Phototherapeutic Keratectomy for Granular and Lattice Corneal Dystrophies at 1.5 to 4 Years

BACKGROUND

The capability of the 193-nm excimer laser to ablate the cornea and to remove opacities and various other corneal diseases in a procedure called phototherapeutic keratectomy (PTK) has been demonstrated. In this study we evaluated the long-term results of PTK for treatment of granular and lattice corneal dystrophies.

METHODS

Four eyes with granular or lattice corneal dystrophy were treated with a mean follow-up of 47.8 months (range, 36 to 58 months). Focal ablations of the central cornea with an ablation zone of 5.5 to 6.0 mm were performed. Ablation depth was 110 microns in three eyes and 140 microns in one eye.

RESULTS

Removal of corneal opacities allowed for improvement in corrected visual acuity in all patients. Mean corneal thickness in the area of pathology decreased from 0.583 mm before surgery to 0.449 mm after surgery. Spherical equivalent of the manifest refraction measurements increased by a mean of +5.09 D. There were no major complications, but all patients developed slight haze. There was a hyperopic shift in three eyes.

CONCLUSION

Our long-term results suggest that PTK is a safe and effective alternative to penetrating and lamellar keratoplasty in patients with granular or lattice corneal dystrophies.

Keratocyte-Populated Collagen Gel as an In Vitro Model of Excimer Laser Keratectomy

BACKGROUND

To develop an in vitro model to study the effects of excimer laser keratectomy on corneal stromal cells, we evaluated two types of collagen gel populated with keratocytes.

METHODS

Keratocyte-populated collagen gels were prepared with type I collagen in 6-well plates or in culture plate inserts, the bottom of which consisted of a nitrocellulose membrane, contained within 6-well plates. The gels were ablated by the 193-nm excimer laser, set to ablate 50, 100, or 200 microns deep, and was observed under a phase-contrast microscope for 2 days.

RESULTS

Keratocytes cultured in collagen gel developed cytoplasmic processes and formed networks of interconnected cells. Cells within the ablated area in the 6-well plates began to lose their cytoplasmic processes and became round approximately 3 hours after excimer laser ablation. These cellular changes were more prominent in the gels ablated to a depth of 200 microns. Cells outside of the ablation zones in the 6-well plates and the culture plate inserts remained intact.

CONCLUSIONS

These results suggest the use of keratocyte-populated collagen gel as an in vitro model of cellular response to excimer laser keratectomy and also suggest that gel prepared in culture plate inserts is the preferred method.

Photorefractive keratectomy for severe postkeratoplasty astigmatism

We performed cylindric corneal ablations with the excimer laser on 12 patients to correct severe, disabling astigmatism after keratoplasty. In some patients, an additional ablation was performed to correct myopia. Patients were followed up for an average of eight months (range, six to 14 months). Uncorrected visual acuity improved in nine patients, and nine of the 12 patients had a decrease in refractive cylinder at last follow-up. The mean preoperative refractive cylinder was 7.0 +/- 3.6 diopters, which decreased to a mean of 3.1 +/- 2.6 diopters at one month (P = .0003) and 4.3 +/- 2.9 diopters at last follow-up (P = .03). Keratometric astigmatism decreased from 7.5 +/- 3.9 diopters preoperatively to 5.2 +/- 3.9 diopters at the last follow-up (P = .001). Mean spherical equivalent was reduced from -7.4 +/- 4.2 diopters preoperatively to -3.3 +/- 4.4 diopters postoperatively (P = .003). Postoperative corneal haze, when present, did not reduce visual acuity. Excimer laser superficial keratectomy thus appears to be safe when used for postkeratoplasty ametropia, although substantial regression may limit its effectiveness in some patients.

Corneal wound healing after excimer laser ablation. Effects of nitrogen gas blower

PURPOSE

To examine the effects of blowing nitrogen gas over the cornea during photorefractive keratectomy.

METHODS

Excimer laser ablations for myopia were performed on rabbit corneas with or without the blowing of nitrogen across the surface of the cornea. All eyes underwent a 5-diopter myopic ablation; in 8 eyes, a ring was used to blow nitrogen gas across the cornea, and, in 8 eyes, the same ring was used, but no nitrogen gas was blown.

RESULTS

Epithelial healing occurred more rapidly in the eyes that were not treated with the gas (3.8 +/- 1.3 days) than in the gas-treated group (6.1 +/- 0.8 days; P = 0.0025). Corneal haze was greater in the group treated with gas. Results of histologic examination showed the ablated area to have a smoother surface when nitrogen was not blown across the cornea surface.

CONCLUSION

Superficial corneal deturgescence produced by the nitrogen gas appears to result in a rougher surface immediately postoperatively with undesirable effects on surface healing, but further studies will be necessary to determine the applicability of these results to humans.

