Interaction of ultraviolet laser light with the cornea.

The effect of pulsed ultraviolet (UV) laser light on the cornea depends on wavelength (photon energy), irradiance (photon flux), and pulse firing rate. At the available excimer laser wavelengths of 193, 249, 308, and 351 nanometers, the authors have varied the irradiance per pulse (10 to 2000 mj/cm2) as well a pulse frequency (1, 10, 25 Hz) and determined the thresholds for coagulation and ablation of the corneal stroma. The latter ablative action creates a groove resembling an incision and was present at all wavelengths studied. The threshold for ablation increased for longer wavelengths and lower pulse frequencies, except for 193-nm exposure, which was characterized by a constant threshold independent of laser pulse rate. The grooves at 193 nm were both biomicroscopically and histologically smooth and no coagulation effects were noted. Some degree of coagulation of adjacent tissues was noted at 249, 308, and 351 nm.

Ablation rates and surface ultrastructure of 193 nm excimer laser keratectomies.

PURPOSE: To determine whether photorefractive keratectomy can be performed at lower energies than are currently employed in clinical trials. METHODS: Fresh pig corneas were ablated using a clinical excimer laser to study the effects of various energy densities (100-200 mJ/cm2) and beam diameters on ablation rates and on the surface ultrastructure of the ablated cornea. RESULTS: A 20-mJ increase in energy density was associated with a 0.03 micron per pulse increase in the ablation rate. A nearly linear increase in the pseudomembrane thickness occurred with increasing energy densities (r2 = 0.83) or decreasing ablation area diameter (r2 = 0.86). CONCLUSIONS: Our findings suggest that fluences less than those currently used in clinical trials (160-180 mJ) are capable of ablating tissue while producing thinner electron-dense pseudomembranes on the corneal surface. The relationship between pseudomembrane thickness and clinical factors such as reepithelialization and postoperative haze remains to be determined. Operating at lower fluences does have the advantages of allowing larger diameter ablations, reducing possible shockwave damage, and reducing the maintenance requirements for the laser.

Quantitation of corneal ablation by ultraviolet laser light.

A surgical role has been suggested for the excimer laser. This followed the discovery of precision etching of biological materials by the laser's ultraviolet emissions. In studying the amount of tissue removed as the laser energy density (irradiance) was varied, we found significant differences for the 193-, 249-, and 308-nm laser emission lines. The most efficient cutting irradiance for each pulse appeared to be about 200 millijoules/sq cm for 193 nm, 1,000 millijoules/sq cm for 249 nm, and 1,500 millijoules/sq cm for 308 nm. The flat slope of the etch depth curve for 193 nm suggests it will be a stable frequency when used clinically.

The role of the vitreous in the intraocular pressure rise after neodymium-YAG laser capsulotomy.

Autologous liquid vitreous injected into the anterior chamber of the phakic owl monkey eye leads to markedly increased intraocular pressure (IOP) peaking at one to two hours. In contrast, neodymium-YAG laser shock waves focused in the center of the anterior chamber of the same animal led to a mild decrease in IOP. Debris produced by laser pulses focused on the residual cortex of owl monkey eyes that had undergone extracapsular surgery failed to increase the IOP. Similarly, injection of dialyzed vitreous did not have any significant influence on IOP. We conclude that the disruption of the integrity of the anterior cortical gel and the subsequent release of a dialyzable intravitreal substance with a molecular weight of less than 10,000 daltons into the anterior chamber may contribute to the IOP rise after surgical discission, including neodymium-YAG laser posterior capsulotomy.

Corneal healing following laser refractive keratectomy.

A 193-nm excimer laser system was used to ablate 4.5-mm optically contoured zones in the corneal stroma of rabbits to achieve optical flattening of 2, 4, 8, and 16 diopters. We studied the effect of edge profile and wound depth on reepithelialization and stromal remodeling using dichlorotriazinyl aminofluorescein, a vital dye that covalently binds to the stromal bed and delineates the boundaries of new collagen synthesis. All the corneas reepithelialized; no subsequent recurrent erosions occurred. All seven corneas that received an ablation of less than 50 microns were clear centrally at 8 weeks. At an ablation depth of approximately 100 microns, opacification and scarring were observed biomicroscopically and histopathologically in two specimens. No evidence of new collagen formation or epithelial hyperplasia was found in any of the seven corneas that remained clear. Stromal remodeling was observed in the two corneas that exhibited scarring.

Localized eosinophilic granuloma (Langerhans' cell histiocytosis) of the orbital frontal bone.

