Paralytic Strabismus presentation slideshare.pptx

Paralytic Strabismus

1. PARALYTIC STRABISMUS NAOBA MUTUM M1

2. DEFINITION : Paralytic Strabismus refers to ocular deviation resulting from complete or incomplete paralysis of one or more extraocular muscles. Complete paralysis is also called palsy and incomplete paralysis is called paresis.

3. ETIOLOGY In many situations, it may not be possible to pinpoint the causes. The lesion may be neurogenic (supranuclear, internuclear, nuclear, or infranuclear), myogenic or at the level of neuromuscular junction. The common causative lesions can be grouped as follows I. Neurogenic lesions A . Congenital lesions: Hypoplasia or absence of nucleus is a known cause of third and sixth cranial nerves palsy. Birth injuries may mimic congenital lesions.

4. B . Inflammatory lesions: These may be in the form of encephalitis, meningitis, neurosyphilis or peripheral neuritis (commonly viral). Nerve trunks may also be involved in the infectious lesions of cavernous sinus and orbit. C. Neoplastic lesions: These include brain tumours involving nuclei, nerve roots or intracranial part of the nerves; and intraorbital tumours involving peripheral parts of the nerves D. Vascular lesions: These are known in patients with hypertension, diabetes mellitus and atherosclerosis. These may be in the form of haemorrhage, thrombosis, embolism, aneurysms or vascular occlusions. Cerebrovascular accidents are more common in elderly people

5. E. Traumatic lesions: These include head injury and direct or indirect trauma to the nerve trunks. F. Toxic lesions: These include carbon monoxide poisoning, effects of diphtheria toxins (rarely), alcoholic and lead neuropathy. G. Demyelinating lesions: Ocular palsy may occur in multiple sclerosis and diffuse sclerosis.

6. II. Myogenic lesions A. Congenital lesions : These include absence, hypoplasia, malinsertion , weakness and musculofacial anomalies of extraocular muscles. B. Traumatic lesions : These may be in the form of laceration, disinsertion, haemorrhage into the muscle substance or sheath and incarceration of muscles in fractures of the orbital walls. C. Inflammatory lesions : Myositis is usually viral in origin and may occur in influenza, measles and other viral fevers.

7. D. Myopathies: These include, thyroid myopathy, carcinomatous myopathy and that associated with certain drugs. Chronic progressive external ophthalmoplegia (CPEO) is a bilateral myopathy of extraocular muscles, which may be sporadic or inherited as an autosomal dominant disorder. III. Neuromuscular junction lesion : It includes myasthenia gravis. The disease is characterised primarily by fatigue of muscle groups usually starting with the small extra ocular muscles, before involving other large muscles .

8. 1.Diplopia : It is the main symptom of paralytic squint. It is more marked towards the action of paralysed muscle. It may be crossed (in divergent squint) or uncrossed (in convergent squint). It may be horizontal, vertical or oblique depending on the muscle paralysed . Diplopia occurs due to formation of image on dissimilar points of the two retinae . CLINICAL FEATURES

9. 2. Confusion . It occurs due to formation of image of two different objects on the corresponding points of two retinae following misalignment of the visual axes of two eyes . 3. Ocular deviation In paralytic strabismus, the primary ocular deviation is incomitant and differs from the secondary deviation. However, with the passage of time there occurs spread of comitance . Primary deviation: It is deviation of the affected eye, when the unaffected eye is used for fixation and is away from the action of paralysed muscle, e.g. if lateral rectus is paralysed , the eye is convergent.

10. Onset of paralytic ocular deviation may be of sudden as seen in trauma and vascular occlusions; or gradual as seen in tumours and multiple sclerosis. Incomitance depending upon gaze . The angle of deviation changes with the direction of gaze. It is greatest in the direction in which maximal activity is required for the involved muscle, i.e. in the diagnostic position of the muscle. For example, in a patient with paresis of right lateral rectus muscle during levoversion , no or little esotropia may be noted in right eye While in primary position, right esotropia is marked, which will become maximum in dextroversion , i.e. deviation will be maximum in the field of action of right lateral rectus muscle.

11. Secondary deviation . It refers to deviation of the unaffected eye seen under cover. When the patient is made to fix with the affected eye. In a recently acquired ocular palsy, the secondary deviation is much greater than the primary deviation . In a recently acquired ocular palsy, the secondary deviation is much greater than the primary deviation. This is due to the fact that the strong impulse of innervation required to enable the eye with paralysed muscle to fix is also transmitted to the yoke muscle of the sound eye resulting in a greater amount of deviation. This is based on Hering's law of equal innervation of yoke muscles.

