Improvements on Littmann's method of determining the size of retinal features by fundus photography.
01 Jun 1994-Graefes Archive for Clinical and Experimental Ophthalmology (Springer-Verlag)-Vol. 232, Iss: 6, pp 361-367
TL;DR: Three new methods of determining Littmann's factor q become available: simply reducing the axial length by a constant 1 · 82 mm; constructing a personalized schematic eye, given additional data; and ray tracing through this eye to extend calculations to peripheral retinal areas.
Abstract: Littmann's formula relating the size of a retinal feature to its measured image size on a telecentric fundus camera film is widely used. It requires only the corneal radius, ametropia, and Littmann's factor q obtained from nomograms or tables. These procedures are here computerized for practitioners' convenience. Basic optical principles are discussed, showing q to be a constant fraction of the theoretical ocular dimension k′, the distance from the eye's second principal point to the retina. If the eye's axial length is known, three new methods of determining q become available: (a) simply reducing the axial length by a constant 1 · 82 mm; (b) constructing a personalized schematic eye, given additional data; (c) ray tracing through this eye to extend calculations to peripheral retinal areas. Results of all these evaluations for 12 subjects of known ocular dimensions are presented for comparison. Method (a), the simplest, is arguably the most reliable. It shows good agreement with Littmann's supplementary procedure when the eye's axial length is known.
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TL;DR: The use of image analysis in the automated diagnosis of pathology (with particular reference to diabetic retinopathy) is reviewed, as well as its role in defining and performing quantitative measurements of vascular topography, and how these entities are based on 'optimisation' principles.
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Cites background from "Improvements on Littmann's method o..."
..., 1994), axial length and ametropia (Littman, 1988; Wilms, 1986), and those utilising all of axial length, anterior chamber depth, lens thickness, keratometry and ametropia (Bennett et al., 1994)....
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...and keratometry (Bengtsson and Krakau, 1992; Littman, 1982), axial length only (Bengtsson and Krakau, 1992; Bennett et al., 1994), axial length and ametropia (Littman, 1988; Wilms, 1986), and those utilising all of axial length, anterior chamber depth, lens thickness, keratometry and ametropia (Bennett et al....
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...Other factors that may need to be taken into consideration include the degree of eccentricity of the measured object from the optical axis (Bennett et al., 1994; Holden and Fitzke, 1988) and camera–eye distance (Arnold et al., 1993; Behrendt and Doyle, 1965; Bengtsson and Krakau, 1977, 1992;…...
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...Other factors that may need to be taken into consideration include the degree of eccentricity of the measured object from the optical axis (Bennett et al., 1994; Holden and Fitzke, 1988) and camera–eye distance (Arnold et al....
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TL;DR: Elucidating the complete range of systemic, environmental, and genetic factors linked with retinal vascular caliber changes may provide critical insight into the etiology, pathogenesis, and natural history of early vascular disease not only in the eye but across the body.
371 citations
TL;DR: Although the normative database provided by OCT has been helpful in identifying ocular diseases involving the RNFL, it may not be reliable in the analysis of myopic eyes and analysis of RNFL thickness in the evaluation of glaucoma should always be interpreted with reference to the refractive status.
Abstract: PURPOSE. To evaluate the relationship between retinal nerve fiber layer (RNFL) thickness measured by optical coherence tomography (OCT) and the axial length/refractive error of the eye. METHODS. A total of 115 eyes of 115 healthy subjects, comprising 75 eyes with high myopia (spherical equivalent [SE] < -6.0 D) and 40 eyes with low to moderate myopia (SE between -6.0 D and -0.5D), were analyzed in this cross-sectional study. Total average and mean clock hour RNFL thicknesses were measured by OCT and compared between the two myopia groups. Associations between RNFL measurements and axial length and spherical equivalent were evaluated by linear regression analysis. RESULTS. The RNFL measurements were significantly lower in the high myopia group compared with those of the low-to-moderate myopia group at 12, 1, and 7 o'clock (right eye orientation). Apart from the temporal clock hours, significant correlations were evident between RNFL measurements and the axial length and spherical equivalent. The average RNFL thickness decreased with increasing axial length (r = -0.314, P = 0.001) and negative refractive power (r = 0.291, P = 0.002). A significant proportion of myopic eyes were classified as outside normal limits, with reference to the normative database. The most frequently abnormal sector was at 2 o'clock, where 16.5% of myopic eyes were outside normal limits. CONCLUSIONS. RNFL measurements vary with the axial length/ refractive error of the eye. Analysis of RNFL thickness in the evaluation of glaucoma should always be interpreted with reference to the refractive status. Although the normative database provided by OCT has been helpful in identifying ocular diseases involving the RNFL, it may not be reliable in the analysis of myopic eyes.
