Excimer laser ablation rate and corneal hydration.

During excimer laser photorefractive keratectomy, dehydration of the cornea begins as soon as the epithelium is removed. Corneal hydration might affect the excimer laser ablation rate, which could affect the accuracy of correction. We studied the effect of corneal hydration on the excimer laser ablation rate in bovine eyes. To control hydration, bovine corneoscleral rims were equilibrated in dextran solutions of varying concentrations. One button trephined from each rim underwent laser ablation. Hydrated tissue ablation rates (amount of collagen, ground substance, and water removed per pulse) and dry component ablation rates (amount of collagen and ground substance removed per pulse) were calculated from mass removed. The hydrated tissue ablation rate at physiologic hydration was 0.40 micron/pulse. As corneal hydration increased, the hydrated tissue ablation rate increased by 5.6 micrograms/cm2/pulse per increase in unit corneal hydration (simple linear regression analysis, P = .0001). The dry component ablation rate decreased linearly by 0.82 microgram/cm2/pulse per unit increase in corneal hydration (simple linear regression analysis, P = .0001). Both clinical data and theoretical arguments imply that dry component ablation rate determines refractive outcome after photorefractive keratectomy. Since the dry component ablation rate increases as the cornea dries, significant dehydration of the cornea before ablation might lead to relative overcorrections of myopia. Surgeons should use a technique that minimizes changes in hydration to maximize the predictability of excimer laser photorefractive keratectomy.

Corneal ectasia as a complication of repeated keratotomy surgery.

BACKGROUND: Staged keratotomy surgery, or "enhancement surgery," may allow a more predictable outcome, but also subjects the patient to additional surgical risks. METHODS: A 39-year-old man underwent astigmatic keratotomy for myopic astigmatism, followed by 12 enhancement procedures for residual astigmatism. RESULTS: These procedures effectively resulted in a double hexagonal keratotomy. The patient's best spectacle-corrected acuity deteriorated to counting fingers. Clinically, a conically-shaped protrusion of the central cornea, Munson's sign, diffuse subepithelial scarring, and central corneal thinning were noted. Penetrating keratoplasty was performed. Histopathologic examination showed central thinning, epithelial edema, disruption of Bowman's layer, marked stromal scarring, and focal areas of endothelial attenuation--findings consistent with keratoconus. CONCLUSION: This case illustrates that multiple keratotomy procedures may result in corneal ectasia in apparently normal eyes and suggests that hexagonal keratotomy may be more likely to cause iatrogenic keratoconus.

Splenomimetic effect of Corynebacterium parvum in fulminant pneumococcemia.

The efficacy of Corynebacterium parvum to stimulate splenic growth and to boost host survival was examined by using adult Sprague-Dawley rats in a highly spleen-sensitive model of fulminant pneumococcemia. Rats were either treated (10 days or 1 hr before or 1 hr after) or not treated with C. parvum; were depleted of complement; underwent partial, total, or sham splenic resection; and then were challenged with either a low (2 X 10(2)) or a high (2 X 10(5)) dose of pneumococci. In the absence of C. parvum, survival (percent and duration) was lowest after total splenectomy and was proportional to remnant spleen weight after partial splenectomy. Although C. parvum treatment sharply increased splenic weight, nucleated cell numbers, and survival, the lowered mortality and improved survival time were independent of spleen weight. The rapidly acting, extrasplenic, splenomimetic protective effect of C. parvum suggests that this class of immunomodulators may be a useful adjunct in managing sepsis associated with defective or absent splenic function.