Mark L. McDermott

Corneal Endothelial Cell Counts After Molteno Implantation

Implantation of a Molteno drainage shunt has been shown to be effective in advanced glaucoma. Foreign substances within the anterior chamber have been known to cause progressive endothelial cell loss. We undertook a study to evaluate the endothelial effects of an indwelling Molteno drainage shunt. Nineteen patients who underwent uneventful implantation of a Molteno drainage shunt for advanced aphakic or pseudophakic glaucoma were followed up. Serial endothelial cell counts were obtained in a masked fashion. During follow-up periods ranging from 5.4 to 25.7 months, endothelial cell loss averaged two cells per square millimeter per postoperative month with a 95% confidence interval of positive seven cells to negative ten cells per square millimeter per postoperative month. No clinically significant progressive trend in endothelial cell loss was seen in patients undergoing uncomplicated Molteno drainage procedures. Larger sample sizes with longer follow-up will be necessary to establish whether a Molteno drainage shunt causes clinically remarkable endothelial cell loss.

Phacoemulsification conditions resulting in thermal wound injury.

Purpose: To determine and model the relationships between phacoemulsification conditions and viscoelastic agents that result in thermal wound injury. Setting: Animal laboratory, Irvine, California, USA. Methods: Mechanical and animal models, various wound sizes, phacoemulsification tips, and dispersive and cohesive viscoelastic agents were evaluated. Settings for phaco power, vacuum, and irrigation levels were controlled within a surgically relevant range. In the mechanical and animal models, incision temperature was assessed as a function of phacoemulsification parameters and time. In the animal model, wound damage was evaluated at the time of surgery. Results: Induced time delays from the onset of phaco power to the onset of irrigation flow caused a thermal rise at the incision site. In these experiments, lack of irrigation and aspiration resulted in the greatest thermal rise and caused wound damage. Both the cohesive and dispersive viscoelastic agents were associated with a delay in the start of irrigation and aspiration, which resulted in similar maximum temperatures. Mathematical models were developed to estimate the maximum incision temperature from the phacoemulsification power, the duration (seconds) of occlusion, the tip gauge and type, and other phacoemulsification parameters. The models predict that under comparable conditions, occlusion with a viscoelastic agent will result in higher incision temperatures than occlusion with a balanced salt solution. Conclusion: Under comparable phacoemulsification conditions, both the cohesive and dispersive viscoelastic agents were associated with elevated temperatures that would be preventable by ensuring irrigation and aspiration flow before the onset of phacoemulsification power.

Phacoemulsification for Cataract Following Pars Plana Vitrectomy

BACKGROUND AND OBJECTIVE To determine if the technical aspects of phacoemulsification for removal of cataract following pars plana vitrectomy differ from those of phacoemulsification for removal of cataract in nonvitrectomized eyes. PATIENTS AND METHODS Twenty-two eyes that had undergone prior vitrectomy were identified through a retrospective review of chart notes and operative records of 1039 consecutive cataract extractions performed by one surgeon. RESULTS Complications of proliferative diabetic retinopathy was the most common indication for prior vitrectomy. The predominant lens change was nuclear sclerosis. Deep anterior chambers with large anterior to posterior excursions of the iris lens diaphragm with simultaneous fluctuation in pupil size occurred during phacoemulsification. The posterior capsule was flaccid and mobile during lens cortex removal. No posterior capsules ruptured. Postoperative visual acuity improved in 91% of the patients. CONCLUSIONS Prior vitrectomy was associated with diabetes and nuclear sclerotic cataract. Phacoemulsification in this patient population was associated with inadequate pupillary mydriasis, superior conjunctival scarring, intraoperative anterior chamber depth, pupil size, and iris lens excursions that hindered phacoemulsification tip placement. Posterior capsules were excessively flaccid, prone to rapid anterior and posterior excursions, and may have plaques that are resistant to intraoperative removal.