12. In a long-standing ocular palsy, the secondary deviation is often much the same as the primary deviation because completion of the muscle sequelae leads to comitance Spread of comitance . A paralytic deviation undergoes several stages. The first stage is characterized by weakness of the paralyzed muscle during which secondary deviation is much more than the primary deviation as described above. Almost immediately following the paresis of an extraocular muscle, the direct antagonist begins to overact.

13. A contracture of the antagonist muscle will develop within days to weeks, if the patient fixes with the unaffected eye. At this point, the deviation may become greater in the field of action of antagonist than it is in the field of action of weak agonist muscle. During the next stage, the deviation will spread into all fields of gaze and become increasingly comitant , i.e. there occurs a gradual spread of comitance to all fields of gaze. Ultimately, it may then no longer be possible to determine the nature of original deviation since the secondary deviation is often much the same as the primary deviation.

14. In most cases, spread of comitance occurs within a few weeks, months or even one to two years after the onset of paralysis. However, the spread of comitance is not a rule; in some cases of paralytic squint, it may not occur at all. Spread of comitance is quite common in infants with sixth nerve palsy, and ultimately patient may present with a comitant esodeviation

15. 4. Ocular movements . Restriction of ocular movements occurs towards the direction of action of the paralysed muscle/muscles. When the paralysis is of recent onset, a careful study of duction and version movements will make the diagnosis on the basis of incomplete movements in the field of action of the paralysed muscle. However, in long-standing cases, development of muscle sequelae,such as contracture of the direct antagonist muscle and secondary inhibitional palsy of the contralateral antagonist muscle, present difficulties in the identification of the paralysed muscle.

16. Under these circumstances, other tests like head-tilt test and Hess screen charting, etc. may be helpful in diagnosing the paralysed muscle. 5. Past Pointing Past pointing also described as false projection or orientation occurs due to increased innervational impulse conveyed to the paralysed muscle during movement in the direction of action of paralyzed muscle. It can be demonstrated by asking the patient to close the sound eye and then to fix an object placed on the side of action of paralysed muscle. Patient will locate it further away in the same direction.

17. For example, a patient with paralysis of right lateral rectus will point more towards right than the object actually is . 6. Nausea, vertigo and dizziness . Nausea, vertigo and dizziness result from diplopia, confusion and false localization. These symptoms are more prevalent in vertical and torsional diplopias than in horizontal diplopia. They do not occur in patients with congenital defects and disappear quickly in children. Adults adapt more slowly.

18. 7. Muscle sequelae. Muscle sequelae refer to changes that take place in the extraocular muscles after some time of the paralysis or paresis of one or more of the extraocular muscles. The speed and extent to which they develop in different patients vary markedly. The exact reasons for this are unknown, but the speed and degree of their development depend partly on which eye the patient uses for fixation.

19. Mostly, the patients use sound eye, however, sometimes the paretic eye may be used for fixation if: (1) it has better visual acuity, (2) it is originally dominant eye, or (3) fixation with paretic eye increases the separation of the double picture (due to secondary deviation) which causes less problem. Muscle sequelae occur to much lesser degrees in patients with congenital paralysis as compared to the acquired paralysis.

20. These occur more in paralysis due to lesions of the nerves than the lesions of muscles and include the following: 1. Overaction of the contralateral synergistic (yoke) muscle Overaction of the yoke muscle develops quickly, when paretic eye is used for fixation and slowly, when the sound eye is used for fixation. This overaction of the yoke muscle is responsible for secondary deviation of the sound eye. With the passage of time, this overaction becomes habitual due to the development of spasm and contracture and remains, even if the original paresis should recover spontaneously.

21. SR IR IO SO IO SO SR IR LR LR MR

22. 2. Contracture of the direct antagonist . After paralysis of a particular extraocular muscle, its direct antagonist is more or less unopposed and thus overact and is responsible for the primary deviation. Within a few weeks, the overacting muscle becomes spastic, contracting more and more leading to a greater angle of deviation. Eventually, this leads to a contracture, an organic change in the muscle in which muscle fibres are replaced by fibrous tissue.

23. 3. Secondary inhibitional palsy of the contra lateral antagonist muscle. It is a manifestation of Hering's law of equal innervation. Since the direct antagonist of the paretic muscle is more or less unopposed, so it will require less than normal innervation for a particular extent of a movement. According to Hering's law, the same innervation will flow to its yoke muscle (which is contralateral antagonist of the paretic muscle). Consequently, the contralateral antagonist of the paretic muscle will exhibit a weakness; which has been called the secondary inhibitional palsy of the contralateral antagonist muscle.

