AUGENHEILKUNDE 212/1998 – ISSUE 4

Abstracts:

S. 190-195 (Art. 340)

Measuring visual acuity: theoretical foundations and computer-assisted instrumentation
Michael Bach, Guntram Kommerell

Psychometric function According to the European stan-dard EN ISO 8596 the Landolt-C in 8 different orientations has to be used to measure visual acuity. With decreasing size of the Landolt-C the hit rate declines from 100% to the chance level of 12.5%. This gradual transition is described by the "psychometric function". The steepest point of the psychometric function is in the middle between 100 and 12.5, i.e., at 56.25%. This point of the psychometric function (approximated by 5 of 8 Landolt-Cs) has been selected as the threshold for visual acuity, because it is there that the visual acuity is influenced least by (incidental) fluctuations. The subject has to answer by forced choice; a response like "I cannot detect anything" is not acceptable.

"Normal" visual acuity cannot be assigned to a certain value, like 1.0 or 6/6. With the standard test procedure, visually healthy, young subjects achieve a visual acuity of about 2.0 or 12/6, while in senior subjects 0,5 (3/6) may be "normal".

Averaging visual acuity Logarithmic, not arithmetic, scaling of visual acuity approximates the perceptual metric. Consequently, visual acuity values may not be averaged arithmetically. Instead, three steps are required: all values have to be converted to logarithms, then averaged, and finally the average can be reconverted. Geometric averaging is equivalent.

"Minimum Angle of Resolution" not necessary MAR is the reciprocal of visual acuity. In many studies, clinical outcome has been assessed using log(MAR). Though statistically correct, this term is unnecessary, as log(acuity) has identical statistical properties. Furthermore, log(MAR) is contra-intuitive as its value becomes smaller when vision improves.

Computer-assisted instrumentation facilitates complying with the EN ISO 8596. For instance, the Freiburg Visual Acuity Test relieves the examiner from observing whether 5 responses have been correct, and that not more than 8 tests are given per level.

Key words Visual acuity - psychometric function - threshold estimation - computer

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S.196-202 (Art. 341)

Objective estimation of the minimum visual acuity by suppression of the optokinetic nystagmus
M. Gräf

Purpose To develop and to validate an objective method which allows an estimation of the visual acuity (VA) of adults.

Methods A horizontal optokinetic nystagmus (OKN) was elicited by a vertical rectangular grating presented on a PC-screen and was recorded by an infrared reflection method. Superimposition of three stationary black detection stimuli was used to suppress the OKN. The size of these stimuli was increased every two seconds, in ten steps. Suppression of the OKN was taken as an indication that the stimuli were detected. The relation between the smallest effective stimulus and logVA was evaluated in 65 healthy volunteers (130 eyes) with artificially reduced VA and 425 cooperative patients (842 eyes) whose VA was reduced due to different etiologies. The tolerance intervals of logVA related to the discrete detection stimuli were calculated.

Results The method allows an estimation of the least expectable VA in steps of 0.8, 0.32, 0.25, 0.12, 0.1, 0.06, 0.03 und 0.025 due to the smallest value of three trials. The least VA of strabismic amblyopic eyes can only be estimated in steps of 0.16, 0.06, and 0.02. A VA > 0.02 can be proven with high reliability.

Conclusion The method allows a clinically useful, objective estimation of the least VA without observer bias in non-amblyopic eyes.

Key words nystagmography - visual acuity - psychophysical methods - functional visual loss - simulation

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S. 203-206 (Art. 271)

Enhanced fixation by unilateral Fadenoperation of medial rectus muscle in amblyopia
Erika de Decker, Wilfried de Decker

Background Strabismus surgery influences the retinal fixation positively, as described by Böhme, Ehrich and Cüppers. Fadenoperation (Posterior Fixation) of the medial rectus muscle allows a more specific approach. It reduces the influence of overadduction and latent-type nystagmus on the retinal fixation, which becomes more central and more controlled after such an operation.

Patients This effect is very marked in infants up to 2 years of age, but still significant in older children with intolerance for occlusion, and motility-dependent excentric fixation. About half of the 70 cases observed reached a useful fixation and acuity by "surgical pleoptics". With respect to the otherwise poor prognosis of these cases, this result can be regarded as favourable.

Key words Amblyopia treatment - surgical pleoptics

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S. 207-211 (Art. 272)

Subjective and objective cyclorotatory changes following a modified Harada-Ito procedure for acquired uni- and bilateral trochlear palsy
Hermann Dieter Schworm, Silvia Horstmann, Theresia Ring, Klaus-Peter Boergen

Background After recently published own investigations on subjective and objective cyclorotatory changes following inferior oblique recession for inferior oblique overaction, it was our aim to determine and to compare subjective and objective cyclorotatory changes following a modified Harada-Ito procedure for acquired trochlear palsy.

