Rectus Pulley Instability as a Cause of Y-pattern Exotropia Revealed by Magnetic Resonance Imaging FREE

Y-pattern exotropia is a rare condition that cannot be corrected by conventional surgical methods used for V-pattern deviation. It has been proposed that lateral rectus (LR) muscle co-contraction during elevation causes Y-pattern exotropia.¹ However, the precise mechanism is still not known. This article describes a patient with Y-pattern exotropia with pulley instability as revealed by magnetic resonance imaging (MRI). The patient's condition was improved through rectus muscle transposition.

An 8-year-old girl had exodeviation during upgaze since age 1 year. She did not have diplopia and had no history of strabismus surgery. She had orthophoria in primary position and downgaze. In upgaze, she had 40 prism diopters (PD) of exotropia at distance and at near. She showed no hyperdeviation in horizontal side gaze but had paradoxical abduction of the normally adducting eye in supraduction (Figure 1A).

High-resolution T1-weighted MRI was performed with a 1.5-T Sigma scanner (GE Healthcare, Milwaukee, Wisconsin). We used a 7.6-cm round surface coil to improve the signal to noise ratio. For obtaining a multipositional MRI, the scanned eye was occluded and the contralateral eye was fixing on the targets at 30° from primary position that were attached to the inside of the scanner magnet. Contiguous MRIs of the 9 cardinal positions of gaze were obtained in the quasi-coronal plane transverse to the axis of each orbit, at 2-mm thickness with a 256 × 192 matrix over a 10-cm² field of view. Digital MRIs were analyzed using the ImageJ program (http://rsbweb.nih.gov/ij/index.html). Analysis of the pulley position was performed as previously described.² The pulley position was estimated as the area centroid 10 mm posterior to the globe center; this was compared with the normal range of the pulley position as reported in our previous study.³

The positions of 4 rectus pulleys of both eyes were normal in primary position. The position of the right LR pulley was displaced downward in attempted abduction, supraducted adduction, and supraducted abduction. The position of the right superior rectus (SR) pulley was displaced laterally in attempted supraduction, supraducted adduction, and supraducted abduction (Figure 2). Thus, the patient showed instability of the right LR and SR pulleys in attempted supraduction and abduction. There was no significant displacement of the position of all rectus pulleys in the left eye. For correction of pulley instability in the right eye, the SR was transposed 1 muscle width nasally and fixed 13 mm posterior to the SR insertion. The LR was transposed 1 muscle width superiorly and fixed 15 mm posterior to the LR insertion. Three months after the operation, 40 PD of exotropia in upgaze decreased to 12 PD and primary-position orthophoria remained (Figure 1B).

A previous study¹ proposed that LR co-contraction during elevation might be the cause of Y-pattern exotropia and stated that LR supraplacement is an effective treatment. As the functional origins of extraocular muscles, pulleys have important effects on extraocular muscle action.⁴ Pulley instability changes the innate pulling direction of the extraocular muscle. Oh et al⁵ reported that pulley instability occurs in some cases of incomitant strabismus. We revealed LR and SR pulley instability in Y-pattern exotropia using high-resolution MRI and then performed LR and SR transposition for the correction of this exotropia.

We suggest that temporal slippage of the SR in upgaze would cause an abducting vector and that inferior slippage of the LR in abduction would cause a depressing vector, resulting in fixation duress to the contralateral inferior oblique muscle. Therefore, the stabilization of pulley positions results in the improvement of Y-pattern strabismus during upgaze. In conclusion, we believe that the pulley instability manifested in our patient is one of the causes of Y-pattern exotropia.

Correspondence: Dr S. W. Park, Department of Ophthalmology, Chonnam National University Medical School and Hospital, 8, Hakdong, Donggu, Gwangju, South Korea 501-757 (exo70@naver.com).

Financial Disclosure: None reported.