24. Perhaps the better term will be 'simulated weakness of the yoke's antagonist' or PAY syndrome: pseudo weakness of the antagonist of a yoke . This underaction of the yoke muscle of the antagonist occurs earlier and is more pronounced, when the paretic eye is used for fixation than when the sound eye is preferred for fixation.

25. Fig. Muscle sequelae following paresis of EOM

26. Fig. Muscle sequelae following paralysis of left superior oblique muscle.

27. Fig. Muscle sequelae following paralysis of right lateral rectus muscle

28. 8. Abnormal head posture . An abnormal head posture is a common feature of the paralytic strabismus. A compensatory head posture does not necessarily develop in every patient with a paresis or paralysis of extraocular muscles. However, when present, it can aid in making the diagnosis. Reasons for abnormal head posture: Abnormal head posture may be adapted for any of the following two reasons: To achieve BSV: Most frequently, an abnormal head posture is adapted to achieve binocular single vision, i.e. to avoid the troubling diplopia and/or confusion.

29. For this purpose, the head is turned into the field of action of the paralysed muscle, and that the eyes are directed by the doll's head phenomenon. This process allows the patient to move his/her limited field of single vision so that it coincides with his/her egocentric (straight ahead) position. In other words, the patient can see the things in front of him/her as single . 2. To achieve wide separation of the two images: Less frequently, patients with paralytic strabismus develop abnormal head posture in order to increase the separation between diplopic images.

30. This occurs in patients who have no useful field of bifoveal single vision but suffer from the constant diplopia. Since they have no choice but to live with it, they attempt to separate the double pictures as far as possible by turning the head in the field of paretic muscle. In this field, the deviation of the involved eye will be maximal and, thus, the 'true' and the 'false' image of objects in front will be maximally separated.

31. COMPONENTS OF ABNORMAL HEAD POSTURE A. In horizontal rectus muscle palsy. If one of the horizontally acting muscles (lateral or medial rectus) is involved, the abnormal head posture will consist of only one component, i.e. face turn towards the action of paretic muscle. For example, in a paresis of right lateral rectus muscle, there will be a face turn to the right side, and in a paresis of right medial rectus muscle, there will be a face turn to the left side.

32. B. In palsy of cyclovertically acting muscles. The superior and inferior recti and the superior and inferior oblique muscles are cyclovertically acting muscles and contraction of any one of them alone would produce a combination of vertical, horizontal and torsional movements. If any one of these muscles is paretic, there will be three components of abnormal head posture as follows: 1. Chin elevation or depression 2. Face turn 3. Head tilt

33. Chin elevation or depression occurs in paralysis/paresis of elevators or depressors of the eye, respectively. By doll's head phenomenon, in chin elevation, the eyes move down and in chin depression, the eyes move up. In this way, the involved eye is brought out of the field of action of paretic elevator or depressor muscle. 2. Face turn : As mentioned above, face turn in the paresis of a horizontally acting muscle (medial/lateral rectus) is towards the action of the paretic muscle. However, in the paresis of one of the cyclovertically acting muscles, the face turn is such that the eyes are brought away from the field in which the muscle has its greatest vertical effect.

34. Thus in the case of superior and inferior recti which have their maximal vertical effect in abduction, the face is turned so that the involved eye is adducted, when the patient looks straight ahead, i.e. in paresis of the vertical recti, the face will be turned towards the affected eye. Since the superior and inferior oblique muscles have their greatest vertical effect in adduction, the opposite is true for them and the face is turned so that the eye with the involved oblique is abducted, i.e. face is turned away from the affected eye (towards left in paresis of oblique muscle of right eye).

35. 3. Head tilt . The head tilt occurs to compensate for the torsion or to help relieve the vertical separation of the double images as follows: • In paresis of a oblique muscle, head tilt occurs to compensate the torsion caused by the direct antagonist of the paralysed muscle. For example, In paresis of right superior oblique, the head will tilt towards left to compensate for the extorsion caused by right inferior oblique muscle.

36. In paresis of a vertical rectus muscle, the head tilt occurs to compensate the torsion caused by the contralateral antagonist of the paralysed muscle. For example, in paresis of right superior rectus, the head will tilt to the right to compensate for the extorsion of the left eye caused by overacting left inferior oblique muscle

37. Fig. Abnormal head posture in EOM Paralysis.

38. OCULAR TORTICOLLIS The abnormal head posture adopted by the individuals with congenital and infantile paralytic squint is sometimes referred to as acquired ocular torticollis, because it resembles an orthopaedic deformity called congenital torticollis. In the later condition, the head cannot be straightened due to organic changes in the neck musculature. In contrast, the ocular torticollis is merely a functional position and the head can be straightened passively. In the early stages, if one eye is occluded, the patient with ocular torticollis will straighten the head.