Photorefractive keratectomy to create toric ablations for correction of astigmatism

Excimer laser photorefractive keratectomy, developed to perform radially symmetric ablations to correct myopic or hyperopic refractive errors, was used to create toric ablations designed to correct cylindrical errors. An expanding slit was created, with no refractive change intended parallel to the slit opening, and central flattening was induced in the meridian in which the slit was expanded. In polymethyl methacrylate blocks, the induced cylinder (as measured with a lensometer) agreed closely (r2 = .99) with intended change, and in plastic corneas, induction of the cylinder could be demonstrated with computer-assisted topographic analysis. Adult pigmented rabbits underwent induction of toric ablations in two-diopter increments; measured keratometric change correlated with desired change (r2 = .87 at 3 weeks; r2 = .89 at 12 weeks). Toric ablations with the excimer laser appear to represent a promising strategy for the correction of cylindrical errors that do not rely on creation of deep corneal incisions, excisions, or compression sutures.

Conjunctival pterygia. Mechanism of corneal topographic changes

Pterygia induce irregular corneal astigmatism that sometimes necessitates surgical removal before the lesion has advanced close to the visual axis. This astigmatism may occur either due to traction generated by the pterygium mechanically pulling on and distorting the cornea, or by the pooling of tears in advance of the pterygium, or both. To evaluate the effect of localized tear pooling on pterygium-induced astigmatism, corneoscope photographs were obtained before and after absorbing excess tears at the pterygium-corneal interface with a cellulose sponge. Pterygia from these patients were then excised and examined with transmission electron microscopy for the presence of myofibroblasts. The absence of myofibroblasts in these lesions as well as the alterations in corneoscopy induced by localized removal of tears at the advancing edge of the pterygia suggest that corneal distortion in the presence of pterygia is caused in part by local changes in the tear film.

Computer-assisted corneal topography in family members of patients with keratoconus

In about 6% of patients with keratoconus, there is a history of familial disease. High keratometric astigmatism and mildly irregular mires observed by placidois disc have been suggested to represent variable forms of expression of a gene in family members of patients with keratoconus. We used a computer-assisted digital videophotokeratoscope to map the corneas of 28 family members of 5 patients with keratoconus. Abnormalities observed in family members included central steepening, greater steepening of the cornea inferior to the apex, and substantial asymmetry in the central dioptric power between the two eyes of the same individual. These findings were similar to, but less severe than, those found in patients with keratoconus and may represent the variable expression of a gene contributing to the development of keratoconus. Pedigree analysis in these families suggests an autosomal dominant mode of inheritance.

Persistence of diurnal fluctuation after radial keratotomy

Three patients were examined before 8:00 AM and after 7:00 PM to determine diurnal changes in uncorrected visual acuity, refractive error, and average central keratometric power at 3 months, 4 years, and 6 years after radial keratotomy. The average absolute diurnal change in the spherical equivalent refraction increased over time: 0.26 diopters at 3 months, 0.75 D at 4 years, and 1.29 D at 6 years. Diurnal change in the absolute average keratometric values were not progressive: 0.35 D at 3 months, 0.27 D at 4 months, and 0.40 D at 6 years. Computer-assisted corneal topographic analysis demonstrated diurnal changes in the topography that may be subtle and not necessarily reflected in the average keratometry values. These limited data suggest that wound healing over several years may not restore stability to the cornea.

Corneal topographic changes after radial keratotomy

A high resolution photokeratoscope using digital video image acquisition and computer graphics was used to describe anterior corneal topography before and after radial keratotomy. After radial keratotomy, the entire cornea is flattened, but this effect is more pronounced centrally. The flat central "optical zone" varies in size, shape, and location. Regional variations in corneal power may explain phenomena observed after radial keratotomy, such as visual acuity that is better than would be expected on the basis of refractive error, and the occurrence of ghost images.

Topographic analysis and visual acuity after radial keratotomy

Using a computerized corneal topographic mapping system that allows detailed examination of the corneal curvature in the vicinity of the visual axis, we identified separate regions of differing corneal curvature in three of 11 eyes after radial keratotomy. In these eyes, the excellent uncorrected visual acuity appeared to be inconsistent with the postoperative spherical equivalent as determined by refraction and corneal curvature by standard keratometer measurements. The distinct regions of corneal curvature appeared to serve as alternative effective optical zones, thus allowing the patients to have excellent visual acuity. In essence, the cornea became a multifocal lens. Although degradation in the contrast of the image as well as monocular diplopia are possible, our patients had no significant complaints.

Morning-to-evening change in refraction, corneal curvature, and visual acuity 2 to 4 years after radial keratotomy in the PERK Study

The authors previously reported morning-to-evening changes in ophthalmic measurements at 3 months and at 1-year after radial keratotomy in a self-selected group of patients in the Prospective Evaluation of Radial Keratotomy (PERK) study. Fifty-two patients included in the earlier study were examined again between 2 1/2 and 4 years after surgery before 10:00 AM and after 5:30 PM on the same day to determine if the changes persisted beyond 1 year after surgery. Between the morning and evening examinations, 31% of the eyes had an increase in minus spherical equivalent power of the manifest refraction of 0.50 to 1.50 diopters; 12% had a change in cylinder power of 0.50 to 1.00 D; 19% had a decrease in uncorrected visual acuity of two to five Snellen lines; and 29% showed central corneal steepening by 0.50 to 1.00 D. Thus, in some patients, morning-to-evening fluctuation persisted until 2 1/2 to 4 years after radial keratotomy.

