Marc M. Whitacre

Effects of anti-inflammatory drugs following cataract extraction.

Cycloplegics, corticosteroids, and nonsteroidal anti-inflammatory drugs have been applied in the treatment of postoperative inflammation following cataract extraction. Of these, topical preparations of nonsteroidal anti-inflammatory drugs, such as ketorolac tromethamine 0.5% and diclofenac sodium 0.1%, offer comparable efficacy to corticosteroids in the reduction of postoperative inflammation, and offer lower risks of adverse events in most patients. Comparative studies of these drugs from the past 5 years are highlighted.

Tolerance of intravitreal povidone-iodine in rabbit eyes

Povidone-iodine is frequently instilled on to the conjunctival surface prior to intraocular surgery in order to prevent septic endophthalmitis. A small amount of povidone-iodine is inevitably introduced into the eye when it is used in this manner. The toxicity of intravitreal povidone-iodine was assessed in rabbit eyes by injecting 0.1 ml of povidone-iodine in concentrations of 0.05%, 0.5% and 5% into the vitreous cavity. The injected eyes were evaluated by clinical examination, anterior segment and fundus photography, endothelial cell counts, electroretinography and histopathology. Compared to control eyes, no changes were observed in all 6 eyes injected with 0.1 ml of 0.05% povidone-iodine solution. 9 of 10 eyes tolerated a concentration of 0.5% with no detectable adverse changes. One eye developed a temporary mild iritis and mild suppression of the ERG. Intra-retinal hemorrhages, edema, arteriolar narrowing and retinal edema were seen one week following injection. Mild retinal necrosis of the same area was seen on histology. All 4 eyes injected with 5% povidone-iodine developed temporary hypotony and iridocyclitis. A dense cataract developed in all eyes. Full thickness retinal necrosis and a profound lasting reduction in the ERG was produced in all of these eyes. No corneal epithelial or endothelial changes were observed in any eye in this series. Low concentrations of intravitreal povidone-iodine likely to be produced by instillation prior to surgery are tolerated by rabbit eyes. The concentrations tolerated by the studied eyes are near reported bactericidal levels.

The effect of Perkins, Tono-Pen, and Schiötz tonometry on intraocular pressure.

We studied the intraocular pressure changes produced in five eye bank eyes by Perkins, Tono-Pen, and Schiötz tonometry performed by experienced and inexperienced personnel. When all users were considered together, Perkins tonometry produced a mean intraocular pressure increase of 0.7 mm Hg, significantly less than the mean increase of 12.1 mm Hg produced by Tono-Pen tonometry (P less than .05) or the mean increase of 16.5 mm Hg produced by Schiötz tonometry (P less than .01). There was no statistically significant difference between the intraocular pressure increase produced by Tono-Pen or Schiötz tonometry. Tonometry performed by inexperienced Tono-Pen users and experienced or inexperienced Schiötz users produced a significantly greater increase in intraocular pressure than that performed by experienced Tono-Pen users (P less than .05), and an extremely significant increase compared to tonometry performed by experienced or inexperienced Perkins users (P less than .01). The marked increase in intraocular pressure produced by Tono-Pen tonometry suggests that hand-held electronic applanation tonometers should be used with caution in eyes with a weakened cornea or sclera.

The Effect of intraocular Gas and Fluid Volumes on Intraocular Pressure

Large increases in the intraocular pressure (IOP) of postoperative gas-containing eyes may require the removal of gas or fluid to reduce the IOP to the normal range. Application of the ideal gas law to Friedenwalds equation provides a mathematical model of the relationship between IOP, intraocular gas and fluid volumes, and the coefficient of scleral rigidity. This mathematic model shows that removal of a given volume of gas or fluid produces an identical decrease in IOP and that the more gas an eye contains, the greater the volume reduction necessary to reduce the pressure. Application of the model shows that the effective coefficient of scleral rigidity is low (mean K, 0.0021) in eyes with elevated IOP that have undergone vitrectomy and retinal cryopexy and very low (mean K, 0.0013) in eyes with elevated IOP that have undergone placement of a scleral buckle and band. By using the appropriate mean coefficient of rigidity, the volume of material to be aspirated to produce a given decrease in IOP can be predicted with clinically useful accuracy.

