Technical note: dimensionless ray transference.

The ray transference matrix completely characterises the first-order optical nature of an optical system including the eye. It is in terms of the transference that quantitative analyses (for example, calculation of an average eye) can be performed. However, the fact that the entries of the transference do not have the same physical dimensions precludes the calculation of the usual scalar value (a Frobenius norm for example) for a change or difference between two optical systems. The purpose of this note is to show how to use the wavelength of the light as a natural unit of length to define a dimensionless transference and so make it possible to calculate a meaningful norm. In most practical applications, some components of the dimensionless transference may dominate unreasonably in the resulting norm in which case suitably weighted norms may be more appropriate. In an appendix, some of the issues are illustrated by application to a lens.

Electrophysiological findings in Stargardt's-fundus flavimaculatus disease.

PURPOSE: To determine the incidence of electrophysiological abnormalities in patients with Stargardt's-fundus flavimaculatus (STGD/FFM) disease. METHODS: A retrospective review was carried out of the hospital records of 46 patients who had undergone a scotopic, single flash photopic and 30 Hz electroretinogram (ERG), pattern ERG (PERG) and electro-oculogram (EOG). RESULTS: Patients were classified in two groups: those with flecks (n = 26) and those without flecks (n = 20). The incidence of abnormalities (amplitude and/or latency) for the two groups was: PERG, 90% and 98%; 30 Hz ERG, 55.8% and 50%; scotopic ERG, 38.5% and 27.5%; and single flash photopic ERG, 26% and 16%, respectively. EOG abnormalities occurred significantly more frequently in the group with flecks compared with the group without flecks: 69% and 42.5% respectively (p < 0.025). Furthermore, in the group with flecks the group mean scotopic ERG b-wave, 30 Hz ERG b-wave and PERG (P50) amplitude were significantly lower than in the group without flecks (p < 0.01). CONCLUSIONS: The most consistent electrophysiological abnormality in STGD/ FFM is the reduction of the PERG. However, EOG, 30 Hz ERG, scotopic and photopic ERG abnormalities can also frequently occur. ERG and EOG abnormalities occur more often in the presence of flecks.

Local connected fractal dimensions and lacunarity analyses of 60 degrees fluorescein angiograms.

PURPOSE: The retinal vascular tree exhibits fractal characteristics. These findings relate to the mechanisms involved in the vascularization process and to the objective morphologic characterization of retinal vessels using fractal analysis. Although normal retinas show uniform patterns of blood vessels, in pathologic retinas with central vein or artery occlusions, the patterns are irregular. Because the generalized box fractal dimension fails to differentiate successfully between normal and abnormal retinal vessels in 60 degrees fluorescein angiograms, the authors have further investigated this problem using the local connected fractal dimension (alpha). METHODS: The authors studied 24 digitized 60 degrees fluorescein angiograms of patients with normal retinas and 5 angiograms of patients with central retinal vein or artery occlusion. The pointwise method estimated the local complexity of the angiogram within a finite window centered on those pixels that belong to the retinal vessels. Color-coded dimensional images of the angiograms were constructed by plotting the pixels forming the object with a color that corresponded to specific values of alpha +/- delta alpha. RESULTS: The color-coded representation allowed recognition of areas with increased or decreased local angiogram complexity. The alpha distributions showed differences between normal and pathologic retinas, which overcomes problems encountered when using the methods of calculating the generalized fractal dimensions. A multivariate linear discriminant function using parameters from the alpha distribution and a further fractal parameter--lacunarity--reclassified 23 of the 24 normal and 4 of the 5 pathologic angiograms in their original groups (total: 92.1% correct). CONCLUSIONS: This methodology may be used for automatic detection and objective characterization of local retinal vessel abnormalities.

Trabeculectomy in uveitis Are antimetabolites necessary at the first procedure?

A retrospective study was undertaken analysing the results of trabeculectomy without antimetabolites in 33 eyes with uveitis compared to 33 eyes with primary open angle glaucoma (POAG). Both groups were matched for surgeon with a median follow-up of 19 months (range 2-44 months) for the uveitis group and 24 months (range 6-92 months) for the POAG group. The overall one and two year success (IOP ± 21 mmHg with or without medication) was 92% and 83% respectively for the uveitis group and 96% for both years for the POAG group (p = 0.241, Mantel-Haenszel test). These results indicate that the success of trabeculectomy in patients with inflammatory glaucoma compares well with that of the POAG group. In the absence of other risk factors, primary trabeculectomy without the use of antimetabolites should be considered as the first choice of surgical treatment for raised intraocular pressure in patients with intraocular inflammatory disease.

