Stigmatic optical systems.

There would appear to be little disagreement on what constitutes an astigmatic system in the case of a thin lens: the cylinder is not zero. A spherical thin lens is stigmatic or not astigmatic. The issue is less clear in the case of a thick system. For example, is an eye stigmatic merely because its refraction is stigmatic (spherical)? In this article, a system is defined to be stigmatic if and only if, through the system, every point object maps to a point image. Every other system is astigmatic. Thus, a system is astigmatic if and only if there exists a point object for which the image is not a point. This article is restricted to linear optics. The optical character of a system is completely determined by the ray transference of the system. The objective here is to find those conditions on the transference for which the system is stigmatic or astigmatic. The result is that, for a stigmatic system, all the 2 x 2 submatrices are scalar multiples of a common orthogonal matrix. For a system to be stigmatic, it is not sufficient that its power be stigmatic. An eye may be astigmatic despite having a stigmatic refraction.     

Corrective saccades: dependence on retinal reafferent signals

The saccadic response to a peripheral step stimulus is composed of a main saccade, and a corrective saccade with a shorter latency. When a single peripheral pulse stimulus is presented with a duration shorter than the latency of the response, the main saccade is not followed by a corrective one, though it is inaccurate. However when a second pulse synchronized to the first saccade is presented within some degrees around the new visual axis, it elicits a saccadic correction with a short latency. If the second pulse is presented at a larger retinal eccentricity, the saccadic correction is performed with a normal latency.The corrective saccade mechanism can be interpreted as a by-pass of decision time at the end of the main saccade if the residual retinal error does not exceed some degrees.     

SCOLIOSIS AND VERTICAL OCULAR MUSCLE PARESIS

Patients with a head tilt from a vertical ocular muscle paresis can often be shown to have an accompanying scoliosis. The scoliosis is usually mild and is postural or compensatory. Although this association is quite common, it is very rarely commented upon, because the relationship between the spinal and the ocular problem is little known. It is important because if such a patient presents with a scoliosis, the patient can be spared extensive investigations if the ocular muscle paresis is of long standing and the Orthopaedic surgeon will know he is, most likely, dealing with a mild scoliosis, and one that has a cause. If the ocular muscle paresis is operated upon early enough it may be possible to remove the head tilt and have a good influence upon the scoliosis. Six patients illustrating these points are discussed.     

Apparent path of a Pulfrich target as a function of the slope of its plane of motion: a theoretical note

A theoretical equation is derived to predict the apparent path of a Pulfrich target moving with constant velocity in a plane that intersects the pupillary plane of an observer. Generally, the predicted apparent path is a hyperbola. However, if the angle between the two planes is zero, or they intersect at a pupillary center, then the predicted apparent path is a line. Other theoretical equations are derived that describe the apparent size and orientation of a line target that is parallel to the pupillary plane and receding from the observer.     

Corneal rust removal by electric drill. Clinical trial by comparison with manual removal.

The dental burr rotated by an electric drill is the quickest, safest, and most precise form of treatment for corneal rust rings. It enables complete removal of the corneal rust at a single treatment and leaves a smooth crater that is no larger than the original rust ring. Pain relief is more rapid after electric drill removal; this is probably related to the complete removal of the rust. Epithelial and stromal healing are marginally faster than after manual removal and the patients' duration of attendance is less. The ideal drill is a slim straight instrument, which rotates dental burrs and is operated by a light finger pressure. A brake which stops drill rotation on lifting the finger is a useful safety feature.     

Flaporhexis: rapid and effective technique to limit epithelial ingrowth after LASIK enhancement

We describe a method of lifting and replacing the laser in situ keratomileusis (LASIK) flap to reduce the incidence of epithelial ingrowth beneath the flap after LASIK enhancement. In the rapid flaporhexis technique, the flap edge is opened by 1 clock hour with a Sinskey hook and the flap is peeled back after the exposed edge is grasped with a forceps. When necessary, further blunt retraction of the flap is performed with a triangular polyvinyl acetate sponge. After ablation and before the flap is replaced, a triangular sponge is used to clear epithelial remnants from the interface. This method consistently produces a smooth epithelial dissection and decreases the possibility that epithelium is retained beneath the flap.     

Two-piece, dual-purpose comprehensive contact lens for vitreous surgery

PURPOSE: To describe a dual-purpose lens that can be used both as a planoconcave direct-image lens and wide-angle lens. This lens provides good resolution, is autoclavable and self-stabilizing. DESIGN: New instrument design. METHOD: This lens has two components (lenses): the inferior element is a modification of the standard, self-stabilizing planoconcave, one-piece acrylic lens with 4 footplates and is used in a similar manner; the superior part is a biconvex glass lens in a high-temperature-resistant plastic casing. The superior plastic casing is screwed on the inferior lens to convert it into a wide-angle lens. The lens is used as a self-stabilizing, planoconcave or wide-angle lens and is sterilized by autoclaving. RESULT: The planoconcave direct-image inferior lens gives a high-resolution image with a field of view of 20 degrees. The wide-angle lens on assembly of the inferior and superior parts gives a static field of view of 80 degrees and a dynamic field of view of 98 degrees. CONCLUSION: The two-piece dual-purpose contact lens is an inexpensive substitute for both planoconcave and traditional wide-angle lens without compromising the stability and quality of the fundus image.     

