title: Masked bilateral superior oblique palsy.
abstract: BACKGROUND
  A diagnosis of masked bilateral superior oblique palsy (MBSOP) is established when signs of SOP appear in the normal eye of a patient after strabismus surgery for SOP in the contralateral eye. Despite the absence of signs of bilaterality before surgery, a palsy will develop in the previously unaffected eye in 10% or more of the patients undergoing surgery. This paper examines the clinical profiles and results of surgical management of 14 patients with MBSOP.
  
  
  METHODS
  We retrospectively analysed the records of all 14 patients with the clinical criteria for MBSOP in the clinical strabismus database of patients treated by the second author between 1979 and 2001. We extracted the history and data from the pre- and postoperative ophthalmic and orthoptic examinations, recorded the surgical procedures and tabulated the postoperative results. The surgical outcome was considered successful if normal head posture was restored, diplopia was eliminated in functional positions of gaze, and ocular alignment was improved to within 5 prism dioptres (PD) of orthotropia.
  
  
  RESULTS
  All 14 patients had presented with seemingly unilateral SOP. The average primary-position hypertropia preoperatively was 17 (range 4-30) PD. The mean excyclotorsion was 5 degrees (n = 12). Most patients (93%) had a head tilt, mild V pattern, moderate inferior oblique overaction and mild superior oblique underaction. Initial surgery consisted of ipsilateral inferior oblique weakening with or without contralateral inferior rectus recession. The average primary-position hypertropia after the first operation (n = 14) was 8 (range 0-15) PD. In the previously masked eye inferior oblique overaction averaged +1.8 and superior oblique underaction -1.1. The average interval from initial surgery to involvement of the contralateral side was 14.9 (range 0.2-52) weeks. The average primary-position hypertropia after the second operation (n = 10) was 1.6 (range 0-10) PD; follow-up averaged 15 (range 0-120) months. Postoperative alignment was excellent (within 6 PD of orthotropia) and binocular vision restored in 9 of the 10 patients.
  
  
  CONCLUSIONS
  Masked superior oblique palsy is difficult to detect before surgical correction of the initially manifest palsy. However, the possibility of an occult contralateral palsy should be considered in all patients undergoing surgery for unilateral SOP. Patients should be informed preoperatively of the possibility of this outcome. When the masked palsy becomes evident, a successful surgical outcome can usually be expected.
authors: Department of Ophthalmology, McGill University Health Centre, Montreal General and Montreal Children's hospitals, Montreal, Que.
authors: Faisal
authors: F
authors: Esmail
authors: Michael
authors: M
authors: Flanders
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D002648:Child; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009801:Oculomotor Muscles / Q000473:pathology; D009886:Ophthalmoplegia / Q000175:diagnosis*; D012189:Retrospective Studies; D013285:Strabismus / Q000175:diagnosis* / Q000601:surgery; D020432:Trochlear Nerve Diseases / Q000175:diagnosis* / Q000601:surgery; D015348:Vision, Binocular