Electrosurgical keratoplasty. Clinicopathologic correlation

A patient with keratoconus who could not tolerate a contact lens underwent electrosurgical keratoplasty. Postoperatively, the central cornea was markedly flattened, but five weeks later the central steepening had recurred. Six months after the procedure, a penetrating keratoplasty was performed. Pathologic examination of the corneal button revealed an intact epithelium and basement membrane. Bowman's membrane was completely destroyed in the treated areas. Stromal scarring extended to involve the anterior two thirds of the corneal stroma. The collagen in this area was contracted and folded. The keratocytes and stromal collagen of the posterior third of the cornea were normal by light microscopy. Descemet's membrane and the endothelium also were normal. Despite the short-lived flattening of corneal topography, this procedure dramatically altered the stromal collagen in the anterior two thirds of the cornea.

Creatine phosphokinase isoenzymes and Trypanosoma cruzi infections

1. Substantial increases in total creatine phosphokinase (CPK) and in isoenzymes from heart (CPK-MB) and skeletal muscle (CPK-MM) were observed during acute infections with the House 510 and House 11 strains of Trypanosoma cruzi. 2. In infections with the reticulotropic Tulahuen strain total CPK levels were lower and the isoenzyme pattern was essentially normal. 3. Gamma-glutamyl transpeptidase was considerably increased in the Tulahuen but not in the House 510 and House 11 infections. 4. These findings are useful in assessing tissue damage during T. cruzi infections and they also demonstrate differences between myotropic and reticulotropic strains which may aid in their taxonomic classification.

Different response of hepatic energy change and adenine nucleotide concentrations to hemorrhagic shock

The adenine nucleotide and mitochondrial metabolisms of the liver were studied in hemorrhagic shock of rats. In reversible shock, the energy charge (ATP+1/2ADP/ATP+ADP+AMP) levels fell more rapidly than the total adenine nucleotide concentrations and decreased to 64% of controls 2 hours after shock. However, the oxidative phosphorylation of isolated mitochondira was only slightly inhibited. The energy charge levels were restored to normal values immediately when the shed blood was reinfused, while the concentrations of adenine nucleotides increased gradually and reached normal at 2 hours later. In irreversible shock which took up 70% or more of the withdrawn blood to maintain pressure of 40mm. Hg, the energy charge, adenine nucleotide concentrations and mitochondrial oxidative phosphorylation decreased considerably and were not restored by the reinfusion of the remainder of shed blood. It is suggested that the energy charge can respond rapidly to changes in an available oxygen and the adenine nucleotide concentrations and that an impairment of mitochondrial oxidative phosphorylation plays an important role in leading to irreversible hemorrhagic shock.

A modified leukocyte nitroblue tetrazolium test in acute bacterial infection

The increased ability to leukocytes to reduce nitroblue tetrazolium (NBT) has been used to detect the presence of systemic bacterial infection. This test has been utilized to evaluate infections and leukocyte dysfunction in children, but has not been extensively applied to traumatized patients or infected volunteers. Moreover, the technic as originally described presented methodologic difficulties. In this study of 889 such patients, a modified NBT test provided excellent differentiation of 63 systemic bacterial infections (NBT score greater than or equal to 10%) from non-infectious fevers, local enteric diseases, and certain viral and plasmodial infections (NBT score less than or equal to 9%). Splenectomy was associated with a transient false-positive score and clinical typhoid fever with a false-negative response

Lack of enhanced oxygen consumption by polymorphonuclear leukocytes on phagocytosis of virulent Salmonella typhi

Human polymorphonuclear leukocytes exhibit an enhanced rate of oxygen consumption during phagocytosis of relatively avirulent strains of Salmonella typhi or Staphylococcus aureus. However, phagocytosis of a virulent strain of Salmonella typhi is not associated with augmented oxygen consumption. The ability of a bacterial strain to alter the postphagocytic rate of oxygen consumption of polymorphonuclear leukocytes may be related to its in vivo virulence.

Structural changes in mitochondria induced by uncoupling reagents. The response to proteolytic enzymes

Interaction of uncoupling reagents with bovine serum albumin markedly inhibited its hydrolysis by proteolytic enzymes. The inhibition presumably is due to conformational transitions in the protein substrate induced by the binding of the ligand-uncoupling reagents. The proteolysis of casein, a protein that does not bind these reagents, was not affected, indicating that the proteinases themselves were not inactivated. In contrast, interaction of uncoupling reagents with freshly isolated rat liver mitochondria enhanced their susceptibility to proteolytic enzymes. This was shown by an increase in the release of ninhydrin-reacting material, by an increase in free acid groups and by a decrease in the turbidity of the mitochondrial suspensions. These effects, although opposite in direction to those obtained with albumin, are also presumed to indicate structural changes in the mitochondrial proteins and a disorganization of the protein-phospholipid complex. It is suggested that such structural alterations are expressed functionally as the uncoupling of oxidative phosphorylation.