Rolf Effert

Objective Measurement of Small Angles of Strabismus in Infants and Children with Photographic Reflection Pattern Evaluation

A new photographic method for the precise measurement of the angle of squint in children and infants is presented. The apparatus consists of a reflex camera with three horizontally aligned flashes and a small fixation light. The subjects are photographed while fixating binocularly or monocularly in the primary or secondary position. Six reflections can be seen on the photograph of each pupil. These are the first and fourth Purkinje images of each light source. From the reflection patterns, the squint angle or the angle kappa or alpha can be computed using a simple formula. A vertical angle of strabismus may be calculated from the same photograph using the same principle. Results from orthotropic and strabismic adults and from children are evaluated to establish reference values. Cooperation from the children is generally very good. The accuracy of the new method is between between 2 and 4.5 prism diopters (between 1 degree and 2.5 degrees), depending on the measuring strategy.

Automatic strabometry by Hough-transformation and covariance-filtering

In order to measure the angle of squint, the eyes of a patient are photographed using two CCD-cameras. Three hashes are activated at the moment of exposure. By measuring the positions of the reflexes in the digital image the squint angle can be determined. This paper presents a segmentation method for the automatic detection of the reflexes without the use of fixed threshold values. With an algorithm based on Hough-transformation the centre of the iris is localized. In the region of interest the reflex positions are located using cross-covariance-filtering. A classification and plausibility control follow to make sure there are no faulty measurements. With 191 clinical images arbitrarily chosen the algorithm was tested showing perfect results. It is already used in clinical practice.<<ETX>>

Self-assessment of angles of strabismus with photographic Purkinje I and IV reflection pattern evaluation

Abstract• Background: Accurate assessment of the angle of strabismus, e.g. of variable angles of strabismus, is crucial in preoperative patient management and is usually performed in a clinical environment. Objective assessment by patients themselves, under everyday conditions, could contribute to a better preoperative work-up. A new objective evaluation procedure for the measurement of manifest angles of strabismus for near and distance fixation by the patient himself is presented. • Methods: To account for the modified experimental setup used for the self-assessment, an amended computation procedure of Purkinje reflection pattern evaluation was developed. For measurement, patients and controls placed their head on a head/chin rest and fixated at 33 cm or 4 m distance in primary position. A reflex camera and three photo flash units were positioned on a special frame underneath the visual axis and in front of the subject so that both eyes could be photographed simultaneously. The cameras remote shutter control was released together with the photo flash units by the properly fixating subject. The angles of strabismus were obtained from the series of pictures through later evaluation of the Purkinje I and IV reflection patterns recorded in the photographs of the eyes.• Results: Measurements of the ocular alignment in two control groups and in a group of strabismic subjects showed satisfactory accuracy of the “self-assessment” method compared to “standard” Purkinje reflection pattern evaluation and orthoptic measurements of the angle of strabismus.• Conclusion: The modified “self-assessment” method can be used for the objective recording of angles of strabismus as needed in the preoperative work-up of patients with variable angles of strabismus, over prolonged periods of time, and outside a clinical setting.

Automatic measurement of the angle of squint by Hough-transformation and covariance-filtering

Describes the image processing method for the automatisation of the measurement of squint using the first and fourth Purkinje reflex. This method is already in use in clinical practice in comparison to the standard method of the cover test. The new method, based on image processing, allows the exact and objective determination of the angle of squint. Children from the age of six months as well as adults can be examined. The automatisation of the previously interactive method is a prerequisite for the screening of large collectives of children for microstrabism. The method uses a two-stage abstraction to identify six reflexes in the iris of each eye. The method based on the Hough-transform and covariance-filtering is robust against image distortions and additional reflexes, as produced by wearing glasses. The reliability of the method has been proven in 191 cases and is tested in routine use now.

Änderung der Lochgröße und des abgehobenen Netzhautbereichs nach Pars-plana-Vitrektomie bei Makulaforamen im Stadium III nach Gass

Bei 15 Patienten mit Makulaforamen im Stadium III nach Gass wurde eine Pars-plana-Vitrektomie mit Membranentfernung vorgenommen. Am Ende der Operation erfolgte eine Auffüllung des Glaskörperraums mit einem 20%-SF-6-Gas-Luft-Gemisch, und die Patienten mußten für mehrere Tage eine Bauchlage einnehmen. Methode: Präoperativ und 4 – 7 Monate postoperativ wurde ein Fundusfoto von der Lochregion und der abgehobenen Netzhautregion mit einer Zeiss-Funduskamera angefertigt. Auf den in digitaler Form vorliegenden Bildern wurden die Fläche des Lochs und die Fläche der abgehobenen Netzhautregion mit einem speziellen, für Meßzwecke ausgelegten Bildverarbeitungsprogramm ausgemessen. Ergebnisse: Bei 7 Patienten kam es zu einer Visusverbesserung, in den restlichen Fällen blieb der Visus gleich oder nahm um 1 Visusstufe ab. In 5 Fällen, also in 33%, konnte eine vollständige Wiederanlegung der Netzhaut am Lochbereich erreicht werden. In 8 Fällen verkleinerte sich die Fläche des abgehobenen Netzhautareals um 22% im Mittel. In 2 Fällen vergrößerte sich die Ablatio. In 2 Fällen war der Rand des Lochs nicht mehr abgrenzbar. In den restlichen Fällen änderte sich der Durchmesser des Lochs um weniger als 10%. Schlußfolgerung: Die Ergebnisse zeigen, daß auch in den Fällen, wo eine komplette Wiederanlegung nicht erreicht werden konnte, mit einer Reduzierung des abgehobenen Netzhautbereichs zu rechnen ist. Dementsprechend beobachtet man eine Abnahme der Metamorphopsien. Der Anstieg der zentralen Sehschärfe ist allerdings weniger deutlich ausgeprägt, so daß wir bezüglich der Operationsindikation im Stadium III zunehmend zurückhaltender geworden sind.In 15 patients with a stage III macular hole a pars plana vitrectomy with membrane peeling was performed. At the end of the operation a gas/air exchange was performed and patients had to lie face down for some days. Methods: Before and 4 – 7 months after the operation a picture of the posterior pole was taken with a Zeiss fundus camera. Pictures were digitized, and the area of the hole and of the surrounding detached retina was measured using a special image analyzer software. Results: In seven cases an improvement of visual acuity was found; in the remaining eight cases visual acuity remained unchanged. In five cases (33%) the retina was totally attached around the hole. In eight cases the detached area had become smaller, in two cases larger, and in two cases we could no longer observe the rim of the hole. In the remaining cases the area of the holes changes by less than 10%. Conclusions: The results show that even in cases where total attachment of the surrounded retina cannot be achieved, reduction of the detached area can be expected in most cases. Simultaneously, a reduction of metamorphopsia can be observed. However, the improvement of visual acuity is not very high. That is why we will perform pars plana vitrectomy in stage III holes only in exceptional cases.