Orbital magnetic resonance imaging of extraocular muscles in chronic progressive external ophthalmoplegia: specific diagnostic findings

Abstract

Introduction: Chronic progressive external ophthalmoplegia (CPEO) is characterized by slowly progressive bilateral ophthalmoplegia and blepharoptosis. Molecular diagnosis is problematic because sporadic mitochondrial DNA deletions can be causative. We sought findings using magnetic resonance imaging (MRI) that might support the diagnosis of CPEO.

Methods: Two men (ages 31 and 47 years) and 3 women (ages 40-49 years) with CPEO and symptom durations of 8 months to 28 years underwent high-resolution (2-mm slice thickness, 312 micron pixels), surface coil, T1-weighted orbital MRI in coronal planes. Images were analyzed quantitatively to determine extraocular muscle (EOM) sizes and were compared with 10 age- and gender-matched normal volunteers, one subject with myasthenia gravis, and with 30 subjects having EOM paralysis caused by oculomotor, trochlear,0 and abducens neuropathies.

Results: EOM function was clinically diminished in CPEO, most markedly for the superior rectus (SR) and levator muscles. All EOMs in CPEO exhibited unusual qualitative T1 MRI signal abnormalities. Unlike the profound EOM atrophy typical of neurogenic paralysis, anterior volumes of medial rectus, lateral rectus, and inferior rectus muscles in CPEO were not smaller than normal (p>0.003). Anterior volumes of the SR muscle-levator complex and superior oblique were significantly reduced (p<0.003). Denervated EOMs exhibited statistically significant volume reduction when compared with normal and CPEO groups. Volume of the SR muscle-levator complex was the same in subjects with CPEO and oculomotor palsies.

Conclusions: CPEO is associated with minimal EOM volume reduction despite clinically severe weakness. This combination of findings may be specific for CPEO and could resolve the diagnostic dilemma in difficult cases.

FIG 1

Coronal T1-weighted MRI of case 1 showing abnormal bright internal signal in multiple EOMs (right MR, right IR, left MR, left LR muscle) in the deep orbit of patient with CPEO. ON: optic nerve.

FIG 2

Coronal T1-weighted MRI in a normal subject and a subject with CPEO. The normal SR-levator complex size in the right orbit contrasts with atrophy in CPEO. Note bright internal signal in the left medial and IR muscles.

FIG 3

Coronal T1-weighted MRI in subject with left abducens palsy showing atrophic left lateral rectus (LLR) compared with normal right lateral rectus (RLR) muscle. No bright internal signal is present in these EOMs.

FIG 4

Coronal T1-weighted MRI in subject with partial right oculomotor nerve palsy. The right medial (MR) and IR muscles are atrophic, as compared with the normal left MR and IR muscles.