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Evaluation of FD2 (Frisby Davis distance) stereotest in surgical management of intermittent exotropia
  1. Abhishek Singh,
  2. Pradeep Sharma,
  3. Digvijay Singh,
  4. Rohit Saxena,
  5. Anudeepa Sharma,
  6. Vimla Menon
  1. Dr Rajendra Prasad Centre for Ophthalmic Sciences, All India Institute of Medical Sciences, New Delhi, India
  1. Correspondence to Dr Pradeep Sharma, Pediatric Ophthalmology and Strabismus, Dr Rajendra Prasad Centre for Ophthalmic Sciences, All India Institute of Medical Sciences, Ansari Nagar, New Delhi 110029, India; drpsharma57{at}


Aim To evaluate Frisby Davis distance (FD2) stereotest for determining the timing of surgical intervention in intermittent exotropia (X(T)).

Methods A prospective case–control study was conducted including 30 patients with X(T) and 30 age-matched controls. Stereoacuity was measured preoperatively and 3 months postoperatively using FD2 for distance and TNO and Randot for near.

Results Preoperative distance stereoacuity was 43.83±35.51 arcsec (median 30 arcsec; range 10–120) which improved postoperatively to 27±33.74 arcsec (median 15 arcsec; range 5–120) (p=0.001). Cases with FD2 stereoacuity worse than 70 arcsec did not show significant improvement. Mean preoperative near stereoacuity by TNO was 94.00±79.48 arcsec (median 60 arcsec) and Randot was 50.33±39.23 arcsec (median 30 arcsec) which improved to 80.00±80.08 arcsec (median 60 arcsec) and 34.17±57.00 arcsec (median 20 arcsec), respectively, after surgery (both p=0.001). The controls had a mean distance stereoacuity of 14.66±4.13 arcsec (median 15 arcsec; range 5–20) and near stereoacuity of 63.00±21.35 arcsec (median 60 arcsec (TNO)) and 23.66±5.07 arcsec (median 20 arcsec (Randot)). There was a significant correlation between FD2 and Randot in the cases but not in controls (p=0.005), however no correlation was found between TNO and FD2.

Conclusions Distance stereoacuity is reduced in X(T) to a greater extent than the near stereoacuity and both improve after surgery. FD2 is useful for deciding timing of surgery and a stereoacuity worse than 20 arcsec is an indication for surgical intervention. A preoperative distance stereoacuity which is worse than 70 arcsec implies a poor prognosis for stereoacuity improvement after surgery.

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