The document discusses paralytic strabismus, including: 1) Hering's law of equal innervation and Sherington's law of reciprocal innervation which are important in diagnosing paralytic strabismus. 2) The sequelae of ocular muscle palsy including overactions and underactions of muscles. 3) Methods for investigating incomitant strabismus including cover tests, motility examination, and Hess screen plots to identify the affected muscle.

1. Paralytic strabismus Movement of the eye, following contracture of a muscle can be considered in terms of the action of an individual, isolated muscle

3. this does not aid in the diagnosis of the affected muscle in paretic situation. the field of action of the muscle should be considered This is the direction in which the muscle’s primary position action has maximal effect.

6. Terms Agonist = prime mover or protagonist Antagonist= muscle having the opposed action Synergist = muscle having the same actions Ipsilateral = on the same side Contralateral = on the opposite side Contracture = increased resistance against passive stretching of the muscle, loss of elasticity

7. Hering’s Law of Equal Innervation equal and simultaneous innervation flows to the synergistic muscles concerned with the desired direction of gaze (applies to voluntary and involuntary eye movements) ie innervation of one eye is equal that of the other eye, resulting in movements of the two eyes that are equal symmetrical and parallel (normally) This law has great practical significance when used in the diagnosis of paralytic strabismus

8. Sheringtons Law of Reciprocal Innervation concerned with the co-ordination of muscle pairs of one eye. Muscle contraction does not increase simultaneously in opposed muscles ie the contraction of each ocular muscle is accompanied by a simultaneous and proportional relaxation of its antagonist.

9. Sequelae of Ocular Muscle Palsy Underaction of the primary affected muscle Overaction of the contralateral synergist Overaction of the ipsilateral (direct) antagonist Underaction of the antagonist of the contralateral synergist (contalateral antagonist) Overaction of the ipsilateral synergist??

10. Overaction of the contralateral synergist always present. This overaction occurs when the affected eye is fixing as a result of increased innervation being required to rotate the affected muscle into its field of action. Due to Herings Law an overstimulation of the contralateral synergist follows This is always the largest overaction in the sequelae.

11. Overaction of the ipsilateral (direct) antagonist can lead to a permanent contracture of the muscle and a loss of elasticity If the patient fixes with the non-involved eye within a few days a contracture will develop in the direct antagonist muscle because the normal contracture of the direct antagonist is unopposed by the weak muscle.

12. Underaction of the antagonist of the contralateral synergist (contalateral antagonist) with the involved eye fixing, the movement of the eye into the field of action of the weak muscles antagonist requires less innervation than normal due to the contracture. Therefore less innervation is supplied to the contralateral antagonist which under-acts.

13. Overaction of the ipsilateral synergist Duke Elder, 1947

14. For example in paralysis of the right superior rectus underaction of right superior rectus overaction of the left inferior oblique overaction of the right inferior rectus underaction of the left superior oblique (overaction of the right inferior oblique)

15. ANATOMICAL CONSIDERATIONS There are 6 extraocular muscles – 4 rectus muscles, 2 oblique muscles Length of each  40 mm, the inferior oblique being slightly shorter 5 muscles arise from the apex of the orbit, the inferior oblique arises form the inferonasal angle of the orbit The 4 recti muscles originate form the apex of the orbit at the level of the Annulus of Zinn The recti muscles are inserted in front of the ocular equator, the obliques are inserted behind Movements occur about 3 primary axes around the centre of rotation – the vertical, horizontal and saggital axes The action of a muscle depends on the angle of its plane and the anterio-posterior axis of the eye. It follows that the action of the muscle may vary according the positions of the globe in the orbit.

16. Medial Rectus

17. Lateral Rectus : abduction

18. Superior Rectus In the primary position the muscle plane of the SR forms an angle of 23  with the optical axis. In this position the SR elevates the globe. Secondary actions are intorsion and adduction. As the eye moves in adduction the SR is now a greater adductor and intortor In 67  adduction the SR is exclusively an intortor. In 23  abduction the SR is a complete elevator. The muscle plane coincides with the optical axis.

19. In primary position muscle plane of the SR forms angle 23°with optical axis. In this position SR Elevates the globe. Secondary Actions intorsion and adduction

21. In 23° abduction, SR complete Elevator, muscle plane coincides with optical axis

22. Superior Oblique Eye in the primary position, the muscle plane SO forms an angle of 54  with the optical axis. The primary action is intorsion The secondary action is abduction and depression. When the eye is adducted 54  the optical axis and muscle plane coincide and the SO is a pure depressor. In abduction the SO is primarily an intortor.

