Masked bilateral SO palsy
Reviewed by Fiona Rowe

This study examined the occurrence of masked bilateral superior oblique (SO) palsy with contralateral inferior oblique (IO) over action requiring further surgical intervention in children undergoing unilateral IO weakening surgery for presumed unilateral SO palsy. This retrospective study included 50 children with a mean age of 5.6 years. All had unilateral IO surgery. The mean follow-up period was 40 months. Nine (18%) developed masked bilateral SO palsy postoperatively causing reversal of initial hypertropia and requiring IO weakening of the unoperated eye. The preoperative primary hypertropia was significantly smaller for masked than unilateral SO palsy. A preoperative primary hypertropia of <5PD was associated with an odds ratio of 8.2 for requiring reoperation. This can be considered a risk for masked palsy. 

Masked bilateral superior oblique palsy in children.
Dotan G, Roarty JD, Baker HD.
JOURNAL OF PEDIATRIC OPHTHALMOLOGY AND STRABISMUS
2014;51(6):333-6.