Assessment of ocular alignment

strabismus , gaze , ocular movements , classification etc
presented by senior optometrist & orthoptician at Sagarmatha Choudhary Eye Hospital, SCEH, LAHAN (NEPAL )

He explain details about the binocular gaze , EOMs, etc & work up of a patient of squint etc.

1. Assessment of Ocular Alignment and evaluation of Eye Movements RAY ARBIND Senior Optometrist SCEH, Eye hospital, Lahan , Nepal Moderator- Dr.Vinit Kumar

2. Positions of Gaze P rimary position of gaze : T he position of the eyes when fixating straight ahead on an object at infinity. For practical purposes, infinity is considered to be 20 ft (6 m), and for this position the head should be straight. Cardinal positions : T hose 6 positions of gaze in which the prime mover is 1 muscle of each eye, together called yoke muscles .

3. Midline positions : S traight up and straight down from primary position. These latter 2 gaze positions help determine the elevating and depressing capabilities of the eye, but they do not isolate any 1 muscle because 2 elevator and 2 depressor muscles affect midline gaze positions.

4. D iagnostic positions of gaze has been applied to the composite of these 9 gaze positions: the 6 cardinal positions, straight up and down, and primary position. For patients with vertical strabismus the diagnostic positions of gaze include forced head tilt to the right and left

5. Definitions Phoria : a latent visual axis deviation held in check by fusion. Tropia : a manifest visual axis deviation. Intermittent tropia : an intermittent visual axis deviation that may exist only in certain gaze positions or target distances. Ductions : monocular movements into various gaze positions; each eye views in the same direction. Vergences : binocular eye movements into opposite gaze directions.

6. INTRODUCTION Subjective methods : useful for cooperative, communicative older patients . Objective methods : must be used in younger patients or those less cooperative.

7. LABORATORY METHODS Most laboratory tests are objective. E lectro- oculogram (EOG) is generated by alterations in direct current between the front and back of the eye as the eye rotate Insulated wire placed in a silicone rubber limbal annulus (eye coil) generates a current in response to a magnetic field; This very precise technique may be used to detect horizontal, vertical, and torsional changes in eye position.

8. CLINICAL METHODS The position of the patient’s head when it is mobilized in free space is carefully noted Atypical head positions may indicate: Restrictive or paralytic strabismus The examiner must differentiate between head turns, tilts, and vertical head positions

9. Patient is seated in the examination chair The patient’s lid position is noted; lid asymmetry is usually found in patients who have vertical strabismus If hypotropia is present and the non-fixing eye is lower than the fixing eye, the lid position is lower in the nonfixing eye; this is termed pseudoptosis if the lid regains normal position when the previously hypotropic eye fixes in primary position. Epicanthal skin folds that extend over the nasal sclera in a small child may simulate esotropia . Vertical displacement of an orbit may simulate vertical strabismus, and hypertelorism may simulate exotropia .

10. OBJECTIVE CLINICAL METHODS Ocular alignment tests can be grouped into 4 basic types: Cover tests, Corneal light reflex tests, Dissimilar image tests, and Dissimilar target tests

11. Cover Tests These objective tests detect and measure horizontal and vertical strabismus, but they cannot measure torsional deviations and detect only some and not all torsional deviations The cover test detects tropias – constant visual axis deviations. The uncovered eye should be observed for movement as its fellow is covered with a paddle, thumb, or remote occluder

12. Cover Tests Necessities for cover testing- Eye movement capability, image formation and perception, foveal fixation in each eye, attention, and cooperation There are 3 types of cover tests: The cover-uncover test, The alternate cover test, and The simultaneous prism and cover test All can be performed with fixation at distance or near

13. The monocular cover-uncover test Most important test for detecting the presence of manifest strabismus Differentiating a heterophoria from a heterotropia As 1 eye is covered, the examiner watches carefully for any movement in the opposite, noncovered eye; such movement indicates the presence of a heterotropia .

14. With movement of the noncovered eye assumed to be absent, movement of the covered eye in one direction just after the cover is applied and a movement in the opposite direction (a fusional movement) as the cover is removed indicate a heterophoria If the patient has a heterophoria , the eyes will be straight before and after the cover-uncover test; The deviation that appears during the test is a result of interruption of binocular vision

15. A patient with a heterotropia -starts out with a deviated eye and ends up (after the test) with either the same or the opposite eye deviated (if the opposite eye is the deviated one, the condition is termed alternating heterotropia ) Some patients with heterophoria the eyes are straight before testing but they dissociate into a manifest deviation( heterotropia ) after the occlusion interrupts binocular vision

16. Small toys are suitable for young children, but bright white lights are avoided as the patient cannot accommodate on the contours of a light. Tropias established by the cover test may be measured using the simultaneous prism and cover test; a prism of appropriate strength held in the appropriate direction is introduced before one eye as its fellow is covered Prism strength is increased until eye movement ceases; this prism strength corresponds to the size of the strabismus. The test is then repeated with the prism before the other eye.

