A magnetic resonance imaging study of horizontal rectus muscle palsies.

The assessment of extraocular muscle function is usually performed indirectly by an examination of ocular motility. We have utilized magnetic resonance imaging, in both cine and static modes, to directly observe the contractility behavior of the extraocular muscles. This noninvasive technique was applied to the study of patients with horizontal rectus muscle palsies. In all cases, the palsied muscle was of a smaller diameter than its normal counterpart. This size differential was noted in all fields of gaze. Additional studies are required to determine the relative contributions of denervation atrophy and loss of tonus to this diminution of muscle size.