39. However, in late stages, secondary scoliosis may occur as a consequence of ocular torticollis and because of it, patient's head may not straighten with monocular occlusion. Further, if secondary vertebral column changes have been allowed to develop, it may no longer be possible to correct the abnormal head posture even by surgically aligning the eyes. Facial asymmetry due to atrophy of the lower side of the face is a feature of both long-standing ocular as well as congenital torticollis, so it cannot be considered a differentiating feature

40. 9. Sensory adaptations. Sensory adaptations such as suppression, abnormal retinal correspondence and amblyopia are, in general, less known with paralytic squint vis-a-vis concomitant squint; perhaps , because of the following reasons: • Patients with paralytic squint can assume abnormal head posture to achieve a single binocular vision which may prevent the occurrence of suppression, ARC and amblyopia. • Patients with paralytic squint have variable angle of deviation in various positions of gaze; while sensory adaptations usually develop in patients who have stable and constant angle of deviation.

41. With the passage of time, the deviation becomes increasingly comitant , as discussed underspread of comitance . Under such circumstances, the patient is unable to maintain fusion in any direction of gaze, and as a result, suppression, ARC and amblyopia become established. Occurrence of comitance in paralytic squint is common but not universal. If the strabismus remains incomitant and onset is during childhood, diplopia in the paretic field of fixation may be prevented by regional suppression. Amblyopia occurs in only those patients of paralytic squint who are unable to maintain simultaneous binocular vision in any direction of gaze and in whom paralysis occurs in early life.

42. Further, sometimes, presence of amblyopia may cause confusion in the diagnosis of paralytic squint. This is because some patients, who prefer to fixate with the paretic eye because of certain reasons mentioned earlier, develop amblyopia in the non-paretic deviated eye.

43. INVESTIGATION OF INCOMITANT SQUINT It should include: evaluation from strabismic point of view and (2) investigations to find out the cause of incomitant squint, such as orbital ultrasonography, orbital and skull computerized tomographic scanning and detailed neurological investigations

44. A detailed history should be taken with reference to following points: 1. Subjective symptoms • Diplopia. Enquiry should be made to ascertain: Onset, constant/intermittent, distance at which diplopia is noticed, relative position of images, field where greatest separation of images occurs, any change since onset, does diplopia disappear, when eye is occluded. HISTORY

45. • Confusion. It occurs due to formation of images of the different objects on the corresponding points of two retina. • Other subjective symptoms which a patient with paralytic strabismus may experience are: Difficulty in focusing, headache, eye strain, general asthenopic symptoms and discomfort from abnormal head posture. 2. Objective symptoms • Constant/intermittent deviation • Abnormal head posture • Ptosis, exophthalmos 3. Any attributed cause 4. Any previous ocular problems and treatment taken 5. General health 6. Family history

46. INSPECTION 1.Ocular posture 2.Abnormal head posture; note its exact components 3.Facial asymmetry 4.Ptosis, exophthalmos

47. Cover test It should be carried out for near and distance; with and without abnormal head posture. The cover test will detect: 1.Presence of any manifest or latent deviation. 2.Type of deviation 3.Incomitance—primary versus secondary deviation 4.Normally fixing eye. Patient usually fixes with the nonaffected eye; but this may be influenced by visual acuity or dominant eye.

48. Ocular movements Investigation of ocular movements is carried out while the patient watches a fixation target, i.e. moved from the primary position into each of the cardinal positions of gaze. Version movements. The examiner compares the movement of the two eyes in all positions of gaze. Symmetric movement indicates that no defect is present. Unequal movements are seen in underactions , overactions and limitations. 2. Duction movements. Monocular movements are of value only in differentiating between a paresis and a total paralysis. Testing for ductions also helps to detect mechanical limitation of movements. 3. Doll's head movements and command move ments testing is of particular use in supranuclear gaze palsies.

49. MEASUREMENT OF DEVIATION Synoptophore method. Major amblyoscope is the best instrument for measurement of deviation in paralytic squint; since measurements are taken to compare the size of the deviation in each of the cardinal directions of gaze while each eye in turn is used for fixation. To make the comparison valid, it is extremely important that the fixation object be moved an equal distance from the primary position in each direction. For this, synoptophore can be adjusted so that the deviation can be measured while the patient is looking at an equal angle from the primary position in all directions of gaze.