In vitro corneal endothelial permeability in rabbit and human: The effects of age, cataract surgery and diabetes

Endothelial permeability was examined in rabbit and human corneas using an in vitro perfusion system with 5(6)-carboxyfluorescein as the permeability tracer. Following endothelial removal, the permeability of de-epithelialized rabbit corneas increased from 3.19 x 10(-4) cm min-1 to 31.21 x 10(-4) cm min-1, and de-epithelialized human donor corneal permeability values increased from 2.26 to 12.85. In human corneas, no correlation was seen between endothelial permeability and donor age, moist chamber storage time, endothelial cell density, coefficient of variation of cell area, or percent hexagonal cells. A positive correlation was found between permeability and time between donor death and enucleation. Donor eyes from three separate populations, having undergone cataract surgery prior to death (aphakic, posterior and anterior chamber implants), all showed a significant increase in permeability. Neither Type I nor Type II diabetes had an effect on the endothelial permeability of human donor corneas.

Therapeutic Uses of Contact Lenses

Therapeutic use of contact lenses is an essential element in ophthalmic care. Materials currently in use include polymethyl methacrylate (PMMA), cellulose acetate butyrate, siloxane-containing polymethacrylates, silicones, and hydrogels. Suitability of a material for therapeutic contact lens use is determined by the physical, chemical, and mechanical properties (notably gas permeability and hydrophilicity, but also lipid absorption and lens movement, among others) and the condition to be treated; fabrication techniques are likewise important, affecting lens diameter and base curve. Selection and fitting of therapeutic contact lenses requires knowledge of how different contact lenses affect corneal physiology, as well as an understanding of the mechanisms whereby a contact lens can be therapeutic. In addition to these topics, general fitting guidelines are discussed, and results of therapeutic lens use in selected clinical situations (including recurrent erosion, metaherpetic ulcers and other epithelial defects, and keratitis sicca, and other dry eye states). Common therapeutic contact lens complications and their treatment are also discussed.

Experimental neodymium:YAG laser damage to acrylic, poly(methyl methacrylate), and silicone intraocular lens materials

PURPOSE To compare neodymium:YAG (Nd:YAG) laser effects on acrylic, silicone, and poly(methyl methacrylate) (PMMA) intraocular lens (IOL) polymers. METHODS Ten Nd:YAG laser exposures were produced in each of 6 implantation-quality acrylic (Alcon MA60BM), silicone (Staar AQ1016), and PMMA (Alcon MC60BM) IOLs under identical conditions. Each polymer type was irradiated at 6 power settings (0.3, 0.5, 1.0, 1.5, 2.0, and 3.0 mJ) and at 2 focal points (midpoint of lens optic and on the posterior surface to which a cellophane membrane was affixed). The linear extent of the damage was measured using light microscopy. Specimens exposed to 1.0 mJ were processed for scanning electron microscopy. RESULTS The damage threshold (> or = 5 microns depth) was 0.3 mJ for silicone and 1.0 mJ for acrylic and PMMA IOLs. At the clinically relevant power levels, 1.0 to 2.0 mJ, the depth of damage in the acrylic polymer was 11.9 to 30.5 times less than the depth in the silicone polymer. Similarly, the depth of damage in the PMMA polymer was 5.4 to 52.6 times less than the depth in the silicone polymer. The morphologic pattern of damage in the silicone IOL showed a deep, irregularly configured trough with meandering tendrils. Acrylic IOL damage morphology consisted of an ameboid-shaped entry site without radiating fractures and mild posterior penetration. Poly(methyl methacrylate) IOL damage consisted of a shallow focal trough with radiating fractures. CONCLUSIONS The silicone IOL polymer had the lowest threshold for laser-induced damage and greater linear extension of damage than the PMMA and acrylic IOL polymers. Poly(methyl methacrylate) and silicone polymers exhibited collateral damage or ejected particulates adjacent to the entry site, whereas the acrylic polymer showed a discrete locus of damage.