Flash and pattern visual evoked potentials in the diagnosis and monitoring of dysthyroid optic neuropathy.

Flash and pattern visual evoked potentials were recorded in 8 patients (13 eyes) with dysthyroid optic neuropathy (DON), diagnosed using the American Thyroid Association classification. All were treated with systemic steroids, but 4 patients (6 eyes) also required orbital decompression. Flash VEP (P2) and pattern VEP (P100) were recorded prior to and 2 weeks after commencing steroid treatment or decompression. Fifteen patients with Graves orbitopathy but without DON, and 20 healthy subjects, acted as controls. Before treatment visual acuity was reduced in 10 eyes and visual fields were abnormal in 5, but the VEP was abnormal in all 13, with the group mean amplitude of P2 and P100 significantly less than controls, and the group mean P100 latency significantly greater than controls. After treatment with high-dose steroids or surgical decompression there were significant improvements in the group mean amplitude of P2 and P100, and significant reductions in P2 and P100 latency; however, individually, improvements in amplitude were more significant than improvements in latency. We conclude that the VEP to flash and pattern stimuli provides a useful diagnostic and monitoring tool in patients with DON, combining objectivity with quantitative analysis.

Size dependent variation in the fractal dimensions of herpes simplex epithelial keratitis.

Dendritic ulcers, the commonest manifestation of herpes simplex epithelial keratitis (HSEK) are fractals. It is likely that their fractal properties alter if they progress to a geographic appearance. This study investigates the relationship of maximum ulcer diameter (Feret's diameter) to area (Dm) and perimeter fractal dimensions (Ds), parameters of the complexity of their areas and outlines respectively. For dendritic ulcers in the size range of 1.6-3.2 mm, Dm = 1.41 +/- 0.06 and Ds = 1.40 +/- 0.04 (mean +/- SD). With increasing ulcer size, a progressive divergence of the values of Dm and Ds occurred, such that values of 1.75 and 1.22 respectively were found at a maximum diameter of 8.4 mm. These results imply that as ulcers enlarge, their outlines become less irregular and they fill more of a 2-dimensional plane. Dm and Ds are useful parameters in quantifying the progression of HSEK from dendritic to amoeboid morphology and could have a role in the assessment of ulcer response to pharmacological intervention. A knowledge of the fractal properties of HSEK may increase understanding of the mechanisms of ulcer formation and viral spread.

Form and behaviour of Haidinger's brushes.

A computer simulation of Haidinger's brushes that is based on the optics of a radially symmetrical array of dichroic elements is presented. The simulation describes the perceived distribution of intensity and contrast of Haidinger's brushes when observed with monochromatic plane polarized light at different orientations. In addition, the behaviour of Haidinger's brushes when the plane of polarization is rotated, and their modification by optical retarders, is reproduced. The similarity between the simulation and the reported behaviour of Haidinger's brushes is consistent with the theory that the phenomenon results from a symmetrical distribution of dichroic pigment molecules within the macula.

Peripheral astigmatic asymmetry and angle alpha.

The association between peripheral astigmatic asymmetry and angle alpha was tested in the present study. Measurements were made in 34 eyes. Peripheral astigmatism was measured over the horizontal meridian using a Zeiss (Jena) Hartinger coincidence optometer and a Canon R-1 autorefractometer. Curves were fitted to the measured data of each eye and the minima determined by differentiation. Angle alpha was estimated by alignment of Purkinje images I (anterior cornea) and IV (posterior crystalline lens). Peripheral astigmatism was found to be symmetrical about a point on the nasal retina. This point departed from the visual axis by 8.8 +/- 7.0 degrees (Hartinger) and 9.4 +/- 9.8 degrees (Canon). Both values were found to be significantly higher than angle alpha 5.0 +/- 1.2 degrees. The results indicate that either peripheral astigmatic asymmetry is due to additional factors such as lack of symmetry in the peripheral curvature of individual optical surfaces, or that there is further misalignment of optical surfaces away from an optical axis.

The role of nasolacrimal intubation in the management of childhood epiphora.

