1.REFRACTIVE CHANGES AFTER POSTERIOR SEGMENT SURGERIES

2. Refractive changes caused by alterations in axial length after retinal surgery. Induced astigmatism after retinal surgery. Refractive changes from alterations in corneal topography after retinal surgery. Refractive changes in children undergoing retinal surgery. Refractive and biometric changes related to silicone oil use. Refractive changes following retinal cryotherapy and panretinal photocoagulation. 3. Refractive changes after retinal surgery may include a hyperopic or myopic shift, as well as induced regular or irregular astigmatism. The significance of these changes has been debated and may be related in part to surgical technique. 4. The eyes of infants and adolescents respond differently to retinal surgery than do those of adults. Retinal surgery in infants and children may have profound effects on ocular development, producing an extreme alterations in postoperative refraction. 5. Early reports focused on alterations in axial length and the resultant spherical changes induced after retinal detachment repair with scleral buckling techniques. Scleral resection reduces axial length and produces a hyperopic shift . 6. Theoretically, scleral buckling should increase the axial length of the eye, producing a myopic shift; however, reported postoperative outcomes have been variable. 7. It was reported that a consistent axial elongation of nearly 1 mm in all eyes examined, with a resultant refractive change of 2.5 D. Elongation occurred in the vitreous cavity, with minimal changes in anterior chamber depth or lens thickness. 8. They found that the type of buckle element and technique directly affected postoperative refraction. Progressive horizontal shortening with a thin solid silicone band (no. 41) increased buckle height and axial length. Thicker elements (no. 287 tire, no.505 sponge, no. 507 sponge) increased axial length when horizontally shortened but decreased axial length when applied by invagination techniques. . 9. It was found that a correlation between buckle type and axial lengthening . The encircling scleral bands increasing axial length by an average of 0.99 mm and producing 2.75 D of refractive change, Non encircling procedures increased axial length by 0.26 mm and induced 0.31 D of refractive change 10. Contrary to the belief of most surgeons, "high equatorial indentations produce a paradoxical shortening of the axial length which changes the induced refractive error toward hyperopia. 11. Induced astigmatism after retinal surgery 12. Highly unpredictable alterations in keratometric power and corneal curvature can be produced by scleral buckling procedures. It is not clear from the literature whether these astigmatic changes are transient or permanent. 13. Induction of severe persistent irregular astigmatism after scleral buckling with episcleral silicone sponges. Radial buckles were significantly more likely to produce astigmatic errors greater than 2 D. It was reported that although small amounts of astigmatism usually represented transient changes, larger amounts of induced astigmatism (>3 D) usually persisted. 14. Also it was reported occurance of astigmatism after pars plana vitrectomy. Most of these cases required lysis of the suture at the vitrectomy site to relieve the astigmatism. Risk factors include total air or gas tamponade and tight wound closure . 15. Induced astigmatism is typically mild and transient, Use of the 25-gauge sutureless vitrectomy system is likely to reduce astigmatism induced by scleral suturing. This system may result in more rapid visual recovery in these patients. 16. Refractive changes from alterations in corneal topography after retinal surgery 17. Although keratometry detects global changes in corneal shape, topography can identify irregular astigmatismfocal alterations in corneal curvature that may not be translated to the entire corneal meridian but nonetheless can diminish visual acuity. 18. These descriptors include the SRI, or surface regularity index, and the SAI, or surface asymmetry index . The SRI is a measure that describes the regularity and optical quality of the central cornea and is correlated with best spectacle-corrected visual acuity . The SRI and SAI values increase, indicating increased surface irregularity and asymmetry . 19. It was found that circular buckles alone induced only 0.4 D steepening over the entire cornea but over 2 D in the central cornea by the first postoperative week. 20. These changes were transient; they became apparent by the first postoperative week but normalized by week twelve in all cases . 21. Topographic analysis has shown two patterns of corneal steepening after the placement of scleral buckles. Encircling buckles produce either uniform central steepening or coupled steepening and flattening in the opposite regions, Local or segmental buckles produce steepening in the corresponding meridian . 22. Vitrectomy had a minimal effect on overall corneal topography; however, it induced 1 to 1.5 D of central corneal steepening, which correlated with the location of the entry port. Suture diameter, wound closure, and cautery were related to corneal curvature changes after pars plana vitrectomy. 23. Recommendations to to lessen these effects are , using small diameter absorbable suture, avoiding excess suture tightening during closure, and minimizing cautery 24. Some loss of best spectacle-corrected visual acuity after scleral buckling procedures or pars plana vitrectomy, previously attributed to macular dysfunction, may, in fact, be due to irregular corneal astigmatism. This is to recommend a delay in prescribing spectacles until 6 or more months postoperatively. 25. Refractive changes in children undergoing retinal surgery 26. Infants and children are most susceptible to large, variable, and fluctuating postoperative refractive results owing to effects on ocular development. Scleral buckling has a profound effect on the development and emmetropization of the eyes . 27. In human infants, it was found that an induced myopia greater than 11 D with scleral buckling. This induced myopia was reduced by one half after division of the buckle . They concluded that a scleral buckle prevented normal ocular development, and that children under the age of 10 years were particularly susceptible to this effect. 28. Refractive and biometric changes related to silicone oil use 29. Silicone oil was chosen as a tamponade owing to its high surface tension, transparency, stability, and relatively low toxicity; however, it induces refractive changes when it occupies the vitreous cavity and it creates difficulties in obtaining accurate measurements for intraocular lens calculations after it is injected into the eye . 