Four patients (aged 5, 8, 14, and 51 years at initial manifestation) with isolated eosinophilic granulomas of the orbital frontal bone displayed short symptomatic periods (two weeks to three months) and some combination of erythema of the lids, a soft, palpable anterior orbital mass, periorbital pain, and osteolytic bone lesions on roentgenography. The bone lesions roentgenographically exhibited irregular, serrated, and sclerotic margins, distinguishing them from the more oval appearance of dermoid cysts. Electron microscopy performed in one case disclosed the presence of Langerhans' granules in the cytoplasm of the histiocytes, indicating that the orbital disease is a mild form of "histiocytosis X" and a benign proliferation of a specific kind of histiocyte--the Langerhans' cell. On follow-up (two to 20 years), after incomplete curettage of two lesions coupled with low doses of postoperative radiotherapy, there was reconstitution of the bone defects, whereas more extensive surgery performed on the other two patients resulted in permanent but subclinical bone defects.

Recognition and differential diagnosis of enlarged extraocular muscles in computed tomography.

We found enlargement of the extraocular muscles in 70 patients out of 603 orbit studies, of whom 310 had exophthalmos. The majority (46 of 70) had the eye signs of Graves' disease. Arteriovenous malformations and carotid cavernous fistulas can cause enlargement of the extraocular muscles by a diffuse increase in orbital venous pressure, Acute orbital myositis can be distinguished from other forms of pseudotumor by the presence of a single enlarge extraocular muscle with associated inflammatory findings which responds to corticosteroid therapy. Neoplasms may invade extraocular muscles or compress their venous drainage causing secondary muscle enlargement. In all these patients the presence of a mass was correctly identified. The ability of the computed tomography scanner to recognize abnormalities of the extraocular muscles represents a significant advance in classification and diagnosis of the causes of exophthalmos.

Excimer laser surgery of the cornea.

The excimer laser, which produces light in the far-ultraviolet portion of the spectrum, allows precise removal of corneal tissue through a photochemical laser-tissue interaction. This interaction is not thermal and does not involve optical breakdown; rather, it directly breaks organic molecular bonds without tissue heating. We used this process of ablative photodecomposition to remove corneal tissue in a series freshly enucleated cow eyes. Applying the far-ultraviolet light in short intense pulses permitted us to control the depth of the incision with great precision. We found that 1 joule/cm2 ablates corneal tissue to a depth of 1 micron. Adjacent tissue suffered no thermal damage and the stromal lamellae adjacent to the incision showed no evidence of disorganization.

Excimer laser therapy for experimental Candida keratitis.

In vivo treatment of experimental Candida albicans keratitis with the excimer laser achieved sterilization on culture and histopathologically in all corneas in which clinically visible infiltration was removed with the laser at the 193-nm wavelength. Treatment at the 248-nm wavelength was not successful in eradicating infection compared with untreated controls. The 193-nm wavelength is highly effective, probably because infected tissue is totally removed by ablative photodecomposition. At the 248-nm wavelength, thermal effects become more dominant and, therefore, fungal elements are incompletely ablated. Light microscopy two days after treatment at the 193-nm wavelength showed healing of ulcerated areas, fine basophilic stippling at the epithelial-stromal interface, and undamaged underlying stroma.

Ultrastructural comparison of conventional surgical and argon fluoride excimer laser keratectomy.

We used scanning electron microscopy to examine the structure of the surfaces created during keratectomy procedures carried out by conventional surgery and ablation by an excimer laser. The walls and the floor of the surgical site were rough with undulations greater than 10 micron in size, caused by broken lamellae and ruptured cells. In contrast, the surfaces produced by the excimer laser were smooth with perturbations of 1 micron or less and were sealed with a pseudomembrane. This smooth, sealed surface produced by the laser is probably responsible for the transparency of the healed cornea in contrast to the scarring produced when a conventional surgical keratotomy heals.

Combined clinical and computed tomographic diagnosis of orbital lymphoid tumors.

Computed tomography has made it possible to make a strongly presumptive preoperative diagnosis of orbital lymphoid tumors, particularly when the radiographic findings are analyzed in conjunction with the clinical features. Twenty-six patients (with 27 orbital lymphoid tumors) had an average age of 57 years and had painless swelling or low-grade proptosis averaging 7.5 months in duration. The computed tomographic findings were highly characteristic. Almost all of the tumors had a retrobulbar or superior orbital component. In both the coronal and axial planes, distinctive growth patterns and contourings were discovered. The lesions molded themselves to preexisting orbital structures without eroding bone or enlarging the orbit. Against the bone, globe, and muscle edges, the lesions featured smooth, sharply demarcated contours with abruptly acute or perpendicular angulations, whereas a streaky profile became apparent as they irregularly infiltrated the retrobulbar fat, reflecting involvement of microfascial structural elements. No distinguishing differences were discovered in the growth patterns of the benign and the malignant tumors.

Pulsating metastatic tumor of the orbit.

