ophthalmic vein; SR-LPS: superior rectus-levator palpebrae superioris complex