preoperatively by using the prism adaptation test. A base-out prism is prescribed to induce artificial divergence. Inducing divergence with a base-out prism causes the patient to converge, and therefore decreases the nys-

12 tagmus, which can then be followed by the corresponding amount of recession−resection procedure [48]. The amount of surgery is based on the prism diopters tolerated by the patient preoperatively.

Spielmann [49] in a retrospective study of 120 patients who underwent artificial divergence surgery found 93 (77.5%) of the patients were orthophoric, 18 patients had exophoria postoperatively, and 9 patients had exotropia. Exotropia was found to be associated with hypermetropia. Spielmann proposed bilateral recession of medial rectus muscle by 5–13 mm depending on the amount of prism determined preoperatively by the prism adaptation test. She recommended 5 mm recession if the fusion was tolerated with 30–40 PD, 7 mm for 50–60 PD, and 8 mm if fusion exceeds 60 PD.

Some patients have a convergence null in addition to the gaze angle null causing the AHP. If the amount of divergence induced by base-out prisms did not satisfactorily correct the AHP, these patients benefitted by a combination of artificial divergence andAnderson–Kestenbaum procedure [48, 50]. The amount of surgery is done for the total prism diopters tolerated by artificial divergence procedure and then the remaining AHP is corrected using the Anderson–Kestenbaum procedure.

Zubcov et al. [48] compared preand postoperative eye movement recording and binocular visual acuities of patients who underwent the Anderson−Kestenbaum procedure (n = 7), artificial divergence procedure (n = 6), and a combination of both procedures (n = 5) in patients with congenital nystagmus. In patients who underwent artificial divergence surgery, only one patient developed 4 PD esophoria postoperatively. Stereopsis improved in four patients. Four patients had a head turn of less than 5°. Binocular visual acuity improved in 50% of the patients by 1–2 Snellen lines. Eye movement recordings showed broadening of the null zone. In patients who underwent a combined procedure, stereopsis improved in two patients and no residual head turn greater than 5° was found. Binocular visual acuity improved by two or more Snellen lines in four of the five patients. Broadening of the null zone was noticed in all patients.

Graf et al. [51] in a retrospective study to analyze the e ects of Kestenbaum surgery and artificial divergence surgery found that artificial divergence surgery when performed alone o ers better correction of AHP than with the Kestenbaum surgery. However, in patients with large AHP, combining both artificial divergence surgery and Kestenbaum surgery gives better results.

12.3.6.5Surgery to Decrease

the Intensity of Nystagmus

In patients who do not exhibit any compensatory mechanism to dampen the nystagmus, various surgeries have been done to dampen the congenital nystagmus. These procedures were referred by Crone [52] as immobilization procedures. Various surgical procedures have been mentioned in the literature. Von Noorden summarized these surgical principles, including large recession of all horizontal rectus muscles, the tenotomy procedure, fixation of the extraocular muscles to the periosteum of the lateral orbital wall, retro-equatorial myopexy of all horizontal rectus muscles, placement of retro-equatorial encircling silicone band over rectus muscles in both eyes and extirpation of horizontal rectus muscles.

Both retro equatorial recession of horizontal rectus muscle and tenotomy procedure have been used more frequently and will be discussed in detail.

Retro-Equatorial Recession of Horizontal Rectus Muscles

Bietti and Bagolini [53], in 1956, first described retroequatorial recession of all four horizontal rectus muscles. Von Noorden and Sprunger [54] performed this procedure on three patients and reported increased acuity in two patients and correction of head posture in one patient. Helveston et al.[55] performed this procedure in ten patients and reported dampening of nystagmus and improvement of visual acuity in 80% of patients.All his patients also reported improvement in visual acuity and head posture. Datta et al. [56] performed surgery on nine patients and reported decreased amplitude in 15 eyes and increased visual acuity in 12 eyes. Boyle et al. [57] in a retrospective review of 18 patients who underwent retro-equatorial recession surgery of horizontal muscle, 50% of patients showed improvement in visual acuity by at least one Snellen line. All patients underwent medial rectus recession of 8–10mm, and bilateral lateral rectus muscle recession of 8–12mm.

Bagheri et al. [58] reported results of 20 patients who underwent horizontal rectus recession surgery. Thirteen patients (76.5%) improved in visual acuity from one to three Snellen lines. AHP improved in most of the patients. Similar results were also documented by other authors, Davis et al. [59] and Atilla et al. [60]. They calculated the amount of recession individually depending on the angle of deviation, head position, and amount of strabismus if present. Recessions performed on the medial rectus were more e ective than recession on the lateral rectus. Thus surgery is planned based on the e ect of recession of the medial rectus muscle rather than the lateral rectus recession. To correct the associated strabismus, the surgical plan is