How much near stereo acuity should deteriorate in intermittent exotropia for it to be considered an indication for surgery? I have a patient with intermittent XT with moderate control and a near stereo acuity of 120 sec arc on TNO test. When should I consider such a patient for surgery?

There is no such "rule". Control of intermittent exotropia is on the basis of motor fusion that responds as an anti-dipolpia mechanism. Stereo acuity is the beneficiary of alignment in patients who have sensory fusion capability. The more time the eye is out and the more difficult it is to control, the more likely surgery should be done. The ability to appreciate stereo acuity is a measure of sensory fusion. This fusion is achieved if the eyes are aligned and the patient has the ability to fuse slightly disparity images coming from eyes situated at either side of the nose and perceive depth. You are correct in suspecting that a prolonged manifest deviation could contribute to the loss of stereo acuity (sensory fusion). However, if a deviation is intermittent, there is no particular reason why stereo acuity should be lost. The main reasons for surgery for an intermittent deviation would be documenting a loss in the ability to control the deviation and symptoms that might arise from the effort to control the deviaton such as asthenopia. Eugene M. Helveston, M.D.