17. The uncover test requires observation of the covered eye as the cover is removed. If that eye is deviated under cover, it may regain fixation or may remain deviated. The former implies the presence of a phoria , a latent deviation held in check by sensory fusion, or an intermittent tropia ; the latter implies a tropia with fixation preference for the fellow eye. Phorias may be detected more directly using the alternate cover test, in which each eye is occluded alternately to dissociate the visual axes maximally.

18. The alternate cover test (prism and cover test) Measures the total deviation, regardless of whether it is latent or manifest

19. Testing should be performed at both distance and near fixation The deviation is quantified by using prism to eliminate eye movement as occluder is switched from eye to eye(prism alternate cover test)

20. It may be necessary to use both horizontally and vertically placed prisms The amount of prism power required is the measure of deviation Two horizontal or 2 vertical prisms should not be superimposed on each other because this can induce significant measurement errors A more accurate method for measuring large deviations is to place prisms in front of each eye, However,It is acceptable to superimpose a horizontal prism on a vertical prism before the same eye if necessary

21. Care must be taken to permit time for each eye to reside behind the cover (the cover must not be ‘fanned’ before the eyes). Appropriately held prisms enable quantitation of the Phoria For most clinical purposes, measurements within 2 D are sufficiently accurate. Cover test measurements are influenced by the presence of eccentric fixation; Its presence must be investigated in patients who have severe amblyopia.

22. The eye behind the prism is the ‘fixing’ eye. If the cause of strabismus is paralytic or restrictive, patients may have greater cover test measurements when the paretic or restricted eye fixes in a given gaze position (secondary deviation) than when the sound eye fixes (primary deviation).

23. Strabismus should be detected and measured in primary position at distance and near fixation and in gaze up, down, right, and left 30° from primary position. The nine ‘diagnostic gaze positions’ include the above plus, Up and right, up and left, down and right, and down and left; these are useful to measure cyclovertical muscle palsies. For oblique muscle dysfunction, measurements are taken with the head tilted 30° right and left at distance fixation

24. The alternate cover test measures the total deviation It doesn’t distinguish between latent ( heterophoria ) and manifest( heterotropia )

25. For more accurate results, Plastic prisms should be held with back surface(the surface closest to patient) in the patients frontal plane,regardless of size of deviation If the patients head is tilted,the prism must be tilted accordingly With incomittant (paretic and restrictive) strabismus,the clinician can measure the primary and secondary deviations by holding the prism over the paretic or restricted eye and the sound eye ,respectively

26. The simultaneous prism and cover test is helpful in determining the manifest deviation without occlusion(only the heterotropia ) The prism is placed infront of deviating eye and covering the fixating eye at the same time Test is repeated using increasing prism power until deviated eye no longer shifts

27. Light Reflex Tests The main tests of this type are the Hirschberg, Modified Krimsky , Bruckner, and Major amblyoscope methods

28. The Hirschberg method Is based on the premise that 1 mm of decentration of the corneal light reflection corresponds to approximately 7degree or 15 ,of ocular deviation of the visual axis Therefore, a light reflex at the pupillary margin is about 2 mm from the pupillary center (with a 4-mm pupil), which corresponds to 15degree, or approximately 30 , of deviation.

29. A reflex in the mid-iris region is about 4 mm from the pupillary center, which is roughly 30degree, or 60Δ of deviation; similarly, a reflex at the limbus is about 45degree, or 90Δ of deviation

30. The Krimsky method Uses reflections produced on both corneas by a penlight and is ideally used at near fixation. The original method involved placing prisms in front of the deviating eye More common modifications today involve holding prisms before the fixating eye or split between the 2 eyes

31. By adjusting the prisms to center the corneal reflection in the deviated eye, it is possible to approximate and quantitate the near deviation

32. Prism deflect light toward their base, but the patient views the light as deflected toward the prism apex The prism diopter is defined as the strength of prism necessary to deflect a light beam 0.4 inches (1 cm) at 3.3 ft (1 m) distance As a tangent function, the prism diopter is not linear and increases in size as the deviation increases, but for small deviations 1° approximates 1.7Δ.