Patients and Methods Eight patients suffering from acquired uni- (np3) or bilateral (np5) trochlear palsy were investigated before surgery and 1 day, 3 days and 4 months after surgery. Subjective cyclodeviation was assessed by Harms' tangent scale. Objective cycloposition was measured by means of fundus cyclometry using an infrared Scanning Laser Ophthalmoscope.

Results The immediate postoperative incyclorotatory effect was 12° in the unilateral group and 18° in the bilateral group. Subjective and objective changes were nearly equal in both groups, with a subjective over-effect of 1°. After two days of binocular stimulation a marked regression of the surgical effect was found which still increased after four months. The long term incyclorotatory effect was subjectively and objectively nearly equal in the unilateral group which showed a relaps of subjective excyclodeviation of 5°; in the bilateral group, the subjective effect was more pronounced than the objective effect, the immediate postoperative over-effect being disappeared.

Conclusions In contrast to our results concerning inferior oblique muscle recession for strabismus sursoadductorius, subjective and objective cyclorotatory changes did not differ grossly following a modified Harada-Ito procedure. Subjective and objective short and long term regression was confirmed which objectively exceeded the amount of over-correction. As the underlying cause mechanical and sensory mechanisms are discussed.

Key words Harada-Ito operation - subjective cyclodeviation - objective cycloposition - fundus cyclometry - trochlear palsy

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S. 212-217 (Art. 270)

Orthoptic problems of macular rotation with and without muscle surgery
Ute Eckardt, Claus Eckardt

Background In spite of new therapeutic approaches the treatment of age related macular degeneration remains unsatisfying. Macular rotation is so far only a rarely employed surgical procedure, where the macula is translocated to an area with intact retinal pigment epithelium. The aim of this study was to elucidate the orthoptic problems of macular rotation and describe possible treatment modalities.

Patients and method Between February and July of 1997 a macular rotation was performed in 8 patients with exsudative macular degeneration, followed by silicone oil removal and implantation of an intraocular lens. In 3 out of 8 patients the oblique muscles were transposed at the beginning of the surgery to produce an excylotropia. In all patients visual acuity for distance and near was determined pre- and postoperatively along with a complete orthoptic status including measuring of cyclotropia. Mean postoperative follow-up was 5.5 months.

Results Due to the unilateral ectopia of the macula all patients presented with a postoperative strabismus with vertical deviation of 2° to 10°. The change of the visual axis resulted in a displacement of the pupillary reflex. The patients described a cyclotropia of 12° to 45° corresponding to the ophthalmoscopically estimated angle of retinal rotation. Patients after surgery of the oblique muscles showed less cyclotropia with an angle of 12° to 20°. They were less disturbed by diplopia and tilted visual impression than those without muscle surgery. All patients exhibited an abnormal head position with chin up and tilt to the side of the operated eye.

Conclusion Macular rotation always causes diplopia because of the unilateral ectopia with cyclotropia and vertical deviation. These problems can be considerably reduced by combining macular rotation surgery with oblique muscles surgery. It may well be that in the future these problems can be completely resolved by additional surgery on the rectus muscles.

Key words macular rotation - age related macular degeneration - cyclotropia - strabismus - diplopia

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S. 218-225 (Art. 344)

Normaccommodative convergence excess: long-term follow-up when treated with bifocals
AK Eckstein, M Fischer, J Esser

Background In patients with normaccommodative convergence excess it is possible to reduce or eliminate the excess of accommodative convergence by adding plus lenses. The resulting reduction of near deviation can lead to an improvement in the quality of binocular vision at near, and also to a better compensation of an esophoria at near. The aim of the paper was to study long term results in patients with small angle esotropia and esophoria and accommodative convergence excess treated by bifocals.

Methodes Clinical data of 91 patients were analysed retrospectively. Among them were 13 patientes with esophoria, 32 patients with microesotropia and 46 with microesotropia and a phoric component. An orthoptic status was performed every three months and at every examination it was tried to reduce the added plus lenses. The mean follow up was 5,6B2,4 years (range: 1.1-13.2).

Results The mean onset of strabismus was similar in all groups: i.e. 2.5 (B1.7) years. The patients received their first bifocals on average 3.4 (B1.9) years later. In 40 of the 91 patients the near addition could be stopped because of sufficient decrease of accommodative convergence excess during the follow-up period. The convergence excess decreased continuously in all patients with esophoria and microesotropia and the additional plus lenses could be stopped on average after 6.4 (3.5-8.4) years (esophoria) and 5.0 (2.6-8.1) years (microesotropia) respectively. In patients with microesotropia and an additional phoric deviation bifocals were only partly successful to reduce the convergence excess. The basic angle decompensated in more than half of the patients (27 out of 46) and was operated in 14 cases by unilateral resection/ recession procedure. After the operation the convergence excess decreased rapidly and the bifocals could be stopped after 4,4 (3.4-7.4) years. In the remaining 19 cases it was possible to reduced the convergence excess with bifocals in 8 patients after about 8,1 (4.1-9.3) years and in some of the remaining 11 cases a Fadenoperation has been suggested.