Hazards of Laser Beam Reflections in Eyes Containing Gas

Three patients suffered photic or thermal laser injuries at unintended sites because of reflection from gas-fluid interfaces in eyes containing gas. Fresnels equations predict the amount of light transmitted or reflected at optical interfaces. Application of Fresnels equations demonstrate that significant laser reflection occurs at gas-fluid or fluid-gas interfaces, especially at high angles of incidence. Photocoagulation across gas-fluid or fluid-gas interfaces should be avoided to reduce the risk of unintended damage from reflected light. If photocoagulation must be performed across an interface, the surgeon should minimize the angle of incidence of the treatment laser beam, use a divergent beam, and consider the location and intensity of the reflected beam. Because of the higher phototoxicity of blue light, photocoagulation with blue-green lasers across gas-fluid or fluid-gas interfaces is undesirable. Proper polarization of the treatment laser beam may significantly reduce laser reflectance.

B-scan Ultrasonography of Eyes Containing Intravitreal Gas

Contact B-scan ultrasonography was performed on postoperative eyes containing intravitreal gas. Correlation of the ultrasonograms with the ophthalmoscopic findings disclosed that intraocular gas had a characteristic appearance on B-scan ultrasonography. Gas-fluid and gas-tissue interfaces were so highly reflective that no structures within or behind a bubble could be visualized. Shadowing, reverberation, and reflection artifacts were prominent, and dominated ultrasonographic findings. If the ultrasonographic characteristics of gas are understood, it is possible to use B-scan ultrasonography to determine if intraocular gas is present, whether there are one or several bubbles, and what percentage of the vitreous cavity is filled by gas. Specular reflection from a gas-fluid interface may be used to examine portions of the eye that might not otherwise be easily seen with ultrasonography.

Noncontact Retinal Photocoagulation at the Slit-Lamp Biomicroscope

Four patients had either ocular conditions that made the use of a contact lens for retinal photocoagulation undesirable, or optical aberrations of the ocular media that made photocoagulation with a conventional contact lens difficult. A hand-held 90-diopter lens permitted retinal photocoagulation to be performed at a conventional slit-lamp biomicroscope laser without corneal contact and assisted in delivering laser burns under physical or optical circumstances that would be difficult or impossible with conventional contact lenses. Four patients without ocular surface or media abnormalities were also treated by this method.

Effect of Buckling Material on Ocular Rigidity

Enucleated human eyes were banded with metal and silicone bands to produce reductions in their diameter of 2 mm and 4 mm. The ocular rigidity produced by each banding material at each diameter was measured in the pressure range of 10 mmHg to 40 mmHg. Metal bands produced mild reductions in ocular rigidity that were significantly (P less than 0.05 to 0.01) lower than the control ocular rigidities in some pressure ranges. Silicone bands produced large reductions in ocular rigidity that were significantly (P less than 0.01) lower than ocular rigidities observed in metal-banded or control conditions in all pressure ranges. The influence of the elastic silicone banding material on ocular rigidity was greater than the influence of altered shape and wall stress that occurred with metal banding.

Photodisruption of Intraocular Gas Bubbles

During gas-fluid exchange bubbles can form in the vitreous cavity. These bubbles can interfere with visualization of the fundus for retinal examination, photocoagulation, or cryopexy. We used the Nd:YAG laser to eliminate these bubbles in vitro and in two patients.

Clinical applications of auto indirect ophthalmoscopy.

A phakic eye that has undergone vitrectomy and gas-fluid exchange produces a real, inverted aerial image anterior to the cornea similar to that produced by a lens for indirect ophthalmoscopy. This image can be used during surgery to perform a variety of maneuvers that would otherwise require a contact prism, high-minus contact lens, or handheld indirect ophthalmoscope lens. Applications of auto indirect ophthalmoscopy include visualization of the retina and vitreous during endophotocoagulation and while using a vitreous cutter.