30. 1-Refractive changes silicone oil has a variable effect on refraction dependent on the phakic status of the eye . Aphakic eyes become less hyperopic by an average of 6 to 7 D . whereas phakic eyes become more hyperopic by an average of 5.5 to 7.6 D . Silicone oil has an index of refraction of 1.405 compared with that of vitreous (1.336). 31. In the aphakic state, silicone oil forms a convex anterior surface relative to the corneal endothelium. This surface decreases the overall hyperopia by acting as a plus lens. In the phakic eye, the silicone oil forms a concave surface behind the natural lens, which acts as a minus lens and increases the overall level of hyperopia. In pseudophakic eyes, silicone oil negates the effect of the intraocular lens (if the refractive index is similar to that of silicone), causing a myopic shift like that of an aphakic eye . 32. The effect silicone oil has on refraction can also be affected by head position . This effect is more pronounced in aphakia, When patients refractions were measured in the supine position and then remeasured in the head-down position, the spherical equivalent increased by approximately 6 D in aphakic eyes. This difference was most likely caused by a shift in the position of the oil bubble. In the head-down position, the anterior surface is less convex than in the supine position, resulting in more hyperopia in the headdown position. 33. In the same study, the cylinder axis was observed to shift 11.5 degrees on average in aphakic eyes and 10.1 degrees on average in phakic eyes with changing head position, but the cylindrical power did not change. These changes can be noted by the patient, who may complain of fluctuations in vision with various activities. 34. Silicone oil has been noted to decrease the accommodation of the lens in phakic patients, and, typically, a +2.00 to 2.50 D bifocal is required in phakic patients when the fellow eye requires none . 35. Biometry of the silicone oilfilled eye To determine the axial length and intraocular lens calculation in a silicone oil filled eye, A-scan ultrasonic biometry requires an adjustment for the slower speed of sound in oil. Sound waves travel at 987 m/s in silicone oil of viscosity of 1000 centistokes versus 1532 m/s in vitreous . Silicone oil also causes poor penetration of the sound wave, complicating measurements further .Because the A-scan uses time in microseconds from the cornea to the retina to determine the length of the eye, if no adjustment is made for the oil, an artificially high axial length is obtained, resulting in the placement of a lower power intraocular lens than necessary and a hyperopic result. 36. To adjust for this effect, the axial length obtained with silicone oil in place can be multiplied by a correction factor of 0.71 . When the A-scan measures the vitreous cavity depth separately, that figure can be multiplied by a correction factor of 0.64 and added to the anterior chamber depth and lens thickness to obtain a true axial length . 37. Another method based on a velocity conversion equation, which provides for the correction of an erroneous measurement that results from the use of an incorrect sound velocity setting. 38. To determine the true axial length (TAL) using the equation, the correct sound velocity (Vc) should be divided by the incorrect sound velocity setting used for the measurement (Vm) and then multiplied by the incorrect (apparent) axial length reading (AAL) . TAL Vc=Vm AAL 39. Owing to shifting of the oil bubble, the patient should be measured upright so the oil fills the eye from lens to macula. The transducer should be oriented with the beam perpendicular to the globe to reduce refraction .If the axial length is out of the range of the A-scan used, the B-scan may be used instead, although this is known to underestimate the true length and may induce more error in long eyes . 40. Posterior staphylomas were found in the eyes with the greatest deviation from predicted values, and it has been suggested that leaving these eyes aphakic, at least initially, may be the best strategy. 41. Because the silicone oil, with its increased refractive index compared with that of vitreous, will remain in direct contact with the posterior capsule, it becomes difficult to predict the postoperative refraction. Owing to the difficulties in obtaining accurate axial length measurements, many authorities have suggested that A-scan ultrasonic biometry be performed before silicone oil injection . 42. In a study by Grinbaum and colleagues [40] in which the A-scan had been completed before oil injection and after scleral buckling, extracapsular cataract extraction and biconvex intraocular lens implantation were successfully completed in eight cases. 43. Refractive changes following retinal cryotherapy and panretinal photocoagulation 44. Loss of accommodation, transient myopia, or both have been reported following retinal cryotherapy and panretinal photocoagulation. Loss of accommodation can be secondary to the retrobulbar block owing to mechanical injury to the ciliary ganglion, its roots, or the short ciliary nerves. 45. Accommodative paresis and myopia have also been reported without retrobulbar block following cryotherapy, laser retinopexy of retinal tears, and panretinal photocoagulation. These symptoms are usually transient and tend to resolve within 5 weeks without treatment. A possible explanation could be damage to the short ciliary nerves during treatment . 46. SUMMARY 47. Retinal surgery can induce significant refractive errors. These errors include spherical changes caused by alterations in axial length after scleral buckle placement and astigmatic changes induced by scleral buckling or pars plana vitrectomy. Focal alterations in corneal curvature can significantly limit postoperative visual acuity when axial length and keratometry values seem relatively normal. Surgical technique may also influence the induction of corneal surface irregularity, especially in highly symptomatic cases. 48. These refractive errors are usually transient, but suture lysis and buckle transection may occasionally be indicated. In very young patients, retinal surgery not only affects refractive outcomes but also alters the course of normal ocular development. The adjunctive use of silicone oil can impose alterations directly, by the oils interaction with the other refractive elements of the eye, and indirectly,through its effects on intraocular lens power calculations for subsequent cataract surgery