50. 2. Prism and cover test. It is an easy method, while carried out with the help of a prism bar. Measurements should be taken with and without abnormal head posture for near and distance fixation, fixing either eye. Measure ments can be made in all the cardinal directions, but for comparison these are not considered very accurate. Since it is not possible to measure at an equal angle from the primary position in all directions; as is possible with the synoptophore .

51. 3. Measurement of torsional deviation can be made with special slides on major amblyoscope , or on adapted Lees screen. Note. In a paresis or paralysis of an extraocular muscle, the deviation will be greatest in the direction of maximal singular action of the muscle while the affected eye is fixating.

52. BIELSCHOWSKY THREE STEP TEST (B3ST)/ PARK’S THREE STEP TEST The classical head tilt test was proposed by Bielschowsky to differentiate between superior oblique palsy in one eye and superior rectus palsy in the contralateral side. However, presently in practice is the three step test as modified by Parks’. It is useful in diagnosing the paresis of any cyclovertically acting muscle. There are in total 8 cyclovertically acting muscles: 4 work as depressors of the eyes, and 4 work as elevators. The two muscles on each eye that are responsible for depression are the inferior rectus and superior oblique, and the two muscles on each eye that are responsible for elevation are the superior rectus and the inferior oblique.

53. As expected, at the onset of a cyclovertical muscle palsy, there will be limitation in the field of action of the paralysed muscle. Shortly thereafter, an overaction in the field of the antagonist muscle will be noted. With time, this overaction will produce a contracture of the antagonist. Thereafter, there will be spread of comitance , so that the amount of deviation will gradually increase and become approximately the same in the all fields of gaze. At this point, based on analysis of duction and version movements of the eye, the diagnosis of cyclovertical palsy becomes impossible. At this juncture, the Parks' modification over Bielschowsky's head tilt test can be quite useful. There are three steps of this test, each of which eliminates half of the remaining potential muscles, leaving only one muscle to be blamed after the three steps.

54. PROCEDURE OF THREE – STEP TEST Step 1 • Perform cover-uncover test in primary position and determine which eye is hyper tropic. If the patient's presenting sign is a hypodeviation , consider it hyperdeviation of the opposite eye. Step 1 reduces the number of affected muscles from 8 to 4. • A right hypertropia (RHT) implies any of the following: – Weakness of depressors of right eye (RIR, RSO), or – Weakness of elevators of left eye (LIO, LSR). • A left hypertropia (LHT) implies any of the following: – Weakness of depressors of left eye (LIR, LSO), or – Weakness of elevators of right eye (RIO, RSR).

55. • Let us assume, for example, the patient being examined has LHT. Draw an oval (with red lines) around the two possible muscle pairs responsible for LHT Step 2 • Determine whether hypertropia (HT) is larger in right gaze or left gaze. • If the LHT is larger in right gaze, it implies weakness of any of the 4 vertically acting muscles in right gaze: – RSR, RIR – LIO, LSO • If the LHT is larger in left gaze, it implies weakness of any of the 4 vertically acting muscles in left gaze, i.e. – LSR, LIR – RIO, RSO

56. Step 3 • Determine, if the HT is larger, when measured during head tilt to the left or right. For proper measurement, the base of the prism should be held parallel to the floor of the orbit and not parallel to the floor of the room. The Maddox rod and correcting prism should be held so that the line and base are parallel to the floor of orbit. If the LHT is larger, when the head is tilted to the right, this implicates any of four muscles that act vertically in right tilt position, i.e. either intorters of right eye (RSR, RSO) or extorters of left eye (LIR, LIO).

57. If the LHT is larger, when the head is tilted to the left, this implicates any of the four muscles that act vertically in left tilt position, i.e. either intorters of left eye (LSR, LSO) or extorters of right eye (RIR, RIO).

58. Fig . Bielschowsky's three-step test (B3ST) in a patient with left superior oblique paralysis. A, step 1; B, step 2; C, step 3

59. Fig. Measurement of deviation using Maddox rod and prism in a patient with right hypertropia. Note, head is tilted to the right and base of the prism is held parallel to the floor of the orbit. Maddox rod is held in such a way that the red line seen is also parallel to the floor of the orbit.

60. Limitations of Park's three-step test This test is quite useful in general, but it is not always diagnostic and can be misleading, especially during following conditions: • In cases of long-standing paresis. • When more than one muscle are paretic, e.g.– Bilateral fourth nerve palsy– Multiple other muscle weakness • In cases with restrictions.– Superior rectus overaction– Superior rectus contracture– Inferior restriction • Dissociated vertical deviation (DVD) • Pulleyheterotopia • Superior rectus palsy • Skew deviation • Prior extraocular muscle surgery

61. DIPLOPIA CHARTING Plotting of diplopia fields is indicated in patients complaining of confusion or double vision. The test is easy to perform provided the patient is co-operative. To perform the diplopia charting, patient is asked to wear red-green diplopia charting goggles; red glass being in front of the right eye and green in front of the left eye. The patient is made to sit with his/her head straight in a semi - dark room and is shown a fine linear light from a distance of 4 ft. The light is moved from primary position into all of the other eight directions of gaze.