Viscoelastic adherence to corneal endothelium following phacoemulsification

Purpose: To compare the residual adherence of viscoelastics to the corneal endothelium following phacoemulsification in an in vitro rabbit model. Setting: Departments of Ophthalmology and Anatomy, Wayne State University, Detroit, Michigan, USA. Methods: Three groups of 10 rabbit eyes each had a lensectomy via phacoemulsification using sodium hyaluronate (Amvisc Plus®, Healon GV®) or sodium chondroitin sulfate‐sodium hyaluronate (Viscoat®) as the viscoelastic agent. After phacoemulsification and cortex removal, a central corneal block was excised, cryofixed, and processed for light and electron microscopy. Viscoelastic thickness was determined by a calibrated reticule on the light microscope or a calibrated measuring program in the electron microscope. The nonparametric statistical test, Kruskal‐Wallis, was used to compare viscoelastic groups. Results: Median phacoemulsification time between viscoelastic agents was not significantly different. Median viscoelastic thicknesses were 13.0 &mgr;m for Amvisc Plus, 0.4 &mgr;m for Healon GV, and 375.0 &mgr;m for Viscoat. Each was significantly different from the others (Kruskal‐Wallis, P < .001). Conclusions: Median thickness of Amvisc Plus, Healon GV, and Viscoat remaining adherent to the corneal endothelium after phacoemulsification was markedly different. Viscoat provided the greatest amount of viscoelastic material adjacent to the corneal endothelium.

Corneal stromal penetration of topical ciprofloxacin in humans

PURPOSE Use of fluoroquinolone agents as single drug treatment for severe bacterial keratitis requires that adequate stromal drug concentrations are achievable that exceed the MIC90 for common corneal pathogens. The purpose of this study, therefore, is to document the human stromal penetration of the fluoroquinolone, ciprofloxacin, in human tissue obtained from patients undergoing penetrating keratoplasty. METHODS Twelve patients, all with intact corneal epithelia, received 0.3% ciprofloxacin (Ciloxan) eyedrops every 15 minutes for the first hour and then every hour for 10 hours before their scheduled keratoplasty. Corneal tissue samples obtained from the excised buttons at the time of surgery were frozen and subsequently analyzed for drug concentration using high-pressure liquid chromatography. RESULTS A mean corneal tissue concentration of 5.28 +/- 3.4 micrograms/g tissue (mean +/- standard deviation) of ciprofloxacin was determined. Tissue levels ranged from 1.43 micrograms/g of tissue to 10.58 micrograms/g of tissue. CONCLUSION Ciprofloxacin penetration was variable in patients with an intact epithelium and, in some cases, did not exceed the MIC90 for some gram-positive organisms. The presence of an epithelial defect is expected to potentiate the penetration of ciprofloxacin to most likely exceed the MIC90 for most corneal pathogens.

Penetrating keratoplasty and intraocular lens exchange for pseudophakic bullous keratopathy associated with a closed-loop anterior chamber intraocular lens

We reviewed the records of 20 consecutive eyes with pseudophakic bullous keratopathy associated with a closed-loop anterior chamber lens that underwent penetrating keratoplasty, total anterior vitrectomy, and anterior chamber intraocular lens exchange. On reexamination, all corneal grafts had remained clear during an average follow-up period of 15 months (range, four to 45 months). Seven eyes (35%) attained a visual acuity of 20/40 or better with spectacle correction; however, 15 of 20 eyes (75%) attained a best-corrected visual acuity of 20/40 or better using a pinhole and a gas-permeable contact lens. In five eyes visual acuity was 20/400 or poorer because of cystoid macular edema, as documented by fluorescein angiography, age-related macular degeneration, or optic atrophy. Localized, minimal peripheral anterior synechiae were present on gonioscopy in seven eyes. The one-piece flexible anterior chamber lens implants used for exchange appeared to be well tolerated in this series of patients.

Tissue Plasminogen Activator and the Corneal Endothelium

• There is a potential risk of bacterial or fungal contamination of the initial preparation, which could then result in contamination of each of the individual doses. It is essential, therefore, that the tissue plasminogen activator be reconstituted under strict aseptic conditions in a sterile hood. As an additional safeguard, we recommend that random samples be cultured for evidence of bacterial contamination before using the divided doses in patients. • • • •