Intraocular photodisruption with picosecond and nanosecond laser pulses: tissue effects in cornea, lens, and retina.

PURPOSE: Nd:YAG laser photodisruption with nanosecond (ns) pulses in the millijoule range is an established tool for intraocular surgery. This study investigates tissue effects in cornea, lens, and retina to assess whether picosecond (ps) pulses with energies in the microjoule range can increase the surgical precision, reduce collateral damage, and allow applications requiring more localized tissue effects than can be achieved with ns pulses. METHODS: Both ps and ns Nd:YAG laser effects on Descemet's membrane, in the corneal stroma, in the lens, and at the retina were investigated in vitro in bovine and sheep eyes and in cataractous human lens nuclei. For each tissue, the optical breakdown threshold was determined. The morphology of the tissue effects and the damage range of the laser pulses were examined by light and scanning electron microscopy. The cavitation bubble dynamics during the formation of corneal intrastromal laser effects were documented by time-resolved photography. RESULTS: The optical breakdown threshold for ps pulses in clear cornea, lens, and vitreous is, on average, 12 times lower than that for ns pulses. In cataractous lens nuclei, it is lower by a factor of 7. Using ps pulses, Descemet's membrane could be dissected with fewer disruptive side effects than with ns pulses, whereby the damage range decreased by a factor of 3. The range for retinal damage was only 0.5 mm when 200 microJ ps pulses were focused into the vitreous. Picosecond pulses could be used for corneal intrastromal tissue evaporation without damaging the corneal epithelium or endothelium, when the pulses were applied in the anterior part of the stroma. The range for endothelial damage was 150 microns at 80 microJ pulse energy. Intrastromal corneal refractive surgery is compromised by the laser-induced cavitation effects. Tissue displacement during bubble expansion is more pronounced than tissue evaporation, and irregular bubble formation creates difficulties in producing predictable refractive changes. CONCLUSIONS: The use of ps pulses improves the precision of intraocular Nd:YAG laser surgery and diminishes unwanted disruptive side effects, thereby widening the field of potential applications. Promising fields for further studies are intrastromal corneal refractive surgery, cataract fragmentation, membrane cutting, and vitreolysis close to the retina.

[Color vision defect. A early sign of Sorsby retinal dystrophy?]. / Farbsinnstörung. Ein Frühzeichen bei Sorsby-Netzhautdystrophie?

In 34 asymptomatic patients from three families at 50% risk of developing Sorsby's fundus dystrophy, colour contrast sensitivity was measured. In 16 the thresholds--mainly of the tritan axis--were raised above the normal values. It is concluded that testing of colour vision is useful in detecting the abnormal genotype. Raised colour contrast sensitivity was not observed before the first fundus or fluorescein angiographic changes in two of the families, while the colour defect occurred in the absence of ophthalmoscopic abnormality in the third family. Therefore, our results support the clinical suggestion that Sorsby's fundus dystrophy is more than one disease.

Study of laser damage to injection-molded diffractive intraocular lenses.

A study of the probability of damage induced in two implant quality injection-molded diffractive intraocular lenses by a commercial Q-switched Nd:YAG laser is presented. The damage thresholds of the diffractive intraocular lenses (0.10 mJ at 10% probability, 0.24 mJ at 50% probability, and 0.90 mJ at 99% probability) did not differ significantly from those of other poly(methyl methacrylate) lenses manufactured with the same technology. These low values (compared to those of other lens types) suggest that care should be taken during laser capsulotomy in eyes implanted with refractive lenses, with special attention given to power losses at the target site due to diffraction effects.

Comprehensive study of damage to intraocular lenses by single and multiple nanosecond neodymium: YAG laser pulses.

The results of an extensive study of the probability of damage induced in implant quality intraocular lenses (IOLs) by a commercially available nanosecond Nd:YAG photodisruptor are presented. These results were compared with the morphology of damage seen on scanning electron microscopy. The differences in the shape of the damage probability curves derived for the different lens groups demonstrate that a single threshold value does not characterize the threshold for IOL damage. We suggest that a standard method using a clinical photodisruptor be used by all workers to derive these curves for all IOLs in clinical use. The clinical relevance of our findings is emphasized.

Sorsby's fundus dystrophy. A light and electron microscopic study.

The light and electron microscopic changes in both eyes of a patient with Sorsby's fundus dystrophy are reported. The most striking finding was a 30-microns thick deposit present within Bruch's membrane that stained positive for lipids. The deposit has several unique features when compared with those found in other retinal dystrophies and in aged eyes. In addition, there was gross loss of the outer retina, the retinal pigment epithelium was discontinuous, and there was atrophy of the choriocapillaris. These findings are discussed in relation to the published clinical observations of other members of this pedigree.

Sorsby's fundus dystrophy. A clinical study.

A survey was undertaken of a family known to have Sorsby's fundus dystrophy. Fifty members were reviewed, and 14 were found to be affected. Many of Sorsby's original conclusions were confirmed, including the pattern of inheritance and age of visual loss. Yellow material was present at the level of Bruch's membrane early in the course of the disease. However, the earliest phenotypic marker was delayed filling of the choriocapillaris. Abnormalities of choroidal perfusion became more profound and extended centrifugally with time. The loss of central vision was commonly due to atrophy of the outer retina and choroid. Subretinal neovascularization was a rare occurrence. The homology between this dystrophy and age-related macular disease underlines the importance of the clinical findings in this family.

A new form of damage to PMMA intraocular lenses by Nd:YAG laser photodisruptors.