62. For each direction, patient is asked to comment on the position, brightness and separation between the red and green images. From the patient's comments, the examiner notes the following points: • Whether horizontal diplopia is homonymous or heteronymous. • Whether the image seen by right eye (red image) is higher or lower than the image seen by the left eye (green image) or vice versa. • In which direction of gaze, separation between red and green images is greatest. • Whether there are any directions in which fusion is present.

63. In a modified test of Franchchetti , instead of red green goggles, a red Maddox rod is placed in front of right eye and white Maddox rod in front of the left eye and the patient fixates on a spotlight which is seen as vertical red line with right eye and vertical white line with left eye. Disadvantages of diplopia plotting test • This test is only qualitative, therefore, it is not possible to comment on the minor changes of the improvement or deterioration from the records of different dates in the same patient. The test requires intelligent patient, especially to comment where the separation is maximum. • It is not possible to perform the test in colour blind patients.

64. • This test is not of use in congenital palsies and those of long-standing onset, because due to deep suppression diplopia cannot be elicited.

65. QUANTITATIVE MEASUREMENTS OF EXTRAOCUOLAR MUSCLE ACTIONS The quantitative measurement of extraocular muscle action is most essential to comment about the paretic muscles and the pathological sequelae of the paralysis, viz. overaction, contracture and secondary inhibitional palsy. Commonly employed tests to have a graphic record of the relative power of extraocular muscles in all directions of gaze are as follows: • Hess screen test • Lees screen test • Lancaster red and green test

66. HESS SCREEN TEST Principle The Hess screen test is based on the haploscopic principle. It utilizes the Hering's law of equal innervation, which states that in all voluntary movements of the eye, equal and simultaneous innervation flows from the brain to the muscles of both eyes concerned in the respective direction of gaze (yoke muscles).

67. Prerequisites Patient should have: 1. Full understanding about what he/she is supposed to do, since the test is purely subjective. 2. Good vision in both eyes. 3. Central fixation. 4. Normal retinal correspondence.

68. Discription of conventional Hess screen Original Hess screen Consisted of a single tangent screen made up of a black cloth 3 ft wide × 3½ ft long, marked by a series of horizontal and vertical red lines. The distance between each line subtends a visual angle of 5°. Fixation points are indicated at the centre of the screen and at the intersections of the 15° and 30° lines by red dots. Thus, the red dots form an inner square of 8 dots along the 15° lines and an outer square of 16 dots along the 30° lines. The inner square represents the 8 cardinal directions of gaze and the outer square the extreme directions of gaze.

69. In the original Hess screen, Indicator consists of a knot tying three green cords together to form the letter Y. The end of central vertical green cord is fastened to a movable black rod 50 cm long. The ends of the other two green cords, forming upper two limbs of the letter Y, are kept taut by black threads that pass through loops to small weights at corresponding upper corners of the screen. This arrangement enables the patient to move the indicator freely and smoothly over the whole surface of the screen in all directions .

70. Procedure Hess screen test The patient wears red-green goggles and sits 50 cm from the commonly used modified wooden Hess screen. The patient now sees the fixation points (red light) with one eye and the indicator (green light of projector) with the other eye. The patient is asked to superimpose the indicator successively on each of the fixation points, and the relative position of the eyes is plotted for each of these directions of gaze on a chart which is replica of the Hess screen.

71. Diagnostic interpretation of the Hess chart The diagnostic interpretation of the Hess chart is done by comparing the two fields, i.e. one, of the left eye plotted while the right eye fixing and other, of the right eye plotted while the left eye is fixing. The interpretation should be done as follows: 1. Compression of the space between the two plotted fixation points indicates underaction of a muscle acting in that direction. 2. Expansion of the space between the two plotted fixation points indicates overaction of the muscle acting in that direction. 3. Smaller field belongs to the eye with the paretic muscle.