33. The angle kappa can affect light reflex measurements. Angle kappa is the angle between the visual axis and the anatomical pupillary axis of the eye If the fovea is temporal to the pupillary axis (as is usually the case), the corneal light reflection will be slightly nasal to the center of the cornea This is termed positive angle kappa and simulates exodeviation .

34. If the position of the fovea is nasal to the pupillary axis, the corneal light reflection will be slightly temporal to the center of the cornea This is termed negative angle kappa and simulates esodeviation

35. Bruckner test The direct ophthalmoscope is used to obtain a red reflex simultaneously in both eyes. If strabismus is present, the deviated eye will have a lighter and brighter reflex than the fixating eye. This test also identifies opacities in the visual axis and moderate to severe anisometropia .

36. Bruckner Test:

37. Subjective Clinical Methods

38. Dissimilar Image Tests 1-target tests in which the image of the target is made artificially dissimilar in both eyes. Because one image falls on the fovea of one eye and on a non foveal region of the other eye in strabismic patients; esotropic patients perceive the images homonymously and exotropic patients perceive them as crossed. All vertical deviations result in vertically crossed diplopia. The 3 most commonly used of these tests are the Maddox rod test, the double Maddox rod test, and the red glass test.

39. The Maddox rod test S pecially constructed device consisting of a series of parallel cylinders that converts a point source of light into a line image When illuminated, these cylinders project a line upon the patient’s retina perpendicular to the groove orientation Because fusion is precluded by the Maddox rod, heterophorias and heterotropias cannot be differentiated. The Maddox rod can be used to test for horizontal and vertical deviations, and, when used in conjunction with another Maddox rod, for cyclodeviations .

40. To test for horizontal deviations, the Maddox rod is placed in front of one eye (for eg,right eye) with the cylindres positioned horizontally The patient fixates a point source of light and then sees a vertical line with the right eye and a point sorce of white light with the left eye If the light superimposes the line, orthophoria is present If the light is to the left of the line,an esodeviation is present If the light is to the right ,an exodeviation is present

41. To measure the amount of deviation the examiner finds the prism that superimpose the point source of line

42. The double Maddox rOd test Is used to determine cyclodeviations Maddox rods are placed in a trial frame or phoropter Positioned in front of each eye, with the rods aligned vertically so that the patient sees two horizontal lines The patient or examiner rotates the axes of the rods until the lines are perceived to be parallel.

43. To facilitate the patient's recognition of the 2 lines, it is often helpful to dissociate the lines by placing a small prism base-up or base-down in front of 1 eye The angle of rotation that causes the line image to appear parallel determines the magnitude and direction ( intorsion or extorsion )of cyclotropia

44. red glass test A red glass is placed in front of the right eye. Same purpose as the Maddox rod test but is not applicable to cyclodeviations The red glass test requires the patient to alert the examiner when the red light viewed behind a red filter before the right eye and a white light viewed with the left eye are superimposed or displaced one from the other

45. Fusion is disrupted by the red glass and thus horizontal and vertical phorias are uncovered and measured; Torsional deviations are not detected by this method. The gaze position of maximal image separation is a clue to the identity of paretic or restricted muscles.

46. Dissimilar Target Tests Based on the patient's response to the dissimilar images created by each eye viewing a different target; the deviation is measured first with 1 eye fixating and then with the other. In contrast to dissimilar image tests, diplopia is created by having the patient observe 2 different targets simultaneously, with each target seen by 1 eye only. Esotropic patients will have crossed diplopia, and exotropic patients homonymous diplopia.

47. Several dissimilar target tests, but the 3 most frequently encountered Lancaster red-green projection test, the Hess screen test, and the major amblyoscope test.

48. The Lancaster red-green test Uses red-green goggles that can be reversed, a red-slit projector, a green-slit projector, and a screen ruled into squares The patient's head is held steady; by convention, the test is begun with the red filter in front of the right eye. The Lancaster red–green test uses a screen marked in 2° increments viewed from a distance of 6.6 ft (2 m). The examiner projects a red slit onto the screen, and the patient is asked to place the green slit so that it appears to coincide with the red slit. The relative positions of the 2 streaks are then recorded

49. The test is repeated for the diagnostic positions of gaze, The goggles are then reversed so that the deviation with the fellow eye fixating can be recorded.