329 citations
TL;DR: The axial length affected the average R NFL thickness, and myopia affected the RNFL thickness distribution, which should be considered in diagnosing glaucoma.
Abstract: PURPOSE To evaluate the effect of myopia on the peripapillary retinal nerve fiber layer (RNFL) thickness measured by Cirrus HD optical coherence tomography (OCT). METHODS Comprehensive ophthalmic examinations were performed, including measurement of visual acuity, refraction, and axial length on 269 subjects (age, 19-26 years) with no ophthalmic abnormality. Further, 200 x 200-cube optic disc scans of the subjects' eyes were obtained with Cirrus HD OCT. The RNFL thickness at 256 points of the RNFL thickness profile and the average RNFL thickness were recorded. The correlations between these values and the axial length and spherical equivalent (SE) of refractive errors were then analyzed by simple linear regression, before and after adjustment of the ocular magnification. RESULTS Before ocular magnification adjustment, the uncorrected average RNFL thickness decreased as the axial length increased and as the SE decreased. However, after the adjustment, the corrected average RNFL thickness exhibited no correlation with the spherical equivalent and a weak positive correlation with the axial length. Myopia also affected the RNFL thickness distribution. As the axial length increased and the spherical equivalent decreased, the thickness of the temporal peripapillary RNFL increased and that of the superior, superior nasal, inferior, and inferior nasal peripapillary RNFL decreased. CONCLUSIONS The axial length affected the average RNFL thickness, and myopia affected the RNFL thickness distribution. High myopes are likely to exhibit different RNFL distribution patterns. Since ocular magnification significantly affects the RNFL measurement in such patients, it should be considered in diagnosing glaucoma.
324 citations
Cites methods from "Improvements on Littmann's method o..."
...82).(25) Further, p is a constant in a telecentric system, and the Cirrus HD OCT system has the same magnification factor as the Stratus OCT system....
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...Various methods have been introduced to estimate the ocular magnification factor on the basis of the degree of ametropia, keratometry, and/or the axial length.(24,25,33,34) We selected the modified axial length method derived from Bennet et al....
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References
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TL;DR: A method is described which allows the real diameter of an object on the fundus to be determined by means of a system of curves which are the result of trigonometrical calculations and which give an approximate value.
Abstract: A method is described which allows the real diameter of an object on the fundus to be determined by means of a system of curves which are the result of trigonometrical calculations and which give an approximate value; only a photograph of the fundus, a measurement of the ametropia and of the radius of the anterior surface of the cornea are required. The results can be corrected with a second graph, which makes use of the length of the optical axis of the eye.
465 citations
"Improvements on Littmann's method o..." refers methods in this paper
...Table 1 Extract from Littmann's Table 1 [ 4 ] (reproduced by permission of the publishers of Klinische Monatsbldtter fiir Augenheilkunde)...
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...This gives a direct reading of q from the grid location representing the known values of ra and A. In a later paper [ 4 ], Littmann offered an alternative to the use of his nomogram....
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Book•
01 Nov 1984
TL;DR: The eye's optical system visual acuity and contrast sensitivity spherical ametropia astigmatism subjective refraction accommodation and near vision ocular motility and binocularVision convergence anomalies of binocular vision - heterophoria and heterotropia stereopsis and the stereoscope.
Abstract: The eye's optical system visual acuity and contrast sensitivity spherical ametropia astigmatism subjective refraction accommodation and near vision ocular motility and binocular vision convergence anomalies of binocular vision - heterophoria and heterotropia stereopsis and the stereoscope the schematic eye subsidiary effects of correcting lenses - magnifying devices anisometropia and aniseikonia ocular aberrations visual examination of the eye and ophthalmoscopy retinoscopy (skiascopy) objective optometers vision screening, new subjective refractors and techniques measurement of ocular dimensions distribution and ocular dioptrics of ametropia entoptic phenomena.
287 citations
Journal Article•
172 citations
"Improvements on Littmann's method o..." refers result in this paper
...[6], whose findings were broadly in line with previous studies, showed that in most refractive states the axial length could vary by as much as 4 mm, sometimes more....
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TL;DR: The result was represented by curves which enable factors used in calculating the true diameter to be determined, but instead of the curves a quadratic equation is used to produce the same factors.
Abstract: In a preceding paper with the same title the result was represented by curves which enable factors used in calculating the true diameter to be determined. Instead of the curves a quadratic equation is used to produce the same factors. The data for the equation are given in two tables.
134 citations
TL;DR: If the positions of the principal points of the crystalline lens are conjectured, its equivalent power and that of the eye can be calculated as described from ocular dioptrics.
97 citations
"Improvements on Littmann's method o..." refers methods in this paper
...This need can now be obviated by a procedure devised by Bennett [1]....
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