72. 4. Non-affected eye shows the larger field expressing the overaction of the contralateral synergist . 5. Fields of similar shape and size are suggestive of comitant deviation, while the fields of dissimilar shape and size indicate incomitance. 6. In the smaller field, the greatest displacement (compression) away from the normal cardinal direction will indicate the paretic muscle underaction ). In many cases, displacement in the direction of the field of the antagonist (due to contracture) may also be seen. 7. In greater field, the greatest displacement (expansion) away from the normal cardinal direction will indicate the overacting muscle (contralateral synergistic or yoke muscle of the paretic muscle). In many cases (especially in those of long duration), there may also be displacement of the field away from the

73. direction of the antagonist of this muscle (due to inhibitional palsy of the contralateral antagonist). LANCESTER RED GREEN TEST The Lancaster red-green test is a haploscopic test. It utilizes a Lancaster red-green screen which is window-shade type of screen that can be rolled up when not in use. The screen contains horizontal and vertical lines forming squares of 7 cm . All the squares are of the same size and the tangential error is not taken into account. While performing the test, the patient's eyes should be in level with the centre zero mark, and he/she can be seated at either 1 or 2 metres . At 2 metres , each square subtends an angle of 2 = 3.5 ; at 1 metre it subtends an angle of 4 =7  

74. The patient is given a red-green reversible goggles (e.g. red glass in front of right eye and green glass in front of left eye) and green flashlight that projects a linear image. The examiner has a similar red flash light and projects the red streak of light on the zero mark on the screen. The patient is asked to superimpose his/her green light on the examiner's red light. This is then repeated in all cardinal directions of gaze. The distance between the streaks of light represents the measurement of the objective deviation provided retinal correspondence is normal. The results are plotted on a chart that is an exact replica of the screen.

75. Since the projected image is a line, the patient's response may indicate the presence of cyclotropia , when his/her streak is tilted. This test is most useful in patients with ocular paralysis and least useful in patients with heterophoria or intermittent heterotropias .

76. DIFFERENTIAL DIAGNOSIS OF INCOMITANT SQUINT Differential diagnosis to be considered in patients with incomitant squint in general are as follows: • Comitant (non-paralytic) versus incomitant (paralytic) squint. •Congenital versus acquired palsies. •Paralytic versus restrictive incomitant squint.

77. COMITANT( NON – PARALYTIC ) VERSUS INCOMITANT SQUINT

78. CONGENITAL VERSUS ACQUIRED OCULAR PALSY

79. COMMONLY EMPLOYED TESTS TO DIFFERENTIATE BETWEEN PALSIES AND RESTRICTIONS . 1. Passive forced duction test (traction test) Steps of the forced duction test (FDT) i . Anaesthesia . In adults and cooperative elder children, FDT can be performed preoperatively under topical anaesthesia with 4% xylocaine instilled every 4 minutes for 4 times. In small and uncooperative children, FDT is done under general anaesthesia during surgery, taking an account of following points:

80. – To remove the effect of tonic innervational factors, the FDT should be performed, when patient has reached stage 3 of anaesthesia – If succinylcholine is to be used, preferably the FDT should be performed while the patient has received an inhalation anaesthetic by mask, but before intubation. Otherwise one will have to wait for at least 20 minutes till the contraction of the extraocular muscles caused by succinylcholine is over. – Pancuronium , a nondepolarizing muscle relaxant, that does not alter the FDT, should be preferred over succinylcholine.

81. ii. Grasping of the globe. After proper anaesthesia, the globe should be grasped near the limbus with either a forceps without teeth or Pierse forceps to avoid tearing of the conjunctiva. Preferably, the globe should be held with the help of two forceps at right angle to the axis in which restriction is to be tested. iii. Passive rotation of the globe. After grasping, the globe should be rotated passively towards the direction of action of suspected weak muscle, e.g. into abduction in patients with lateral rectus weakness versus mechanical restriction involving medial aspect of the globe, taking following precautions:

82. When FDT is being performed under topical anaesthesia, patient should be instructed to look at his/her hand held in the direction in which the eye is to be rotated by the forceps. This will help in avoiding the effect of tonic innervational factor. – Care should be taken not to push the globe into the orbit posteriorly, since this may conceal a restriction of the movement resulting in a false negative FDT. • To test the restrictions in the field of action of recti, the globe should be rotated, up, down, medially or laterally. • To test the restrictions in the field of action of oblique muscles, the globe should be rotated both down and in, and up and in.

83. INTERPRETATION OF FORCED DUCTION TEST 1. Forced duction test is labelled negative , if no resistance is encountered during passive rotation and the examiner can rotate the globe to its full extent. A negative FDT implies that the motility defect is clearly caused by paralysis of the weak muscle. 2 . Positive FDT is labelled , if a resistance is encountered during passive rotation of the globe. With a feeling of resistance, if the examiner can rotate the globe no further than the patient voluntarily can, the motility defect is purely due to mechanical restriction.