50. The Hess screen test Contains a tangent pattern displayed on a dark grey background. Red lights that can be individually illuminated by a control panel indicate the cardinal positions of gaze within a central field (15° from primary position) and a peripheral field (30°); each square represents 5° of ocular rotation. The eyes are dissociated by the use of reversible goggles incorporating a red and a green lens, the red lens in front of the fixating eye and the green lens the non-fixating eye. Red points of lights are illuminated at selected positions on the screen. The patient holds a green pointer, and is asked to superimpose a green light over each red light in turn. In orthophoria the two lights should be more or less superimposed in all positions of gaze. The goggles are then reversed and the procedure repeated. Software is available that facilitates the plotting of a Hess chart using a standard desktop computer screen.

51. Performing Hess Screen Test:

52. major amblyoscope method uses separate target illumination, which can be moved to center the corneal light reflection. The amount of deviation is then read directly from the scale of the amblyoscope The instrument is adjusted to the patients interpupillary distance in the usual manner, the correcting spectacles are worn. Targets are used which ensure foveal fixation. The subjective angle is determined and the readings taken from the prism diopter scale. Minus lenses usually-3DS are inserted in the lens holder of the instrument and the measurement is repeated.

53. Assessment of Eye Movements

54. Ocular Rotations Generally, when eye movement are assessed, versions are tested first. The examiner should pay particular attention to the movements of both eyes in to the 9 diagnostic positions of gaze. Limitations of movement into these positions and asymmetry of excursion of the 2 eyes should be noted. Spinning the child, or provoking the doll's head phenomenon,may be helpful in eliciting the vestibular -stimulated eye movements. If versions are not full, duction movements should be tested for each eye separately

55. Convergence Alignment at near is usually measured at 13 inch (33 cm) directly in front of the patient in the horizontal plane. Comparison of the alignment in the primary position at both distance and near fixation helps assess the accommodative convergence ( synkinetic near) reflex. The near point of convergence is determined by placing a fixation object at 40 cm in the midsagittal plane of the patient's head. As the subject fixates on the object, it is moved toward the subject until 1eye loses fixation and turns out.

56. The point at which this action occurs is the near point of convergence. The eye that is able to maintain fixation is considered to be the dominant eye. The normal near point of convergence is 8-10 cm or less. This determination does not distinguish between fusional and accommodative convergence, And any heterophoria that is present should be taken into account and corrected for algebraically

57. Accommodative convergence/accommodation ratio The accommodative convergence/accommodation (Ac/A) ratio is defined as the amount of convergence (in prism diopters) per unit change in accommodation(diopter) There are 2 methods of clinical measurement The gradient method and The heterophoria method

58. The gradient method Arrives at the AC/ A ratio by dividing the change in deviation in prism diopters by the change in lens power An accommodative target must be used, and the working distance (typically at 1/3 m or 6 m) is held constant. Plus or minus lenses ( eg , + 1, +2, +3, - 1, -2, -3) are used to vary the accommodative requirement. This method measures the stimulus AC/A ratio, which is not necessarily identical to the response AC/ A ratio. The latter can be determined only with the use of an optometer that records the change in accommodation actully produced.

59. The heterophoria method The distance and near deviations are used,along with the interpupillary distance,to calculate AC/A ratio If the patient is more exotropic or less esotropic at near, Too little convergence, or a low AC/ A ratio, is present; If the patient is more esotropic or less exotropic at near,a high AC/A ratio is present. In accommodative esotropia,an increase of esotropia of 1O or more from distance to near fixation is considered to represent a high AC/ A ratio.

60. An abnormally high AC/ A ratio can be managed optically, pharmacologically, or surgically. For example, plus lens spectacles for hyperopia reduce accommodation and therefore reduce accommodative convergence. This principle is the mainstay of the medical management of esotropia . Bifocals reduce or eliminate the need to accommodate for near fixation. This optical management may be used for excess convergence at near-that is, an esodeviation greater at near. Underplussed or overminused spectacles create the need for greater-than-normal accommodation.

61. This excess accommodation creates more accommodative convergence and is occasionally used to reduce an exodeviation . Long-acting cholinesterase inhibitors ( eg , echothiophate iodide) can be used to decrease accommodative convergence. These drugs act directly on the ciliary body, facilitating transmission at the myoneural junction. They reduce the central demand for accommodative innervation and thus reduce the amount of convergence induced by accommodation

62. Fusional Vergence Vergences move the 2 eyes in opposite directions. Fusional vergences are motor responses used to eliminate horizontal, vertical, or torsional image disparity. They can be grouped by the following functions: Fusional convergence eliminates bitemporal retinal disparity and controls an exophoria Fusional divergence eliminates binasal retinal disparity and controls an esophoria .