Nasolacrimal intubation has been advocated to obviate the need for dacryocystorhinostomy (DCR) for childhood epiphora which fails to resolve despite apparently successful probings. Twenty-eight intubations were attempted on children falling into this category. Of these, 25 were anatomically successful intubations (3 having had to be abandoned because of difficulties in retrieving the silicone tubes from the nose). Twenty patients (80%) had complete resolution of symptoms, 2 (8%) had improvement of symptoms such that no further intervention was necessary and 3 (12%) proceeded to DCR. A greater likelihood of a good outcome was seen if the tubes were left in situ for 6 months or more. We suggest that primary nasolacrimal intubation (that is nasolacrimal intubation without DCR) should be the next step in the management of childhood epiphora which fails to resolve after two probings. This approach may avoid a DCR in over 80% of children.

Comparison of the Keeler Pulsair 2000 non-contact tonometer with Goldmann applanation.

The Pulsair 2000 non-contact tonometer (Keeler Ltd, UK) is compared with the Goldmann applanation tonometer. Data from 80 eyes were acquired by four experienced observers. A linear regression analysis showed the relationship between the instruments to be: Pulsair = 0.66 + 0.95 Goldmann. Individual components of variation were analysed by analysis of variance which indicated a significant variation in the slope of the regression equation due to observers (p = 0.02) but not to the order in which topical anaesthesia was administered. Differences between two Pulsair instruments were of marginal significance (p = 0.07). The intercept of the regression equation was unaffected by any of the components of variation. Seventy-nine per cent of averaged intraocular pressure measurements obtained with the Pulsair 2000 fell on or within +/- 3 mmHg of those measured with the Goldmann tonometer. It is concluded that the Pulsair 2000 can provide clinically useful measurements of intraocular pressure.

Fractal analysis of the normal human retinal fluorescein angiogram.

The fractal dimension of the retinal vasculature and isolated venous and arterial trees down to a caliber of 40 microns was estimated in 23 routine fluorescein angiograms of normal retinas. Fractal dimension was determined with a method based on the box counting theorem. This method is less susceptible to the radial architecture of the retinal vascular tree than those previously reported (mass-radius relation and density-density correlation function). Two scale ranges with different fractal dimension were consistently present. The estimated fractal dimensions showed no significant difference between isolated arterial and venous trees which is not supported by previous reports. This method was designed for simple application in a clinical setting.

Keratometry and post-operative astigmatism.

Total post-operative refractive astigmatism and keratometric corneal astigmatism were determined in patients following cataract surgery. By comparing the results of the two methods, the value of keratometry as a simple and quick procedure for identification of surgically induced astigmatic errors was established. It is proposed that this method should be used before final refraction in an attempt to increase clinic efficiency and reduce further appointments resulting from surgically induced astigmatism.

Effect of iris displacement on oblique astigmatism in aphakic eyes.

Aphakic eyes possess considerably less oblique astigmatism than phakic eyes. The possibility that posterior iris displacement after lens extraction accounts for the observed reduction in oblique astigmatism is tested in this study. Oblique astigmatism was calculated for a phakic and an aphakic schematic eye. With no iris displacement little reduction of oblique astigmatism occurred contrary to experimental findings. However, introduction of varying levels of iris displacement reduced this aberration to values found experimentally. Posterior iris displacement (mean = 1.19 mm) was confirmed in vivo by pre- and postoperative anterior chamber pachometry in eight patients.

Visual acuity and contrast sensitivity in cataract: summation and inhibition of visual performance.

Patients exhibiting uniocular cataract often report an improvement in vision on closing their cataractous eye. Such qualitative evidence suggests the presence of binocular inhibition--the converse of binocular summation (that is the normal superiority of binocular over monocular vision). To quantify the extent of inhibition in cataract, binocular and monocular visual acuity and contrast sensitivity were measured in 28 patients. Twelve patients showed binocular inhibition for visual acuity whilst 11 showed inhibition for contrast sensitivity measured at four cycles per degree (c.deg-1). Contrast sensitivity for 8 c.deg-1 targets was further recorded in a subset of 14 patients in whom seven showed inhibition. In patients who demonstrated inhibition, the mean decrement in performance for visual acuity was 13%. For contrast sensitivity at 4 and 8 c.deg-1 the mean decrement was 25% and 32% respectively. The clinical significance of these findings is discussed with respect to the assessment of visual function and management of the cataract patient.

Stress birefringence in the human cornea.

Stress-induced birefringence is used in engineering to determine the distribution of mechanical stress in experimental models. The phenomenon has previously been reported in the cat cornea but no data for the human exists. A qualitative description of stress-induced birefringence in the human cornea in vitro is presented. Circularly polarised light is then used to detect the phenomenon in the post-operative human cornea in vivo.