84. However, with a feeling of resistance, if the examiner can passively rotate the globe beyond where the patient can voluntarily rotate it, but not to its full extent, the motility defect is a combination of mechanical restriction and agonist muscle weakness. The restriction noted in the positive FDT may be one of the following types : i . Leash restriction is caused by the mechanical factors such as marked scarring of Tenon's capsule and conjunctiva, contracture of an extraocular muscle and/or entrapment of muscle or its facial sheath on the side of globe opposite the limited field of rotation. The globe can be passively rotated freely up to a point after which tethering effect of restriction does not allow the globe to move further any more. Such a restriction is not only felt but can also be seen as a taut string of conjunctiva (String sign).

85. ii. Reverse leash restriction . The tethering effect of restriction is similar to leash restriction as described above. However, the mechanical factors responsible for tethering are marked shortening of conjunctiva and Tenon's capsule, marked posterior scarring of orbital tissues or a tight posterior fixation suture used in Faden's operation on the same side of globe in which rotation is limited. ii. Elastic restriction is caused by an early contracture of a muscle following paresis of its agonist, co-contraction of extraocular muscles due to effect of succinylcholine and orbital cellulitis. In contrast to the leash and reverse leash restriction (in which globe can be rotated to a point after which tethering effect of restriction does not allow the globe to move further any more), in elastic restriction there occurs a

86. partial resistance over the entire range of ocular movement which can be overcome by an increased force. 2. Exaggerated traction test : It is a modified forced duction test which is performed to estimate the tightness in superior oblique (SO) and inferior oblique (IO) muscles. Procedure: For checking tightness of RSO, the eyeball is grasped near the limbus at 6 and 9 O'clock positions, as described in FDT. To perform this test, the eyeball is first pushed in the orbit and then elevated, adducted and rolled back and forth by extorting and intorting the globe across the tendon. During this manoeuvre , if the eyeball jumps across the tendon, tightness of the superior oblique is indicated.

87. Tightness of the inferior oblique is also tested in the similar manner, except that instead of elevating and adducting, the eyeball is pushed down and nasally. 3. Spring-back balance test It is a continuation of the FDT, when performed under general anaesthesia. It is of specific use in patients who are suspected (after FDT) of having mechanical restriction and not a weak muscle. In this test, after holding near the limbus, eyeball is rotated back and forth vigorously for 2–3 times and then released suddenly. After settling, normally, the globe comes to rest in straight ahead position. However, in the presence of a significant mechanical restriction, the eyeball will be drawn towards the direction of the mechanical pull, e.g. the eyeball will be adducted, if the cause of mechanical restriction is located medially.

88. 4. Active force generation test In this test, eyeball is stabilized with the forceps applied at the limbus under topical anaesthesia and patient is asked to move his/her both eyes in the direction of the muscle to be tested. For example, if right lateral rectus muscle is to be tested, patient is asked to move his/her eyes in dextroversion . During this movement, the force generated by the contracting muscle of the eye being tested is transmitted through the forceps to the examiner's fingers. From the feel of the transmitted force, examiner can judge subjectively whether the contracting muscle is weak or normal .

89. For objectively quantifying this test, calibrated forceps are available which indicate the amount of force generated in grams. A normally acting muscle generates a force of 60–80 g in extreme gaze. This test is quite useful in diagnosing the weak muscle. However, it can only be performed in alert and co-operative patients. 5. Lid fissure changes on eye movements • Narrowing of lid fissure along with globe retraction is seen in restrictive squints, as in Duane’s retraction syndrome. • Lid fissure widening and a relative proptosis is noted in paralytic squint as the patient looks into the field of action of paretic rectus muscle.

90. 6. Electro-oculographic measurement of saccadic velocity Saccades are sudden, jerky conjugate eye movements, that occur as the gaze shifts from one object to another. These movements bring the object of regard quickly on the fovea with an average velocity of 250°/second in the field of action of the muscle concerned. Measurement of saccadic velocity with the help of specially designed electro-oculographic (EOG) recorder can help in differentiating muscle restrictions from the muscle weakness. The saccadic velocity is decreased in paretic muscle, while it is near normal in mechanical muscle restrictions.

91. 7. Positional tonometry It has been reported that intraocular pressure rises from the compression of a non-relaxing stiff muscle, when attempts are made to move the eye into the field of its antagonist. Perkin's hand held applanation tonometer or Digilab Pneumo tonometer can be used to measure the IOP in different gaze positions. A pressure increase of over 5 mm Hg in a particular field of gaze is indicative of a restriction