The damage that Nd:YAG laser radiation can cause to plastic (PMMA) IOLs during pseudophakic capsulotomy is well documented in the literature. This damage is the result of direct plasma action on the lens material. We report here another form of damage to PMMA IOLs which is more subtle and does not result from plasma action in the plastic. Even when the irradiance within the IOL is well below the threshold for optical breakdown, the PMMA can be damaged if irradiated by many laser pulses. This subthreshold damage is therefore cumulative. Its importance, likelihood of occurrence and ways to minimise this are considered.

Neodymium: YAG laser iridotomies--short-term comparison with capsulotomies and long-term follow-up.

This study addresses three aspects of anterior segment Nd:YAG laser treatment--acute endothelial damage, as assessed by endothelial specular photomicrography (ESP), acute and long-term intraocular pressure (IOP) changes, and long-term iridotomy patency. The acute ESP and IOP changes in 26 eyes (21 patients) after Nd:YAG laser iridotomies were compared to 39 eyes (37 patients) after Nd:YAG laser capsulotomy. Similar endothelial damage occurred in both groups, although less damage was noted in the group of 9 eyes in which capsulotomies were undertaken in the presence of an intraocular lens. In a parallel study 53 eyes (44 patients) were followed for a mean of 83 weeks (19 months) from the time of Nd:YAG iridotomy. There were no late closures and no late rises in IOP. The level of acute IOP rise after treatment did not predict long-term IOP. We conclude that Nd:YAG iridotomy is an effective procedure in the long-term, and that both iridotomy and capsulotomy are accompanied by noteworthy acute endothelial changes and intraocular pressure rises.

Sorsby's pseudoinflammatory macula dystrophy--Sorsby's fundus dystrophies.

The findings are presented on the updated Kempster pedigree with Sorsby's fundus dystrophy. The study confirms the features described in other families: autosomal dominant inheritance with complete penetrance, loss of central vision due to subfoveal ingrowth of new vessels, and progressive peripheral chorioretinal atrophy. By contrast to other reports the family in the current study have peripheral retinal dysfunction, a deposit of a subretinal yellow material throughout the fundus and a tritan colour defect, all prior to the loss of central vision; in some patients there was loss of central vision from atrophic disease, rather than from ingrowth of subretinal new vessels; and, there was a different temporal progression of the central subretinal neovascular complex. These features suggest the possibility of genetic heterogeneity.

Nd:YAG laser photodisruption: an experimental investigation on shielding and multiple plasma formation.

In an ocular model plasmas were induced by Q-switched, commercially available Nd:YAG lasers that operated in low order or fundamental mode. Plasma shielding was measured and analysed in relation to energy input, number of plasmas and plasma evolution. Near breakdown threshold, attenuation or shielding of the laser beam resulting from absorption and scattering by the plasma is low and is characterized by high variability from shot to shot. More importantly, shielding does not increase when multiple plasmas are formed, possibly due to a competitive mechanism between these plasmas. Further evidence of this mechanism was obtained from measurement performed using a streak camera. The effectiveness of shielding is scarcely changed by an increase in the cone angle of the incident radiation. In the light of these experiments and other published data, safety guidelines for the clinical use of the Nd:YAG laser have been formulated.

Study of the temporal and spatial dynamics of plasmas induced in liquids by nanosecond Nd:YAG laser pulses. 1: Analysis of the plasma starting times.

We report on a theoretical and experimental study of the temporal and spatial dynamics of plasmas produced in liquids by single Nd:YAG laser pulses of nanosecond duration. This study was motivated by the increasing attention paid to the phenomenon of optical breakdown and to its related effects on tissues and media in connection with microsurgical techniques developed for ophthalmology and urology. Streak camera recordings of the emission from laser-induced plasmas were taken in distilled and tap water in controlled irradiation conditions. From streak recordings, plasma starting times as a function of the axial distance from focus, the overall length of the plasma column, plasma lifetimes, and plasma absorption were derived and analyzed. In this first paper we analyze the curves of plasma starting time, as a function of the irradiation parameters and of the properties of the medium. We show that a model obtained by upgrading the theory of the moving breakdown allows accurate interpretation of the experimental observations.

Study of the temporal and spatial dynamics of plasmas induced in liquids by nanosecond Nd:YAG laser pulses. 2: Plasma luminescence and shielding.

We describe and analyze the temporally and spatially resolved luminescence and attenuation characteristics of plasmas induced in liquids commonly accepted as models for ocular media by high-irradiance Nd:YAG laser pulses of nanosecond duration. Measurements of plasma generated in different irradiation conditions, performed with the aid of a streak camera, show that, when expansion of the plasma column toward the incoming beam occurs, it coincides with quenching of the luminescence exhibited by plasma regions located closer to the focus, where breakdown has first occurred. Evidence for this effect is given both with impurityfree media, where plasma expansion occurs in a continuous and regular way, and with impurity-rich media, where plasma columns are composed either of unevenly distributed plasma regions or of single plasmas with irregular breakdown starting patterns. Experimental findings have been analyzed using a model of spatially distributed plasma shielding, in combination with a previously developed model of moving breakdown, that well interprets, in liquids, the spatially dependent breakdown starting times.

Retinal detachment following Nd:YAG posterior capsulotomy.

Five hundred and eighty-two patients who underwent Nd:YAG laser posterior capsulotomy at Moorfields Eye Hospital were reviewed retrospectively. Twelve patients (2 per cent), nine of whom were previously myopic, subsequently developed rhegmatogenous retinal detachment. Comparison with other studies suggests there is no greater risk of retinal detachment associated with Nd:YAG laser capsulotomy than with surgical discission. The relevance of the damage mechanisms of Nd:YAG lasers are discussed.