Conclusion While wearing bifocals the accommodative convergence excess decreased completely in patients with esophoria and microesotropia. In the condition with markedly reduced binocular vision and a large phoric component at far and near, the convergence excess decreased only in some of the patients while wearing bifocals. Conventional strabismus surgery to reduce the basic angle has a positive influence. A Fadenoperation is only necessary in a few cases.

Key words bifocals - convergence excess - esophoria - microesotropia - long-term follow-up

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S. 226-233 (Art. 348)

Fixation disparity in the Zeiss Pola-Zeigertest (Clock hand Test): not representative for eye position under natural viewing conditions
Jürgen Gerling, Miriam Ball, Thomas Bömer, Michael Bach und Guntram Kommerell

Background According to certain findings obtained with the Zeiss Polatest®, H. J. Haase defined a "Fixation Disparity Type One". In this diagnosis, the "Zeigertest" is particularly important. The Zeigertest consists of a central ring presented to both eyes for fixation, a vertical clock hand presented to the right eye and two markings at the six and twelve o'clock positions presented to the left eye. All parts are surrounded by a binocularly visible frame. Subjects with a "Fixation Disparity Type One" see a misalignment between the clock hand and the peripheral markings. We investigated (1) whether the perceived misalignment correlated with an objective deviation of the eyes from orthovergence and (2) whether subjects with a "Fixation Disparity Type One" had a deviation of the eyes from orthovergence when looking at a natural, i.e., fully fusionable object.

Subjects and methods Out of 303 medical students, 10 subjects with a "Fixation Disparity Type One" were selected and asked to indicate the perceived alignment or misalignment in the Zeigertest with a laser pointer. Two subjects without fixation disparity served as controls. The position of both eyes was recorded using the search coil technique. One of the 10 subjects with "Fixation Disparity Type One" had to be excluded due to excessive blinking.
Experiment 1: In the beginning all parts of the Zeigertest were presented to both eyes (natural viewing condition). Then, the object for one of the eyes was switched off leaving the frame as the only fusional stimulus. The outcome variable was a refixation movement of the other eye. This experiment is similar to the unilateral cover test.
Experiment 2: In the beginning all parts of the Zeigertest were presented to both eyes (natural viewing condition). Then, the original Zeigertest was switched on (clock hand presented only to the right eye, peripheral markings only to the left eye). The outcome variable was a change of vergence.

Results Experiment 1: A significant refixation movement did not occur in any of the subjects. Experiment 2: In all 9 subjects with "Fixation Disparity Type One" the vergence changed significantly between 2.4 and 14.9 arcmin. The change of vergence correlated significantly with the angle of the perceived misalignment between clock hand and peripheral markings.

Conclusion A fixation disparity ascertained at the Zeigertest does not indicate a fixation disparity under natural viewing conditions.

Key words fixation disparity - Polatest - search coil technique

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S. 234-239 (Art. 273)

Measurements regarding the influence of spectacle lenses on the determination of the angle of strabismus with Purkinje reflexes and with the cover test
Jean-Cyriaque Barry, Andreas Backes

Background The alternating prism and cover test is the conventional test for the measurement of the angle of strabismus. The error induced by the prismatic effect of glasses is typically about 27-30%/10 D. Alternatively, the angle of strabismus can be measured with methods based on Purkinje reflex positions. This study examines the differences between three such options, taking into account the influence of glasses.

Materials and methods The studied system comprised the eyes with or without glasses, a fixation object and a device for recording the eye position: in the case of the alternate prism and cover test, a prism bar was required; in the case of a Purkinje reflex based device, light sources for generation of reflexes and a camera for the documentation of the reflex positions were used. Measurements performed on model eyes and computer ray traces were used to analyze and compare the options.

Results When a single corneal reflex is used, the misalignment of the corneal axis can be measured; the error in this measurement due to the prismatic effect of glasses was 7.6%/10 D, the smallest found in this study. The individual Hirschberg ratio can be determined by monocular measurements in three gaze directions.

Conclusions The angle of strabismus can be measured with Purkinje reflex based methods if the fundamental differences between these methods and the alternate prism and cover test, and if the influence of glasses and other sources of error are accounted for.

Key words objective measurement of the angle of strabismus - accuracy - Purkinje reflections - computer ray tracing - model eye

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