63. Vertical fusional vergence controls a hyperphoria or hypophoria . Torsional fusional vergence controls incyclophoria or excyclophoria . Fusional vergences can be measured by using a haploscopic device (major amblyoscope ),a rotary prism, or a bar prism, and gradually increasing the prism power until diplopia occurs. Accommodation must be controlled during fusional vergence testing .

64. NORMAL FUSION VERGENCE AMPLITUDES Testing Distance (m) Convergence (D) Divergence (D) Vertical Vergence (D) 6 14 6 2.5 0.025 38 16 2.5

65. Fusional vergences can be altered by the following: Compensatory mechanisms : As a tendency to deviate evolves,the patient gradually develops a larger-than-normal fusional vergence for that deviation. Very large fusional vergences are common in compensated,long -standing vertical deviations and in exodeviations .

66. Change in visual acuity : Improved acuity improves the fusional vergence mechanism The treatment of reduced vision may change a symptomatic intermittent deviation to an asymptomatic heterophoria . State of awareness : Fatigue, illness, or drug and alcohol ingestion may decrease the fusional vergence mechanism,converting a heterophoria to a heterotropia , Orthoptics : The magnitude of the fusional vergence mechanism (mainly fusional convergence) may be increased by exercises. This treatment works best for near fusional convergence, particularly for the relief of the symptoms of convergence insufficiency

67. Optical stimulation of fusional vergence : In controlled accommodative esotropia , reducing the strength of the hyperopia or bifocal correction induces an esophoria that stimulates fusional divergence. The power of prisms used to control diplopia may be gradually reduced to stimulate a compensatory fusional vergence

68. To test fusional convergence, the prism is held base-in before a patient’s eye; to test fusional divergence, the prism is held base-out. Positive vertical vergence requires a base-down prism placed before the right eye, and negative vertical vergence requires one placed before the left eye

69. MECHANICAL TESTS OF EYE MOVEMENT LIMITATION Patients who have duction limitations may suffer from paralysis or paresis, or mechanical restriction of full duction movement, or both. Forced duction testing and active force generation test may help to differentiate.

70. Forced Duction Test The forced duction test is an attempt by the examiner to move a patient’s eye farther in a given direction than the patient can move it. Topical anesthetic is placed on the appropriate limbal location (generally 180° away from the duction limitation) with a small cotton swab and the limbal conjunctiva is grasped firmly with a toothed forceps. The patient is asked to rotate the eye fully in the direction of the limited duction .

71. An attempt is then made by the examiner to rotate the eye beyond the position attained by the patient while avoiding globe retraction Care must be taken not to abrade the cornea. Patients who have pure nerve palsy exhibit no restriction to full movement by the examiner; patients who have pure restriction ( dysthyroid orbitopathy , entrapment of ocular contents after blowout fracture) exhibit restricted movements (sometimes termed a positive forced duction test).

72. Some patients initially have a pure nerve palsy, but contracture of the antagonist muscle results in secondary mechanical restriction of movement. Suction cup devices have been developed for examiners who are wary of using toothed instruments at the limbus ; A cotton swab may be a sufficient tool in some patients. Forced duction testing of oblique muscles may be performed, but two forceps are used and the globe is depressed forcibly into the orbit. To test tension in the superior oblique tendon, the globe is grasped in the limbal meridian of the tendon (the 2:30 o’clock position in the right eye, the 10:30 position in the left) and 180° away. The globe is rocked clockwise and counterclockwise over the taut tendon and the tendon tension is evaluated; experience with normal tendons is essential to obtain an accurate evaluation. To test tension in the inferior oblique muscle, the globe is grasped at the 4:30 position limbus and 180° away in the right eye, at the 7:30 position limbus and 180° away in the left eye; the globe is rocked similarly and the muscle tension evaluated.

73. Active Force Generation Test Active force generation testing may be used to evaluate the ability of a muscle to move the eye against a resisting force. The forceps is placed at the limbus of the anesthetized globe in the meridian of the muscle whose duction is limited and the patient requested to rotate the eye in the direction of the limited duction ; the examiner judges through the forceps the relative amount of force generated. Strain gauges have been devised that enable quantitation of this force. In Fig-patient has maximal adduction in the left eye. The examiner can gauge the relative abduction strength as the patient attempts to move her eye to theleft .