A 47-year-old white man in apparent good health had diplopia and swelling of the right upper eyelid. Ocular examination showed proptosis of the right eye, together with a large, pulsatile, collapsible mass simulating a vascular neoplasm and involving the right temple as well as the right upper outer quadrant of the orbit. Biopsy of the orbital tumor disclosed a clear-cell carcinoma compatible with a renal primary tumor; subsequent laboratory examination revealed the offending tumor in the left kidney. Renal carcinomas may metastasize to the globe or to the orbit before the primary tumor is recognized. Pulsatile exophthalmos acquired in middle life associated with significant bone destruction represents a constellation of findings most consistent with a metastatic tumor, probably renal carcinoma, caused by the exceedingly rich vascularization of these metastatic deposits.

Combined clinical and computed tomographic diagnosis of primary lacrimal fossa lesions.

We studied 39 patients who had solid mass-lesions primary in the lacrimal gland by computed tomography and reviewed their clinical histories. Twenty-three patients had either inflammatory conditions (16 cases) or lymphoid tumors (seven cases), with average symptomatic periods of less than a year. In this group, soft-tissue contour analysis in the axial and corneal projections demonstrated diffuse, compressed, and molded enlargements of the lacrimal gland in an oblong fashion, and there were no associated bone defects. Sixteen parenchymal benign or malignant tumors (six benign mixed tumors, one schwannoma, and nine malignant epithelial tumors) exhibited rounded or globular soft-tissue outlines and were frequently associated with contiguous bone changes. The benign tumors had smooth encapsulated outlines at their margins, whereas the malignant tumors displayed microserrations indicative of infiltration. The patients with the benign mixed tumors had had symptoms, on the average, for more than a year, whereas those with epithelial malignancies became symptomatic or had a preexisting benign mixed tumor that became exacerbated in periods of less than six months. Contour analysis of the soft-tissue mass depicted in coronal and axial tomograms is a valuable adjunct that leads to more accurate preoperative diagnosis when combined with a radiographic search for bone changes and the clinical history. Once a diagnosis regarding the presumptive lesional family has been made preoperatively, corticosteroid therapy may be instituted for acute inflammation and biopsies through the eyelid should be performed for suspected chronic inflammations, lymphoid lesions, or epithelial malignancies. A lateral orbitotomy without prior biopsy should be performed for rounded, well-encapsulated masses of long duration that are likely to be benign mixed tumors.

Corneal ablation profilometry and steep central islands.

BACKGROUND: Photorefractive keratectomy with large diameter ablations using a uniform laser beam has produced central undercorrections, or "steep central islands" in patients, as seen with videokeratography. METHODS: Using a custom optical profilometer to measure corneal ablation profiles and a VISX excimer laser system, we measured the effect of ablation algorithms, diameter, depth, and dioptric correction on enucleated porcine eyes and living rabbit eyes. Our profilometer was verified using a 43.00 diopter (D) spherical surface and a 35.00 and 43.00 D bicurve test surface as a model for the ablated cornea. RESULTS: The profilometer measured the test surfaces to within 3 microns of predicted values. Photorefractive keratectomies showed over-ablation peripherally and under-ablation centrally which increased with ablation diameter and dioptric correction. Fixed diameter ablations 2 to 6 mm in diameter and 10 to 80 microns deep showed stromal ablation rates vary spatially but not with ablation depth. These spatially variant ablation profiles were used to re-engineer the ablation algorithm and to produce photorefractive keratectomies with improved sphericity. CONCLUSIONS: Steep central islands are caused by the spatial variance of tissue ablated with a uniform laser beam irradiance. This aberration can be corrected by modifying the laser ablation algorithm to correct for the spatial variance of stromal ablation.

Night vision testing in unoperated eyes.

BACKGROUND: We previously described a simple test which evaluates image degradation in post-excimer laser (PRK) patients under scotopic conditions. After refractive surgery, corneal haze, ablation zone decentration, ablation zone/pupillary diameter disparity, and under-correction each result in a characteristic pattern on the Night Vision Recording Chart. METHODS: Using the same method, further studies evaluated night vision image degradation in 118 un-operated emmetropic, myopic, hyperopic, and astigmatic eyes and in 26 contact lens wearers. RESULTS: Scotopic image degradation increases with myopic refractive error, image displacement increases with astigmatism, and contact lens wearers have more image degradation that with spectacle correction. CONCLUSION: Our Night Vision Recording Chart offers a simple, reproducible method to characterize image degradation under scotopic conditions.

Photokeratitis from 193 nm argon-fluoride laser radiation.

The threshold for photokeratitis at 193 nm was obtained for the rabbit cornea using an ArF excimer laser. Because ablation occurs at a level below that for photokeratitis, it was necessary to expose the cornea to a lengthy series of low-energy exposures. It is concluded that the 193 nm photons have such a shallow penetration depth, being limited to the outermost epithelial cells, that classical photokeratitis occurs from the fluorescence emitted at the corneal epithelial absorption site. An intact tear film may help to protect the cornea from low-level, scattered 193 nm laser radiation.