Combined Cataract Extraction and Glaucoma Surgery

An approach to the surgical management of eyes with concomitant cataract and glaucoma is outlined, and the author's experience in 73 consecutive cases is described. Cataract extraction alone is recommended when glaucomatous damage is minimal, and the intraocular pressure is controlled with a low-dose, well-tolerated medical regimen. When the glaucoma is uncontrolled on maximum tolerable medical therapy and poses an immediate threat to vision, a two-stage procedure of filtering surgery with subsequent cataract extraction is preferred. Between these two extremes of glaucoma control are those cases for which a combined operation is felt to be indicated, especially when cataract surgery is planned in an eye with borderline glaucoma control and/or moderate to advanced glaucomatous damage. The preferred combined approach is cataract extraction with a guarded filtering procedure, and a simplified technique to accomplish this is described.     

Nucleus cryoinversion for extraction of highly complicated cataracts

We describe a technique for cataract extraction in vitrectomized eyes that uses secured removal of the nucleus with no manipulation of the capsular bag. After a large scleral tunnel is prepared and a capsulorhexis created, a 20-gauge retinal cryoprobe enveloped in a plastic or a silicone sleeve is introduced into the anterior chamber. The cryoprobe engages the nucleus by a "freeze-grip" and rotates it 180 degrees so that the nucleus is elevated into the anterior chamber with no force applied to the capsule and the cryoprobe supporting it from below. The cryoprobe is replaced by a vectis under the nucleus and with the help of another instrument from above, the nucleus is removed. Aspiration of cortical material under low-pressure fluid maintenance of the anterior chamber completes removal of the cataract. An intraocular lens is then implanted in the ciliary sulcus for maximum capsule support.     

Human cataractous lens nucleus implanted in a sheep eye lens as a model for phacoemulsification training

We describe a realistic and inexpensive experimental cataract model for phacoemulsification training. After a capsulorhexis is performed, a deep cavity in the lens of an enucleated sheep eye is formed by phacoemulsification through a lateral incision. An undamaged human cataractous lens nucleus obtained by extracapsular cataract extraction is inserted in the preformed cavity, resting in the center of a cortex cushion. Phacoemulsification training is performed through a corneal tunnel incision. The experimental model is prepared with a human cataractous lens nucleus of the preferred hardness, simulating nuclear phacoemulsification in humans.     

The subjective quality (SQF) of Bausch and Lomb SoflensTM.

The visual acutiy of a person is examined in terms of his response to a unique target. This target is a sequence of black bars on a white background, where the spacing of the bars and their width is governed by a log periodic function. The generation of these targets is performed by a PDP-8 computer connected to an X-Y plotter. There are six sets of bars which vary in contrast and frequency. In a clinical situation, an observer notates the position where he can no longer resolve the bar pattern. A recroding of these positions produces a Subjective Quality Function (SQF) curve for the individual. On fitting with soft contact lenses, a new SQF curve is generated. A comparison of the SQF for soft lenses with that for spectacle corrections is given.     

The effect of stimulus strength on the speed and accuracy of a perceptual decision

Both the speed and the accuracy of a perceptual judgment depend on the strength of the sensory stimulation. When stimulus strength is high, accuracy is high and response time is fast; when stimulus strength is low, accuracy is low and response time is slow. Although the psychometric function is well established as a tool for analyzing the relationship between accuracy and stimulus strength, the corresponding chronometric function for the relationship between response time and stimulus strength has not received as much consideration. In this article, we describe a theory of perceptual decision making based on a diffusion model. In it, a decision is based on the additive accumulation of sensory evidence over time to a bound. Combined with simple scaling assumptions, the proportional-rate and power-rate diffusion models predict simple analytic expressions for both the chronometric and psychometric functions. In a series of psychophysical experiments, we show that this theory accounts for response time and accuracy as a function of both stimulus strength and speed-accuracy instructions. In particular, the results demonstrate a close coupling between response time and accuracy. The theory is also shown to subsume the predictions of Piéron's Law, a power function dependence of response time on stimulus strength. The theory's analytic chronometric function allows one to extend theories of accuracy to response time.     

New disposable cystotome for capsulorhexis

Circular tear capsulotomy, or capsulorhexis, is becoming an increasingly popular method of anterior capsulotomy. We describe a new cystotome with a simple tip modification which is useful for performing a circular tear capsulotomy. The cystotome is a 27 gauge disposable device that is similar to a conventional cystotome used for a can-opener-type capsulotomy except for its unique but simple tip modification. The beveled tip is first bent at 90 degrees and then twisted on its main axis so that the beveled cutting edge is parallel (i.e., in the same axis) to the shaft of the cystotome. We also briefly describe its method of use.     