74. Special Tests Motor Tests Special motor tests include forced ductions , active force generation, and saccadic velocity. Saccadic velocity can be recorded using a special instrument that graphically records the speed and direction of eye movement This test is useful to differentiate paralysis from restriction A paralyzed muscle generates a reduced saccadic velocity throughout the movement of the involved eye, whereas a restricted muscle produces an initially normal velocity that rapidly decelerates when the eye reaches the limit of its movement

75. Assesment of the field of Single binocular vision May be tested on either a Goldmann perimeter or a tangent screen. These tests are useful for following the recovery of a paretic muscle or for measuring the outcome of surgery to alleviate diplopia. A small white test object is followed by both eyes in the various cardinal positions throughout the visual field. When the patient indicates that the test object is seen double,the point is plotted.

76. The examiner then repeats the same procedure until he or she has plotted the entire visual field,noting the area in which the patient reported single vision and the area of double vision. The field of binocular fixation normally measures about 45deg – 50deg from the fixation point except where it is blocked by the nose

77. 3-Step Test The 3-step test is an algorithm that can be used to help identify the paretic cyclovertically acting muscle. There are 8 cyclovertically acting muscles: 4 work as depressors (2 in each eye), and 4 work as elevators (2 in each eye). The 2 depressors of each eye are the inferior rectus (IR) and superior oblique (SO) muscles; the 2 elevators of each eye are the superior rectus (SR) and the inferior oblique (IO) muscles.

78. Step 1 If the case is one of paralysis, determine which eye is hypertropic by using the cover-uncover test). Step 1 narrows the number of possible underacting muscles from 8 to 4. Draw an oval around these 2 muscle groups

79. Step2 Determine whether the vertical deviation is greater in right gaze or in left gaze. In this example, the deviation is larger in left gaze.

80. This implicates 1 of the 4 vertically acting muscles used in left gaze. Draw an oval around the 4 vertically acting muscles that are used in left gaze At the end of step 2, the 2 remaining possible muscles (1 in each eye) are both intortors or extortors and both superior or inferior muscles (l rectus and 1 oblique).

81. Step 3 Known as the Bielschowsky head-tilt test, Final step involves tilting the head to the right and then to the left during distance fixation Head tilt to the right stimulates intorsion of the right eye (RSR, RSO) and extorsion of the left eye (LIR, LIO). Head tilt to the left stimulates extorsion of the right eye (RIR, RIO) and intorsion of the left eye (LSR, LSO). Normally, the 2 intortors and the 2 extortors of each eye have opposite vertical actions that cancel each other. If 1 intortor or 1 extortor is weak, it cannot act vertically,and the vertical action of the other ipsilateral torting muscle becomes manifest.

82. To continue with example When head is tilted to right,the right eye moves upward as it attempts to intort to maintain fixation,increasing vertical deviation This suggest vertical action of right superior rectus muscle is unoppsed,indicating right superior oblique is weak and paretic

83. Prism Adaptation Test The patient is fitted with prisms of sufficient magnitude to permit alignment of the visual axes. In many cases, this step provokes a restoration of sensory binocular cooperation in a form of fusion and even stereopsis. This technique simulates orthotropia and possibly offers some predictive value of whether fusion may be restored when the patient undergoes surgical alignment.

84. In some patients, however (especially those with acquired esotropia ), placement of such prisms increases the deviation. In such cases, anomalous retinal correspondence based on the objective angle may drive the eyes to maintain this adaptive alignment even with prismatic correction. After wearing such prisms, the patient returns with a greater angle of deviation.

85. The patient is reexamined every 1-2 weeks and given larger prism correction, if needed, until the deviation no longer increases. Surgery is then performed on the new, larger, prism-adapted angle. The Prism Adaptation Study demonstrated a smaller undercorrection rate, about 10%, when surgery was based on this deviation compared to standard surgery

86. references Ophthalmology, Yanoff . 4 th Edition American academy of Ophthalmology( paediatric ophthalmology and strabismus), 2016-2017 Clinical ophthalmology, 8 th Edition, Kanski Albert & Jakobiec , volume 4, 3 rd Edition Strabismus simplified,2 nd edition,Pradeep Sharma

87. Thank you

88. Because the right superior oblique is weak, the vertical action of the right superior rectus is unopposed, Contraction of this muscle in an attempt to incycloduct the eye results in an upward movement of the right eye, thus increasing the vertical deviation. Because the oblique muscles are minor elevators and depressors, the difference in vertical alignment of the eyes will be smaller during head-tilt testing when there is a paresis of the vertical rectus muscles as compared to the oblique muscles.