Reversal of dysthyroid optic neuropathy following orbital fat decompression.

AIMS: To document the successful treatment of five patients with dysthyroid optic neuropathy by orbital fat decompression instead of orbital bone decompression after failed medical therapy. METHODS: Eight orbits of five patients with dysthyroid optic neuropathy were selected for orbital fat decompression as an alternative to bone removal decompression. Treatment with systemic corticosteroids and/or orbital radiotherapy was either unsuccessful or contraindicated in each case. All patients satisfied clinical indications for orbital bone decompression to reverse the optic neuropathy. High resolution computerised tomographic (CT) scans were performed in all cases and in each case showed signs of enlargement of the orbital fat compartment. As an alternative to bone decompression, orbital fat decompression was performed on all eight orbits. RESULTS: Orbital fat decompression was performed on five patients (eight orbits) with optic neuropathy. Optic neuropathy was reversed in all cases. There were no cases of postoperative diplopia, enophthalmos, globe ptosis, or anaesthesia. All patients were followed for a minimum of 1 year. CONCLUSIONS: In a subset of patients with an enlarged orbital fat compartment and in whom extraocular muscle enlargement is not the solitary cause of optic neuropathy, fat decompression is a surgical alternative to bony decompression.

Pupil diameter and the principal ray.

Placement of the surgical zone is critical in refractive procedures that alter a portion of the corneal curve. An improperly centered optical zone may produce glare, decrease best corrected visual acuity, and decrease contrast sensitivity. For proper placement, the new surface should be centered around the line of sight, which is the principal ray from the object of regard that passes through the image of the patient's pupil as projected on the cornea. This point is not necessarily at the geometric center of the cornea and is found by locating the center of the pupil while the patient is maintaining fixation coaxially with the surgeon. However, the pupil does not dilate concentrically and its geometric center moves as the pupil diameter changes. We have found a shift up to 0.7 mm in the geometric center of the pupil as it dilates. Therefore, centration of an ablated or a radial keratotomy zone is most efficiently done when the diameter of the modified corneal optical zone is centered around the line of sight and is superimposed upon the entrance pupil. This will minimize extension of the edge of the large pupil beyond the ablated zone and reduce unwanted secondary optical effects from degrading vision.

Enlargement of the photorefractive keratectomy optical zone.

PURPOSE: To evaluate the safety and efficacy of a photorefractive keratectomy (PRK) re-treatment procedure that enlarges the optical zone and treats undercorrection. SETTING: Rotterdam Eye Hospital and Medisch Centrum Alkmaar, The Netherlands. METHODS: This study evaluated 16 eyes that had PRK for myopia with the Summit excimer laser that resulted in a mean undercorrection of -2.82 diopters (D). Patients also reported impaired night vision including difficulty in driving, halos, and stray light and ghost images. These phenomena persisted after spectacle correction of residual refractive error, necessitating further treatment with a larger ablation zone. With a VISX 20/20 excimer laser, the optical zone was enlarged to 6.0 mm and undercorrection treated with a 6.0 mm ablation. RESULTS: At 13.5 months after re-treatment, mean reduction in myopia was 1.70 D, resulting in a residual undercorrection of -1.08 D. In seven eyes, final refraction was within 1.00 D of emmetropia. Only two patients continued to report night-driving problems. CONCLUSIONS: Re-treating undercorrections combined with enlarging the ablation zone resulted in a reduction in myopia from a mean of 2.82 to 1.08 D. Subjective reports of halos and stray light images were decreased in all cases.

Mid-infrared lasers for endoscopic surgery. A new class of surgical lasers.

Unlike the Nd:YAG, the CO2 laser produces well circumscribed tissue removal with little unwanted damage because of its high absorption coefficient relative to tissue water. Unfortunately, conventional quartz fibers cannot transmit wavelengths greater than three microns, well below the ten micron wavelength of the CO2 laser. Thus, fiberoptic transmission of CO2 laser requires use of exotic materials that lack the excellent properties of quartz fibers and that, in some cases, are toxic or chemically unstable. However, there are large peaks in the infrared absorption curve of water in the two to three micron region. Lasers that operate in this mid-infrared region should permit CO2-like precision of cutting, along with excellent transmission through quartz fiberoptics. Two mid-infrared lasers with a fiberoptic delivery system, the Er:YAG (2.94 micron) and the Tm-Ho-Cr:YAG (2.15 micron) have been evaluated with human colon tissue in vitro and with rabbit gastric tissue in vivo. Histologic sections reveal excellently well localized lesions with minimal underlying thermal damage. Depth of penetration is controllable and occurs in very small increments. There is little spreading of the lesions at the 24 hour mark resulting from secondary thermal damage. Based on these facts, the authors think that these lasers will be of benefit to endoscopic surgeons.