23. Eye in primary position Muscle plane SO forms angle 54° with optical axis. Action is intorsion Secondary actions abduction and depression RE

24. When globe adducted 54°, Optical axis and muscle plane coincide, SO is pure depressor RE

25. RE In abduction SO is primarily an intortor

26. Inferior Rectus With the eye in the primary position the IR forms an angle of 23  with the optical axis. Thus the relationship is the same as the SR except that it is an inferior rectus and therefore depresses the eye Secondary actions = extortion and adduction. As adduction increases the depressor ability decreases and extortion increases. In 67  abduction IR = exclusively extortor In 23  abduction IR = exclusively a depressor

27. Inferior Oblique When the eye is in the primary position, the muscle plane of the IO forms an angle of 51  with the optical axis. the relationship is almost the same as the SO, except by virtue of the muscles anatomical position, the action is extorsion. Secondary actions are abduction and elevation When the eye is adducted 51  the muscle is still an extortor but the elevator action increases In abduction the IO extortion ability increases.

28. For right eye read from the top For left eye read from the bottom

29. Rule for the secondary actions of the vertical muscles RADSIN = Recti ADduct Superiors INtort

30. INCOMITANT STRABISMUS : CLASSIFICATION AND INVESTIGATION Definition: a strabismus where the angle or degree of the deviation varies in different directions of gaze OR with each eye fixing (ie the secondary deviation is greater than the primary).

31. Classification According to the underlying cause: (i) neurogenic (ii) mechanical (iii) myogenic Also may be congenital or acquired

32. Congenital Usually isolated defects in otherwise healthy individuals Sometimes familial Specific cause unknown, presumed due to a developmental anomaly in the anatomy or functioning of the extra-ocular muscles or their innervating nerves Some are associated with serious developmental neurological defects eg hydrocephalus & cerebral palsy Gradually become more comitant as Px gets older Not likely to respond to orthoptic treatment

33. Acquired Caused by injury or disease of the ocular motor system eg trauma, inflammation, Vascular Space-occupying lesions Metabolic disorders NB It is vital that we distinguish between longstanding/ static acquired disorders and recent onset/active ones

34. INVESTIGATION OF INCOMITANCY History and Symptoms General observation of the patient Cover Test with and without AHP Motility Hess Screen Plots

35. History and Symptoms In general symptoms are likely to be marked and dramatic in recent onset incomitancy, with the time of onset known. In longstanding cases, they are less marked and intermittent due to the intervention of suppression, and the patient may not complain of symptoms. Diplopia – present in most incomitancy, usually with a vertical element. May only be present in one gaze direction. May be intermittent in longstanding cases. Establish type, gaze direction, constant/intermittent, associated symptoms, with or without Rx, diurnal variation, onset, any change since onset, previous history of diplopia, compensating factors, how troublesome.

36. History and Symptoms Abnormal head posture – patient more likely to be aware of this in recent onset. Blurred vision Difference in pupil size Other symptoms due to underlying disease (eg HA, malaise, weight loss, appetite changes, fatigue, muscle tremor etc) Injury to head or orbital regions may be reported – not necessarily recent. Includes during birth delivery (eg forceps) and surgical trauma (eg strabismus surgery)

37. General observation of the patient Note the presence of the following:

38. Abnormal head posture Purposes (I) to place the eyes in the position of least deviation to enable BSV (ii) to centralise the field of BSV (iii) to avoid direction of gaze where there is discomfort or pain (iv) occasionally to separate diplopic images widely (atypical)

39. Components Face turn – to the left or right. Chin elevation or depression Head tilt – occurs for two reasons

40. Face turn – to the left or right. This can indicate an anomaly of the medial rectus or lateral rectus muscle Places the eyes away from the direction of the underaction & into the position of the least deviation ie turn towards affected muscle.

41. Chin elevation or depression to place the eyes in position of least deviation eg in A and V syndromes to avoid discomfort eg chin is often elevated in Pxs with dysthyroid eye disease, who can find it uncomfortable to look up. Also in Browns syndrome.

42. Head tilt : occurs for 2 reasons in vertical deviations (without cyclotropia)) to level up the diplopic images In cyclotropia (mainly a function of the obliques) eg SO palsy results in excyclotropia due to loss of intorting effect, hence tilt away from affected side so does not have to intort IO palsy results in incyclotropia; head tilt towards affected side.

43. Head tilt : occurs for 2 reasons SR and IR palsies: direction of tilt is inconsistent; may be as result of overaction of the contralateral synergic muscle rather than primary affected muscle Head tilt may be combined with face turn and chin elevation/depression AHP is less obvious in longstanding deviations due to increased comitancy and suppression. Absence of AHP does not mean absence of incomitancy. AHP may remain due to habit.