Familiäre exsudative Vitreoretinopathie

Familial exudative vitreoretinopathy (FEVR) is an autosomal dominant inherited disease with a penetrance of nearly 100% and is mostly found in Asia. The most important sign is a temporal avascular retina with pathologically changed vessels combined with a temporal dragging of the disk and a heterotopy of the macula. In contrast to retinopathy of prematurity, only full-term children without oxygen therapy are affected. Clinical severity is highly variable. A stage of an asymptomatic gene carrier with peripheral ischemia only visible on angiography up to a massive deterioration of vision because of a combined tractional and rhegmatogenous retinal detachment is possible. All stages with intra- or subretinal exudates require therapy. First-line therapy is the coagulation of the leaky vessels to reduce the exudation. In addition cryocoagulation is possible. Advanced stages with complex retinal detachments usually require a vitrectomy to prevent further loss of visual function.     

Visual acuity with reversed-contrast charts: I. Theoretical and psychophysical investigations.

When the contrast polarity of a visual acuity chart is reversed by using bright letters on a dark background instead of dark on light, the shape and height of the resolution feature in the retinal image is identical but inverted compared with the normal situation. However, the contrast is different in the two cases because the background light level, which is a dividing factor when contrast is calculated, is much less when only the letters are bright. To the extent that contrast is a limiting factor in visual acuity, reversed-contrast acuity would be expected to be better, and proportionally more so in eyes where light scatter and aberrations widen and flatten the point-spread function. In a careful psychophysical study of Landolt C resolution, the minimum angle of resolution was found to be significantly smaller for white letters on a dark background than for the traditional dark on bright situation.     

Therapeutic contact lenses: the role of high-Dk lenses

Currently, the armamentarium of contact lenses that can be used for therapeutic effect provides a wider selection of lenses than ever before. If the therapeutic goal is protection and healing of the corneal epithelium, epithelial or stromal edema is best avoided, and the selection of a high-Dk silicone hydrogel (balafilcon A, lotrafilcon A) lens or a very thin membrane-type lens (crofilcon) is the best choice. If the goal is surface protection as well as stimulation of stromal wound vascularization, selection of a low-water content, thick, hydrophilic lens is the better option. If the patient is prone to lens loss or requires frequent replacement of the therapeutic lens, a prudent economic decision is to select a daily disposable moderate-water content lens. Specific circumstances may mandate the selection of a specific therapeutic lens. Patients with a prior history of active giant papillary conjunctivitis may be better served by the use of a crofilcon glyceryl methacrylate lens, which has a lower incidence of this complication. Patients who have dry eye may benefit from a higher-water content lens if adequate unpreserved tear supplementation is provided with or without punctal occlusion. The options when selecting a therapeutic contact lens are wider than ever before. Although the new generation of high-Dk lenses promises fewer limiting problems of vascularization and infection, one can use the older traditional therapeutic lenses when induced vascularization of the cornea is needed or when an economic necessity exists. Not all of the available lenses are FDA approved for therapeutic use, and such wear is an off-label use. The patient should be informed of the goal of therapy as well as the benefits and risks of therapeutic contact lenses.     

Floppy eyelid syndrome: a case report and clinical review.

Floppy eyelid syndrome is a clinical entity associated with a chronic papillary conjunctivitis which is resistant to topical therapeutic agents. The disorder is usually found in older, obese males with upper eyelids which are loose, rubbery, and easily everted. This paper is a review of the clinical characteristics and treatment of the syndrome and includes a case of a 16-year-old male with floppy eyelid syndrome, one of the youngest reported.     

A new indirect ophthalmoscope.

A hand-instrument is described, which serves the purpose of direct and indirect ophthalmoscopy. One part of the instrument, which may be used alone, is a direct ophthalmoscope; it comprises as a distinctive feature a mirror which is vertically movable. The other part is an attachment comprising magnifying lenses, a system for viewing the fundus periphery, a movable fixation target, a system for seeing the patient's eye, and a demonstration mirror. Both parts together form a light, easy-to-handle unit for monocular indirect ophthalmoscopy.     

Fehlerquellen bei der Goldmann-Applanationstonometrie

Reliable measurements of the intraocular pressure (IOP) are necessary for glaucoma patients. The measuring procedure, anatomical changes of the eye or extraocular influences can lead to errors during applanation tonometry. The IOP is overestimated if measured through the wrong eyepiece of the slit lamp, if a force is interfering with the tonometer arm during the measurement, if the lid comes into contact with the tonometer tip, if blepharospasm occurs, if there is lid retraction or if a Valsalva maneuver is present. An underestimation of the IOP occurs if staining with fluorescein is absent or insufficient, if the illumination is not bright enough, if a corneal stromal edema is present, after LASIK, during accommodation, during repeated measures within a few minutes or during systemic blood pressure drop. Discrepancies of the IOP in both directions can appear if the calibration of the tonometer is irregular, if abnormal central corneal thickness or astigmatism is present. For correct measurements calibrations are advised once or twice a year.