Page 32 - Practical-Refraction-English

Subjective Refraction C Binocular Balance Having determined the refractions of the right and left - alternate occlusion: this involves occluding first eyes separately under monocular conditions, it is impor- 1) Dissociate the two eyes by: tant to ensure that these refractions correspond well one eye then the other and rapidly continuing to alter- under binocular conditions. This is the aim of binocular nate this occlusion such that the patient sees with both balance. The spherical component is adjusted as neces- eyes but never simultaneously. During this test, the sary to equalize the accommodative effort of the two eyes, patient should never be placed in binocular vision condi- so that the retinal images of both eyes may simulta- tions where both eyes see the target at the same time. Practical Refraction In particular at the beginning of the test (that is at the neously be in focus. If this is not the case, asthenopia may end of monocular subjective refraction), occlude the eye result, as accommodation is destabilized. that is open before uncovering the eye that is still closed. Initially, the patient should be placed in (incomplete) bino- - vertical prism: this involves placing a total of 6 cular vision conditions in such a way that both eyes see base down right to dissociate the eyes, split between the separate views of the same test (simultaneous monocular eyes (3 BDR and 3 BUL) so the effect of the prism len- vision). With each eye seeing the same image separately, ses on the quality of vision is equal for each eye. The the right and left eyes’ vision can be compared and the introduction of this prism results in two images: the best refractive balance found. higher image is seen by the right eye, the lower by the left eye and thus the patient may compare the two ima- ges/eyes. Various methods can be used to achieve such “simulta- neous monocular” vision conditions. The two eyes are dis- - polarizing filters/lenses: this method achieves sociated such that either (i) both eyes see the same target dissociation by the use of polarized targets and polari- but never simultaneously, or (ii) each eye sees a different zing lenses of mutually perpendicular orientations. image of the same target and both are viewed simulta- Targets may include letters or polarized duochrome neously. The patient is then asked to compare the clarity charts. of the two images. If one is seen more clearly than the other, plus lenses are added to the eye seeing the clearer 2) Fog binocularly by +0.50 D: Vision is reduced image, until the two eyes see equally clearly. If there is slightly and such blurred conditions enable the patient never a point at which the patient sees equally clearly with to make a comparison more easily. both eyes, the dominant eye should be favoured and left slightly clearer. 3) Ask the patient to compare the images (which will be slightly blurred) seen by the right and left Note that most binocular balance techniques can be per- eyes and indicate which eye sees more clearly (image formed only when the patient has equal visual acuity in less blurred) both eyes; only certain techniques allow some binocular balance to be achieved when the visual acuities are 4) Equalise the vision (equalise the blur) of both unequal (for example, the doubled duochrome method). eyes by further fogging the eye which sees more clearly (less blurred). Do this by adding plus lenses in +0.25 D steps until both eyes see equally. If both eyes never see Procedure: equally, favour the dominant eye (leave it slightly clea- 1. dissociate the two eyes 2. add +0.50 D fog rer) so that the refractive correction respects the natural 3. have the patient indicate which eye sees more clearly ocular dominance. 4. add plus lenses in front of this eye until the both eyes see equally (If both eyes never see equally, favour the 5) Remove +0.50D from both eyes, place the patient in full binocular vision conditions (both eyes open, vie- dominant eye (leave it slightly clearer)). 5. Remove +0.50 D from both eyes. wing the same target) and check the level of vision, bino- cularly. Note that binocular balance may be performed at both distance and near (see below). 32 Copyright © 2008 ESSILOR ACADEMY EUROPE, 13 rue Moreau, 75012 Paris, France - All rights reserved – Do not copy or distribute.