Corneal topography may be affected by surgery to structures other than the cornea, although techniques usually aim to minimise this. In these cases, assessment of corneal shape can be useful in the investigation of otherwise unexplained visual loss and in the development of new techniques.
In trabeculectomy, excessive scleral cautery causes collagen contraction with local flattening and associated steepening of the paracentral cornea. The partial-thickness scleral flap and full-thickness sclerostomy beneath it can cause similar changes to peripheral corneal incisions: flattening if the incision is long and the sutures are too loose, steepening if the sutures are too tight and a spiral or steep/flat pattern with irregular astigmatism if one side is sutured more tightly than the other or if there is horizontal misalignment. A large anteriorly situated drainage bleb can collect a tear meniscus at its junction with the cornea, causing apparent flattening, similar to a pterygium.
After vitreous or retinal surgery, altered topography should be considered as a cause of reduced visual acuity. Of local explants, radial plombs tend to induce greater corneal steepening than circumferential scleral buckles, but both can produce an asymmetric bow tie with high astigmatism. Encircling bands generate peripheral corneal flattening, with central steepening, which contributes to a myopic shift, as does elongation of the axial length. Both external and internal approaches can cause swelling of the conjunctiva, leading to peripheral corneal flattening due to dellen or meniscus formation, as adjacent to pterygia or blebs.
Strabismus surgery can generate variable effects on corneal astigmatism, which tend to be short-lived. Horizontal muscle surgery tends to produce vertical corneal steepening, and a tear meniscus adjacent to conjunctival swelling can be associated with the appearance of local flattening. Effects can be minimised by attention to good technique, including the length of the suture bites and tension in the sutures.
Corneal topography Trabeculectomy Scleral flap Sclerostomy Bleb Vitreoretinal surgery Explants Encircling bands Strabismus surgery
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