44. Cover Test with and without AHP

45. Cover Test: comparison of deviation with each eye fixing primary deviation = non strabismic eye fixing Secondary deviation = strabismic eye fixing The detection of a larger movement in one eye than the other helps to identify the weak member of the yoke muscle pair. Due to Hering’s Law of equal innervation when one eye is covered the fixing eye will determine the amount of innervation transmitted to both eyes Excessive innervation is required to move and maintain the eye in the primary position

46. Cover Test: comparison of deviation with each eye fixing The same amount of innervation flows simultaneously to the yoke muscle in the sound eye and the angle of deviation is larger when the patient uses the eye with the weak muscle for fixation than when the patient fixates with the sound eye.

47. Motility – check Both eyes move smoothly and follow the target across the horizontal, and there is no narrowing of the palpebral apertures There is a corresponding lid movement accompanying the vertical movements, and there is no A and V pattern There is no restriction of the movement of the eye in any direction of gaze Aid detection of incomitancy by: asking for subjective doubling Observing corneal reflexes

48. Motility – check Observing symmetry of lid positions Using red/green diplopia goggles Combining with cover test in different positions of gaze Measure the degree of incomitancy by : PCT Maddox Rod Hess screen methods Past Pointing – due to a disturbance of absolute localisation

49. 3 step technique (more reliable in fairly recent paresis) Right Hypertropia Either RSO, RIR, LSR, LIO underacting

50. 3 step technique (more reliable in fairly recent paresis) R Hypertropia greater in dextroversion Either RIR, LIO underacting

51. 3 step technique (more reliable in fairly recent paresis) R Hypertropia greater in levoversion Either RSO, LSR underacting

52. 3 step technique (more reliable in fairly recent paresis) Tilt head to right shoulder RE central, LE depressed = LIO

53. 3 step technique (more reliable in fairly recent paresis) Tilt head to left shoulder LE central, RE elevated = RIR

54. 3 step technique (more reliable in fairly recent paresis) Has the patient got a right or left hypertropia? Is the hypertropia greater in right or left gaze? Is the hypertropia greater with a head tilt to the right or left? Based on Bielschowsky head tilt test (next week)

55. Hess Screen Plots Compare the size of the fields the affected eye corresponds to the smaller field – the involved eye shows the greatest under-action and smallest over-action compared with the non-involved eye Greater difference in size will be found in mechanical or recent onset neurogenic deviations In longstanding neurogenic defects the full sequelae is present – spread of incomitance = smaller difference in plots between eyes Mechanical vs Paralytic Strabismus Mechanical (and recent onset palsy) only shows first and second parts of the sequelae Mechanical deviations failure is more abrupt – making the defect small in the central field in comparison to the outer field

56. Lateral Rectus Greatest defect occurs on attempts to abduct the paretic eye Usually esotropia in the primary position Head turn towards palsied side

57. Medial rectus Isolated paresis is rare Greatest defect occurs when palsied eye adducts Usually exotropia in primary position Head turn is towards sound side

58. Superior Rectus Isolated palsy is usually congenital Paretic eye is affected in elevation and abduction Elevation is limited in the primary position, but normal on adduction Paretic eye is hypotropic in the primary position Frequent torticollis, but not of good diagnostic value (head held up and top palsied side or tilting of head to the paretic side) Usually associated with weakness of the levator (psuedo-ptosis – usually disappears when the paretic eye fixates) Contralateral inferior oblique will overact

59. Inferior Rectus Isolated palsy is rare and usually congenital Greatest deviation on attempts to look downward with the paretic eye in abduction Unopposed antagonist (SR) causes the paretic eye to be incyclotropic and hypertropic in the primary position Pseudoptosis in the sound eye Head turns down and to the paretic side with tilt to the sound side

60. Inferior Oblique Least likely of the muscles innervated by the 3 rd cranial nerve to be paralysed Greatest deviation when the patient attempts to elevate the adducted eye Overaction of the unopposed antagonist (SO) causes incyclotropia Nearly always congenital Head posture is fairly characteristic with head inclined towards the paretic side and face turned towards the sound side Often confused with Browns Tendon Sheath Syndrome

61. Superior Oblique Greatest defect with the eye adducting in depression Overaction of the antagonist inferior oblique causes the paretic eye to be hypertropic in the primary position Often secondary contracture of the IO occurs to a large degree so that it appears as the most prominent sign Head position is characteristically tilted towards the uninvolved side and the chin is depressed