Ординатура / Офтальмология / Английские материалы / Manual of Squint_Ahuja_2008

polarizing filter. Any abnormality in conceding the lines will reveal the aniseikonia. Meridional aniseikonia can also be measured.

Space Eikonometer

The best technique so far devised is of space eikonometer. This is based upon the fact that when the incongruity of the ocular image differs from normal, anomalous spatial localization must necessarily result. If spatial localization is removed from accessory aids and uniocular clues and the patient has to rely solely upon desparities of the image of the two eyes. Any image desparity is measured by neutralizing the displacement of image with iseikonic lenses. The sensitivity of the instrument is up to 0.05 percent of the image desparity.

Maddox Rod Test

Two Maddox rods were placed before the two eyes to obtain a binocular image of two vertical streaks from two muscle lights, the streaks will appear at unequal distances from the observer if aniseikonia is present at the axis of 90°. If aniseikonia is at 180° then three lights will appear, on in center of each line. Thus, aniseikonia in horizontal meridian can be known space eikonometer is the most satisfactory technique. Here target slides for amblyoscope bases on Ames vertical and cross with the subject fusing the two slides while the arms of synaptophore are slowly diverged about 0.250 at each interval. At certain point the image will break as the lines on the side of the eye receiving larger image will occur as fusion break. This should be repeated for confirmation. In front of the eye receiving small image as determined as 5% aniseikonic lens is placed and the test is repeated, if the result is revised then the 5% lens is reduced gradually untill a neutral point is reached, when the breaking of the lines occur at the same time and to the same amount on each slide. To measure aniseikonia over 5% to axis 1.05 slide is used with 5% lens and test is repeated.

Aniseikonia

Symptoms are both subjective and visual. Patients with difference in image size may feel visual symptoms as well as asthenopic symptoms. Aniseikonis interferes with the smooth functioning of visual discomfort, eye stain, burning, itching, blurred vision, diplopia fixation difficulty

and squint. Fusion mechanism demands adjustment of visual axes of the two eyes so that images, fall, not on exactly corresponding point but on non-corresponding areas thus eyes are whipped up to accurate focusing fixation and fusion.

Patients may develop neurotic symptoms like tenseness, irritability, vertigo, headache and exhaustion. Patient may be having gastric disturbance like gastritis, nausea, vomiting and indigestion are also associated symptoms in aniseikonic patients.

Large number of cases show a partial and local suppression which takes place only at fovea, patient often feels slanting of the surface and ups and down on waling aniseikonia and may be compensated after the use of glasses.

Small differences in size of the retinal image of the two eyes are not generally appreciated and it is likely that these do not impair binocular vision. As a general rule differences upto 5% can be compensated by the plasticity of visual perceptive mechanism but such compensation may impair the effectiveness of depth perception. Stereopsia markedly improves after correction of such disparity when the difference is in excess of this and or compensatory power is poor binocular vision becomes difficult or been impossible. Suppression and amblyopia may develop at an early stage in such cases. If however, binocular vision has already been well-established and sudden marked aniseikonia may be introduced (as in monocular aphakia) diplopia and other consequences may develop which have to be appropriately dealt with tolerance to aniseikonia can also be helpful in maintaining the binocular vision and preventing amblyopia.

Abnormal difference between the size and/or shape of the ocular images in a horizontal direction deranges the apparent position of objects in visual field. This causes an apparent horizontal rotation of the visual field and may affect the fusion process. There much as fusion because in general depth perception from the disparity of images in each eyes.

In higher degrees, aniseikonia causes imperfect binocular vision, but in lower degrees in earlier life it causes eye strain. It was accordingly observed in the investigation on aniseikonia and fusion that large majority of cases with convergence insufficiency had poor tolerance to aniseikonia and those with better fusion had better tolerance. Gradually increasing aniseikonia steudily reduced fusion range and affected fusion ultimately.

MANAGEMENT

Small degree of aniseikonia may be corrected by iseikonic lenses. These lenses cause magnification without introducing appreciable refractive power by changing the beam of rays passing through them. Magnification in one or in all meridian can be given in iseikonic lenses to suit the regular aniseikonia.

The fact that as we approaches the modal point of the eye, the magnification approaches unity, prompted the idea of contact lenses. Contact lenses reduce aniseikonia considerably and binocular function can also improve with contact lenses.

Aniseikonia is disturbing in monocular saphakia even with contact lenses though binocular functions are fairly good residual aniseikonia has been affectively reduced by the prescription of a combination of over powered contact lens and minus spectacle glass. It is tolerated well and improves the binocular function and therefore prevents onset of amblyopia.

18 Nystagmus

Rhythmic rapidity to and fro movement of the eyes is called nystagmus. Type of nystagmus described based one certain characteristics like rate (rapid or slow), amplitue (corse or fine), direction (horizontal, vertical or rotational), and type of movements (pendular or jerky). Nystagmus is pendular where eye movements in each direction are equal. On the other hand it is called jerky when there is slow component in one direction and fact component in opposite direction.

SPECIFIC TYPES

I. Congenital: It is usually present at birth but may be noticed by parents within a few months. Most commonly it is a horizontal nystagmus. Acromatopia and hypophasia of the optic nerve may be present. Usually the nystagmus reduced is of totally absent in a certain direction of gaze the patient’s null point. Visual acuity is best tested at null point.

Latent nystagmus is type of congenital jerk nystagmus appearing on attempted fixation, when other eye is covered. Rapid irregular random eye movement in all direction of gaze are seen in cases of laber’s congenital amaurosis or hypoplastic optic nerve. Head nodding and head taking is seen more frequently in congenital that acquired nystagmus.

Spasmus nutans is a condition featuring nystagmus, head nodding and toticallie with onset between the age of 4 to 12 months.

II.Down beating nystagmus: When fast phase of nystagmus is downwards.

III.Up-beating nystagmus: When fast phase of nystagmus is upwards. This is a pattern of ocular movements in which the one eye elevates while the other depresses, usually accompanied by intorsion and extorsion on elevation and depression respectively.

IV. Periodic alternating nystagmus: It is jerky type of nystagmus which shows rhythmic change in direction and amplitude.

V.Physiological nystagmus: There 2 forms of phenomenon-optokine nystagmus and caloric nystagmus. Optokinetic nystagmus result when a person gazed at a succession of object-moving fast in one direction for example, looking at the outside object, through a window of a fast moving train-rail road nystagmus. The eyes follow one object slowly and then return quickly to fixate at the next object. When warm or water is irrigated in the external auditory canal, convention correct are produced in the semicircular canal resulting

in calonic nystagmus.

In cold water irrigation the induced has a fast phase in the direction opposite ear. If warm is used the fact phase will be on the same side.