Prior to strabismus surgery, an important and seemingly obvious question should be asked: “Why are we operating?” Is our treatment goal to establish binocular fusion, eliminate diplopia, expand the field of binocular vision, correct a compensatory head posture, or simply to improve cosmetic appearance? Establishing the goals prior to surgery helps us clarify indications for surgery, and formulate a logical treatment plan. The plan that is made should be the one that is best for the patient—not just the plan that is best for correcting the angle of deviation.

The indications for surgery should be based on the patient’s needs: either binocular function or cosmetic appearance (Table 2.1). Urgent surgery is indicated to reestablish binocular fusion in a child with an esophoria that has recently broken down to a tropia. The family should be told that surgery is indicated to regain binocular fusion and not just to improve the cosmetic appearance. In contrast, surgery for a long-standing sensory esotropia secondary to a blind eye is cosmetic, as there is virtually no potential for binocular fusion. In this case, the indication for surgery should be based on the cosmetic desires of the patient. In some cases, it may be difficult (or even impossible) to determine the binocular potential. For example, an older child with equal vision and a history of esotropia since infancy may or may not have binocular fusion potential. I tend to give these patients the benefit of the doubt and treat them as if they have fusion potential.

Understanding the functional goal also helps direct the surgical plan. Esotropic patients with fusion potential generally require large amounts of surgery, more than the standard surgical numbers (see Chap.​ 4). A plan based on standard surgery in these patients routinely results in undercorrection. Esotropic patients without binocular fusion potential are ill served by planning for “more” surgery, however, as a consecutive exotropia will inevitably increase over time, and an exotropia is a poor cosmetic outcome. In these patients without fusion potential, it is better to do less surgery, as a small residual esotropia is more stable and has a better appearance than a consecutive exotropia. Consideration of the functional outcome also influences the selection of the type of surgery. Monocular recession-resection surgery produces incomitance, which is not optimal in a fusing patient, as incomitance can cause diplopia in eccentric positions of gaze. Monocular surgery on the blind eye, however, is the procedure of choice for sensory strabismus, to protect the only seeing good eye. These are but a few examples that demonstrate the importance of considering the potential for binocular fusion when planning strabismus surgery. Table 2.2 lists some important signs that indicate the potential for binocular fusion.

Prior to surgery, it is helpful to establish a specific strabismus diagnosis. Most cases of strabismus can be classified into a type, such as partially accommodative esotropia, intermittent exotropia, Duane’s syndrome–esotropia type 1, congenital superior oblique palsy, or Brown’s syndrome. At times, it may be difficult to determine the exact etiology of the strabismus, and an MRI of the head and orbit may be indicated. If the cause is still unknown after a complete evaluation, then it is appropriate to operate for the strabismus pattern, taking into account the ductions, versions, and the presence of incomitance.

Strabismus surgery corrects ocular misalignment by slackening a muscle (i.e., recession), by tightening a muscle (i.e., resection), or by changing the insertion site of the muscle, thus changing the direction of pull or vector of force (i.e., transposition).